Oxygen evolution reaction (OER) electrocatalysts with high activity, high stability, and low costs are needed for proton-exchange membrane (PEM) electrolyzers. Based on the high cost and limited supply of iridium, approaches that result in iridium-based OER catalysts with increased catalytic activity are of significant interest. We report a carbon-free, self-supported hydrous iridium−nickel oxide two-dimensional nanoframe structure synthesized by thermal treatment of iridium-decorated nickel oxide nanosheets under reducing conditions and subsequent chemical leaching in acid. The catalyst nanoarchitecture contains an interconnected network of metallic iridium−nickel alloy domains with hydrous iridium oxide and nickel oxide located in the surface region. The electrochemical oxidation step maintains the three-dimensional nanoarchitecture and results in a thin (∼5 Å) oxide/hydroxide surface layer. The temperature used for thermal reduction was found to strongly affect the catalyst surface structure and OER activity. Using a lower thermal reduction temperature of 200 °C was determined to provide a higher relative surface concentration of hydroxides and nickel oxide and result in higher OER activities compared with materials treated at 300 °C. The 200 °C-treated hydrous iridium−nickel oxide electrocatalyst showed 15 times higher initial OER mass activity than commercial IrO 2 , and the activity remained 10 times higher than IrO 2 after accelerated durability testing. Density functional theory (DFT) calculations and analysis of the experimental Tafel slopes support that the second electron transfer step is the rate-limiting step for the reaction. The DFT calculations demonstrate that Ni substitution on the IrO 2 surface lowers the activation energy for adsorbed intermediates of the second electron transfer step of the OER reaction. This work establishes that noble metal-decorated metal oxide nanosheets can be transformed into high surface area, carbon-free electrocatalyst nanostructures with high catalytic activities and molecular accessibility and reveals the importance of using controlled thermal reduction temperatures to alter the surface structure and OER activity.

Electrochemical oxygen reduction reaction (ORR) catalysts that have both high activities and long-term stabilities are needed for proton-exchange membrane fuel cells (PEMFCs) and metal-air batteries. Two-dimensional (2D) materials based on graphene have shown high catalytic activities, however, carbon-based materials result in significant catalyst degradation due to carbon oxidation that occurs at high electrochemical potentials. Here, we introduce the synthesis and electrochemical performance of metallic 2D nanoframes which represent a new approach to translate 2D materials into unsupported (carbon-free) electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts. Metallic Ni-Pt 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated Ni(OH) nanosheets. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-Ni alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The inclusion of Ni within the Pt structure resulted in significantly smaller Pt lattice distances compared to those of Pt nanoparticles. On the basis of its unique local and extended structure, the ORR specific activity of Ni-Pt 2D nanoframes (5.8 mA cm) was an order of magnitude higher than Pt/carbon. In addition, accelerated stability testing at elevated potentials up to 1.3 V showed that the metallic Ni-Pt nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Analysis of the ORR electrochemical reaction kinetics on the Ni-Pt nanoframes supports that at low overpotentials the first electron transfer is the rate-determining step, and the reaction proceeds via a four electron reduction process. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications.

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Obtaining acidic bifunctional oxygen electrocatalysts that simultaneously provide high activity and high stability for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remains a significant challenge and need for unitized regenerative fuel cells and metal-air batteries. We report that bimetallic nickel-platinum/cobalt-iridium two-dimensional (2D) nanoframes provide significantly higher ORR and OER activities compared with platinum-iridium oxide (Pt-IrO 2 ). The metallic alloy 2D nanoframes (NiPt and CoIr) were synthesized by thermal reduction of noble-metal-decorated transition-metal hydroxide nanosheets followed by chemical leaching. The 2D nanoframes utilize noble metal (Pt, Ir)−non-noble metal (Ni, Co) interactions to alter the atomic structure, and the unsupported nanostructure provides a carbon-free matrix with three-dimensional (3D) accessibility to the catalytically active sites. Within the cyclic voltammograms, hydrogen adsorption/desorption features on Pt were suppressed within Pt-IrO 2 but were clearly observed within NiPt-CoIr which is attributed to the different size and shape of the nanoframes relative to Pt and IrO 2 that result in less interparticle interaction within NiPt-CoIr compared with commercial Pt-IrO 2 . Rotating disk electrode testing showed that the 2D nanoframe bifunctional oxygen electrocatalysts showed significantly higher ORR mass activity, OER mass activity, and round-trip efficiency compared with Pt-IrO 2 . Over repeated mode switching between ORR and OER potential ranges using an accelerated durability testing protocol, the NiPt-CoIr 2D nanoframe electrocatalysts showed lower ORR stability but improved OER stability compared with Pt-IrO 2 . Bimetallic 3D structures with controlled size, shape, surface structure, and morphology provide the opportunity to design bifunctional catalysts with improved activity and stability.