journal of electrochemical science and engineering

ISSN: 1847-9286 Open Access Journal www.jese-online.org

Journal of Electrochemical

Science and Engineering

J. Electrochem. Sci. Eng. 3(3) 2013, 91-135

Volume 3 (2013) No. 03 pp. 91-135

IAPC

http://www.jese-online.org/

J. Electrochem. Sci. Eng. 3(3) (2013) 91-135 Published: June 1, 2013

Open Access : : ISSN 1847-9286

www.jESE-online.org

Special issue dedicated to the 6th European Summer School on Electrochemical Engineering Guest Editor: Zoran Mandić, University of Zagreb, Faculty of Chemical Engineering and Technology, Zagreb, CROATIA

Contents

EDITORIAL ALIYA TOLEUOVA, VLADIMIR YUFIT, STEFAAN SIMONS, WILLIAM C. MASKELL and DANIEL J. L. BRETT A review of liquid metal anode solid oxide fuel cells (Review) ............................................................................ 91

QUENTIN MEYER, SIMON BARASS, OLIVER CURNICK, TOBIAS REISCH, DANIEL J. L. BRETT A multichannel frequency response analyser for impedance spectroscopy on power sources ............... 107

STEFANIA GIORDANO, MARIANGELA LONGHI, LEONARDO FORMARO, HERMES FARINA and GIUSEPPE DI SILVESTRO Electrochemical behaviour of PES ionomer and Pt-free catalyst for PEMFCs .......................................... 115

ANNABEL FERNANDES, EDITE CATALÃO, LURDES CIRÍACO, MARIA J PACHECO and ANA LOPES

Electrochemical treatment of leachates from sanitary landfills ............................................................. 125


J. Electrochem. Sci. Eng. 3(3) (2013)


www.jESE-online.org

EDITORIAL

This issue of the Journal of Electrochemical Science and Engineering is dedicated to the 6.

European Summer School on Electrochemical Engineering (ESSEE6) held in Zadar, Croatia,

September 16-21, 2012. As a triennial event the European Summer School of Electrochemical

Engineering aims to raise awareness of the importance of electrochemical engineering in the

various aspects of technological applications as well as to help students and engineers to

successfully meet the challenges they encounter in their careers.

Thanks to the participation of world-recognized professor and teachers, a warm and stimulating

atmosphere was created in Zadar, enabling students and other participants to acquire basic and

advanced knowledge and skills of electrochemical engineering theory and practice. Close to 70

students, mostly from European countries but also from other parts of the world, participated in

ESSEE6.

Most of the participants exhibited their current work in the form of poster presentation which

attracted a great interest among professors and other students. The posters showed a content of

high scientific level and revealed how much efforts students had to made in pursuing their work

and career. The topics of their work covered almost all fields in the contemporary electrochemical

engineering including electrochemical energy storage and conversion, electrochemical engineering

in the environmental protection, corrosion engineering, industrial electrochemistry and

electrochemical reactor design.

In this issue of the Journal of Electrochemical Science and Engineering a small selection of four

papers representing students’ work is published. One review and three original scientific papers

give a general overview of the scientific content of the Summer school and prove high scientific

and engineering achievements of the authors. I wish authors and all other participants of ESSEE6 a

successful and brilliant career and a significant contribution to the field of electrochemical

engineering.

Zoran Mandić

doi: 10.5599/jese.2013.0032 91

J. Electrochem. Sci. Eng. 3(3) (2013) 91-105; doi: 10.5599/jese.2013.0032


www.jESE-online.org

Review

A review of liquid metal anode solid oxide fuel cells

ALIYA TOLEUOVA*,**, VLADIMIR YUFIT***, STEFAAN SIMONS*,****, WILLIAM C. MASKELL*,***, DANIEL J. L. BRETT*,

*Electrochemical Innovation Lab, Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK **School of Engineering, Nazarbayev University, 53 Kabanbay Batyr Ave, Astana 010000, Republic of Kazakhstan ***Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK ****International Energy Policy Institute, UCL Australia, 220 Victoria Square, Adelaide, Australia Corresponding Author: E-mail: [email protected]; Tel.: +44(0)207 679 3310; Fax: +44(0)207 383 2348

Received: December 31, 2012; Published: June 12, 2013

Abstract This review discusses recent advances in a solid oxide fuel cell (SOFC) variant that uses liquid metal electrodes (anodes) with the advantage of greater fuel tolerance and the ability to operate on solid fuel. Key features of the approach are discussed along with the technological and research challenges that need to be overcome for scale-up and commercialisation.

Keywords Solid oxide fuel cell; liquid metal anode; direct carbon fuel cell.

Introduction

The world’s growing energy demands have led to increasing environmental and resource

availability concerns. In this regard, stabilization of increasing anthropogenic CO2 emissions is one

of the most urgent issues associated with global climate change. According to the International

Energy Agency (IEA), fossil fuels will remain the primary energy sources, representing more than

75% of the overall growth in energy use from 2007 to 2030 [1]. Moreover, carbon-free energy

technologies are unable to meet global demands in the foreseeable future [2]. Therefore,

continued use of fossil fuels in a ‘low-carbon economy’ necessitates the reduction of CO2

emissions by improving process efficiency. Notwithstanding the reduction in greenhouse gas

emissions brought about by high efficiency, fuel cells generate carbon dioxide free from diluting

nitrogen which can be sequestered at lower cost compared with the exhaust products from most

electricity generation based upon combustion of fossil fuels.


mailto:[email protected]

J. Electrochem. Sci. Eng. 3(3) (2013) 91-105 LIQUID METAL ANODE SOLID OXIDE FUEL CELLS

92

While coal remains the most available and abundant fuel source around the world (60% of all

global sources), highly efficient fuel technologies are required to minimize the severe

environmental impact of electric energy production from coal [3]. In this regard, several studies

[4–7] have presented novel concepts for the direct electrochemical oxidation of carbon (coal or

biomass) for highly efficient electricity generation using Liquid Metal Electrode Solid Oxide Fuel

Cells (LME SOFCs). This new class of fuel cell offers the advantage of high efficiency with the

benefits of greater fuel tolerance, the concept being particularly well suited to the direct use of

solid fuel.

High temperature solid oxide fuel cells (SOFC) are highly efficient electrochemical energy

conversion devices capable of converting the chemical energy stored in gaseous fuels (including

hydrogen, alkanes, alcohols, alkenes, alkynes, ketones, etc.) into electrical power [8]. The

operational efficiency of such systems is higher than conventional heat engines since the process

is electrochemical and not constrained by the Carnot efficiency limitation. Typical efficiencies

obtained from SOFCs are in the range of 50-60% (LHV). The technology is highly scalable with

systems in the hundreds of kilowatts demonstrated. The SOFC is increasing attention as a clean

and efficient power-generating technology. However, durability and performance degradation due

to impurities contained in hydrocarbon fuels, such as sulfur and other trace elements, continue to

be a concern [4]. Contaminants may react with the anode materials via various mechanisms to

decrease electrochemical reaction rates by increasing charge transfer resistance and result in

mechanical failure of materials [9]. In order to mitigate the influence of fuel impurities on cell

degradation, the supplied fuel should undergo pretreatment using a variety of adsorbents and

filters [10]. Alternatively, substantial improvements in anode materials are necessary to make the

anode more tolerant of fuel contaminants. One approach to achieving this is to use a liquid metal

anode solid oxide fuel cell (LMA SOFC). This allows operation on a variety of carbonaceous fuels –

gaseous or solid, while remaining highly tolerant towards fuel contaminants.

Some of the earliest work on liquid metal electrodes was reported by Cell Tech Power, who

used liquid tin as their anode material of choice [5,6,11,12] and achieved a power density of 170

mW cm−2 on hydrogen and JP-8 fuel [4]. The projected efficiency of such a system operating on

coal is 61 % [13].

However, LMA SOFC technology is not yet well-understood. The nature of the cells makes

experimental investigation challenging and the processes occurring in the melt are difficult to

study in situ. There is a certain inconsistency between published studies in terms of the

experimental arrangement and little is known about the mechanism of fuel oxidation within the

liquid metal media.

Being a comparatively novel system, LMA SOFC technology is some way from being fully

commercialized due to technical challenges unique to this class of fuel cell. The objective of this

review paper is therefore to provide a summary of emerging LMA SOFC technologies, discuss the

advantages of the approach and set out the research and technological challenges that need to be

addressed and overcome.

Background to fuel cells

Fuel cells are electrochemical energy conversion devices that convert chemical energy in fuel

directly into electricity (and heat) without involving the process of combustion. The technology is

highly efficient, can be applied to a range of fuels (depending upon the type of fuel cell), quiet in

operation (the fuel cell itself has no moving parts) and scalable from mW to MW. As such, fuel

A. Toleuova et al. J. Electrochem. Sci. Eng. 3(3) (2013) 91-105

doi: 10.5599/jese.2013.0032 93

cells are considered as one of the most promising technological solutions for sustainable power

generation. Fuel cells can be used in a broad range of applications, including: transportation,

residential combined heat and power (CHP), large scale distributed power generation and battery

replacement uses.

Fuel cells come in a range of architectures and material sets, but central to all is the electrolyte

onto which there is attached the anode and cathode electrodes on each side. Figure 1 illustrates

the simplistic operation of a fuel cell (using the SOFC as an example). The cell is constructed from

two porous electrodes separated by an electrolyte. Oxygen or air is supplied to the cathode (also

referred to as the ‘air electrode’) where electrochemical reduction of the oxygen takes place to

form oxide ions (O2-) that migrate through the electrolyte, to the anode (also referred to as the

‘fuel electrode’), and oxidize the fuel (hydrogen in this case) releasing water, heat and electrons

that travel around an external circuit to the cathode and do useful work.

Figure 1. Schematic operation of an individual fuel cell

(taking a solid oxide fuel cell as an example).

Different types of fuel cells

Fuel cell types vary from one to another in operating parameters and technical characteristics

(e.g. power density, efficiency, etc.). However, the fundamental feature of the fuel cell which is

different, and indeed names the fuel cell type, is the electrolyte. There are at least five main fuel

cell types: Phosphoric Acid Fuel Cell (PAFC), Proton Exchange Membrane Fuel Cell (PEMFC),

Alkaline Fuel Cell (AFC), Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC). In the

first two types, hydrogen ions (protons) flow through the electrolyte from anode to cathode to

react with oxygen, and produce water; whereas in the last three types (Alkaline, Molten and Solid

Oxide fuel cells) anions (OH-, CO32-, O2-) migrate through the electrolyte from cathode to anode to

react with fuel and similarly produce water (Figure 2). Fuel cell electrolytes are electronically

insulating but ionically conducting, allowing certain types of ions to transport through them.


94

Figure 2. Flow of ions through different fuel cell electrolyte types.

Fuel cell applications

The most common types of fuel cell systems and their main characteristics, including fuel

efficiency, operating temperature, lifetime, etc. are summarised in Table 1. The type of fuel cell

and the range of operating temperatures are primarily related to the electrolyte material. Hence,

the first three types (AFC, PAFC, and PEMFC) are considered as low temperature fuel cells, and the

other two types (MCFC and SOFC) require high temperature. Low temperature fuel cells have the

advantage of rapid start-up time but necessitate precious metal electrocatalysts and high purity

hydrogen as fuel.

