Advancing water oxidation from catalyst design to membrane electrolysis

Abstract

Electrochemical water splitting driven by the off-peak electricity is a promising technology for the large-scale green hydrogen production. The most well-established alkaline liquid water electrolyzers (AWEs) suffer from limited operating current density attributed to relatively high internal resistance and easy gas crossover of thick and porous diaphragm. Membrane electrode assembly (MEA) technologies with highly conductive proton/anion-exchange membranes are developed to overcome the shortage of AWEs. Unfortunately, proton-exchange membrane water electrolyzers (PEMWEs) suffer from high cost with noble-metal catalyst, e.g. IrO2, and anion-exchange membrane water electrolyzers (AEMWEs) suffer from a relatively low efficiency and stability without appropriate anode catalysts. To tackle these challenges, three OER electrocatalysts were developed for the anode in PEMWEs and AEMWEs. Firstly, the “quasi-glassy” Y-doped RuO2 was developed with hybrid structure of nanograins and amorphous domains due to the mismatch of ionic radii between Y3+ and Ru4+ (Y3+: 0.9 Å; Ru4+: 0.62 Å), which possesses larger electrochemical surface area and exposes more low-coordination Ru atoms, enhancing the OER kinetics and current density. The optimized catalyst, Y0.3Ru0.7O2 demonstrated an extremely low overpotential of 170 and 220 mV at 10 and 100 mA cm-2, respectively, which also exhibited an outstanding performance in PEMWEs with the voltage of 1.65 V at 1 A cm-2 and the degradation rate of 87 μV/h at 0.5 A cm-2 for 300-hour operation, demonstrating its enormous potential for the application in PEMWEs. Secondly, a highly efficient and stable OER catalyst, Sr0.3Pr0.7Fe0.25Co0.75O3, was developed via in-depth mechanistic pathway design and synthesis of dual-site co-doped perovskite SryPr1−yFexCo1−xO3. The catalyst shows much superior OER performance in alkaline solutions with a high intrinsic activity (35 times higher than the commercial IrO2). It presented an outstanding performance in AEMWEs, with the cell voltage of only 1.91 V to achieve 1 A cm-2. Moreover, the catalyst exhibited a high operation stability for 40 h with a degradation rate of 1.9 mV/h at 0.5 A cm-2, outperforming all the other catalysts in the pure-water AEMWEs reported to date. Thirdly, the metallic-conductive LaNiO3-based perovskite oxides were demonstrated as highly effective anode catalysts. The electrical conductivity and active phases of perovskites can be modulated by adjusting the ratios of Sr/La at A site and Co/Ni at B site, respectively. The optimized perovskite, Sr0.1La0.9Co0.5Ni0.5O3, presented excellent performance in AEMWEs with the cell voltage of 1.97 V to achieve 2 A cm-2, which approaches the performance of KOH-fed AEMWEs with noble-metal catalysts (~ 1.8 V at ~2 A cm-2). This work provides critical insights for applying highly conductive perovskite anode catalysts in pure-water-fed AEMWEs. Overall, regardless of the cost, the developed Y0.3Ru0.7O2 is the most promising one for commercial application. The two developed low-cost alkaline OER catalysts-Sr0.3Pr0.7Fe0.25Co0.75O3 and Sr0.1La0.9Co0.5Ni0.5O3 showed promising development potential and space for AEMWEs with significant performance improvements, but the degradation rate of AEMWE cell is a little bit high due to the ionomer degradation at the catalyst-ionomer interface. Further optimization of catalyst-ionomer interface is needed to satisfy the requirements of industrial applications.