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.

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.

Proton-exchange membrane water electrolyzers produce hydrogen from water and electricity and can be powered using renewable energy; however, the high overpotential, high cost, and limited supply of the oxygen evolution reaction (OER) electrocatalyst are key factors that hinder wide-scale adoption. Ruthenium oxide (RuO2) has a lower overpotential, lower cost, and higher global supply compared with iridium oxide (IrO2), but RuO2 is less stable than IrO2. As an approach to improve the catalytic stability, we report the effect of titanium substitution at different concentrations within nanoscale RuO2, Ru1–x Ti x O2 (x = 0–50 at. %), on the structure, OER activity, and stability using combined experiments and theory. Titanium substitution within rutile RuO2 affects the electronic structure, resulting in regions of electron accumulation and electron depletion at the surface, and shifts the d-band and O 2p band centers to higher binding energies. Calculations show that the effects of Ti on the electronic structure are highly dependent on not only the concentration but also the specific dopant location. From electrochemical testing and analysis of the electrolyte and simulations, titanium substitution at low concentrations (12.5 and 20 at. %) improves catalyst stability and lowers Ru dissolution. Experiments of OER activity agree with the theory that Ti substitution results in a higher overpotential when averaging over all adsorption sites. Theoretical analysis shows that specific sites predominately act as catalytic sites for the OER, while metal dissolution occurs at different sites. Specifically, OER has the lowest barriers at penta-coordinated Ru sites, while hexa-coordinated Ru sites have the lowest energetic barriers for dissolution.

Proton exchange membrane unitized regenerative fuel cells (URFCs) combine the ability to both produce power (in fuel cell mode) and generate fuel and oxidant (in electrolysis mode) from the same cell. The further development and utilization of URFCs is hindered by the high costs and low stability of bifunctional oxygen electrocatalysts. Distributing the oxygen electrocatalyst on a high surface area support can reduce the loading of the active noble metal (e.g., platinum and iridium) and influence the efficiency and stability of the catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Carbon, which is typically used as a support material, is highly unstable under the conditions required for OER which makes carbon unfeasible for long-term use within URFCs. Therefore, alternative carbon-free catalyst supports are needed. Niobium oxide, Nb2O5, is stable in the oxidative potentials and highly corrosive acidic conditions of URFCs; however, the low electronic conductivity of Nb2O5 significantly limits its use as a catalyst support material. We investigated approaches to obtain high surface area and conductive niobium oxides, NbOx, as stable supports for bifunctional oxygen electrocatalysts. The effects of synthesis conditions, drying methods, and temperature/atmosphere treatments on the structure, morphology, surface area, and electronic conductivity were evaluated. The structure, morphology, composition, and surface area were determined by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and nitrogen physisorption analysis. Sol-gel synthesis and supercritical drying resulted in a NbOx aerogel with high surface area and porous morphology. Thermal treatment of the NbOx aerogel under H2 increased the electronic conductivity of NbOx when compared to the as-prepared NbOx. Different methods to deposit noble metals on the NbOx support were evaluated. The NbOx-supported catalysts were tested as bifunctional ORR/OER electrocatalysts to determine their activity and stability. The development supported bifunctional oxygen electrocatalysts that can provide lower noble metal loadings, higher activity and improved stability under harsh acidic conditions and highly oxidizing potentials furthers the development of URFCs with lower cost, improved performance, and enhanced durability.

The complex reaction mechanisms and dissolution pathways that drive oxygen evolution reaction on metal and metal oxide surfaces under acidic conditions challenge the development of a highly active, highly stable, and low cost catalyst for proton-exchange membrane (PEM) water electrolyzers. Currently, ruthenium-based materials present lower overpotentials, lower cost and higher global supply compared to iridium-based materials, though they are also considerably less stable. In order to elucidate a way to improve RuO2 stability under the harsh conditions during water electrolysis, we use a density functional theory (DFT) approach to investigate the effects on catalyst structure, OER activity and stability of transition metal substitution within Ru1-xMxO2 (M = Ti, Zr, Nb, Ta, Cr) at different atomic concentrations. Calculations show that M substitution within rutile RuO2 affects the electronic structure resulting in regions of electron accumulation and depletion at the surface and shifts the Ru d-band and O2p band centers, which are highly dependent on dopant characteristics and doped site. Moreover, dissolution calculations on the Ru1-xTixO2 surfaces show that Ti substitution alter the metal dissolution pathway energetics and thermodynamics bringing stability to the catalyst in terms of reducing material loss. Theoretical XRD patterns, Ru d-band and O p-band center calculations, and activation and reaction energy trends are in excellent agreement with experimental results that includes X-ray diffraction analysis, high resolution transmission electron microscopy images, X-ray photoelectron spectroscopy of core and valence bands, and rotating disk electrode measurements. In this presentation, we focus on the theoretical and computational aspects. The evaluation of the thermodynamics and kinetics of the water splitting, and oxygen evolution mechanism is done with spin-polarized plane-wave DFT calculations, while Ab Initio Molecular Dynamics (AIMD) simulations allow following and complementing the understanding of the dynamics of the initial steps of water dissociation on the various M and Ru sites. Electronic structure changes on the surface due to the presence of M before and during the reaction are analyzed based on the density of states and local magnetic moments, and the activation energies are obtained from the climbing Nudge Elastic Band method. Dissolution calculations are carried out with constrained-AIMD simulations using the slow-growth approach within the “Bluemoon ensemble” as implemented in VASP.