Table 1. Technical characteristics of the five main fuel cell types (1 - [14]; 2 - [15]; 3 -[16])

Fue

l Ce

ll Ty

pe

Op

era

tin

g

tem

pe

ratu

re 1

,2,C

0

Po

we

r d

ensi

ty1

,2,

mW

/cm

2

Typ

ical

sta

ck s

ize

3

Fue

l Eff

icie

ncy

2 , %

CO

to

lera

nce

1

Life

tim

e2

× 1

00

0, h

r

Applications2,3; Capital Cost2, $/kW

Advantages3 Disadvantages3

PAFC 150-200 150-300 400 kW 100 kW module

55 Poison (<1%)

> 40 Distributed Power; 3000 $/kW

- Higher temperature enables CHP;

-Increased tolerance to fuel impurities

- Pt catalyst; - Long start up time; - Low current and

power;

PEMFC 50-100 300-1000

(3502) < 1-100kW 45-60

Poison (<50ppm)

> 40

- Backup power; - Portable power; - Distributed gene-

ration; - Transportation; - Specialty vehicles;

>200 $/kW

- Solid electrolyte reduces corrosion & electrolyte mana-gement problems;

- Low temperature; - Quick start-up

- Expensive catalysts; - Sensitive to fuel

impurities; - Low temperature

waste heat

AFC 90-100 150-400 10-100 kW 40-60 Poison

(<50ppm) > 10

- Military; - Space; >200 $/kW

- Cathode reaction faster in alkaline electrolyte, leads to high performance;

- Low cost components

- Sensitive to CO2 in fuel and air;

- Electrolyte management

MCFC 600-700 100-300 0.3 – 3 MW

300 kW module

60-65 Fuel > 40

- Electric utility; - Distributed power

generation; 1000 $/kW

- High efficiency; - Fuel flexibility; - Can use a variety of

catalysts; - Suitable for CHP;

- High temperature corrosion and breakdown of cell components;

- Long start-up time; - Low power density

SOFC 800-1000 250-350 1kW-2MW 55-65 Fuel > 40

- Baseload power generation;

- Auxiliary power; - Electric utility; - Distributed

generation; 1500 $/kW

- High efficiency; - Fuel flexibility; - Can use a variety of

catalysts; - Solid electrolyte; - Suitable for CHP&CHHP; - Hybrid/GT cycle

- High temperature corrosion and breakdown of cell components;

- High temperature operation requires long start up time


doi: 10.5599/jese.2013.0032 95

Liquid Metal Electrode Electrochemical Systems

The generic construction of a LMA SOFC is illustrated in Figure 3. The arrangement is ostensibly

identical to a conventional SOFC except that the anode is a molten metal. Fuel is fed to the

electrode and oxidized products are generated in the same way. The difference is that both

gaseous and solid fuel can be fed directly and effectively converted with no pre-treatment stage.

Figure 3. Operation of liquid metal electrode reactor in fuel cell and electrolyser mode.

Figure 3 also shows the potential for operation in electrolyser mode. This concept has the cell

fed with CO2 and steam (H2O) that are electrochemically converted to synthesis gas (a mixture of

hydrogen and carbon monoxide), that can be used to make liquid fuel (i.e. via the Fischer-Tropsch

process) or used in a fuel cell directly [17].

Liquid Metal Anode SOFCs

The liquid metal electrode resides in a layer between the fuel chamber (gaseous or solid fuel)

and the solid electrolyte (Figure 4). The oxygen reduction reaction occurs at the

cathode/electrolyte interface resulting in oxygen ions, which then migrate though the solid

electrolyte, typically represented by yttria-stabilised zirconia (YSZ), to the liquid metal anode. The

O2- ions react electrochemically with the liquid metal, generating metal oxide which is the active

species for the oxidation of the fuel (gaseous or solid - carbon and hydrogen in the diagram),

producing carbon dioxide or water respectively. However, the exact mechanism occurring and the

species involved in the liquid metal anode media are not well defined and depend upon the metal

used.

The molten metal blocks direct contact of electrolyte with gaseous impurities and hence

reduces electrolyte degradation by inhibiting the reaction between contaminants and electrolyte

[4]. Moreover, the fuel contaminants can become a fuel source themselves as they undergo

electrochemical oxidation. All of the electrolyte surface in contact with the anode is available to

supply a flux of oxide ions for reaction - there is no need to engineer a complex ceramic metal

(cermet) electrode with optimized triple phase boundary (TPB) as is necessary in conventional

SOFCs [18,19].


96

Figure 4. Schematic showing operation of LMA SOFC with

(a) hydrogen and (b) solid carbon, as the fuel.

Operation of Liquid Metal Anode SOFCs

The liquid metal anode acts as an intermediary for the oxidation of fuel [4]. Taking a molten

metal (M) and hydrogen as a fuel, the reactions at the LMA-SOFC anode follows the generic

mechanism:

M + xO2- M[Ox]M + 2xe- Reaction (1)

xH2 + M[Ox]M M + xH2O Reaction (2)

Reaction (1) proceeds at the liquid anode-electrolyte interface, whereas reaction (2) occurs at

the fuel-anode interface. The oxide ions (O2-) are delivered from the cathode via the electrolyte.

[Ox]M is a state of oxygen within the liquid metal anode. The state of oxygen in the metal varies

with operating conditions, and hence ‘x’ is used to represent this uncertainty. When using tin,

oxygen may form oxide or suboxide or remain dissolved in the melt [20]. The state of oxygen may

vary, though it has no particular effect on the overall reaction, which is the sum of Reactions (1)

and (2):

H2+ O2- H2O+ 2e- Reaction (3)

Reaction (3) is applicable to any liquid metal anode SOFC system including Sn or Cu [21].

Thermodynamics of LMA SOFC operation

The maximum electrical voltage generated by an electrochemical system can be determined via

the standard equilibrium electrode potential (Eeq), expressed by the Nernst Equation (1) as follows:

0i i ilneq

RTE E v a

nF Equation (1)

Where ai and νi are the activities (fugacities for gases) and stoichiometric factors of the species i

(the stoichiometric factors νi taking positive and negative values for oxidised and reduced species

respectively), R is the molar gas constant, 8.314 J mol-1 K-1, T is absolute temperature and Eo is the

standard electrode (equilibrium) potential of the reaction that can be defined from the

equation (2):

00 rG RT

EnF

Equation (2)


doi: 10.5599/jese.2013.0032 97

where ΔG0 r is the Gibbs free energy change of electrochemical reaction, n – number of electrons

transferred during electrochemical reaction and F is Faraday’s constant, 96,485 C mol-1.

Figure 5 shows the standard equilibrium potentials of several metal-metal oxide couples as a

function of temperature. The data for the figure was generated using the chemical reaction and

equilibrium software HSC Chemistry 6.1 (Outotec, Finland).

It can be seen that the cell voltage is expected to decrease with increasing temperature, as a

result of the negative entropy change associated with the reactions. However, the voltage

obtained under operating conditions (with a net current flowing) is difficult to predict. In addition

to the usual loss mechanisms associated with reaction kinetics, ohmic and mass transport

limitations, cell voltage will depend on the actual reaction at the anode: whether it is directly with

dissolved fuel or the oxidation of the metal to oxide (electrochemical reaction) followed by

oxidation of the hydrogen by the oxide (chemical reaction).

If the reaction is directly with the fuel then conventional fuel cell reaction OCVs will hold, which

are generally higher than those for the reaction with the metal. Therefore, from a thermodynamic

perspective, it may be preferable for the metal to have a low affinity for oxygen and a high

solubility of fuel and oxygen.

Figure 5.Effect of temperature on standard equilibrium cell potentials for various metal

oxidation reactions.

Battery Effect

The use of LMAs can have an additional positive impact on fuel cell performance by creating the

so-called ‘battery’ buffering effect. It is well known that SOFCs (similar to other fuel cells) can be

detrimentally affected by power demand surges during operation, since such surges can cause fuel

starvation, which in turn may cause irreversible structural changes at the electrodes. The presence

of a certain amount of LMA acts as a metal-oxygen battery, where the metal itself is oxidised to

provide sufficient load [4,5,7]. The oxidised metal can be reduced again by the fuel fed into the

anode when normal load is resumed.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

500 700 900 1100 1300 1500

Stan

dar

d e

qu

ilib

riu

m p

ote

nti

al /

V

Temperature / K


98

Liquid Tin Anode SOFC

Liquid tin anode SOFCs (LTA SOFC) have been demonstrated with a variety of fuels including

gaseous, solid and liquid carbonaceous fuels that did not require any processing, reforming or

pretreatment to remove sulfur. The power density reported was >160 mW cm-2 with hydrogen,

and 120 mW cm-2 with JP-8 fuel[20]. Operated at 900 C, a liquid tin layer of 6 mm was exposed to

humidified H2 gas (3 % H2O). From the analysis of transient characteristics time, the oxygen (oxide)

diffusion coefficient of 1×10-3 cm2 s-1 was derived from test results. The value differs significantly

from previously measured literature values [20]. Oxide diffusivity is one of the most critical

fundamental parameter that determines cell performance. Inconsistencies in literature reported

values call for further work to develop robust methods for the determination of this parameter.

Physical and chemical properties of tin

Abernathy et al. make the case for tin as an ideal metal for LMA SOFCs [4]. A comparison table

(Table 2) summarises basic physical properties and abundances of various metals that can be used

as a liquid anode. The metal should have relatively low melting point (simplifying operational

conditions) and high boiling point. Tin has a low vapour pressure (9.87×10-4 Pa at 1000 C) and

hence, low volatility [12], i.e. the anode is not vaporised into the fuel exit stream during operation.

Being technically easier to operate with, Sn remains the most commonly used anode material for

such LA SOFC systems, though some researchers are seeking better anode materials and suggest

the use of metal alloys based upon tin.

Such factors as metal abundances in the Earth’s crust, annual production and associated prices

also have to be taken into consideration. Research studies so far have shown results working with

tin, copper and bismuth. Other metals may not be suitable due to hazards (Sb), high price (Ag) or

low abundance (In).

