Impact of Cathode Current Collector on High Temperature PEM Water Electrolysis

1. Introduction

Polymer Electrolyte Membrane Water Electrolysis (PEMWE) is a hydrogen production method, which has advantages on conversion efficiency, compactness and gas purity. Also, PEMWE is expected to be applied to load leveling of electricity between consumer demands and supply from renewable energies ^[1].

  Higher conversion efficiency has been realized in two strategies: higher temperature operation with new composite electrolyte membranes and development of current collector. The new membrane can endure high temperature up to 120°C, and a high temperature operation can reduce overpotentials, leading to further performance^[2]. Developing the current collector is concentrated on anode side, major source of overpotentials exist. Optimizations of current collector on its hydrophobicity and stricture play a key role on water/gas management and improve performance^[3].

This study evaluates the current collector in cathode if it changes the performance of PEMWE especially under high temperature condition. As well know, high temperature tends to dehydrate PEM and results in low performance, although temperature dependence of non-leaner overvoltages predicts their reduction. This study aims to suppress the dehydration with possible candidates of current collector in cathode. Impact of hydrophobicity in cathode current collector on the dehydration is experimentally evaluated through IV characteristic and high frequency resistance.

2. Experimental apparatus and method

The PEMWE cell consists of a CCM, porous current collectors, and separators with flow channels. The CCM was fabricated by spraying catalyst ink on Nafion® 117 membrane and hot-pressing. Iridium oxide (IrO₂powder, type IV, Tokuriki Co., Japan) was used for anode catalyst, and a commercial 46% Pt/C (Tanaka Kikinzoku Japan) was for cathode catalyst.

  As for cathode current collector, carbon paper (SGL Co., Germany, 20BA, thickness 0.2mm) or SUS316L sheet (Nikko Techno, fiber diameter 35μm, thickness 0.3mm) was selected. The two collectors have different hydrophobicity. The carbon paper indicates rather hydrophobic characteristic. On the other hand, The SUS sheet shows opposite characteristic. In the case of carbon paper, two layers of it was built in the cell considering the thickness of seal material and contact resistances. Ti mesh (Nikko Techno Co., Japan, fiber diameter 20μm) was used for anode current collector.

3. Results and discussion

Fig.1 and Fig.2 show the measured I-HFR and I-V characteristics during PEM electrolysis with different cathode current collectors at 80°C and 100°C. The HFR in the case of carbon paper had lower value, suggesting lower ohmic resistance. Corresponding to I-HFR characteristic, the I-V characteristic in the carbon paper case indicated lower cell voltage. The cell voltage in the case of carbon paper is smaller than that in the case of SUS sheet.

As expected, the carbon paper, which has rather hydrophobic property, hydrates the PEM and make its ionic resistance low. The hydrophobic carbon paper placed in cathode may work to block the water transport driven by electro-osmosis from anode to cathode. It also enhances the back diffusion of water, resulting in the hydration of PEM and lower resistance.

  However, this mechanism, in which PEM is hydrated with hydrophobic carbon paper, is not enough to explain the cell voltage difference by 110 mV at 2 A/cm²appeared in Fig. 2. The ohmic resistance difference of 10 Ω*cm2 at 2 A/cm2 appeared in Fig.1 predicts a cell voltage difference 20 mV, which is 90 mV smaller than that in I-V characteristic.

  A possible explanation to the gap of 90 mV is the overvoltage reduction in anode. Generally, the overvoltage in anode is larger than that in cathode. Also, the water content in anode catalyst layer impacts the overvoltage in anode. These concern and the water back diffusion with carbon paper in cathode, above mentioned, can suggests that the water back diffusion from cathode may reach to the anode catalyst layer and hydrate the ionomer in the anode catalyst layer even though cell is operated at high temperature. This mechanism may explain the gap of 90 mV, where the water back diffusion reduces the overvoltage in anode.

References

[1] Egil Rasten, Georg Hagen, Reidar Tunold. Electrocatalysis in water electrolysis with solid polymer electrolyte. Electrochimica Acta 48, (2003), 3945-3952

[2] V.Baglio, R.Ornelas, F.Matteucci, F. Martina, G. Ciccarella, I. Zama, L.G. Arriaga, V.Antonucci, and A.S. Aricò. Solid Polymer Electrolyte Water Electrolyser Based on Nafion-TiO₂Composite Membrane for High Temperature Operation. Fuel Cells 09(2009), No.3 247-252.

[3] Chul Min Hwang, Masayoshi Ishida, Hiroshi Ito, Tetsuhiko Maeda, Akihiro Nakano, Atsushi Kato, Tetsuya Yoshida. Effect of titanium powder loading in gas diffusion layer of a polymer electrolyte unitized reversible fuel cell. Journal of Power sources 202(2012)108-113.