pefc single cell test: setup preparationarakilab.ynu.ac.jp/summer2017/text/squadrito...

Consiglio Nazionale delle Ricerche10th International summer school on Advanced Studies of Polymer Electrolyte Fuel Cells

PEFC SINGLE CELL TEST:

SETUP PREPARATION

CNR-ITAE, salita per S. Lucia sopra Contesse, 5 – 98126 – Messina (Italy)

E-mail: [email protected] phone +39-090624274 fax +39-090624247

Gaetano SquadritoEnergy Systems Research Group

Istituto di tecnologie avanzate per l’Energia “Nicola Giordano del CNR (CNR-ITAE)

10th International summer school on

Advanced Studies of Polymer Electrolyte Fuel Cells

Yokohama National University

20th – 25th August, 2017


SUMMARY

• Introduction: PEFC Membrane Electrode Assembly (MEA)

• Gas Diffusion Layer (GDL)

• Catalyst Layer : Gas Diffusion Electrodes and Catalysed Membranes

• MEA Assembling

• The experimental set Up

• Concluding remarks


According to the IEC-62282-1* - Terminology

Polymer electrolyte fuel cell – PEFC - It is a fuel cell that employs a

polymer with ionic exchange capability as the electrolyte.

Usually the name PEFC (or PEMFC) is referred to H2 and reformate

gas fueled cells, but PEFCs can be feed with different fuels and can

have different applications. Each one of these require a different MEA

architecture to reach the better performances.

In recent years, increasing the PEFC operating temperature over 100

°C became a very important target, and alcaline version has been

proposed also. For this purpose new ion conducting membranes are

under evolution.

* IEC 62282-1 TS: Fuel cell technologies – Part 1: Terminology


Usually, the MEA component are prepared individually and then stacked and pressed together in a single piece.

Anode macroporous layer

Membrane

Anode Microporous layer

Anode Catalyst layer

Anode Gas diffusion layer

Cathode macro-porous layer

Cathode Microporous layer

Cathode Catalyst layer

Cathode Gas diffusion layer

Any single cell experiment on PEFC materials or components starts with the

Membrane Electrode Assembly (MEA) preparation, the PEFC heart.

Then our first target is a good quality MEA production,

by a repeatable preparation method.


MEA components’ functions.

We have to transport reactant fluids to the catalyst layer and toremove the products, in fluid form also. The fluids can be gases(H2, air, vapors), liquids (reactant/exhaust solutions, condensedwater), or mixing of the two.

Good electronic conduction mustbe assured between the reactionsites and the external circuit.

The electrolyte membrane mustguarantee electronic insulation,avoiding reactant mixing andallowing a good ion transport.

Electrode structure must maximise the catalyst utilisation.


PEFC have not unique architecture.

The architecture depends by the supplied fuel andoxidant, operating conditions, application and otheroperational factors.

While, the configuration can be resumed as a couple ofporous electrodes sandwiched on a solid electrolytemembrane based on a conducting polymer (acid or basic,usually H+,Na+ or OH-).

The ending question is: this MEA running or not?

The MEA testing will consist in performing a number oftests on it to verify its functionality, to study its responseby changing operative condition, to verify the long termstability …………….


The MEA manufacturing is a complex matter.

In the follow the discussion is centered on the most used

laboratory procedures, with just few notes about other

techniques.

For a wide and detailed description the following lectures are

suggested: the reviews published by Litster and McLean

(2004), Metha and Cooper (2003) and Wheeler and Sverdrup

(2008), or one of the fuel cell handbooks recently published.

Our target is the experiment, not material preparation.

Then only gas diffusion layer and catalyst layer

preparation, and MEA assembling are considered.


• Its function is to provide a ion conductive path,separating the reactant gases and avoidingelectronic conduction between electrodes.

• Usually it is proton conducting.

• The standard electrolyte material in PEFCsbelongs to the fully fluorinated Teflon®-basedfamily similar to that produced by E.I. DuPont(Nafion®).

• A continue effort is made in new membranesdevelopment, to reduce cost and/or increaseperformances, especially in direct fueled PEFC,were reactant cross over must be minimized, andin high temperature hydrogen PEFC, to increasestability.

For our purpose the membrane is a material to be used as it is supplied.