Table 2. Common metals properties and abundances, prices (adapted from [4])

Metal Tm / C Tb /, C Price*,

$/kg Abundance in Earth’s

crust, ppm Reported experimental studies using metal and/or metal alloys

Aluminum 660 2520 1.7 83,000 -

Antimony 631 1587 5.0 0.20 [22], [23]

Bismuth 271 1564 16.3 0.063 [21], [24–27]

Cadmium 321 767 2.7 0.10 -

Copper 1085 2563 5.2 79 [21]

Indium 157 2073 390 0.05 [23]

Lead 328 1750 1.7 7.9 Pb - [23];Sn-Pb alloy-[25];

Sn-Pb-Bi alloy-[25],

Tin 232 2603 13.9 2.5 [4–6], [11–13], [20], [24], [26–28]

Silver 962 2163 471 0.079 -

Zinc 420 907 1.7 79 - *The metal prices are taken from US Geological Survey Minerals Information Team, Mineral Commodity Summaries 2010

The following table (Table 3) shows the change of thermophysical properties of tin at a range of

temperatures, including the solubility of oxygen, hydrogen and sulfur. Electrical resistivity of tin at

1000 C has a reasonable value of 67.1 µΩ cm compared to the resistivity of nickel (53.1 µΩ cm)

used in conventional ceramic metal (cermet) SOFC anodes [12].


doi: 10.5599/jese.2013.0032 99

Table 3. Thermophysical properties of tin (adapted from [4])

Property Temperature, C Value

Surface tension 232 5.44 mN cm-1

Viscosity 232 1.85 mN s m-2

Expansion on melting 232 2.3 %

Density 800 6580 kg m-3

1000 6480 kg m-3

Resistivity 800 62.1 µΩ cm

1000 67.1 µΩ cm

Gas solubility-oxygen 536 0.000018 wt %

750 0.0049 wt %

Gas solubility- hydrogen 1000 0.04 cm3 H2 / 100 g Sn

1300 0.36 cm3 H2 / 100 g Sn

Gas solubility - sulfur 800 4.5 wt%

1000 8.0 wt%

The density of tin and nickel at room temperature are 7.28 g cm-3 and 8.90 g cm-3 respectively.

This makes tin an even more preferable metal on the basis of weight [12]. High solubility of sulfur

in the melt is an attractive property of tin, allowing ‘dirty’ fuels to be used without sulfur removal

or fuel reforming.

Fuels Investigated

Various fuels have been studied for LTA SOFCs including biodiesel [5], military fuels, such as JP-8

[6,11,13], gasoline, methane, etc. All these studies confirm the LTA SOFC fuel flexibility. Figure 6

summarises measured efficiency of LTA SOFC operated on different fuels [6].

Figure 6. Measured efficiency of LTA SOFC over the range of fuels [6]

Electrochemical Characterisation of LTA SOFCs

The first studies on LTA SOFC developed by Cell Tech Power LLC were reported in 1998 [12] and

subsequently mainly focused on applying LTA SOFC for distributed power generation using natural

gas. The next step was to test alternative fuels such as waste plastics and military logistic fuels

(e.g. JP-8) applicable for portable applications, followed by studies where biomass and coal are


100

considered to be directly converted in LTA SOFC. Tao et al. [12] initially developed the Gen 3.0 cell

and stack and demonstrated a power density of 40 mW cm-2 with JP-8 fuel. Cell performance was

significantly improved in the next generation cell - Gen 3.1, which was lighter by a factor of four.

The operational power density was 120 mW cm-2 with JP-8 fuel that contained 1400 ppm of

sulphur [12]. This study confirmed the operation of LTA SOFC without fuel desulfurization or

preprocessing. To overcome existing drawbacks of the previous cell design (Gen 3.0) an alumina

porous separator was introduced to increase fuel mass transport to liquid tin. Prior to testing with

JP-8, Gen 3.1 was tested with hydrogen to ensure proper cell operation (Figure 7 (left)).

Figure 7. Power curves for tin based cells operating on hydrogen and JP-8 (left);

durability test on JP-8 (V, W) (right) [12]

At equal operating conditions the cell fed with JP-8 showed 70-80% of the cell performance

with hydrogen: the maximum current density for hydrogen was 316 mA cm-2 and 220 mA cm-2 for

JP-8, corresponding to a maximum power density of 153 mW cm-2 and 120 mW cm-2 respectively.

In terms of durability, Gen 3.1 with JP-8 demonstrated an average efficiency of 41.3% at 76 mW

cm-2 for 1 hour. The whole durability test is illustrated in Figure 7 (right). The next generation of

cells aims to reach >200 mW cm-2 power density with the military fuels. To achieve this a more

detailed evaluation of separator properties such as porosity and stability for long-term operation is

still needed [12]. It has been pointed out that interactions between the liquid tin anode and fuel

have not been systematically studied and that better modelling and a fundamental study is

needed.

Direct conversion of biodiesel has been demonstrated using the LTA SOFC [5]. In an approach

similar to that adopted for JP-8, a tubular LTA SOFC cell was tested for 4.5 h with biodiesel (B100)

prepared from virgin and waste cooking oils [5]. Peak power and current density values of

117 mW cm-2 and 217 mA cm-2 and overall cell efficiency of 40% were reported. The results of the

study also verify the fuel flexibility of the LTA SOCF system.

An interesting approach of using a separate electrochemical reactor coupled to an external

chemical reactor with liquid tin anode SOFC was recently proposed by Colet Lagrille et al. [26]. The

system oxidizes carbon directly in fuel cell mode and produces H2 in the electrolyser mode. The

schematic of two separate reactors is illustrated in Figure 8. The approach has the advantage of

allowing the ‘combustion’ (fuel oxidation) process and the electrochemical oxidation of the melt to

be optimized in two separate reactors; and it also improves mass transport losses. The

disadvantage is that pumping of liquid metal between the reactors is necessary.


doi: 10.5599/jese.2013.0032 101

Figure 8. (a) Schematic of a LMA SOFC coupled to an external combustion reactor,

(b) Schematic of a LMA Solid Oxide Electrolyser (SOE) coupled to an external combustion reactor [26]

This study also reports a 2D model of a micro-tubular hollow fiber fuel cell to predict

distribution of oxygen concentration within the liquid tin anode [26]. The diffusion coefficient of

oxygen in tin was assumed, the authors highlighted the need for accurate determination of this

parameter for reliable model prediction.

Alternative metals and metal-alloy anodes for LME SOFC

Other metals and metal alloys have been demonstrated as interesting anode materials capable

of direct carbon oxidation in novel SOFCs. As summarised earlier in Table 2, the most common

liquid metal anodes are made of: tin, bismuth, antimony, lead, copper, as well as indium and

silver.

Liquid Copper Anode SOFCs

Successful performance of LME direct carbon fuel cell (DCFC) has been demonstrated using a

molten copper anode operating at 1373 K [21]. The overall conversion efficiency measured at

maximum power density was 62.5 %.

Design of this system was different to those discussed earlier in the way that the cathode-

supported YSZ tube was immersed into liquid copper media. Figure 9(a) shows the schematic

presentation of the system [Pt, Cr Cermet, Cu-O (l) +C // (Y2O3) ZrO2//La0.9Sr0.1 (Mn, Fe, Co)O3-δ O2

(air), Pt]. The fuel (coke particles) was added via an alumina inlet tube and stirred thoroughly with

a stirrer; the entire cell was suspended in a vertical furnace. The OCV of the system was measured

to be 1.2 V, and generated a maximum power density of 1.7 W cm-2 (2.25 A cm-2 at 0.75 V) as

shown in Figure 9(b).

Despite good overall performance, liquid copper SOFCs are still difficult to operate. The

relatively high vapour pressure of copper, compared to tin and other metals, is a challenge for

practical operation. In addition, copper anodes necessitate very high operating temperatures

(m.p. = 1357.8 K), which inevitably complicates operation of the system.


102

Figure 9: (a) Schematic of liquid copper anode SOFC,

(b) Polarisation and power density curves for liquid copper anode SOFC[21]

Liquid antimony

Liquid antimony has been assessed as an anode for DCFCs [29]. Operating at 973 K and

employing a Sc-stabilized zirconia (ScSZ) electrolyte, the system, incorporating an air electrode but

with no input of fuel, has demonstrated an open-circuit voltage (OCV) of 0.75 V for the redox

couple Sb - Sb2O3. Having an anode electrode resistance of 0.06 Ω cm2, the operated power

density was 350 mW cm-2. As expected, the addition of sugar char immediately increased the cell

voltage as the Sb2O3 was reduced with fuel, preventing formation of a metal oxide layer on the

electrolyte surface. Scaling up of such liquid antimony DCFC technology may require an increase in

the rate of reaction between fuels and antimony oxide, as some carbonaceous fuels (carbon black,

rice starch) have not demonstrated any interaction with liquid Sb [29].

Liquid bismuth

Liquid bismuth has been reported to have an inferior performance to tin anodes [25]. The OCV

was reported to be slightly lower than theoretical (440 mV): 424 mV with hydrogen supplied and

408 mV without [25]. Being ion-conductive, bismuth oxide may solve the problem with oxide

transport blockage through YSZ-anode media [26]. However, operating at lower power densities,

liquid bismuth SOFCs require in-depth investigation and are not considered to be a feasible

alternative to SOFCs with a tin anode [21,22] .

Other metal anodes for LME SOFCs

Indium (In), lead (Pb), antimony (Sb) [23] and silver (Ag) [30] have also been examined in

battery and fuel cell modes (if feasible) to test their anodic efficiency in LME SOFCs. Similar to tin,

the performance of indium was adversely affected by formation of metal oxide at the electrolyte

interface when higher current was drawn. This was not the case for Pb and Sb, where the melting

points of their metal oxides were lower than the operating temperature. Direct oxidation of coal

has also been proposed to operate in a liquid iron anode SOFC [31].

The application of a liquid silver anode for DCFCs has been experimentally explored by

Javadekar [30] who achieved an OCV of 1.12 V with charcoal. Liquid silver has lower melting point

and higher oxide solubility compared to other metals (e.g. copper) and therefore is advantageous.

Another approach was to use a Ag-Sb alloy, where the high solubility of oxide in silver might be

combined with the low melting point of Sb, to improve overall anode performance [30].

(b)


doi: 10.5599/jese.2013.0032 103

Metal alloy anodes

In an effort to optimize anode properties, various alloy systems have been investigated. The

work involving the pure metal anodes (bismuth) mentioned above was developed further to

explore anodes of liquid tin-lead and liquid tin-lead-bismuth alloys in battery mode followed by

direct oxidation of coal [25]. For the tin-lead alloy, performance degradation (from 11.7 mW cm-2

to 10.1 mW cm-2) occurred within 13 hours, with an increase in performance to 14 mW cm-2 when

coal was introduced. The addition of bismuth to tin-lead alloy decreased the overall anode

effectiveness.

Uncertainties and technological challenges of LME SOFC

Prior to commercialization of liquid metal anode solid oxide fuel cells, greater understanding of

the electrochemical reactions, redox and transport processes within liquid metal electrodes is

needed [20]. The following research challenges are particularly highlighted: the nature of oxidised

species in liquid metal electrode media; transport of oxygen or oxidized species in LME SOFC;

reaction kinetics between oxide and fuel within liquid anode; kinetics of oxygen transfer at liquid

metal anode - electrolyte interface; oxygen (oxide) diffusivity / transport through liquid metal

layer and solubility of oxide and fuels in the melt.

As argued by Abernathy et al. [4], improvement of oxygen transport within liquid metal media

can be achieved by increasing oxygen solubility through alloying. Wetting characteristics of liquid

metal with the electrolyte and fuel oxidation kinetics need also to be considered.

Most of the work so far has been focused on the testing of single cells, which limits its further

application. In this regard, it would be beneficial to model the performance of LME SOFCs stacks

and prototype based on derived design guidelines.

In order to be able to model and later design practical LME SOFC systems the information on

oxygen solubility and transport in liquid metals is essential as it is a central process that may limit

the performance of the fuel cell. Fundamental investigation of these interactions within a liquid

anode, as well as analysis of other physical metal properties with temperature (e.g. vapour

pressure, melting point, surface tension, contact angle, etc.) will be beneficial in designing and

building durable and stable fuel cells.

