Electronic supporting information for:

Hydrogen Production with a Simple and Scalable Membraneless Electrolyzer

Glen D. O’Neil, Corey D. Christian, David E. Brown, and Daniel V. Esposito*

Dept. of Chemical Engineering, Columbia University, New York, NY 10027

*To whom correspondence should be addressed: de2300@columbia.edu

Table of contents:

S1. Preparation and evaluation of catalysts for hydrogen and oxygen evolution reactions.

S2. Characterization of voltammetric hydrogen sensors.

S3. Measurement of product gas crossover using gas-chromatography.

S1. Preparation and evaluation of catalysts for hydrogen and oxygen evolution reactions.

Titanium mesh electrodes were used as substrates for Pt catalyst electrodeposition. The substrates were first cleaned by application of a +2.5V pulse for 3 seconds, and a -2.5 V pulse for 60 seconds in 0.5 M H2SO4 to remove carbonaceous impurities and the TiO2 layer, respectively. By first cleaning the substrates electrochemically, the reproducibility of catalyst electrodeposition was improved. Note that a separate solution of H2SO4 was used for Ti cleaning, and was not used for characterization of the electrodeposited catalysts in order to eliminate possible contamination from Ti. Platinum electrodeposition was performed by cycling the potential between +0.3 and -0.7 V vs. Ag|AgCl in an electrolyte containing 3 mM K2PtCl4 in 0.5 M NaCl which was de-aerated for 30 minutes with purified N2 before electrodeposition. Representative CVs of the electrodeposition are shown in Fig. S1.

Figure S1: Representative electrodeposition curves for K2PtCl4 in 0.5 M NaCl on titanium mesh electrodes. Potential was swept between 0.3 and -0.7 V vs. Ag|AgCl, with the scan initiated at open circuit potential. The arrows indicate the effect of an increasing number of scans.

Figure S2: Optical image of platinized titanium mesh (left) and scanning electron micrograph of the same sample showing the morphology of the electrodeposited platinum nanoparticles.

The electrodeposited Pt catalyst was characterized using linear sweep voltammetry (10 mV s-1 scan rate). Tafel plots were constructed from the LSVs and are presented below (Fig. S3). The Tafel parameters were: βHER = 38.8 ±2.2 mV, jo,HER = 6.6 x 10-4 A cm-2; βOER = 133.2 ± 9.0, jo,OER = 1.8 x 10-7 A cm-2.

Figure S3: Tafel analysis of electrodeposited Pt/Ti catalysts. (a) HER and (b) OER in de-aerated 0.5 M H2SO4.

The stability of the catalysts was evaluated by repeated cycling using CV in 0.5 M H2SO4 at a scan rate of 100 mV s-1. Each catalyst showed excellent stability over the observed potential window, with high activity for HER and moderate activity for OER, as shown in Fig. S4.

Figure S4: Stability of HER and OER reactions on the platinized titanium mesh electrodes in de-aerated 0.5 M H2SO was evaluated by cycling the electrodes 100 times between -0.25 and +2.5 V vs. RHE.

S2. Characterization of voltammetric hydrogen sensors.

In order to measure hydrogen selectively in the presence of oxygen, the sensor electrode must be operated at a potential where the hydrogen oxidation reaction is favorable, but the oxygen evolution is not. Fig. S5 shows the results of a series of experiments conducted to determine the operating potential of the sensor electrode in order to measure hydrogen in the presence of oxygen. The experimental setup was identical to that presented in Fig. 6a of the main text.

The black curve is collected without a potential applied between the cathode and anode, and shows a CV which is characteristic of a Pt electrode in acid, with the hydrogen under potential deposition and desorption evident at ~+0.1 V vs. Ag|AgCl, and the Pt oxide stripping peak at ~+0.6 V. The red curve was collected with the sensor downstream of the cathode with -0.3 V vs. Ag|AgCl applied. With a reference electrode present, -0.3 V is sufficient to drive HER and produce a hydrogen-rich environment. This is clear from the red trace in Fig. S7, where an increase in anodic current from HOR is apparent throughout the entire potential window. When the same experiment is repeated with the sensor placed downstream of the anode, as shown in the blue trace in Fig. S5, an increase in cathodic current caused by oxygen reduction is observed below at potentials more negative than +0.5 V vs. Ag|AgCl. However, from +0.7 - 1.0 V the CV response is nearly identical to the control experiment. From the three curves in Fig. S5, it is clear that by operating the sensor at +0.8 V vs. Ag|AgCl, hydrogen can be measured selectively in the presence of oxygen.

