Revealing the role of double-layer microenvironments in pH-dependent oxygen reduction activity over metal-nitrogen-carbon catalysts.

A standing puzzle in electrochemistry is that why the metal-nitrogen-carbon catalysts generally exhibit dramatic activity drop for oxygen reduction when traversing from alkaline to acid. Here, taking FeCo-N6-C double-atom catalyst as a model system and combining the ab initio molecular dynamics simulation and in situ surface-enhanced infrared absorption spectroscopy, we show that it is the significantly distinct interfacial double-layer structures, rather than the energetics of multiple reaction steps, that cause the pH-dependent oxygen reduction activity on metal-nitrogen-carbon catalysts. Specifically, the greatly disparate charge densities on electrode surfaces render different orientations of interfacial water under alkaline and acid oxygen reduction conditions, thereby affecting the formation of hydrogen bonds between the surface oxygenated intermediates and the interfacial water molecules, eventually controlling the kinetics of the proton-coupled electron transfer steps. The present findings may open new and feasible avenues for the design of advanced metal-nitrogen-carbon catalysts for proton exchange membrane fuel cells.

Understanding Alkaline Hydrogen Oxidation Reaction on PdNiRuIrRh High-Entropy-Alloy by Machine Learning Potential.

High-entropy alloy (HEA) catalysts have been widely studied in electrocatalysis. However, identifying atomic structure of HEA with complex atomic arrangement is challenging, which seriously hinders the fundamental understanding of catalytic mechanism. Here, we report a HEA-PdNiRuIrRh catalyst with remarkable mass activity of 3.25 mA µg-1 for alkaline hydrogen oxidation reaction (HOR), which is 8-fold enhancement compared to that of commercial Pt/C. Through machine learning potential-based Monte Carlo simulation, we reveal that the dominant Pd-Pd-Ni/Pd-Pd-Pd bonding environments and Ni/Ru oxophilic sites on HEA surface are beneficial to the optimized adsorption/desorption of *H and enhanced *OH adsorption, contributing to the excellent HOR activity and stability. This work provides significant insights into atomic structure and catalytic mechanism for HEA and offers novel prospects for developing advanced HOR electrocatalysts.

Oxygen-Inserted Top-Surface Layers of Ni for Boosting Alkaline Hydrogen Oxidation Electrocatalysis.

Precisely tailoring the electronic structures of electrocatalysts to achieve an optimum hydroxide binding energy (OHBE) is vital to the alkaline hydrogen oxidation reaction (HOR). As a promising alternative to the Pt-group metals, considerable efforts have been devoted to exploring highly efficient Ni-based catalysts for alkaline HOR. However, their performances still lack practical competitiveness. Herein, based on insights from the molecular orbital theory and the Hammer-Nørskov d-band model, we propose an ingenious surface oxygen insertion strategy to precisely tailor the electronic structures of Ni electrocatalysts, simultaneously increasing the degree of energy-level alignment between the adsorbed hydroxide (*OH) states and surface Ni d-band and decreasing the degree of anti-bonding filling, which leads to an optimal OHBE. Through the pyrolysis procedure mediated by a metal-organic framework at a low temperature under a reducing atmosphere, the obtained oxygen-inserted two atomic-layer Ni shell-modified Ni metal core nanoparticle (Ni@Oi-Ni) exhibits a remarkable alkaline HOR performance with a record mass activity of 85.63 mA mg-1, which is 40-fold higher than that of the freshly synthesized Ni catalyst. Combining CO stripping experiments with ab initio calculations, we further reveal a linear relationship between the OHBE and the content of inserted oxygen, which thus results in a volcano-type correlation between the OH binding strength and alkaline HOR activity. This work indicates that the oxygen insertion into the top-surface layers is an efficient strategy to regulate the coordination environment and electronic structure of Ni catalysts and identifies the dominate role of OH binding strength in alkaline HOR.

In Situ Synthesis of NiCoP Nanoparticles Supported on Reduced Graphene Oxide for the Catalytic Hydrolysis of Ammonia Borane.

Developing highly efficient and stable noble-metal-free catalysts toward catalytic hydrolysis of ammonia borane (AB) for hydrogen storage is highly desirable, but still remains challenging. We report a simple and in situ co-reduction approach to synthesize bimetallic NiCoP nanoparticles (NPs) supported on reduced graphene oxide (rGO). Thanks to the strong electronic interaction between Ni, Co, and P, the as-synthesized Co89.8 Ni10.2 P11.7 /rGO catalyst exhibits superior catalytic performance towards hydrolysis of AB, with the turnover frequency value (TOF) of 18.6 min-1 , which is about 2.5 times higher than that of NiCo/rGO.

Carbon Encapsulated Hollow Co3O4 Composites Derived from Reduced Graphene Oxide Wrapped Metal-Organic Frameworks with Enhanced Lithium Storage and Water Oxidation Properties.

Transition metal oxides have received great attention for boosting the performances for lithium-ion batteries and oxygen evolution reaction (OER). Here, hollow Co3O4 nanoparticles encapsulated in reduced graphene oxide (rGO) ( h-Co3O4@rGO) were synthesized through a two-step annealing process of graphene oxide wrapped zeolitic imidazolate framework-67 (ZIF-67@GO) precursors. By taking advantage of the enhanced conductivity, high dispersity, high surface area, and unique hollow morphology derived from the GO-wrapped protecting annealing strategy, the as-synthesized h-Co3O4@rGO composite not only exhibits a reversible capacity as high as 1154.2 mAh g-1 at 500 mA g-1 after 100 cycles and high rate performance (746 mAh g-1 at 3000 mA g-1) but also displays superior OER performance with an overpotential of 300 mV to obtain 10 mA cm-2.