MEA materials: Polymer Electrolyte Membrane

Home made composite

membrane


• The function of the catalyst is to improve the reaction rate increasingcell efficiency. Usually, it is supported on a conductive material. Thisallow to increase the active area and to reduce the catalyst loading.

• In previous lessons catalyst production and characterization havebeen exhaustively treated.

For our purpose also the catalyst is a material to be used as it is supplied.

Materials: Catalyst

TEM image of a commercial

Pt/carbon catalyst

TEM image of a home made Pt on

carbon nanotube Catalyst


Other Materials

In the following we will see that a number of other materials arenecessary for MEA preparation, within these:

• Solvents: usually alcohols and distilled/deionised water.

• Hydrophobic agents: Teflon® or FEP suspensions.

(these are used also as binder)

• Pore Formers: compounds that during components (GDL or Catalystlayer) preparation are inserted to create a network easy to beremoved after the MEA preparation leaving, in this way, a network ofpores / channels.

• Sealing materials for gaskets.


SUMMARY




• MEA Assembling




• It provides reactant gas access from flow-field channels to

catalyst layers, and passage for removal of product water

from catalyst-layer area to flow-field channels; including in-

plane permeability to regions adjacent to flow field lands;

Gas Diffusion Layer (GDL)

• It provides electronic and thermal conductivity between

bipolar plates and catalyst layer including in-plane

conductivity to regions adjacent to channels;

Gas

Water

ElectronLandChannel

Anode side scheme


• It provides also mechanical support to the MEA, especially

when thin membranes are used, and a pressure difference

between the anode and cathode gas channels is present.

Summarising the requested properties are:

Good mechanical properties, especially againstcompression.

Good electrical and thermal conductivity, especiallythrough the plane.

High porosity and good hydrophobic properties.

Chemical and mechanical stability in operativeconditions.


Usually it is constituted by 2 layers.

• Macro-porous substrate or support layers (SL);

• Micro-porous layer (MPL) coated onto the SL.

The addition of MPL onto SL improves the water

gas management of the whole GDL, and allow a

better contact between the Catalyst Layer and

the SL.

MPL typical pore size: 100-500 nm

SL typical pore size: 10-30 µm

Typical thickness: SL= 100-300 µm

MPL= 5-50 µm

Gas Diffusion Layer (GDL) - Structure

A Carbon fiber papernormally used as SL

Carbon /PTFE Microporous layer


As can be seen in scanning electron microscope images of two gas-diffusion-media substrates, the carbon-fiber paper is bound bywebbing (carbonized thermo-set resin), whereas no binder is neededin the carbon cloth due to its woven structure.

(a) Inner structure of aCarbon fiber paper, notwet proofed.

(b) Carbon cloth, Textron Avcarb1071 HCB. Not wet proofed

Not woven carbon fiber paper is widely used as SL,

woven carbon cloth is used also.


GDL and flow field interactstrongly, the GDL supply asecondary path for reactantgas flow and water removal.

Water condensate in macro-pore of SL and is removedby the secondary gas flowalso.

Visualisation of the crossflow between adjacentchannels in serpentineflow filed (left).

Resulting stream lines(down).

(CFD simulations carried out atCNR-ITAE)


Hydraulic driven flow model: theliquid water only enters the largestpores after hydrophobic surfacetension is overcome. The small poresremains free for gas transport. Smallpores of GDL will be filled only if theflow channel is flooded.

Scheme of hydraulic driven flow modelof liquid water in GDL (*)

Idealised schematic of the process (**)

(*) E. Kimball et al., AIChE J. 54 (2008) 1313

(**) S. Litster et al., J. Pow. Sources 154 (2006) 95


Step 1

• Carbon fiber paper fabrication by pre-pregging, molding, carbonization,

graphitization.

• Carbon cloth fabrication by carbonaceous fiber production, (fiber

weaving), fiber oxidation, (fiber weaving), graphitization

• Macro-porous support acquisition from producers.

GDL Fabrication

Step 2

• Support wet proofing (if requested).

• Micro-porous layer application (if requested) by screen printing, spray, roll

coating, and so on.

• Thermal treatment.


PTFE or FEP are usually used.

The most common application

method is the immersion or dip

coating:

• the diffusion media is dipped into a

polymer suspension,

• excess suspension is dripped off,

• the remaining solvent is removed

by oven drying,

• finally the green is heated up to

250-300°C to sinter the polymer

particles and fix the FEP/PTFE to

the fiber surface.