Other important factors requiring investigation include: aspects concerned with the

introduction of solid fuel into the melt; optimization of thickness of the melt; prevention of

unfavorable formation of metal oxide tending to result in blocking of the electrolyte; fundamental

studies concerned with transportation of the oxide from the generation zone at the interface with

the electrolyte to the reaction zone at the interface with the fuel (relevant aspects include melt

thickness, diffusion of oxidised species and convection in the melt); effect and optimization (which

may include surface structuring) of contact area between the liquid anode and electrolyte.

Engineering considerations include: matching of electrolyte composition to temperature of

operation; choice of electrolyte thickness based upon optimization between mechanical

properties and ionic conductivity; agitation of the melt; generation of suitable electrolyte surface

structures; cell design; scale-up of laboratory cells to pilot and then to commercial plant.

Conclusions

LMA SOFCs are a highly efficient option for direct electrochemical conversion of solid fuels into

electrical energy. A range of possible metal anodes have been reviewed with tin the material on

which most development work has been performed. Despite numerous studies, the understanding


104

of the reaction kinetics at liquid metal-fuel and liquid metal-electrolyte interfaces, as well as

oxygen diffusivity through the liquid metal layer needs further systematic investigation.

Acknowledgements: The authors wish to thank Nazarbayev University and the Government of Republic of Kazakhstan for the BOLASHAK International Scholarship for Aliya Toleuova and the EPSRC Supergen Fuel Cells programme (EP/G030995/1) for supporting Brett’s research.

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[8] S. Singhal, K. Kendall, High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications, Elsevier, Oxford, UK, 2003, p. 89–92.

[9] T. S. Li, W. G. Wang, J. Power Sources 196 (2011) 2066–2069. [10] D. J. Brett, M. N. Manage, E. Agante, N. P. Brandon, E. R. Brightman, J. C. Brown, I. Staffell,

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471.

© 2013 by the authors; licensee IAPC, Zagreb, Croatia. This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license

(http://creativecommons.org/licenses/by/3.0/)

http://creativecommons.org/licenses/by/3.0/

doi: 10.5599/jese.2013.0033 107



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Original scientific paper

A multichannel frequency response analyser for impedance spectroscopy on power sources

QUENTIN MEYER, SIMON BARASS, OLIVER CURNICK*, TOBIAS REISCH*, DANIEL J. L. BRETT

Electrochemical Innovation Laboratory, Centre for CO2 Technology, Department of Chemical Engineering, University College London, UK *Intelligent Energy Ltd, Loughborough, Leicestershire, UK Corresponding Author: E-mail: [email protected]; Tel.: +44(0)207 679 3310; Fax: +44(0)207 383 2348

Received: December 19, 2012; Published: June 12, 2013

Abstract A low-cost multi-channel frequency response analyser (FRA) has been developed based on a DAQ (data acquisition)/LabVIEW interface. The system has been tested for electric and electrochemical impedance measurements. This novel association of hardware and software demonstrated performance comparable to a commercial potentiostat / FRA for passive electric circuits. The software has multichannel capabilities with minimal phase shift for 5 channels when operated below 3 kHz. When applied in active (galvanostatic) mode in conjunction with a commercial electronic load (by discharging a lead acid battery at 1.5 A) the performance was fit for purpose, providing electrochemical information to characterize the performance of the power source.

Keywords LabVIEW, multi-channel impedance, electrical circuit, fuel cell, lead acid battery.

Introduction

Electrochemical impedance spectroscopy (EIS) is a powerful diagnostic technique that has been

the subject of significant technical development over the last fifty years [1–3]. It has proven to be

particularly useful in the fields of electrochemistry, corrosion [4], primary and secondary batteries

[5,6] and fuel cells [7–9]. Advances in electronics and reduction in the cost of hardware has made

the technique increasingly popular. However, EIS is still mainly used as a high-end lab-based

diagnostic that requires relatively expensive hardware. This has hindered its uptake as a diagnostic

tool used in routine industrial analysis or for on-line measurements of electrochemical power

systems. For applications that would benefit from multichannel input (on different cells in a fuel

cell stack, for example) the cost and complexity multiplies. Therefore, there is a need for a low-

cost, modular approach for electrochemical impedance spectroscopy analysis.



J. Electrochem. Sci. Eng. 3(3) (2013) 107-114 FREQUENCY RESPONSE ANALYSER FOR IMPEDANCE SPECTROSCOPY

108

Alternative methods for performing multichannel EIS analysis have been attempted previously

using Matlab GUI Builder [10] and a built-in 10 channels frequency analyser. Single channel EIS

using commercially available DAQ (data acquisition) hardware and a LabVIEW interface has been

attempted for fuel cell applications [11], but multichannel metrology has yet to be reported.

This paper details the development of UCL-FRA, a low-cost EIS system based on the use of a com-

mercially available DAQ interface and LabVIEW software (DAQ/LabVIEW interface). Issues relating to

the successful application of the technique are discussed, results of its operation in galvanostatic

mode applied to a battery are presented and operation in multi-channel mode described.

Experimental

Software

The software used for data collection and processing was developed using LabVIEW 2011

(National Instruments) and was based on the use of VIs (virtual instruments) in the Sound and

Vibration Toolkit library for generating and analyzing AC waveforms. The coding methodology

required special consideration to account for low frequency detection, simultaneous sampling and

multichannel responses. The multichannel response was achieved by outputting single discrete

sine waves sequentially across the frequency range of choice, and measuring on 5 response

channels simultaneously. The software is currently capable of 5 response channels, but can be

modified to a higher number of channels to suit the application, provided that the performance of

the hardware is adequate to ensure robust data collection. The processed signals are displayed as

Nyquist and Bode plots in real time as the data is collected; the time for the software to process

the signal does not impact on the rate at which the measurement can be made. An entire

measurement from 10 kHz to 0.1 Hz with 10 points per decade, takes about 400 s, which is

comparable to other commercial devices.

The signal sampling frequency is limited by the hardware used. High frequency sampling

improves the accuracy of the measurement when analyzing high frequency signals. However,

when sampling low frequencies, where the minimum sampling time is the inverse of the signal

frequency, the data buffer becomes saturated. It is also necessary to respect the Nyquist-Shannon

principle for robust data sampling [12]; e.g. for a maximum bandwidth of 10 kHz, the sampling

rate cannot be smaller than 20 kS / s. In practice, to avoid aliasing, a clock speed 10 times greater

than the maximum speed of the measurement was used for each channel (100 kS/s). Therefore,

the DAQ hardware suitable for this application needs at least 500 kS/s, for 5 response channels.

Hardware

A USB data acquisition card (USB 6363, National Instrument) was used for data collection and

signal generation. The DAQ has 16 bit resolution, with 32 input channels (16 differential), 4 analog

outputs, a sampling rate of 1 MS/s and an output rate of 2.86 MS / s. It was used in the ±5 V range

(or lower), with a resolution of 153 μV (i.e. 10 V / 216) . All the response signals were recorded in

differential mode.

The AC perturbation was generated as a single sine wave, switching between discrete levels

from high to low frequency. The AC+DC stimulus and response signals are both recorded by the

DAQ and processed using a Fast Fourier Transform (FFT) to convert the signals from the time

domain to the frequency domain.

Figure 1 shows the set-up used in this work. Electrical impedance measurements on ‘dummy

cells’ were performed in potentiostatic mode, where the stimulus voltage is imposed to the

Q. Meyer et al. J. Electrochem. Sci. Eng. 3(3) (2013) 107-114

doi: 10.5599/jese.2013.0033 109

system and the current response is recorded using the 22 series resistor as a shunt resistor. The

electrochemical impedance measurements on the battery were performed in galvanostatic mode;

the stimulus current is controlled using an analogue control port on the load unit and recorded

using a current transducer.

Figure 1. Electrical connection schematic for (a) electrical and (b) electrochemical impedance spectroscopy. Black line represents the voltage stimulus from the DAQ; blue the voltage as a

proxy for the current and red the actual voltage from the cell.

Measurement procedure for passive components in potentiostatic mode

An electric circuit (dummy cell) was used for electrical impedance testing. This was composed

of a 22 Ω resistor R1 in series with an RC parallel combination (4.7 μF capacitor C1 and 200 Ω

resistor R2). This circuit was connected to the DAQ card to enable single and multiple channel

response. The series resistor R1 is used as a shunt to derive the current through the circuit.

Figure 2. (a) Electrical circuit comprising the ‘dummy cell’; (b) connections for measuring voltage across the shunt resistor in single and (c) multiple-input mode. AO: analog output. AI: analog input.


110

Measurement procedure for electrochemical impedance in galvanostatic mode

In order to trial the system on an electrochemical power source, a commercially available lead

acid battery (6 V, 10 Ah, Yuasa) was tested. An electronic load (6060B Agilent) with a bandwidth of

10 kHz was used to control discharge rate. The load is equipped with an analogue input capable of

remotely controlling the current that is passed [13], a 0-10 V signal from the DAQ can be

converted into a 0-60 A current by the load unit. A current transducer, with a frequency

bandwidth of 150 kHz (ITS-ULTRASTAB-60, LEM) capable of zero flux detection was used to record

the AC and DC current, and convert it to a voltage across a resistor. The voltage of the battery and

the voltage across the burden resistor are then recorded at the same time; consequently the

transfer function can be extracted, as described in Figure 1(b). The load unit has a bandwidth of 10

kHz, and therefore some attenuation could be expected for frequencies higher than 2 kHz. For

comparison with a commercially available high current potentiostat an IviumSTAT A11700 (10 V,

10 A, Ivium, Netherlands) was used. For both systems the AC perturbation was equivalent to

75 mA imposed across a frequency range of 10 kHz – 1 Hz.

Results and Discussion

Assessment of DAQ hardware as a function generation and frequency response analyser.

It is important to ensure that there are no artifacts or significant phase shifts introduced by the

hardware/software combination. Figure 3(a) shows that when the input and output of the DAQ are

connected together there is agreement between the two in terms of the frequency generated and

measured. Figure 3(b) shows a Bode plot of magnitude and phase. It can be seen that the

magnitude of the signal is constant across the range and that there is only a small phase shift

introduced above 1 kHz (<0.03 degrees at frequencies up to 10 kHz).

Figure 3. (a) Frequency generated versus recorded frequency sweep, from 10 kHz to 0.5 Hz. (b) Phase and Magnitude for the impedance between two channels recording the same signal.

Electrical impedance: single channel mode

The EIS system was tested in single channel mode on the circuit shown in Figure 2(b). Figure 4

shows Nyquist and Bode plots comparing the response obtained from the UCL system, the

commercial (Ivium) frequency response analyzer and the model simulation based on the

components used in the circuit. It can be seen that excellent agreement is obtained between each,

over the entire frequency range (10 kHz to 0.1 Hz). This demonstrates that the system is suitable

for performing EIS on passive (non power source) systems.


doi: 10.5599/jese.2013.0033 111

Figure 4. (a) Nyquist and (b-c) Bode plots for a single response channel using UCL-FRA and

commercial Ivium systems compared with the modeled transfer function. Frequency sweep between 10 kHz and 0.1 Hz, AC amplitude 0.1 V for a DC signal of 2 V with 10 frequencies per decade.

