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It is highly desirable to synthesize and deploy low-cost and highly efficient catalysts for the oxygen evolution reaction (OER) to catalyze water splitting. We show that N/C doped amorphous iridium oxide combines the benefits of nano-size (approximately 2 nm), which results in exposure to large active surface areas and features of oxygen defects, which make for an electronic structure suitable for the OER. Systematic studies indicate that the OER activity of the iridium oxide catalyst is accelerated by the effect of the structure and chemical state of the iridium element. Remarkably, the N/C doped amorphous iridium oxide catalyst shows a lower cell voltage of 1.774 V at 1.5 A cm−2, compared with IrO 2 (1.847 V at 1.5 A cm−2), and it can maintain such a high current density for over 200 h without noticeable performance deterioration. This work provides a promising method for the improving OER electrocatalysts and the construction of an efficient and stable PEM water cracking system. • Carbon nitride was used as additive and dopant to synthesize N/C doped amorphous iridium oxide. • N/C doped amorphous iridium oxide with large surface area and an oxygen-defective feature. • N/C doped amorphous iridium oxide exhibits superior electrochemical activity and higher ECSA than pure iridium oxide. • Electrolyzer achieved 1.774 V@1.5Acm−2 of PEMWE excellent durability over 200 h. [ABSTRACT FROM AUTHOR]

The design of the anode catalyst layer (ACL) is crucial to the performance and stability of PEM electrolyzers. In this work, the ACL was designed into three sublayers, each different in ionomer content and IrO 2 /TiN X ratio. The high ionomer content of the subset layer will reduce the utilization of the active sites of the catalytic layer, but based on the improvement of the kinetic performance and proton conductivity near the membrane interface, increased mass transport can be achieved. A higher Ir loading near the gas diffusion layer (PTL) and less Ir loading near the membrane interface can improve cell performance, especially at high current densities. The abundance of bubbles produced by oxygen evolution reduces the three-phase reaction boundary, which results in a low utilization rate of the catalyst close to the membrane. In contrast, more Ir in the catalytic layer near the PTL interface can alleviate the effect of mass transfer performance and improve the performance. The internal voltage loss of the optimal MEA is 21.5% lower than the conventional single-layer catalyst layer MEA at 3 A cm‐2. [Display omitted] • Gradient design is implemented through three sublayers construction of anode catalyst layer. • Synergistic gradient distribution of IrO 2 /TiN X ratio and ionomer content. • The Ir content is gradient distributed in the catalyst layer thus the Ir utilization is improved. • Gradient ionomer distribution improves kinetic properties and proton conductivity. • Optimization of MEA can reduce overpotential by up to ∼234 mV at 3 A/cm2. [ABSTRACT FROM AUTHOR]

The development of cost-effective and highly efficient iridium-based catalysts for the anode of proton exchange membrane water electrolyzers (PEMWEs) is urgently required. This study utilized a straightforward wet-chemical method to produce IrNi nanoparticles that were supported on titanium nitride (TiN), where the ultra-small particle size of 1.9 nm for IrNi nanoparticles and the structural support provided by TiN ensure the full exposure of catalytic sites. The electronic interaction between Ir-Ni and Ir-TiN enhances the intrinsic activity of the catalytic sites. The developed catalyst exhibits excellent oxygen evolution reaction (OER) performance, with an overpotential of just 267 mV at a current density of 10 mA cm−2, and a mass activity of 1.07 A mg Ir −1 at 1.55 V versus reversible hydrogen electrode, which is more than 14 times that of commercial IrO 2. Furthermore, its catalytic performance is validated in a PEMWE single cell, along with stable operation for 100 h. The proposed design of the supported iridium-based alloy catalysts in this study presents a novel and referential method for developing anode catalysts in PEMWEs. [Display omitted] • Ultrafine IrNi nanoparticles supported on titanium nitride are synthesized through a facile wet-chemical strategy. • Strong interaction of Ir-Ni and Ir-TiN contributes to remarkable OER activity. • 40-Ir 2 Ni/TiN exhibits an overpotential of 267 mV at 10 mA cm−2 for OER and a mass activity of 1.07 A mgIr−1 at 1.55 V vs. RHE. • With 40-Ir 2 Ni/TiN as the anode catalyst, the single cell voltage of PEMWE reaches 1.708V at 1 A cm−2. [ABSTRACT FROM AUTHOR]

Anode catalytic layer (ACL) plays a vital role in developing a highly efficient proton exchange membrane water electrolyzer, yet its reaction activity and mass transfer at high operating current density remain a challenge. To address this issue, an innovative configuration design of ACL with three sub-layers is constructed in the form of hierarchical pore size distribution. The hierarchical structure of ACL with increasing pore size away from the membrane exhibits superior electrochemical performance (2.043 V @ 3 A cm−2), which is 163 mV lower than that of ACL without pore-forming treatment. The mercury intrusion porosimetry results demonstrate that the porosity increases from 38.8% to 52.3% after the pore-forming treatment. Electrochemical characterization analyses further indicate that the hierarchical structure of ACL enhances active site amount, kinetics, and mass transfer. Therefore, the pore-forming treatment for the anode catalytic layer is a simple and efficient approach to achieving superior electrolysis performance. [Display omitted] • Anode catalytic layers with varied pore-forming are designed. • Effects of adding pores former for electrochemical activities are studied. • After pore-forming, porous anode catalytic layers show enhanced mass transport. • Effects of hierarchical pore size distribution are revealed. [ABSTRACT FROM AUTHOR]

[Display omitted] • N defective g-C 3 N 4 as highly efficient support of IrO 2 is demonstrated. • Strong electronic interaction emerges between IrO 2 and N defective g-C 3 N 4 support. • IrO 2 /N-CN exhibits distinctly lower kinetics loss and sufficient durability. • Optimizing support effect benefits to improve electrocatalytic performance of IrO 2. Herein, a strategy is reported for boosting the support effect between IrO 2 nanoparticle and g-C 3 N 4 (CN) support via regulating CN with the introduction of N defect, thus improve the intrinsic OER activity of IrO 2 in PEM electrolysis. N defective graphite carbon nitride (N-CN) reveals distinctive physicochemical characteristics relative to CN. The mass activity and specific activity of IrO 2 /N-CN are 7.86 times and 1.75 times relative to IrO 2 , respectively. Polarization curves exhibit the optimal OER activity of 1.778 V for IrO 2 /N-CN at 1.6 A cm−2, which is substantially smaller than that of IrO 2 /CN (1.824 V) and IrO 2 (1.846 V). Meantime, IrO 2 /N-CN maintains good stability during the 300 h stability test at the current density of 1.6 A cm−2. DFT calculations and experimental results reveal that outstanding OER activity and sufficient stability are contributions to the increment of active site accessibility and the moderate adsorption of oxygen intermediates. This work provides a feasible strategy for designing and optimizing highly efficient catalysts in electrocatalytic. [ABSTRACT FROM AUTHOR]