Reducing the Ir−O Coordination Number in Anodic Catalysts based on IrO_x Nanoparticles towards Enhanced Proton‐exchange‐membrane Water Electrolysis

Abstract

Due to the robust oxidation conditions in strong acid oxygen evolution reaction (OER), developing an OER electrocatalyst with high efficiency remains challenging in polymer electrolyte membrane (PEM) water electrolyzer. Recent theoretical research suggested that reducing the coordination number of Ir‐O is feasible to reduce the energy barrier of the rate‐determination step, potentially accelerating the OER. Inspired by this, we experimentally verified the Ir‐O coordination number’s role at model catalysts, then synthesized low‐coordinated IrOx nanoparticles toward a durable PEM water electrolyzer. We first conducted model studies on commercial rutile‐IrO2 using plasma‐based defect engineering. The combined in‐situ EXAFS analysis and computational studies clarify why the decreased coordination numbers increase catalytic activity. Next, under the model studies’ guidelines, we explored a low‐coordinated Ir‐based catalyst with a lower overpotential of 231 mV@10 mA cm‐2 accompanied by long durability (100 h) in an acidic OER. Finally, the assembled PEMWE cell delivers a low voltage (1.72 V) at a current density of 1 A cm‐2 as well as excellent stability exceeding 1200 h (@1 A cm‐2) without obvious decay. This work provides a unique insight into the role of coordination numbers, paving the way for designing Ir‐based catalysts for PEM water electrolyzers.

State-of-the-Art In Situ Technologies for Unravelling the Dynamic Structure Evolution during Acidic Oxygen Evolution Reaction

“…In contrast, to IrO x , catalysts with the lower CN seemed to exhibit higher performance than those rutile type. 27 In situ EXAFS spectra revealed that the Ir−O bond in low-coordinated IrO x would become shorter from 1.66 to 1.60 Å along with raising the applied bias from the OCV to 1.6 V, while nearly no change could be observed over rutile-IrO 2 (Figure 7e…”

“…Based on comparing the transition of the hybridization between the O 2p and the Ru 4d t 2g /e g orbitals from the measurement of O K-edge by in situ soft XAS, the local coordination state further demonstrated that the oxygen vacancies were only formed in RuO x , confirming that OER was carried out in AEM pathway over SnRuO x while in LOM over RuO x (Figure d). In contrast, to IrO x , catalysts with the lower CN seemed to exhibit higher performance than those rutile type . In situ EXAFS spectra revealed that the Ir–O bond in low-coordinated IrO x would become shorter from 1.66 to 1.60 Å along with raising the applied bias from the OCV to 1.6 V, while nearly no change could be observed over rutile-IrO 2 (Figure e–g).…”

“…(g) Potential dependence of the Ir–O bond length of the plasma-treated IrO 2 and rutile-IrO 2 . Reproduced with permission from ref . Copyright 2023 John Wiley and Sons.…”

“…23 More detailed information could be provided by in situ synchrotron radiation X-ray absorption spectroscopy (XAS) via detecting the steady electronic structure and coordination environment information faster and more sensitive, so that the structure−activity relationship could be clarified in detailed (Figure 1d,e). 24−26 One of the most recent discoveries found that the low-coordinated IrO 2 would undergo site oxidation and a decrease in Ir−O bonds, which could strengthen the adsorption of *O and accelerate the deprotonation of *OH, resulting in a 42 times increase in mass specific activity at 1.53 V. 27 Another powerful technology adopted for the OER mechanism determined is differential electrochemical mass spectrometry (DEMS), which enables real-time product analysis of the reaction. 28 When the lattice 16 O of the catalysts have been labeled by 18 O, DEMS could distinguish 32 O 2 , 34 O 2 , and 36 O 2 in the product gas immediately, 9 so that the reaction pathway in AEM, LOM, or other advanced synergic mechanisms could be demonstrated.…”

Immiscible Ruthenium–Cadmium Alloy for Acidic Oxygen Evolution Reaction

“…To date, iridium (Ir) and its oxides are usually reported as acidic OER catalysts due to their robust activity and excellent durability. − However, the extreme scarcity and high cost of Ir greatly limit its commercialization. Instead, ruthenium (Ru) with 1/8 price of Ir, present significantly higher activity than Ir catalysts, which makes it a more ideal alternative for acidic OER catalysts. , Nevertheless, the activity and stability of Ru are barely satisfactory due to its excessively strong adsorption of O-related intermediates, leaving space for the enhancement of catalytic activity and stability.…”