Directional electronic tuning of Ni nanoparticles by interfacial oxygen bridging of support for catalyzing alkaline hydrogen oxidation

Interfacial Electronic Interactions Induced by Self-Assembled Amorphous RuCo Bimetallenes/MXene Heterostructures for Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (NO3RR) is an attractive green route to generate valuable ammonia and remove nitrates in industrial processes. However, under the intense competition of hydrogen evolution reactions (HER), it is a key challenge to improve the selectivity and reduce the energy consumption of the nitrate reduction reaction. Herein, a unique amorphous RuCo Bimetallenes confined on Ti3C2Tx-MXene (RuCo/Ti3C2Tx) is reported as a highly efficient NO3RR catalyst, showing a remarkable Faradaic efficiency for ammonia of 94.7% at -0.2 V versus reversible hydrogen electrode (RHE), with the corresponding high ammonia yield rate of 98.8 mg h-1 mgcat -1 at -0.6 V versus RHE. Significantly, the RuCo/Ti3C2Tx heterostructures are able to operate stably at 1 A cm-2 for over 100 h under membrane electrode assembly (MEA) conditions with a stabilized NH3 Faraday efficiency. In-depth theoretical and operando spectroscopic investigations unveil that the in situ generation of heterojunction via interfacial Ru/Co─O bridges can induce charge redistribution through Ru/Co─O-Ti structure and modulate the electronic structure of RuCo Bimetallenes, significantly promoting *H production and the adsorption and activation of reactants/intermediates, while suppressing HER, thereby boosting NO3RR performance. This study offers a new insight the metal-support interaction for the development of high-performance electrocatalysts.

D-Orbital-Modulated Ruthenium Embedded within Functionalized Hollow MXene Networks for Enhanced Hydrazine-Assisted Hydrogen Production

Electrochemical green hydrogen production via water splitting is an attractive and sustainable pathway; however, the sluggish kinetics of anodic oxygen evolution reaction is still a critical challenge. In this study, an effective electrocatalyst engineering approach is demonstrated by preparing an innovative hybrid of ruthenium d-orbitals-regulated nanoclusters embedding within functionalized hollow Ti3C2 MXene networks (Ru0.91Ni0.09-N/O-Ti3C2) to promote the hydrazine-assisted hydrogen production. A specific charge redistribution is revealed, locally concentrating at interfaces derived from stable Ru(Ni)-N/O-Ti coordination and d-p orbital hybridization. The charge transfer effect from Ni to Ru within Ru0.91Ni0.09 structure and Ru0.91Ni0.09 to N/O-Ti3C2 tailors electronic features of Ru sites to enable reasonable adsorption/desorption toward reactant intermediates. The Ru0.91Ni0.09-N/O-Ti3C2 requires an overpotential of only 29.3 mV for cathodic hydrogen evolution and a low potential of -29.9 mV for anodic hydrazine oxidation to reach 10 mA cm-2, showing excellent stability. The hydrazine-assisted hydrogen production system based on Ru0.91Ni0.09-N/O-Ti3C2 electrodes delivers small cell voltages of 0.02 V at 10 mA cm-2 and 0.92 V at industrial current level of 1.0 A cm-2. This work may open a new electrocatalysis strategy from lab scale to industry for robust and efficient green hydrogen production.

Enhanced Water Splitting Electrocatalysis with Heterointerfacial Cobalt Phosphide In Situ Supported on a Cellulose-Derived Carbon Aerogel

The active site density, intrinsic activity, and supporting substrate of cobalt phosphide catalysts are vital to their performance in alkaline water electrolysis. In this work, a CoP/Co2P loaded on cellulose nanofiber-derived carbon aerogels (CP/CCAs) bifunctional electrocatalyst with a three-dimensional network and heterostructure is illustrated through sequential facile hydrothermal, freeze-drying, and phosphorylation processes. The three-dimensional network of carbon aerogels derived from cellulose nanofibers reveals a specific surface area of 183.41 m2/g, greatly enriching the active sites and facilitating the electron and mass transportation. Besides, interactions between CoP/Co2P with a heterogeneous structure and carbon aerogels modulate the electronic structure to enhance the intrinsic activity of the catalysts. Benefiting from these advantages, CP/CCAs-2 demonstrates a superior oxygen evolution reaction activity (η10 mA cm-2 = 277 mV) over the benchmark RuO2 and a moderate hydrogen evolution reaction performance (η10 mA cm-2 = 63 mV). Density functional theory calculations further reveal that the coupling of CoP/Co2P and cellulose-derived carbon aerogels promotes the water adsorption and activates the H-O bond. As a result, the alkaline electrolyzer assembled with CP/CCAs-2 both as a cathode and an anode shows a low cell voltage of 1.59 V at a current density of 10 mA cm-2 and good stability. This work provides a strategy for phosphide electrocatalysts composited with green carbon carriers for overall water splitting.

Enhancing molecular oxygen activation by nitrogen-doped carbon encapsulating FeNi alloys with ultra-low Pt loading

Modulating the electronic structure of noble metals via electronic metal-support interaction (EMSI) has been proven effectively for facilitating molecular oxygen activation and catalytic oxidation reactions. Nevertheless, the investigation of the fundamental mechanisms underlying activity enhancement has primarily focused on metal oxides as supports, especially in the catalytic degradation of volatile organic compounds. In this study, a novel Pt catalyst supported on nitrogen-doped carbon encapsulating FeNi alloy, featuring ultrafine Pt nanoparticles, was synthesized. This catalyst demonstrated exceptional catalytic activity (92%), recyclability, and water tolerance for the deep oxidation of formaldehyde at room temperature. Structural analyses and theoretical calculations revealed a directional electron transfer from FeNi alloy to Pt, even there is no direct contact between them. This electron penetration effect, mediated by carbon, conferred electron-rich properties to Pt, leading to the activation of molecular oxygen by elongating O-O bond length (1.405 Å). Consequently, efficient formaldehyde removal was achieved with an ultra-low Pt loading. This investigation offers a novel perspective on modulating the electronic structure of Pt by engineering a unique EMSI effect between a nonoxide support and active species, thereby enabling efficient oxygen activation for air purification.

