Elsevier

Electrochimica Acta

Volume 100, 30 June 2013, Pages 242-248
Electrochimica Acta

Influence of pore structural properties of current collectors on the performance of proton exchange membrane electrolyzer

https://doi.org/10.1016/j.electacta.2012.05.068Get rights and content

Abstract

The effect of pore structural properties (porosity and pore diameter) of current collectors in proton exchange membrane (PEM) electrolyzers on electrolysis performance was experimentally evaluated by using various titanium (Ti)-felt substrates with different porosities and pore diameters (measured by capillary flow porometry) as the anode current collectors. The current–potential (j–ΔV) characteristics were measured, and overpotential analysis was performed based on the j–ΔV characteristics and the cell resistance (Rcell) data. The results showed that (1) the effect of the decrease in water supply on the membrane resistance due to produced gas bubbles is limited when the mean pore diameter of the anode current collector is less than 50 μm, but might appear at the concentration overpotential, and (2) enhancing the uniform and sufficient contact between the current collector and the electrode reduces not only the contact resistance but also the activation overpotential.

Introduction

Hydrogen is expected to play an important role as an energy carrier in the future. When combined with state-of-the-art energy devices, hydrogen can be used as fuel in almost every application where fossil fuels are being used today, but without harmful emissions. Thus, “capturing” renewable energy with hydrogen is critical for establishing a “hydrogen society” in which energy produced by unstable renewable sources can be stored as various stable forms of hydrogen such as compressed, liquefied, or metal hydrides [1]. Among the several methods developed for hydrogen production from renewable energy sources, water electrolysis is the most practical and flexible technology [2], [3]. In particular, proton exchange membrane (PEM) water electrolyzers have recently attracted attention due to their wide range of current density and high conversion efficiency compared to conventional alkaline water electrolyzers [4], [5], [6], [7], [8].

Fig. 1 shows a schematic cross-section of a PEM electrolyzer. The configuration of a PEM electrolyzer is similar to that of a PEM fuel cell (PEMFC) in that it consists of a membrane electrode assembly (MEA), current collectors, and bipolar plates with flow channels. (Not shown in the figure are several other components needed in the cell set-up, such as bus plates, manifolds, and end plates.) In a PEM electrolyzer, the PEM serves as the electrolyte as well as the barrier between the hydrogen and oxygen. Because PEM is acidic, half-cell reactions during electrolysis can be expressed as follows:H2O2H++12O2+2eat anode2H++2eH2at cathode

Liquid water is introduced at the anode and dissociated into molecular oxygen, protons, and electrons. Solvated protons formed at the anode migrate through the membrane to the cathode where they are reduced to molecular hydrogen. During migration of protons through the membrane, electro-osmosis drag occurs in which water molecules accompany the protons through the membrane from the anode to cathode due to an electric field. Thus, due to this dragged water, the PEM must be kept wet without an outside water supply to the cathode, and water as a reactant is supplied only at the anode during typical operation of a PEM electrolyzer.

Current collectors are porous media placed between the MEA and bipolar plate at both electrode sides. The two major roles of a current collector are similar to those of a gas diffusion layer (GDL) of a PEMFC, namely, electric conduction between the electrode and the bipolar plate and efficient mass transport of liquid and gas between the electrode and the flow channels. In a typical PEMFC, carbon paper or carbon cloth is used as the GDL at both sides of the electrodes. However, in a PEM electrolyzer, carbon material cannot be used for either the electrode (i.e., catalyst layer) or for the GDL (i.e., current collector) of the oxygen side (anode), because anodic potential tends to corrode carbon material during electrolysis operation. Thus, in a PEM electrolyzer, the anode current collector is typically titanium (Ti) in the form of sintered porous media, expanded screen mesh, or felt (unwoven fabric).

In practice, the anodic activation overpotential during PEM electrolysis is much larger than the cathodic activation overpotential [9], [10], [11]. At the anode of a PEM electrolyzer, liquid water is transferred through the current collector from the flow channel and dissociated into molecular oxygen. Produced oxygen gas diffuses back to the flow channel by diffusion through the anode current collector. Liquid water acts as a reactant in the anode reaction while simultaneously humidifying the membrane to maintain high proton conductivity. Therefore, from the point of view of mass transport, the anode current collector has two major requirements: (1) sufficient supply of liquid water to the electrode and (2) prompt release of oxygen gas from the electrode. Tanaka et al. [12] applied different Ti crossbar-type current collectors for PEM electrolysis and investigated the relation between a geometrical parameter (interval of bus bar) and the current distribution. By using a plate of sintered Ti-powder as the GDL of the electrolyzer, Grigoriev et al. [13] experimentally determined that the optimum pore size of a GDL is 12–13 μm.

Contrary to the anode reaction, the cathode reaction does not require liquid water, although liquid water is transferred (by electro-osmosis) from the anode and is accompanied by protons in the membrane. Thus, hydrogen gas and liquid water are simultaneously transported to the channel through the current collector during electrolysis operation. Because the activation overpotential of the cathode reaction is small, the effect of the properties of the cathode current collector on the cell performance is limited.

In the present study, the effect of structural properties of current collectors on electrolysis performance was investigated comprehensively by focusing on anode current collectors. First, various samples of Ti-felt with different structural properties (porosity and fiber diameter) were prepared as anode current collectors. Then, polarization curves and cell resistance were measured. Finally, based on these measurement results and overpotential analysis, the relation between the structural properties of the anode current collector and the electrolysis performance was evaluated.

Section snippets

Experimental set up

The experimental set-up of the PEM electrolyzer and the balance of plant (BOP) used here were previously described [14]. The electrolyzer was a small single cell (active electrode area of 27 cm2) that had the commonly used PEMFC configuration, consisting of an MEA, porous current collectors, and separators (bipolar plates) with flow channels. The MEA used here was designed for a unitized reversible fuel cell (URFC) and was developed by Takasago Thermal Engineering Co. and Daiki Ataka Engineering

Temperature dependency of membrane resistance

Fig. 2 shows the measured current–potential (j–ΔV) characteristics of the standard cell (Ely1) at Tcell = 60, 70, and 80 °C, and shows the average Rcell at each j. Rcell has two components: membrane resistance (Rmem), which is related to the proton transport resistance in the polymeric membrane, and the electric resistance (Rele), which is due to the electric resistance of the cell components (bipolar plate, current collector, electrode layer) and the electric contact resistance between each cell

Conclusions

The effect of changes in pore structure of the anode current collector on PEM electrolysis performance was experimentally investigated using a small cell with various Ti-felt substrates of different fiber diameter (ϕ) and porosity (ɛ). The current–potential (j–ΔV) characteristics were measured (after reproducibility was confirmed), and overpotential analysis was performed based on the j–ΔV characteristics and the cell resistance (Rcell) data.

Results revealed that produced gas bubbles hinder the

Acknowledgments

The authors gratefully acknowledge the financial support from the Ministry of Economy, Trade and Industry (METI) of Japan. H. Ito thanks to Dr. Chul Min Hwang of the University of Tsukuba for his kind supports.

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