Wet Proofing

A representation of industrialdip coating process.

Usually, polymer

loading is controlled by

adjusting suspension

concentration.

To coat one side only of

the diffusion media,

techniques such as

spraying or air brushing

are well suited.


Micro-porous layer application

• An ink containing PTFE and carbon is prepared by stirring.

Content of 10 – 40 % of PTFE are used.

• The ink is applied to the SL.

• After drying, the GDL is (pressed and) heated above to

300°C to sinter the polymer particles and fix the PTFE to the

grains surface.

• Increasing PTFE content the hydrophobicity increases, but the

porosity is reduced.

• PTFE is an insulator, excess of PTFE results in electric

resistance increase.

• Pay attention, the micro-porous layer must be on the surface

of the SL, excess of penetration could reduce the effectiveness

of GDL.


SUMMARY




• MEA Assembling




The catalyst layer, or active layer, is

the location where electrochemical

reaction takes place. It is in direct

contact with the membrane and the

gas diffusion layer.

It can be prepared onto the gas

diffusion layer or on the membrane.

In either case, the goal is to place as

more catalyst particles (shown as

black ellipses) as possible, in close

contact to the ion conductor, by

maintaining good gas accessibility

and water removal paths.

Catalyst Layer (CL)


Carbon

Polymer

Catalyst

Water

Oxygen

Protons

PTFEElectrons

•Gases coming from GDL

must reach the catalyst

sites: porous structure.

What kind of structure is needed?

•At the anode, H2 generate

2H+ and 2e-: conductive

path for e- to GDL and for

H+ to the membrane.

•At the cathode, O2, 4H+ and

4e- must be at the same

place at the same time to

form water: conductive path

for e- from GDL and for H+

from the membrane.

• Produced water must be removed: drain channels path


Catalyst layer is made directly on GDL, catalyst particles are bonded

together by PTFE. In first formulations micro porous layer was not

present, this last was introduced in 1990-2000 for matching porosity

difference between the two layers, reducing mass transfer limitations.

Also the introduction of electrolyte inside the catalyst layer, by

brushing a inomer solution, was an evolution of first formulation.

This addition allowed a Pt loading reduction from 4 to 0.4–0.6

mg/cm2, with a Pt utilization of about 20%.

Teflon bonded structure

Nafion solution 5 wt%

Teflon-bonded catalyst layer

Carbon paper or carbon cloth

Teflon-bonded carbon layer

Proton Exchange Membrane


The present convention in fabricating catalyst layers for PEFC is to employ

thin-film method introduced by Wilson (1993).

In this method the hydrophobic PTFE traditionally employed to bind the

catalyst layer is replaced with hydrophilic perfluorosulfonic ionomer (Nafion).

Thus, the binding material in the catalyst layer is the same of membrane

material resulting in both ion conduction and Pt utilization increasing.

Teflon-bonded carbon layer

Carbon paper or carbon cloth

Nafion inter-mixed catalyst layer

Proton Exchange Membrane

Thin film structure

The thin film structure can be build on the GDL or directly on the membrane.


Two dimensional representation of the

catalyst layer structure.

(a) Content of ionomer too low: not

enough catalyst particles with ionic

connection to membrane.

(b) Optimal ionomer content: electronic

and ionic connections well

balanced.

(c) Content of ionomer too high:

catalyst particles electronically

isolated from diffusion layer.

Experimental works, at low temperatureswith conventional catalysts (10-30% Pt/C )showed the existence of an optimumNafion content of about 33 wt% (*).

(*) E. Passalacqua et al., Electrochimica Acta 46 (2001) 799


The catalyst layer can be applied directly onto membrane. The usual

procedure for CCM fabrication is the DECAL:

• the catalytic ink is applied onto inert support (blank);

• after drying we have the bake;

• the bake is hot pressed onto membrane;

• finally the blank is peeled out.

Catalyst Coated Membrane (CCM)


Layers preparation.

Both GDL and Catalyst layer are prepared starting from an ink:

for MPL of GDL a mix of carbon and hydrophobic/binding agent, withor without a pore former,

for CL a mix of Catalyst and Nafion, with or without a pore former orstabilising agents.

The steps to prepare these layers are:

• Ink preparation.