Electrical impedance in multi-channel mode

Operation in multi-channel mode imposes more of a challenge on the DAQ system. Lower cost

DAQ hardware uses a single clock that triggers each of the channels sequentially. A phase shift

between each of the channels is therefore possible at high frequencies and care should be taken

when introducing additional channels to the measurement. Higher end hardware, with dedicated

clocks on each channel, avoids this potential artefact. Figure 5 shows the response from 5

channels on the DAQ system (Z1 – Z5) connected to the circuit described in Figure 2(c). In the

Nyquist projection the system shows little difference between each channel across the full

frequency range, so allowing an accurate measurement of the real resistances in the circuit. In the

Bode projection it can be seen that some phase difference is introduced at high frequency.

However, below 3 kHz this difference is less than 3 degrees.

Overall, these results suggest that this DAQ/LabVIEW interface system can be used for multi-

channel EIS data acquisition over the frequency range of 3 kHz to 0.1 Hz.


112

Figure 5. (a) Nyquist plots and (b-c) Bode plots showing the response from 5 channels, sampled concurrently, using the circuit described in Figure 2(c) with UCL-FRA. Frequency sweep between

10 kHz and 0.1 Hz, AC amplitude 0.1 V for a DC signal of 2 V with 10 frequencies per decade.

Electrochemical impedance spectroscopy on power sources

Power sources are typically very low impedance devices and notoriously challenging for FRAs. A

lead acid battery was tested (in discharge) to assess the performance of the system in active mode

when incorporating an electronic load. Figure 6 shows the Nyquist and Bode (phase and

magnitude) responses for the UCL and commercial (Ivium) systems. Discharge from the battery

was performed in galvanostatic mode at 1.5 A using the set-up of Figure 1 (b). The measurement

was achieved with a lowest frequency of 1 Hz, to avoid discharging the battery too much due to

the longer measurement time at low frequencies.


doi: 10.5599/jese.2013.0033 113

The Nyquist plot shows that there is a difference (shift) in the real impedance between the

DAQ/LabVIEW system and that from the Ivium analyser. However, the form of the Nyquist plot is

very similar for each, exhibiting the shape of two depressed semi-circular arcs, representing two pro-

cesses with different time constants (RC combinations). This is typical of an EIS response for a bat-

tery of this kind [14]. The systematic difference in real resistance between the two (<10 m) is attri-

buted to the different leads and connectors used for each system causing different ohmic resistance.

The Bode plots in Figure 6(b-c) show that the two magnitudes are very similar across the range,

although the one recorded with the UCL-FRA exhibits a constant lower resistance associated with

the different lead configuration. The two phases remain in close agreement up to a frequency of

~3 kHz. Above this frequency, the bandwidth limitation of the load starts to become apparent.

Figure 6. Electrochemical impedance response for UCL-FRA and commercial FRA (Ivium) in the form of: (a) Nyquist, and Bode, (b) phase and (c) magnitude for a lead acid battery being

discharged at 1.5 A. Frequency sweep between 10 kHz and 1 Hz, AC amplitude 75 mA for a DC signal of 1.5 A with 10 frequencies per decade.


114

Conclusion

A DAQ/LabVIEW interface has been developed capable of performing frequency response

analysis for electrochemical impedance spectroscopy of low-impedance power sources. The

system was tested using electrical circuits to demonstrate that multi-channel data acquisition is

possible (here using 5 channels) without significant phase shift between the channels up to 3 kHz.

EIS on a battery discharging at 1.5 A compared favorably with a commercial FRA; the main

difference being a systematic shift in real impedance and high frequency inductance, both of with

can be attributed to differences in the cabling used for each arrangement.

This method is attractive for applications where a low cost solution is required for measuring

electrochemical power systems and a multichannel input is needed. For example, performing

individual cell EIS on fuel cell stacks or on-board diagnostics.

Acknowledgements: The authors would like to acknowledge University College London and Intelligent Energy Ltd. for supporting the work of Meyer, the EPSRC Supergen Fuel Cells programme (EP/G030995/1) and EPSRC Flexible Fuel Cell project (EP/G04483X/1) for supporting Brett’s research.

References

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doi: 10.5599/jese.2013.0035 115



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Electrochemical behaviour of PES ionomer and Pt-free catalyst for PEMFCs

STEFANIA GIORDANO, MARIANGELA LONGHI, LEONARDO FORMARO, HERMES FARINA and GIUSEPPE DI SILVESTRO

Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi, 19, 20133, Milano, Italy

Corresponding Author: E-mail: [email protected]; Tel.: +39-50314226; Fax: +39-50314300 Received: January 5, 2013; Revised: April 10, 2013; Published: June 12, 2013

Abstract Proton Exchange Membrane Fuel Cells (PEMFCs) represent promising technologies to the world economy, with many applications and low environmental impact. A most important aspect concerning their widespread implementation is the cost of polymeric membranes, typically perfluorinated membranes and platinum-based catalytic electrode materials, all of which are necessary to promote electrode reactions, thus increasing fuel cell energy efficiency. In this work, we present some data about non-fluorinated polyetheresulphone (PES) membranes and Pt-free catalysts, as possible substitutes of the above materials. Their electrochemical behaviour in oxygen reduction reaction in acidic media are investigated and compared with available reference materials.

Keywords ORR, Pt-free catalyst, PES, PEMFC.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) have achieved significant progress over the

past few decades. They are considered one of the most promising fuel cell technologies for both

stationary and mobile applications owing to their high energy efficiency, convenient operation,

and environmentally friendly characteristics. The main objective in fuel cell technologies is to

develop low cost, high-performance and durable materials [1-6]. At present, platinum is the best

cathode catalyst for oxygen reduction reaction (ORR) in PEMFCs; however, because of the scarcity

and cost of the metal, there is a strong effort to find alternative metals or alloys with similar

activity [7-10]. Promising advances have been made with new composites (non-precious

metals/heteroatomic polymers), pyrolysed metal porphyrins (cobalt or iron porphyrins viewed as

the most promising precursors) or bio-inspired materials. As they are capable of combining high

oxygen-reduction activity with good performance, these materials appear viable alternative

catalysts for ORR [11-17]. In PEMFC, catalysts are commonly supported on proton conductive



J. Electrochem. Sci. Eng. 3(3) (2013) 115-123 PES IONOMER AND Pt-FREE CATALYST FOR PEMFCs

116

membranes. The most common material of this kind is DuPont’s Nafion®, a perfluorinated

sulphonic acid polymer with many noteworthy features: good conductivity up to 80°C, which

decreases beyond 80°C by dehydration; however, this also has a high cost. Extensive research has

been done to produce cheaper membranes to replace Nafion®. New electrolyte membranes

obtained by grafting with styrene and sulphonic acid look promising and further development is

underway to improve their performance [18-20].

Considering the above mentioned critical aspects, in this work we are looking for new materials

for PEMFCs applications, i.e. non-fluorinated polymeric membranes and platinum-free catalysts.

As possible Nafion substitutes, we synthesised non-fluorinated polyetheresulphones (PES) with

different degrees of sulphonation. Polymers were obtained starting from different ratios of

sulphonated and non-sulphonated co-monomers. Varying the monomers ratio it was possible to

obtain polymers with different values of ion exchange capacity, swelling, hydrodynamic volume

and rigidity. PES membranes have been characterised by thermogravimetric analysis (TGA) and

differential scanning calorimetry (DSC) in order to study thermal stability and water retention. On

the side of Pt-free catalysts, the one presented below was synthesised by a sugar pyrolysis route in

the presence of a nitrogen precursor and a non-precious transition metal. Thiamine was chosen as

the precursor because, besides nitrogen, it also contains sulphur that may be a useful carbon-

doping element [21]. Iron was used as the transition metal. To increase catalyst surface area and

to control porosity in the mesoporosity range, a hard template method was adopted using a silica

gel in water [22]. Catalyst surface area and porosity were determined by BET and BJH theory,

respectively. The electrochemical behaviour of the PES membrane and thiamine catalyst in oxygen

reduction (ORR) conditions were investigated and compared to reference materials, i.e. Nafion

1100 and EC-20 catalyst (20 % Pt dispersed on Vulcan XC72R). An ORR study was carried on in

acidic media by cyclic voltammetry using a rotating disk electrode (RDE).

Experimental

Materials

Glucose, thiamine, Fe(II) acetate, glacial acetic acid, sodium hydroxide, silica (Silica Gel 60 HR),

HCl (37 %), H2SO4 (95-97 %), ethanol, N-methylpyrrolidone (NMP), dimethyl acetamide (DMAc),

and Nafion® 1100 (5 wt. % suspension) were purchased from Aldrich and used as received. HClO4

and K2CO3, dimethyl sulphoxide (DMSO) were obtained from Fluka and Carlo Erba, respectively.

High purity water from a MilliQ system (Millipore) was used. Nitrogen and oxygen (5.5 and 5 nines

respectively) were purchased from Sapio.

Catalyst synthesis

The Pt-based catalyst EC-20 was used as received. Home-made catalyst carbon was obtained by

the following procedure. Thiamine was added to a nearly saturated glucose-in-water solution

(1.68 mol L-1) in a 1:10 molar ratio. Thiamine dissolution was aided by equimolar glacial acetic acid.

Thereafter, acetate iron salt was added (0.96 wt. % on total amount of non-water reactants). Ten

millilitres of the solution was stirred with 4.3 g of silica in order to form a gel. Then, the suspension

was loaded in a quartz reactor, degassed with nitrogen and inserted in a preheated vertical oven

at 600°C for one hour to carbonise the precursors as fast as possible. Silica was removed in

3 M boiling sodium hydroxide followed by repeated carbon washing/filtering. Products were dried

in nitrogen (100°C, 24 hours) and finely ground. Materials were heat-activated in a second step at

900 °C (ramping at 6°C min-1, three hours standing) under constant nitrogen flow.

S. Giordano et al. J. Electrochem. Sci. Eng. 3(3) (2013) 115-123

doi: 10.5599/jese.2013.0035 117

Catalyst specific surface area and porosity

Surface area and porosity were determined using a TriStar® II 3020 apparatus (Micromeritics

Instrument Corporation system). Before measurement, samples were outgassed at 150 °C for

4 hours under nitrogen in a FlowPrep 060 degas system. Surface area was determined by low

temperature BET (Brunauer, Emmett and Teller) N2 adsorption. Pore size and pore size distribution

were calculated by the Barrett, Joyner, Halenda (BJH) method.

Polyetheresulphones (PES) synthesis

PES copolymers were synthesised by a co-polycondensation method in NMP solution at high

temperatures in the presence of K2CO3. Different ratios of sulphonated and non-sulphonated co-

monomers were used. PES membranes were prepared by casting from DMAc solutions.

PES thermal analysis

Thermal stability and PES water retention were determined by TGA (using a Perkin Elmer TGA

400 system, T = 25-900 °C, 20 °C min-1) and DSC (using a Mettler Toledo DSC 1 system,

T = 25-400°C, 20 °C min–1). Data were recorded in N2. Similar data were recorded for Nafion 1100.