Construction of Mo2TiC2Tx MXene as a Superior Carrier to Support Ru-CuO Heterojunctions for Improving Alkaline Hydrogen Oxidation

The sluggish anodic hydrogen oxidation reaction (HOR) of the hydroxide exchange membrane fuel cell (HEMFC) is a significant barrier for practical implementation. Herein, we designed a catalyst of Mo2TiC2Tx MXene-supported Ru-CuO heterojunctions (named as Ru-CuO/MXene). The XPS spectra and the d-band center data of the different amounts of Cu of the Ru-CuO/MXene suggested that there existed a strongly electronic metal-support interaction between the active species and the substrate with MXene as the excellent carrier. Furthermore, the in situ electrochemical experimental results (operando electrochemical impedance spectroscopy and in situ electrochemical Raman spectroscopy) and density functional theory (DFT) calculations showed that Ru and CuO were the optimal adsorption sites for surface species *H and *OH, respectively, endowing Ru-CuO/MXene with the ability to weaken the hydrogen binding energy (HBE) and strengthen the hydroxide binding energy (OHBE). Remarkably, the optimized catalyst modified an impressive HOR activity in the 0.1 M KOH electrolyte with a kinetic and an exchange current density of 7.63 and 1.37 mA cm-2 at 50 mV overpotential (vs. RHE), respectively, which were 1.98- and 1.2-fold higher than those of commercial Pt/C (20 wt %). Finally, the as-prepared catalyst also exhibited superior durability and exceptional CO antipoisoning ability in 1000 ppm of the CO/H2-saturated 0.1 M KOH electrolyte. This work provides an inspiring strategy to design high-activity HOR electrocatalyst-based metallic Ru in alkaline environments.

Unconventional hcp/fcc Nickel Heteronanocrystal with Asymmetric Convex Sites Boosts Hydrogen Oxidation

Developing non-platinum group metal catalysts for the sluggish hydrogen oxidation reaction (HOR) is critical for alkaline fuel cells. To date, Ni-based materials are the most promising candidates but still suffer from insufficient performance. Herein, we report an unconventional hcp/fcc Ni (u-hcp/fcc Ni) heteronanocrystal with multiple epitaxial hcp/fcc heterointerfaces and coherent twin boundaries, generating rugged surfaces with plenty of asymmetric convex sites. Systematic analyses discover that such convex sites enable the adsorption of *H in unusual bridge positions with weakened binding energy, circumventing the over-strong *H adsorption on traditional hollow positions, and simultaneously stabilizing interfacial *H2O. It thus synergistically optimizes the HOR thermodynamic process as well as reduces the kinetic barrier of the rate-determining Volmer step. Consequently, the developed u-hcp/fcc Ni exhibits the top-rank alkaline HOR activity with a mass activity of 40.6 mA mgNi -1 (6.3 times higher than fcc Ni control) together with superior stability and high CO-tolerance. These results provide a paradigm for designing high-performance catalysts by shifting the adsorption state of intermediates through configuring surface sites.

Metal-ligand dual-site single-atom nanozyme mimicking urate oxidase with high substrates specificity

In nature, coenzyme-independent oxidases have evolved in selective catalysis using isolated substrate-binding pockets. Single-atom nanozymes (SAzymes), an emerging type of non-protein artificial enzymes, are promising to simulate enzyme active centers, but owing to the lack of recognition sites, realizing substrate specificity is a formidable task. Here we report a metal-ligand dual-site SAzyme (Ni-DAB) that exhibited selectivity in uric acid (UA) oxidation. Ni-DAB mimics the dual-site catalytic mechanism of urate oxidase, in which the Ni metal center and the C atom in the ligand serve as the specific UA and O2 binding sites, respectively, characterized by synchrotron soft X-ray absorption spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, and isotope labeling. The theoretical calculations reveal the high catalytic specificity is derived from not only the delicate interaction between UA and the Ni center but also the complementary oxygen reduction at the beta C site in the ligand. As a potential application, a Ni-DAB-based biofuel cell using human urine is constructed. This work unlocks an approach of enzyme-like isolated dual sites in boosting the selectivity of non-protein artificial enzymes.

CO-Tolerant Hydrogen Oxidation Electrocatalysts for Low-Temperature Hydrogen Fuel Cells

The severe performance degradation of low-temperature hydrogen fuel cells upon exposure to trace amounts of carbon monoxide (CO) impurities in reformate hydrogen fuels is one of the challenges that hinders their commercialization. Despite significant efforts that have been made, the CO-tolerance performance of electrocatalysts for the hydrogen oxidation reaction (HOR) is still unsatisfactory. This Perspective discusses the path forward for the rational design of CO-tolerant HOR electrocatalysts. The fundamentals of the CO-tolerant mechanisms on commercialized platinum group metal (PGM) electrocatalysts via either promoting CO electrooxidation or weakening CO adsorption are provided, and comprehensive discussions based on these strategies are presented with typical examples. Given the recent progress, some emerging strategies, including blocking CO diffusion with a barrier layer and developing non-PGM HOR catalysts, are also discussed. We conclude with a discussion of the strengths and limitations of these strategies along with the perspectives of the major challenges and opportunities for future research on CO-tolerant HOR electrocatalysts.