• Ink deposition of the desired surface (or surface coating).

• Layer stabilisation by drying and/or mechanical or thermal

treatments.


• Procedures for catalyst layer preparation are usually based on a

catalyst ink deposition generally obtained by spray, doctor blade, ink

jet, decal.

• The ink requested density depends on the deposition method and on

the substrate of application. But the components of the ink are the

same: catalyst (Pt on carbon), ionomer, solvent (water and

alcohols).

• Pore former can be added to increase catalyst layer porosity, and

surfactants can be added to stabilise the ink.

• Usually ionomer is supplied in alcoholic solution, a water wetting of

the catalyst before the ionomer solution addition is suggested.

Ink/paste preparation (catalyst layer)


Solution and colloidal methods

Due to its chemicalstructure, Nafion forms asolution in solvents withƐ>10, a colloidal solutionwith 3<Ɛ<10, and aprecipitate with Ɛ<3. (*)

(*) M. Uchida et al., J. Electrochem. Soc. 145 (1998) 3708

Typical solvent is isopropyl alcohol, Ɛ= 18.3, addition of water do notchange the situation (Ɛ= 80.1). => SOLUTION

Normal-butyl acetate (Ɛ = 5.01) was proposed => COLLOID

The two preparation methods are similar, but two different catalyst layer structure are obtained.


(*) S.-J. Shin et al., J. Pow.. Sources. 106 (2002) 146

Proposed structure for catalyst layer obtained by solution (a) andcolloidal (b) methods (*).

The catalyst layer thickness doubling from solution to colloid method.

Pt/C agglomerates increases from 550 to 736 nm, when sprayingapplication is used.


Deposition / Coating methods

Laboratory

Industry

Pictures realised at CNR-ITAE.


Each preparation technique have advantages and disadvantages.

Coating Methods Advantages Disadvantages

Automated Spray /

Air brushingGood performance

Ink loss

Low speed

Screen printing Low costLow Q.C.

Ink loss

Bar coating SimplicityLow Q.C.

Ink Loss

Slot Die High Speed CCM difficult

Ink jet printingGood deposition

control

Ink stability

Ink formulation

Today spay is largely used in laboratory practice, but if it is manual

the layer quality depends on the operator ability and experience.


3D printingToday there is a lot of interest in 3D printing. Up to now I have notinformation about large scale application of this technic to PEFC MEApreparation. There are some publications about PEFC and SOFCapplication.

• Advantages: 3D control of the architecture, good deposition control.

• Disadvantages: today the resolution is too poor for PEFC, necessityof a post treatment for template material removing, limits in ratioactive / template materials.

Thermo polymer based 3D printer.

Liquid ink based 3D printer.

Image sources: WEB.


(*) a possible mix: Water, isopropilic alcohol, pore former/plasticizer.

Catalyst layer preparation

Pt/C catalyst

(sometime with

5-10% PTFE)

MixingNafion solution

(ionomer solution)

Dripping drop by

drop in NBA solvent

Colloidal methodBasic inkSolution method

Addition of

Solvent solution (*)

Ultrasonic stirring

(30-90 min)

Ultrasonic stirring

(60-90 min)

Ink application

Spray, paint ….

Drying, thermal treatment, mechanical

treatment (cold or hot pressing)

Ink after stirring

Ultrasonic Bath

Treatment

Deposition

Thermal treatment


SUMMARY




• MEA Assembling




Usually MEA is obtained by hot pressing a sandwich amembrane between two GDE or a CCM between two GDL.

Hot pressing is applied to reach a better contact at the MEAcomponent interface and to obtain a single piece.

Hot pressing at 110 - 130°C for 2-4 minutes under acompaction pressure of 20-40 kg cm-2 is usual, but bothhigher and lower pressure, time and temperature conditionshave also been reported in literature.

MEA Production

Plates

Warming up

Inserting Components

and Closing Plates Applying Pressure


Please, verify accurately the press plate planarity andparallelism: we work with very thin layers.

Excess in pressure application will damage the components,principally the GDL. Carbon cloth is less sensitive than carbonpaper.

Membrane and catalyst layer can be also damaged by themixing pressure + temperature: excess of dehydration,structure changes.

Application of pre-gaskets to the membrane out of the activearea is suggested.

In this phase also gaskets could be built on or assembled.

