Anion-Exchange Membrane Water Electrolyzers

Abstract

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60–80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni­(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm2, respectively.

Impact of Catalyst Reconstruction on the Durability of Anion Exchange Membrane Water Electrolysis

“…Hydrogen, a clean energy carrier, is considered as a potential substitute for fossil fuels. Among all the hydrogen production technologies, AEMWE has developed rapidly in recent years. − As a water electrolysis technology, the advantage of AEMWE is its capability to use nonprecious metal electrocatalysts in both cathodes and anodes due to its alkaline environment, as well as a zero-gap cell structure to effectively reduce the ohmic polarization of the electrolyzer. ,− …”

Metal-Compound-Based Electrocatalysts for Hydrogen Peroxide Electrosynthesis and the Electro-Fenton Process

“…61−64 The O 2 solubility and diffusion limitations can primarily be addressed by the use of catalyst-loaded hydrophobic gas diffusion electrodes (GDEs) and flow cells to deliver a constant flow of O 2 gas directly to the catalyst surface at the three-phase boundary. Such benefits of the engineered electrochemical devices, which have been well studied for water-splitting electrolyzers 65 and CO 2 electroreduction devices, 66 are starting to be exploited for practical electrosynthesis of H 2 O 2 . 61−64 In a recent work, a GDE coated with a layer-templated CoSe 2 (sc-CoSe 2 ) catalyst was run in a flow cell (Figure 10a) and achieved a large H 2 O 2 partial current density up to 60 mA cm −2 (Figure 10b) for high-rate and selective H 2 O 2 production in recirculated 0.5 M H 2 SO 4 .…”

Highly Efficient and Durable Anion Exchange Membrane Water Electrolyzer Enabled by a Fe–Ni3S2 Anode Catalyst

“…[1,2] Nowadays, there are two commercially available water electrolysis systems: alkaline water electrolyzer (AWE) and proton exchange membrane water electrolyzer (PEM-WE). [3][4][5] However, AWE shows poor efficiency due to the high impedance of the electrolyte and diagram, while the large-scale application of PEM-WE is restricted by the high capital cost of noble metal-based catalysts and the device. To overcome these shortcomings, anion exchange membrane water electrolyzer (AEM-WE) has developed as a novel approach for large-scale hydrogen production owing to its compact design similar to PEM-WE and usage of non-noble hydrogen evolution reaction (HER) and oxygen evolving reaction (OER) catalysts, yet is still impeded by poor stability and low energy efficiency.…”