Figure S5: Effect of hydrogen and oxygen on the cyclic voltammetry of the Ti/Pt sensor electrode in 0.5 M H2SO4. Scan rate: 100 mV s-1; flow rate: 4.3 mL s-1

In order to determine how the rate of hydrogen production effects the current measured at the sensor, the potential applied to the cathode and anode was varied from -0.24 V vs. Ag|AgCl where HER is kinetically limited, (Fig. S6, black curve) to -0.5 V where the reaction occurs rapidly (Fig. S6, green trace). By comparing the black, red, blue and green traces it is clear that as the overpotential for HER is increased the sensor current increases. The two traces at -0.4 V and -0.5 have similar limiting current values, likely do to saturation of H2 in the catholyte stream. A control experiment, where -0.5 V was applied to the cathode, and the sensor electrode was placed downstream of the anode shows a significant increase in noise, but maintains a constant current (grey trace). The important result of these experiments is that by operating the sensor at +0.8 V, hydrogen can be selectively measured in the presence of oxygen.

Figure S6: Effect of hydrogen concentration on the amperometric response of the Ti/Pt sensor electrode in 0.5 M H2SO4.

Section S3. Measurement of product gas crossover using gas-chromatography.

A schematic of the experimental setup used for collection of gasses is shown in Fig. 7a in the main text. Two calibrated glass tubes (McMaster-Carr) were fitted with rubber Septa and placed over each of the outlets from the electrolyzer. The glass tubes were fitted with gas-tight rubber septa, which allowed for sampling of the product gasses using a gas-tight syringe (Hamilton). GC measurements were performed using a gas chromatograph using a thermal conductivity detector (TCD) using He as a carrier gas. Method calibrations were performed by sampling different volumes using Hamilton gas-tight syringe with pure hydrogen (Airgas, 99.9999%). The calibration points were 1, 5, 25, 50, 75, 100% H2 with the balance consisting of air from the laboratory ambient.

Figure S7: Chromatograph showing the H2 peak on both the cathode and anode side collected at 25.2 cm s-1.

The peaks for hydrogen (tr = 12.25 min) were integrated and converted to concentration using the method calibration. The concentrations of hydrogen in the hydrogen and oxygen sides were used to calculate product crossover according to the method used by Hashemi et al.:1

(S1)

Typically, gas crossover is defined as the amount of O2 in H2 tube to the total amount of gas in H2 tube. Thus, our measured value of 97.2% should serve as a lower bound of the H2 purity, since the relative pressure and diffusion coefficient of H2 are larger compared to O2. Additionally, the experiment was conducted with a recirculating electrolyte stream in which small H2 bubbles that are not captured in the H2 collection tube have a 50% chance of ending up in the anode collection channel on subsequent passes.

Reference

1. S. M. Hosseini Hashemi, M. A. Modestino, and D. Psaltis, Energy Environ. Sci., 8, 2003 (2015).

1

0 1

-0.10

-0.05

0.00

0.05

(a)

Potential / V vs. RHE

log |J| / mA cm

-2

-0.4-0.20.00.20.40.6

1.64

1.66

1.68

1.70

1.72

1.74

1.76

1.78

1.80

(b)

Potential / V vs. RHE

log |J| / mA cm

-2

-0.50.00.51.01.52.02.5

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Cycle 1

Cycle 10

Cycle 25

Cycle 50

Cycle 100

J

/ A cm

-2

E / V vs. RHE

-0.20.00.20.40.60.81.0

-0.3

-0.2

-0.1

0.0

0.1

Control

In catholyte

In anolyte

sensor current / mA

potential / V vs. Ag|AgCl

202530354045

0.0

0.1

0.2

0.3

E

app

for HER

-0.24 V

-0.3 V

-0.4 V

-0.5 V

control

sensor current / mA

time / s

12.1512.2012.2512.3012.35

-15

-10

-5

0

H

2

side

O

2

side

signal / mV

retention time / min

-0.50.00.5

-4

-3

-2

-1

0

1

2

3

current / mA

E / V vs Ag|AgCl

Increasing number

of sweeps