Electrochemical characterization

Before measurement, all glassware was first washed in HCl (37 %), then H2SO4 (95-97 %) and

finally rinsed with MilliQ water. The working electrode was a rotating disk electrode (RDE-EDI 101

Radiometer) with a glassy carbon tip (cross-sectional area of 0.07 cm2) embedded in a Teflon

sheath. Before use, the carbon tip was gently cleaned with soft sandpaper, polished with diamond

powder (Aldrich) and finally degreased with ethanol. The thin film rotating disk electrode method

(TF-RDE) was used to immobilise small catalyst amounts onto the polished carbon tip. To improve

reproducibility the electrode was initially conditioned by cycling in O2 saturated solution for 50

min (10 mV s-1 without electrode rotation) in the potential range E = –0.275/+0.800 V vs. Ag/AgCl.

Measurements were thereafter recorded in a range of rotation rates (300–1600 rpm) at 5 mV s-1.

When performing measurements on Pt-containing-materials, a Pt counter-electrode was used

(0.6 cm2); a glassy carbon rod (5.5 cm2) was used instead for Pt-free materials. I/E recordings were

obtained by means of an Amel 7050 Potentiostat in a standard three-electrode electrochemical

cell with an Ag/AgCl external reference electrode in 3 M NaCl. All potentials are reported on the

Normal Hydrogen Electrode scale (NHE). Measurements were carried out in 0.1M HClO4 solution

at room temperature.

The electrode preparation consisted of two successive deposition steps: the first with a catalyst

suspension in water (ink) and the second with an ionomer solution as catalyst binder. In the first

step, 10 mg of catalyst was dispersed in 1 mL water and sonicated for 30 min; 7 µL of this mixture

was pipetted onto the electrode tip and dried in a bottom-up position under a tungsten lamp. In

the second step a few µL of an ionomer solution (see details below) were deposited onto the

catalyst layer. This operation was found to be critical, especially in the case of the homemade PES

ionomer which, besides being insoluble in water, was also found to be less effective in “gluing” the

catalyst layer onto the RDE tip. Because of solubility, organic solvents had to be used for PES;

these were also good wetting agents for the RDE Teflon sheath. This feature is unfortunate

because it may hamper the accurate determination of the ionomer mass per catalyst surface area,

depending on the more or less effective wetting by ionomer solutions of the Teflon sheath that

surrounds the graphite tip. Therefore, different ionomer deposition procedures were adopted

depending on the ionomer used. For Nafion, deposition was straightforward because water is the


118

Nafion suspension solvent and does not wet Teflon. For PES, several solvents were tested (DMAc,

NMP, DMSO). Among them NMP and DMSO were found to be suitable; for DMAc a compromise

had to be obtained by mixing DMAc with water in various ratios.

Detailed ionomer deposition conditions were as follows: for Nafion, 7 L of a Nafion solution in

water (3.5 10-4 g mL-1) was deposited on the electrode tip giving rise to a final Nafion loading of

3.51 10-5 g cm-2; for PES, 4 L of a PES solution (10-2 g mL-1) was used with a final PES electrode

loading of 5.7 10-4 g cm-2. The adopted PES solvent and solvent composition will be specified when

reporting electrochemical results.


Catalyst surface area and porosity

After heat-activation at 900°C, the catalyst surface area was about 800 m2 g-1, which is lower

than that after the first heat treatment at T = 600°C. Fractional pore volume distributions were

found to be essentially independent of heating temperature. The pore size distribution shows a

maximum (70 %) at about 40-50 nm; however, there was a sizeable presence of micropores

(10 %). The high percentage of 40 nm mesopores is presumably useful to ORR behaviour, providing

easy diffusional access to the reactive catalyst surface and to the Nafion ionomer units [23, 24].

PES and Nafion thermal analysis

TGA: To increase the PES water storing capability, the virgin polymer was conditioned in HCl

and HClO4 (compare the red and green curves in Figure 1). It can be noted that the two acidic

conditionings give indistinguishable results. Figure 1 also reports the PES behaviour without acid

conditioning (light blue curve). As seen in the figure, acidic conditioning definitely decreases the

polymer water retention. Figure 2 reports a comparison between PES and Nafion 1100, with both

of them HCl conditioned (red and blue curve, respectively). The most notable feature is the greater

PES mass loss in the respect of Nafion up to the crossing point of the curves, at T 480 °C. At

higher temperatures, the Nafion mass losses become much greater than the PES ones. In a further

aspect, the PES mass loss up to T 100-120 °C, which is the upper project temperature for PEMFCs

operation, is greater than for Nafion. At high temperatures, Nafion behaviour can be interpreted

by references to literature data. According to Wilkie et al. [25], Almeida and Kawano [26, 27]

fluorocarbon polymers exhibit high thermal stability and decompose by a first step (290–400 °C)

that may be associated with a polymer desulphonation process, then by a second step

(400–470 °C) related to side-chain decomposition and, finally, by a third one (470–560 °C), due to

the PTFE backbone decomposition. In comparison, Figure 2 shows similar thermal degradation on

PES after the completion of water removal at T = 200-220 °C. At present, no details are available

on PES decomposition processes.

DSC: Figure 3 shows some features of the virgin PES (see the black curve) and after acidic

conditioning (the red curve). In the black curve, a rather weak endothermic maximum extends

over a wide temperature range (T 50-170 °C, ΔH = -120 J g-1), followed at higher T by a second

sharp endothermal peak that, by comparison with a pure DMAc sample, is due to the removal of

residual reaction solvent (DMAc). The sample behaviour is strongly affected by acidic conditioning

(red curve). As the most relevant feature, a stronger endothermic process appears in the above-

mentioned temperature range (T 50-170°C, with a sharp maximum at T = 110 °C, ΔH = -480 J g-1).

By comparison with TGA results in Figure 1, the process is due to water removal. By the

integration of Figure 3 curves, the water amount released from the acid conditioned samples is


doi: 10.5599/jese.2013.0035 119

approximately four times greater than that from the unconditioned ones. At higher temperatures,

the water removal is followed by a second, more diffuse endothermic process ending at T 200°C.

In comparison, the Nafion behaviour (see Figure 4) is characterised by a sequence of

comparatively weaker endothermal (ΔH = -120 J g-1) processes with a first maximum at T 100°C

and a second, barely distinguishable one at T 180°C. By reference again to the TGA results in

Figure 1, both of these processes are attributed to water removal. Overall, from a PEMFC

operative viewpoint, the water amount retained in PES is favourably greater than in Nafion and is,

moreover, more thermally stable.

Figure 1. TGA curves of different PES acidic treatment

recorded at 20 °C min-1 from 25 to 900 °C in N2 atmosphere. (a) Light blue curve is relative to non-conditioned PES; (b) red curve for HCl conditioned;

(c) green curve for HClO4 conditioned.

Figure 2. TGA curves obtained in the same

conditions reported in Fig. 1. (a) Red curve is relative to PES membrane conditioned in HCl; (b) blue curve is relative to Nafion membrane conditioned in HCl.

Figure 3. DSC curves recorded from 25 to 400 °C,

heating rate of 20 °C min-1 in N2 atmosphere. (a) Virgin PES is the black curve;

(b) PES HCl conditioned is the red one.

Figure 4. DSC curves recorded in the same condition

of Figure 3. Membranes are HCl conditioned: (a) Nafion is the green curve; (b) PES is the red one.

Electrochemical characterisation

The following results report net oxygen reduction currents, corrected for background residual

currents recorded in N2. Results from different working electrodes (catalysts, ionomers) are

normalised with respect to the geometrical electrode surface area.

Nafion 1100 and EC-20 were used as external references to evaluate the homemade materials,

PES and Pt-free catalyst. Relevant reference data in O2 saturated solution are reported in Figure 5

for many RDE rotation rates. An extended region of limiting current is present and depends on the


120

RDE rotation rate. The onset potential is about 0.9 V vs. NHE, which is very near to the value of

massive platinum [28]. By comparison, Figure 6 reports ORR data for EC-20 catalyst with a PES

ionomer layer. For the steeper current increase with decreasing electrode potential, ORR results

obtained with Nafion are clearly better than PES ones. This difference may be due to the greater

loading of PES than Nafion (see the Experimental), or to a greater Nafion acidity/conductivity in

respect of PES. Despite many attempts, we were unable to decrease the PES amounts due to

increasing mechanical instability of the catalyst layer. As a further main feature, PES causes an

apparent slope decrease of j/E plots in the mixed control region (E 0.9-0.7 V), and lower limiting

current density values at more negative potentials. Figure 7 reports Koutecky-Levich plots for the

used Nafion and PES ionomer onto the used Pt catalyst (EC-20). To avoid graphical crowding, the

figure is only based on results in the limiting current region (E = 0.535 V vs. NHE).

Figure 5. Voltammetric data for EC-20 with Nafion binder. RDE rotation rates are shown in the figure

label. (0.1 M HClO4; v = 5 mV s-1; T = 25 °C).

Figure 6. Voltammetric data obtained for EC-20 with PES binder (DMSO only). RDE rotation rates

are shown in the figure label. (0.1 M HClO4; v = 5 mV s-1; T = 25 °C).

Nafion data are interpolated by a straight line with an almost exactly zero intercept at the axes

origin and a slope from which, also taking into account numerical uncertainties, the number of

stoichiometric exchanged electrons, n, ranges from 3.6-3.8.

By comparison, PES data give rise to a straight line with a similar slope, which is in accordance

with a comparable kinetic hindrance from diffusion in solution. From this slope, n is 3.8; however,

there is a greater intercept that, as mentioned above, might be due to the greater PES amount

necessary.

Figure 8 shows the Tafel plots of EC-20/Nafion and EC-20/PES electrodes obtained from

Koutecky-Levich plots for many potentials of the investigated range. For Nafion results, a first,

extended Tafel region, with a slope of ca. -60 mV dec-1, begins at somewhat more cathodic

potentials than the ORR onset and ends at E 0.83 V. A second, much shorter, Tafel section with a

higher slope (ca. -120 mV dec-1) can be specified, although with some difficulty, at more cathodic

potentials. These features are in accordance with data from the literature [27-30]. The PES

behaviour is characterised by a curved Tafel plot whose slope continuously increases with

increasing over-potential.

Figure 9 shows results for EC-20 electrodes with a PES binder layer deposited using various

solvents. As mentioned in the Experimental section, many attempts were necessary to balance a

good PES solubility in a given solvent with low affinity of the resulting solution for the RDE Teflon

sheath. As shown in the figure, results outline a single response in which the behaviour of each


doi: 10.5599/jese.2013.0035 121

electrode closely superimposes on the others. This uniformity in behaviour presumably outlines a

range of similar interactions between the PES ionomer, solvent and catalyst.

Figure 7. Koutecky-Levich plots for EC-20 catalyst

ORR currents at 0.535 V vs. NHE obtained (a) from Figure 5 for Nafion (blue line);

(b) from Figure 6 for PES membrane (red one).

Figure 8. Tafel plots for EC-20 catalyst obtained

(a) from Figure 5 for Nafion (blue curve); (b) from Figure 6 for PES membrane (red one).

