Improving electrochemical performance of PEM water electrolyzer by optimizing side-chain structure and content of ionomer

Abstract

As the proton transport channel and binder within the catalytic layer (CL), the physicochemical properties of the ionomer can affect the CL microstructure and performance of the membrane electrode assembly. In this paper, we select ionomers with different side-chain lengths and investigate the effects of the side-chain structure and content of the ionomers on the performance of membrane electrode assembly (MEA). Electrochemical tests show that at a mass ratio of 10 wt.% of ionomer/Ir (I/Ir), long-side-chain (LSC) ionomer exhibits the best performance (2.141 [email protected] A/cm2, while short-side-chain (SSC) ionomer is 2.208 [email protected] A/cm2). The MEA containing LSC ionomer shows better electrochemical performance than the SSC at the same I/Ir mass ratio, especially at high current density. The MEA containing LSC ionomer has a larger average pore size and porosity, which indicates that it may have better mass-transfer properties. From the analysis of voltage loss, it can be seen that LSC ionomers have a smaller ohmic impedance and mass transfer resistance than SSC ionomers. In conclusion, LSC ionomers are more conducive to water-gas transport, which can provide excellent water electrolysis performance. This article focuses on the optimization of ionomer side chains and content, which can enhance PEM water electrolysis performance at lower cost.

1. Introduction

The widespread consumption of fossil fuels in recent decades has led to severe environmental problems due to carbon dioxide and other greenhouse gas emissions. With the rapid growth of global energy demand, various energy sources have been proposed, including hydropower, wind energy, solar energy, nuclear energy, and hydrogen energy. Hydrogen energy offers a potential solution for meeting global energy demand with minimal pollution.

As a major source of green hydrogen production, proton exchange membrane water electrolyzer (PEMWE) technology is the most promising technology for large-scale hydrogen production because of its high proton conductivity, high hydrogen volume fraction (>99.99%), low gas crossover and high operating pressure. However, PEMWE has a high cost of components and materials, and small application scale causes high assembly costs, resulting in an overall high cost. To make PEMWE economically viable, material costs should be reduced and system performance should be improved.

The catalytic layer (CL) of membrane electrode assembly (MEA) is an essential site for conducting electrochemical reactions and is a critical parameter in determining electrochemical performance. The ideal CL has efficient catalyst activity and utilization, maintains high porosity, and constructs efficient mass transfer channels. As one of the critical materials of catalytic layers, ionomers play an essential role in the electrochemical performance of CLs by constructing proton transport channels and acting as a binder to maintain the CL structure. Physicochemical properties of ionomers cause different effects on the CL microstructure and MEA properties, such as ionomer morphology, coverage and ionomer content. Therefore, to achieve low noble metal loading while maintaining high catalytic performance, it is beneficial to understand the interaction mechanism between ionomers and catalysts and analyze the effect of ionomers on the structure and performance of the CL of MEA.