Some suggestions


SUMMARY




• MEA Assembling




Section of single cell testing hardware

MEA Testing

To verify MEA performances we need to insert it into anappropriate embodiment.

Experiments on materials and new architectures are conductedin single cell before to go to stack application.


Electronic

Load

Temperature

Control

Gas conditioning

Flow

Control

Pressure

Control

H2

Air

O2

outlet

The test bench

Inlet

Bench control and

Data acquisition


A protocol is simply a recipe for performing something.

The experimental protocol.

Why writing a protocol?

• to be sure that we have both a clear idea of how we will do the

experiment and that we will have all the materials that are needed for

the experiment.

• A scientist usually writes his/her protocol in a laboratory notebook.

• Following the completion of the protocol, the next step is to perform

the experiment. This include: experimental apparatus preparation, data

acquisition and analysis.


• Purpose: the question/hypothesis we are trying to answer/verify.

• Materials: all (major) materials and instruments needed to carry out the

experiment.

• Methods: how many experimental sets, how will we measure the effect we

wish to study, how long will the experiment last, other…. Methods should be

explicitly stated or referenced so that a third person has all the information he

needs to know to be able to repeat our experiment and verify results.

Experimental protocol components

• Controls: the relevant control(s) to be performed before, during and after the

experiment. Remember: controls must not affecting the test results.

• Data Interpretation: What will be done with the data once it is collected?

Data must be organized and summarized (tables, graphs, statistic analyses) so

that the scientist himself, and other researchers can determine if the purpose

has been answered/supported or negated.


The experimental protocol have the same structure of a

scientific publication.

Writing a good testing protocol simplify result publication.

TITLE: scientific publication

Authors ………….

Abstract ……….

Introduction = Purpose

Experimental = Materials and methods

Results and Discussion = Data interpretation

Conclusions


• PEFC testing standard protocol are needed both to promote and quantify

scientific advancement, and to provide PEFC industries with a method to

qualify commercial products.

From Lab to Industry: Standards & protocol harmonisation

• European Union supported these efforts with FCTestNet (FP5) and FcTestQua

(FP6) projects under. Today a similar process is developing for SOFC and

Electrolysers (Reverse fuel cells). Other actors in this field are: USFCC,

NEDO, JARI

Testing procedures

definition

Testing procedures

validation


The Technical Committee 105 of the International Electrotechnical

Commission (IEC) since 2000 started the development of a series

of normative and recommendation documents regarding the fuel

cells systems (mainly about safety and testing).

For PEFC testing: “Fuel cell technologies – part 7.1: Single cell test methods for

polymer electrolyte fuel cell (PEFC)” [IEC/TS 62282-7-1 ].

In this document the instrumentation requirements and reference procedures for

testing methods and results reporting are defined.

You are suggested to consider this document as a reference for the testing methods and

procedures to be used in your experimental protocol.

The 2016 revision has been recently published.

Just for information, the Technical Specification “Fuel cell technologies – part 7.2:

Single cell/stack performance test methods for solid oxide fuel cells (SOFC)”

[IEC/TS 62282-7-2], has been published in 2015 (first edition).


SUMMARY




• MEA Assembling




CCM based

Support layer

Carbon paper

Carbon cloth

ApplicationMP layer

Carbon + PTFE

GDLGDE based

Catalyst application to GDL

Pt/C + Nafion (+ PTFE)

Catalyst application to

Membrane

Pt/C + Nafion (+ PTFE)

Membrane

Membrane addition GDL addition

Hot Pressing

MEA production


Reference materials and components

•To insulate the “effects” of the new component on PEFC weneed to insert it in a set of well know materials andcomponents.

It is useful to have a benchmark for each component and for the overall MEA.

•This will have two functions: supplying a reference level andallowing a periodic control of the experimental set up.

•In daily laboratory practice we will have to test “new”materials or components, then the MEA “must be” homemade.


CNR-ITAE, salita per S. Lucia sopra Contesse, 5 – 98126 – Messina (Italy)

E-mail: [email protected] phone +39-090624274 fax +39-090624247

Gaetano SquadritoEnergy Systems Research Group

Istituto di tecnologie avanzate per l’Energia “Nicola Giordano del CNR (CNR-ITAE)

pefc single cell test: setup preparationarakilab.ynu.ac.jp/summer2017/text/squadrito...

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