The Role of Discrepant Reactive Intermediates on Ru-Ru2 P Heterostructure for pH-Universal Hydrogen Oxidation Reaction

Developing highly efficient electrocatalysts for hydrogen oxidation reaction (HOR) under alkaline media is essential for the commercialization of alkaline exchange membrane fuel cell (AEMFC). However, the kinetics of HOR in alkaline media is complicated, resulting in orders of magnitude slower than that in acid, even for the state-of-the-art Pt/C. Here, we find that Ru-Ru₂ P/C heterostructure shows HOR performance with a non-monotonous variation in a whole pH region. Unexpectedly, an inflection point located at pH≈7 is observed, showing an anomalous behavior that HOR activity under alkaline media surpasses acidic media. Combining experimental results and theoretical calculations, we propose the roles of discrepant reactive intermediates for pH-universal HOR, while H* and H₂ O* adsorption strengths are responsible for acidic HOR, and OH* adsorption strength is essential for alkaline HOR. This work not only sheds light on fundamentally understanding the mechanism of HOR but also provides new designing principles for pH-targeted electrocatalysts.

Competitive Adsorption: Reducing the Poisoning Effect of Adsorbed Hydroxyl on Ru Single-Atom Site with SnO2 for Efficient Hydrogen Evolution

Ruthenium (Ru) has been theoretically considered a viable alkaline hydrogen evolution reaction electrocatalyst due to its fast water dissociation kinetics. However, its strong affinity to the adsorbed hydroxyl (OH_ad ) blocks the active sites, resulting in unsatisfactory performance during the practical HER process. Here, we first reported a competitive adsorption strategy for the construction of SnO₂ nanoparticles doped with Ru single-atoms supported on carbon (Ru SAs-SnO₂ /C) via atomic galvanic replacement. SnO₂ was introduced to regulate the strong interaction between Ru and OH_ad by the competitive adsorption of OH_ad between Ru and SnO₂ , which alleviated the poisoning of Ru sites. As a consequence, the Ru SAs-SnO₂ /C exhibited a low overpotential at 10 mA cm^-2 (10 mV) and a low Tafel slope of 25 mV dec^-1 . This approach provides a new avenue to modulate the adsorption strength of active sites and intermediates, which paves the way for the development of highly active electrocatalysts.

Interfaces Decrease the Alkaline Hydrogen-Evolution Kinetics Energy Barrier on NiCoP/Ti3C2Tx MXene

Heterointerfaces can adjust the adsorption energy with intermediates in the transition state for a much decreased kinetics energy barrier (E_a). One typical transition metal phosphide, NiCoP grains (∼5 nm in size), was anchored on a Ti₃C₂T_x MXene monolayer (∼1 nm in thickness) to boost the kinetics toward alkaline hydrogen evolution reaction (HER). General electrochemical experiments at different temperatures give a small E_a of 31.4 kJ mol^-1, showing a 22.1% decrease compared to its counterpart NiCoP nanoparticles (40.3 kJ mol^-1). Impressively, the overpotential of NiCoP@MXene dramatically decreases from 71 mV to 4 mV at 10 mA cm^-2 when the temperature increases from 25 °C to 65 °C. On a single NiCoP@MXene sheet, scanning electrochemical microscopy (SECM) tests also give a very close value of E_a = 31.9 kJ mol^-1, with a relative error of ∼1.6%. Density functional theory (DFT) calculations confirm the interface between NiCoP and MXene can effectively decrease the energy barrier of water dissociation by 16.0%. The three kinds of studies on macro, micro/nano, and atomic scales disclose the interfaces can reduce the kinetics energy barrier about 16.0-22.1%. Besides, the photothermal effect of MXenes can easily raise the catalyst temperature under vis-NIR light, which has been applied in practical scenarios under sunlight for energy savings.

Atomically Dispersed Cu Sites on Dual-Mesoporous N-Doped Carbon for Efficient Ammonia Electrosynthesis from Nitrate

The industrial Haber-Bosch process for ammonia synthesis is extremely important in modern society. However, it is energy intensive and leads to severe pollution, which has motivated eco-friendly NH₃ synthesis research. Electroreduction of contaminant nitrate ions back to NH₃ is an effective complement but is still limited by low NH₃ yields and nitrate-to-NH₃ selectivities. In this study, the electrochemical nitrate reduction reaction (NTRR) is carried out over a single-atom Cu catalyst. Atomically dispersed Cu sites anchored on dual-mesoporous N-doped carbon framework display excellent NTRR performance with NH₃ production rate of 13.8 mol NH 3  g_cat ^-1  h^-1 and NO₃ ^- -to-NH₃ faradaic efficiency (FE) of 95.5 % at -1.0 V. Cu-N-C catalyst can sustain continuous 120 h NTRR test in the simulated NH₃ synthesis scenarios with large current density (about 200 mA cm^-2 ) and amplified volume of NO₃ ^- solution (9 times). Theoretical calculations reveal that atomically dispersed Cu₁ -N₄ sites reduce the energy barrier of potential-determining step in NTRR and promote the decomposition of primary intermediate in NO₃ ^- -to-N₂ process. These findings provide a guideline for the rational design of highly active, selective and durable electrocatalysts for the NTRR.