Figure 9. Voltammetric data obtained for EC-20 with different PES/solvent deposition:

(a) yellow curve, DMAc/water 1:2 vol; (b) blue, NMP; (c) red, DMSO. (0.1 M HClO4; v = 5 mV s-1; T = 25 °C; ω = 1600 rpm)

ORR results for the thiamine sample catalyst with a Nafion binder layer are reported in Figure

10 for various RDE rotation rates. Although not exciting, this behaviour is acceptable overall, with

a noteworthy and unfavourable shift of the curves towards more negative potentials in respect of

the external reference EC-20.

Figure 11 reports the behaviour of the thiamine sample with a PES, instead of Nafion, binder. A

dramatic worsening in behaviour is observed in comparison to Figure 10. This becomes more

evident by comparison with a similar result for the PES/EC-20 electrode assembly (the relevant

curve of Figure 11 is taken from Figure 6).

In the last figure, PES in itself or the PES/solvent mixture used behaves as a real “killer” for the

homemade catalyst, while still affording an acceptable response when used on EC-20, even though

less satisfactory than Nafion. This brings to light a complexity of the many interactions that may

occur among separate components necessarily involved in building up a final, well behaving

PEMFCs electrode catalytic assembly.


122

Figure 10. Voltammetric data obtained for thiamine catalyst with Nafion.

RDE rotation rates are shown in the figure label. (0.1 M HClO4; v= 5 mV s-1; T = 25 °C).

Figure 11. Voltammetric data obtained for PES

membrane (in DMSO only) for (a) EC-20 (red curve) and (b) thiamine catalyst (black curve).

(0.1 M HClO4; v = 5 mV s-1; T = 25 °C; ω = 1600 rpm).

Conclusions

The present homemade catalyst, synthesised by the pyrolysis of sugar thiamine mixtures, is

characterised by unsatisfactory ORR features. It is, however, an element of the catalyst family that

is currently under investigation as a substitute for Pt.

Similarly, the reported PES sample belongs to a polymer class that is being investigated for

favourable water retention and thermal stability. Improvements may likely concern ionic

conductivity in PEMFC operational conditions.

It can be also mentioned that the present homemade catalyst and ionomer afford acceptable

results when separately tested with one or another reference material. Their ORR response fails

completely when used together. This shows that, in view of real PEMFC applications, many mixed

interactions have to be taken into account and optimised in detail.

Acknowledgements: Financial support from Cariplo Foundation (Project 2010-0588 “Non

fluorinated polymeric membranes and Platinum-free catalytic systems for Fuel Cells with low cost

and high efficiency” ) are gratefully acknowledged.

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Electroanal. Chem. 647 (2010) 29-34




doi: 10.5599/jese.2013.0034 125



www.jESE-online.org


Electrochemical treatment of leachates from sanitary landfills

ANNABEL FERNANDES, EDITE CATALÃO, LURDES CIRÍACO, MARIA J PACHECO and ANA LOPES UMTP and Department of Chemistry, University of Beira Interior, 6201-001 Covilhã, Portugal Corresponding Author: E-mail: E-mail:[email protected]; Tel.: +351275329259; Fax: +351275319730 Received: December 19, 2012; Published: June 12, 2013

Abstract The electrochemical treatment of leachate samples from a Portuguese intermunicipal sanitary landfill was carried out using anodic oxidation. The treatment was performed in a pilot plant that possesses an electrochemical cell, with boron-doped diamond electrodes, working in batch mode with recirculation. The influence of the applied current density and the flow rate on the performance of the electrochemical oxidation was investigated. Current density was decreased by steps, during the degradation, in order to study this effect on the efficiency of the process. For the assays run at equal flow rate and initial current intensity, chemical oxygen demand (COD) removal seems to depend mainly on the charge passed and the variation of the current density during the anodic oxidation process can reduce the energetic costs. An increase in the recirculation flow rate leads to an increase in the organic load removal rate and a consequent decrease in the energetic costs, but it decreases the nitrogen removal rate. Also, the bias between dissolved organic carbon and COD removals increases with flow rate, indicating that an increase in recirculation flow rate decreases the mineralization index.

Keywords Landfill leachate treatment; BDD; anodic oxidation.

Introduction

Leachate generation is an inevitable consequence of the deposition of solid wastes in sanitary

landfills. It is the result of rainwater percolation through wastes, that extracts and brings with it

several pollutant materials dissolved and in suspension [1]. Sanitary landfill leachate composition

is very complex and depends mainly on the type of solid wastes that are deposited, the climatic

conditions and the age of the sanitary landfill [2]. Inadequate leachate management involves

considerable risks, particularly contamination of water resources, at the surface and groundwater,

and soils [1].

A common treatment for sanitary landfill leachates comprises biological reactors with

nitrification/denitrification steps, followed by membrane technologies. However, due to variability



J. Electrochem. Sci. Eng. 3(3) (2013) 125-135 ELECTROCHEMICAL TREATMENT OF LEACHATES

126

in the quality and quantity of leachate throughout the life span of the treatment plant, these

conventional treatments become ineffective. Thus, it is necessary to implement technologies that

can be adjusted to the in situ needs [3]. Electrochemical technologies have shown high efficiency

in the elimination of persistent pollutants and several studies have described the application of

electrochemical methods in wastewater treatment [4-10].

A promising electrochemical method that can be used in wastewater treatment is the anodic

oxidation. Despite several different materials are being used as anodes in the oxidation of

persistent pollutants, the best results are obtained with boron-doped diamond (BDD) anodes, due

to their unique chemical, electrochemical and structural stabilities that allow their use at high

potentials, where most organic pollutants can be oxidized [11-13]. There are already several

reports describing the application of electrochemical oxidation with BDD anodes for the treatment

of landfill leachates [14-24].

Cabeza and co-workers [14,15] reported the application of electrochemical oxidation process,

using a BDD anode, to treat raw leachates and biologically and physicochemically pre-treated

leachates from a municipal landfill site. Experimental results showed very high chemical oxygen

demand (COD) and ammonia removals, although ammonia removal was slower than that of COD.

They also observed that when additional chloride ions were provided, the treatment efficiency

increased. Anglada and co-workers [17,20,21] also studied the effect of the applied current density

and of the initial concentration of chloride ions, as well as the influence of other operating

conditions, such as treatment time and initial pH, in the electrochemical oxidation of landfill

leachates, using a BDD anode. They have shown that when high current densities are applied, a

change in the mechanism of the organic matter oxidation occurs and that organic matter and

ammonia oxidation are highly influenced by the applied current density [17]. Also, they have

reported that the concentration of chloride has an effect on the oxidation of ammonia and that

chloride ions compete with organic matter to be oxidized at the anode. It was found that some

chlorinated organic compounds are formed as a result of organic matter oxidation and their

concentration increase continuously with treatment time [21]. Acidic conditions were found to

favour the formation of haloacetonitriles and haloketons. A kinetic modelling of the

electrochemical removal of ammonium and COD from landfill leachates was proposed in literature

[23-24]. Authors found that the use of BDD anodes promotes the generation of hydroxyl radicals,

while the high content of chloride induces the simultaneous formation of free chlorine,

responsible for the ammonium indirect oxidation and for the formation of undesirable products

such as chloramines, chlorate and perchlorate. Chlorine evolution is enhanced at lower COD

concentrations. During this process, ammonium removal leads to the formation of nitrogen gas

and nitrate as the main oxidation products.

In this work, the influence of the raw leachate dilution on the electrochemical degradation of a

biologically pre-treated leachate from a sanitary landfill, using a BDD anode, was assessed and it

has shown that mineralization of the organic matter improves with the dilution of the leachate

[22]. However, an increase in the dilution greatly increases the energy consumption.

The aim of this work was to study the influence of flow rate and applied current density, carried

out with multiple step electro-oxidation, on the performance of the electrochemical oxidation of

raw leachate from a sanitary landfill. The energy consumption in the different experimental

conditions tested was also assessed.

A. Fernandes et al. J. Electrochem. Sci. Eng. 3(3) (2013) 125-135

doi: 10.5599/jese.2013.0034 127

Experimental

The leachate samples used in this study were collected at a Portuguese intermunicipal sanitary

landfill site, in the equalization tank, before any kind of treatment. Samples characterization is

presented in Table 1.

Experiments were conducted in a semi-pilot plant operating in batch mode with recirculation,

at room temperature and natural pH, without adding background electrolyte. A BDD DiaCell 100

electrochemical cell, with an electrode area of 70 cm2, and a DiaCell-PS1500 power supply, with

automatic polarity reversal, were used. In all assays, automatic polarity reversal occurred every

minute. Different current densities, between 50 and 200 mA cm-2, and different flow rates,

between 100 and 950 L h-1, were tested in sample volumes of 5, 10 or 15 L. During the degradation

process, current density was kept constant or decreased by steps, in order to study this effect on

the efficiency of the process. Potential differences between anode and cathode were registered

throughout the experiments in order to determine energetic consumptions. All assays were

performed in duplicate.

Degradation tests were followed by chemical oxygen demand (COD), dissolved organic carbon

(DOC), total nitrogen (TN), total Kjeldahl nitrogen (TKN) and ammonia nitrogen (AN).

Table 1. Physicochemical characteristics of the raw leachate.

Parameter Medium value SD*

COD, g L-1 8.9 0.8

BOD5, g L-1** 1.3 0.3

BOD5/COD 0.15 0.05

DOC, g L-1 3.5 0.4

TN, g L-1 2.8 0.2

TKN, g L-1 2.4 0.2

AN, g L-1 2.2 0.3

cChloride / g L-1 4.5 0.3

cSuspended Solids / g L-1 0.7 0.1

cDissolved Solids / g L-1 16.6 0.1

pH 8.3 0.2

Conductivity, mS cm-1 29.1 1.0 *SD - Standard Deviation; **BOD5 – Biochemical oxygen demand

COD determinations were made using the closed reflux and titrimetric method [25]. DOC and

TN were measured in a Shimadzu TOC-V CSH analyser. Before DOC and TN determinations,

samples were filtrated through 1.2 µm glass microfiber filters. TKN and AN were determined

according to standard procedures using a Kjeldatherm block-digestion-system and a Vapodest 20s

distillation system, both from Gerhardt [25].


The effect of the applied current density on the rate of electrochemical oxidation was studied

by performing the electrodegradation assays at three different current intensities, 4, 7 and 14 A, at

a flow rate of 360 L h-1, and using leachate volumes of 15, 5 and 5 L, respectively. Figure 1 presents


128

the results of the normalized COD variation with time and with specific charge passed for these

electrodegradation assays. Specific charge was calculated as It/V, in C L-1, where I is the current

intensity, in A, t is the time, in s, and V is the leachate volume, in L.

Figure 1. (a) Normalized COD variation with time for the electrodegradation assays performed at different current intensities, at a flow rate of 360 L h-1. (b) Normalized COD variation with

specific charge for the electrodegradation assays performed at different current intensities, at a flow rate of 360 L h-1. Error bars refer to the standard deviation of the COD mean values.