Selective Electrooxidation of Biomass-Derived Alcohols to Aldehydes in a Neutral Medium: Promoted Water Dissociation over a Nickel-Oxide-Supported Ruthenium Single-Atom Catalyst

The biomass-derived alcohol oxidation reaction (BDAOR) holds great promise for sustainable production of chemicals. However, selective electrooxidation of alcohols to value-added aldehyde compounds is still challenging. Herein, we report the electrocatalytic BDAORs to selectively produce aldehydes using single-atom ruthenium on nickel oxide (Ru₁ -NiO) as a catalyst in the neutral medium. For electrooxidation of 5-hydroxymethylfurfural (HMF), Ru₁ -NiO exhibits a low potential of 1.283 V at 10 mA cm^-2 , and an optimal 2,5-diformylfuran (DFF) selectivity of 90 %. Experimental studies reveal that the neutral electrolyte plays a critical role in achieving a high aldehyde selectivity, and the single-atom Ru boosts HMF oxidation in the neutral medium by promoting water dissociation to afford OH*. Furthermore, Ru₁ -NiO can be extended to selective electrooxidation of a series of biomass-derived alcohols to corresponding aldehydes, which are conventionally difficult to obtain in the alkaline medium.

The Underlying Molecular Mechanism of Fence Engineering to Break the Activity-stability Trade-off of Catalysts

Non-precious-metal (NPM) catalysts often face the formidable challenge of a trade-off between long-term stability and high activity, which has not yet been widely addressed. Here we propose distinct molecule-selective fence as a promising novel concept to solve this activity-stability trade-off. This unique fence has the characteristics of preventing poisonous species from invading catalysts, but allowing catalytic reaction-related species to diffuse freely. We applied this concept to construct CoS2 layer with the function of molecular selectivity on the external surface of highly active Co doped MoS2, achieving a remarkable catalytic stability towards alkaline hydrogen evolution reaction, along with a further optimized activity. In situ spectroscopy technologies uncovered the underlying molecule mechanism of the CoS2 fence for breaking the activity-stability trade-off of the MoS2 catalyst. This work offers valuable guidance for rationally designing efficient and stable NPM catalysts.

An Overlooked Natural Hydrogen Evolution Pathway: Ni2+ Boosting H2 O Reduction by Fe(OH)2 Oxidation during Low-Temperature Serpentinization

Natural hydrogen (H₂ ) has gained considerable attentions as a renewable energy resource to mitigate the globally increasing environmental concerns. Low-temperature serpentinization (<200 °C) as a typical water-rock reaction is a major source of the natural H₂ . However, the reaction mechanism and the controlling step to product H₂ remained unclear, which hinders the further utilization of natural H₂ . Herein, we demonstrated that the H₂ production rate could be determined by the Fe(OH)₂ oxidation during low-temperature serpentinization. Moreover, the co-existence of Ni²⁺ could largely enhance the H₂ production kinetics. With the addition of only 1 % Ni²⁺ , the H₂ production rate was remarkably enhanced by about two orders of magnitude at 90 °C. D₂ O isotopic experiment and theoretical calculations revealed that the enhanced H₂ production kinetics could be attributed to the catalytic role of Ni²⁺ to promote the reduction of H₂ O.

Hierarchical Architectures Based on Ru Nanoparticles/Oxygen-Rich-Carbon Nanotubes for Efficient Hydrogen Evolution

Highly active and durable electrocatalysts are essential for producing hydrogen fuel through the hydrogen evolution reaction (HER). Here, a uniform deposition of Ru nanoparticles strongly interacting with oxygen-rich carbon nanotube architectures (Ru-OCNT) through ozonation and hydrothermal approaches has been designed. The hierarchical structure of Ru-OCNT is made by self-assembly of oxygen functionalities of OCNT. Ru nanoparticles interact strongly with OCNT at the Ru/OCNT interface to give excellent catalytic activity and stability of the Ru-OCNT, as further confirmed by density functional theory. Owing to the hierarchical structure and adjusted surface chemistry, Ru-OCNT has an overpotential of 34 mV at 10 mA cm^-2 with a Tafel slope of 27.8 mV dec^-1 in 1 M KOH, and an overpotential of 55 mV with Tafel slope of 33 mV dec^-1 in 0.5 M H₂ SO₄ . The smaller Tafel slope of Ru-OCNT than Ru-CNT and commercial Pt/C in both alkaline and acidic electrolytes indicates high catalytic activity and fast charge transfer kinetics. The as-proposed chemistry provides the rational design of hierarchically structured CNT/nanoparticle electrocatalysts for HER to produce hydrogen fuel.

The Electronic Metal-Support Interaction Directing the Design of Single Atomic Site Catalysts: Achieving High Efficiency Towards Hydrogen Evolution

It is still of great difficulty to develop the non-platinum catalyst with high catalytic efficiency towards hydrogen evolution reaction via the strategies till now. Therefore, it is necessary to develop the new methods of catalyst designing. Here, we put forward the catalyst designed by the electronic metal-support interaction (EMSI), which is demonstrated to be a reliable strategy to find out the high-efficiency catalyst. We carried out the density functional theory calculation first to design the proper EMSI of the catalyst. We applied the model of M1-M2-X (X=C, N, O) during the calculation. Among the catalysts we chose, the EMSI of Rh1TiC, with the active sites of Rh1-Ti2C2, is found to be the most proper one for HER. The electrochemical experiment further demonstrated the feasibility of the EMSI strategy. The single atomic site catalyst of Rh1-TiC exhibits higher catalytic efficiency than that of state-of-art Pt/C. It achieves a small overpotential of 22 mV and 86 mV at the at the current density of 10 mA cm^-2 and 100 mA cm^-2 in acid media, with a Tafel slope of 25 mV dec^-1 and a mass activity of 54403.9 mA cm^-2  mg_Rh ^-1 (vs. 192.2 mA cm^-2  mg_Pt ^-1 of Pt/C). Besides, it also shows appealing advantage in energy saving compared with Pt/C (≈20 % electricity consuming decrease at 2 kA m^-2 ) Therefore, we believe that the strategy of regulating EMSI can act as a possible way for achieving the high catalytic efficiency on the next step of SACs.