It can be observed (Fig. 1a) that, for the assays performed with equal leachate volume COD

removal rate increases with current density, which points to electrolysis operating under charge

transfer control. In fact, for a single-compartment electrolytic reactor similar to the one used in

this work, operating at flow rates of 200 and 600 L h-1, mass transport coefficients, km, of 1.39x10-5

and 1.5x10-5 m s-1, for 200 L h-1, and 2.2x10-5 m s-1, for 600 L h-1, are presented in literature [26-28].

With these km values from literature, limiting currents between 10.4 and 16.5 A were obtained,


doi: 10.5599/jese.2013.0034 129

showing that at least two of the assays presented in Fig. 1 started at current limited control

conditions.

According to the model previously proposed in the literature for electrolysis under current

limited control [29], i.e., at maximum current efficiency, the trend of COD during electrochemical

oxidation can be predicted by Eq. (1), where F is the Faraday constant, 96485 C mol-1, and V is the

volume of the samples treated, in m3. Thus, theoretical slopes of COD vs. time, I/4FV, can be

calculated for each of the assayed current intensities.

0

ICOD COD

4 t t

FV (1)

The comparison between these theoretical slopes and the experimental curves (Fig. 1a), for

equal recirculation volume, shows that the discrepancy between experimental data and predicted

slopes slightly decreases with current density. This can be explained if one assumes that the

degradation process happens also by indirect oxidation. The increase in the leachate recirculation

volume also seems to contribute to an increase in the efficiency of the process, due to the lower

ratio electrode area/treated volume. In fact, when the volume is increased, keeping the same

anodic area, the quantity of the compounds that are more easily degraded and that behave ideally

augments. Thus, their concentration is kept higher for longer times when the recirculation volume

is increased.

The effect of applied current on the trend of the COD with the specific charge consumed during

the treatment (Fig. 1b) is less pronounced than the effect on the variation of COD vs. time. For

equal leachate volume, an increase in current density leads to a more efficient use of the electric

charge, since the experimental curve for 14 A is closer to the theoretical prediction. However,

since higher current densities imply higher potentials, although the electric charge is more

efficient the energetic consumption can be higher. Figure 1b also shows that an increase in the

leachate recirculation volume approaches the experimental results to the theoretical prediction.

To try to improve the current efficiency, assays were performed with successive decreases in

current density, by steps, during the oxidation process, at different recirculation flow rates. In

Figure 2 it can be observed the variation of normalized COD with specific charge for the assays run

at constant (14 A) and variable current density (5 h at 14 A + 5 h at 7 A + 10 h at 4 A), at a flow rate

of 360 L h-1. COD removal seems to depend only on the charge passed. Variation of normalized

COD with time (Fig. 2, inset) shows that during the first five hours, where the applied current

density was equal, no difference can be seen in the COD removal rate. But, when the applied

current density decreases, in the assays with steps, a decrease in the COD removal rate can be

observed.

The influence of the recirculation flow rate in the electrochemical oxidation performance of the

assays that were run with current density decreased by steps during the experiment was also

studied. Figure 3 shows the normalized COD variation with the specific charge consumed for the

assays performed with three or four current density steps at different recirculation flow rates:

three steps, 5 h at 14 A + 5 h at 7 A + 10 h at 4 A, flow rates of 160 and 360 L h-1, leachate volume

5 L; four steps, 4 h at 14 A + 4 h at 10.5 A + 4 h at 7 A + 4 h at 4 A, flow rates of 100 and 950 L h-1,

leachate volume 10 L.


130

Figure 2. Normalized COD variation with specific charge passed and with time (inset) for the

electrodegradation assays performed at constant and variable current intensity, at a flow rate of 360 L h- 1, with a leachate volume of 5 L. Error bars refer to the standard deviation of the

COD mean values.

The theoretical curves at these conditions are also presented. A slight variation in the trend of

the COD depletion was observed, pointing to better removals at higher recirculation flow rates.

The same behaviour is observed when normalized COD variation with electrolysis time is plotted

(Fig. 3, insets). The discrepancy between experimental values and theoretical curves, after the first

step of the assays, indicates a high loss in current efficiency that increases when current density is

further decreased during the steps process.

Comparing the discrepancy between experimental values and theoretical curves for three and

four steps, it can be concluded that charge efficiency is higher when four steps are applied,

although this fact must be also related with the higher recirculation volume of leachate used in the

four steps experiments.

Nitrogen removal was also assessed. In Figure 4 are plotted the normalized variation with time

of ammonia nitrogen and of total nitrogen. Both parameters present similar behaviour of that

described for COD in these assays, i.e., a decrease in the applied current density, in the steps

assays, leads to a decrease in the nitrogen removal rates. It can be seen that, for the experiments

performed at 14 A, an increase in the removal rate is observed after eight hours assay. This fact is

consistent with previous reports from other authors [23], which indicate that while BDD anodes

promotes the generation of hydroxyl radicals, the high content of chloride induces the

simultaneous formation of free chlorine, causing indirect oxidation of ammonium. In fact, this

leachate presents high chloride concentration (4.5 g L-1), thus enhancing the chlorine evolution at

lower COD concentrations, justifying the increase in the nitrogen removal rate when COD levels

are lower. In contrast to what was observed with COD removal, nitrogen removal is higher when

the recirculation flow rate is lower. In fact, at higher flowrates COD oxidation is favoured, chlorine


doi: 10.5599/jese.2013.0034 131

evolution, that is a competitive reaction, is delayed as a consequence and thus it influences and

slows down the rate of ammonium removal.

In order to analyse the energy consumption, the specific energy consumptions, Esp, in

W h/ g COD removed were calculated, by means of Eq. (2):

CODsp

UI tE

V

(2)

where U is the cell voltage, in V, resulting from the applied current intensity I, in A, t is the

duration of the electrolysis, in h, V is in m3 and COD is the removed COD, in g m-3, during t.

Figure 3. Normalized COD variation with specific charge and with time (inset) for

electrodegradation assays performed with (a) three current density steps, with a leachate volume of 5 L and with (b) four current density steps, with a leachate volume of 10 L. Error bars

refer to the standard deviation of COD mean values.


132

Figure 4. (a) Normalized ammonia nitrogen variation with time for the electrodegradation assays performed at constant current density and at three and four current density steps.

(b) Normalized total nitrogen variation with time for the electrodegradation assays performed at constant current density and at three and four current density steps.

Figure 5 reports the specific energy consumption as a function of the time for the different

assays performed. The specific energy consumption seems to increase with current density

(Fig. 5a), which is a consequence of the increase in potential when the current density is increased.

When constant current density was imposed (Fig. 5a), there is an increase in the energy

consumption during the first part of the assay, followed by a decrease. This behavior must be due

to the different types of compounds that are present and that are not degraded simultaneously,

being first degraded those that are present in higher concentration and, among them, those who

have higher diffusion coefficients. The introduction of steps, although leads to a overall decrease

in the energetic consumption, did not present the expected results in terms of specific energy

consumption, since it leads to more irregular consumptions rather than lower consumptions


doi: 10.5599/jese.2013.0034 133

(Fig. 5b). For these assays, an increase in the recirculation flow rate seems to slightly decrease the

Esp (Fig. 5c and 5d). On the other hand, the increase in the leachate volume being recirculated

really decreases the specific energy consumption, since the values in the yy’ axis are much lower in

Fig. 5d (10 L) than in Fig. 5c (5 L).

Figure 5. Evolution of specific energy consumption with time for (a) electrodegradation assays

performed at different current densities, at a flow rate of 360 L h-1 (b) electrodegradation assays performed at constant and variable current density, at a flow rate of 360 L h-1 (c)

electrodegradation assays performed with three current density steps at different recirculation flow rates, with a leachate volume of 5 L (d) electrodegradation assays performed with four

current density steps at different recirculation flow rates, with a leachate volume of 10 L.

The removals in COD, DOC, TN, TKN and AN for all assays performed with current density

decreased by steps, as well as the medium specific energy consumption, are presented in Table 2.

This table includes also the results obtained in the assay performed at constant current intensity of

14 A and 360 L h-1 recirculation flow rate, in order to allow comparison between assays performed

with and without reduction in the current intensity during the assay. The apparent discrepancy

between absolute and percentage values presented in Table 2 is due to the variation of the

experimental determinations of those parameters for the different assays, due to the complexity

and heterogeneity of the leachate suspension. Data reported confirm the previous analysis,

showing that for both multiple step designs, with 3 or 4 current density steps, and for a wide range

of recirculation flow rate, from 100 to 950 L h-1, an increase in the recirculation flow rate increases

COD removal rate and decreases nitrogen removal rate (TN, TKN and AN). Also, it can be seen that

DOC removals are always lower than COD removals and these differences increase with flow rate,

indicating that a decrease in the flow rate increases the mineralization index. Regarding the energy


134

consumption, an increase in the recirculation flow rate leads to a decrease in the medium energy

consumption, mainly because COD removal rate increases with recirculation flow rate.

Table 2. COD, DOC, TN, TKN and AN removals and medium specific energy consumption for assays performed with one, three and four current density steps at different recirculation flow rates.

Parameter

Experimental conditions 16 h (14 A)

V = 5 L; t = 16 h 5 h (14 A) + 5 h (7 A) + 10 h (4 A)

V = 5 L; t = 20 h 4 h (14 A) + 4 h (10.5 A) + 4 h (7 A) + 4 h (4 A)

V = 10 L; t = 16 h

360 L h-1 160 L h-1 360 L h-1 100 L h-1 950 L h-1

COD Removal g L-1

% 5.42 69

3.11 41

4.09 50

2.56 25

2.57 34

DOC Removal g L-1

% 1.35 44

0.50 19

0.94 30

0.61 15

0.18 6

TN Removal g L-1

% 1.23 48

1.04 39

0.93 35

0.59 19

0.36 15

TKN Removal g L-1

% 1.72 72

1.11 53

1.06 45

0.83 35

0.33 15

AN Removal g L-1

% 1.66 80

0.99 60

0.95 45

0.78 32

0.24 14

spE / kW h (kg COD)-1 90.1 106.0 80.9 55.7 49.5

Conclusions

The anodic oxidation was used to treat leachate from an intermunicipal sanitary landfill and the

following conclusions can be drawn:

Organic load removal rate increases with applied current density. This happens mainly

because, due to the high organic load content, the electrochemical processes are under

current control most of the assay period.

An increase in the recirculation flow rate leads to an increase in the organic load removal

rate. However, it decreases the nitrogen removal.

By reducing the current density along the anodic oxidation process it is possible to reduce

energetic costs. Similar results can be obtained by increasing the recirculation flow rate.

DOC removals are always lower than COD removals and these differences increase with flow

rate. Thus, a decrease in flow rate seems to increase the mineralization index.

Thus, although huge variations can be found in the composition of leachates from sanitary

landfills, the anodic oxidation, performed with a BDD anode, can be an alternative/complement to

treat this kind of wastewaters. Also, the variation found in the medium specific energy

consumption shows that it is possible to optimize the process in order to reduce energy costs.

Acknowledgements: Financial support from FEDER, Programa Operacional Factores de Competitividade – COMPETE, and FCT, for the projects PTDC/AAC- AMB/103112/2008 and PEst-OE/CTM/UI0195/2011 of the MT&P Unit and for A. Fernandes grant to SFRH/BD/81368/2011.

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