Advancing electrocatalytic reactions through mapping key intermediates to active sites via descriptors

Co-doped Bi24O31Br10 with photoswitchable Br, O binary vacancies synergistically provide dynamic active sites for N2 reduction

Ultra-thin two-dimensional bismuth oxyhalide with rich surface vacancies is an ideal material for photocatalytic nitrogen reduction reaction (NRR) of ammonia synthesis. However, the repair of these vacancies during the reaction invalidates its unique local microenvironmental advantage as an active site. Herein, ultra-thin Co-doped Bi24O31Br10 (Co-BOB) nanosheets with photo-switchable Br, O binary vacancies were synthesized by a hydrothermal method. The surface Br and O vacancies were generated under light and filled again with migrated Br- and O in solution at air atmosphere under dark. The Br vacancies serve as a medium for electron accumulation and transfer facilitating interlayer charge transfer. The O vacancies generate charge delocalization and lead to the local electron-deficient site, which promotes the adsorption and activation of N2. In addition, density functional theory calculation show that N2 reduction follows an alternating association pathway producing ammonia on the surface of Co-BOB. The photogenerated vacancies reduce the energy barrier of the first proton-coupled electron transfer process acting as a rate-determining step. The ammonia production rate of 5 % Co-BOB is 120.3 µmol g-1 h-1, which is 6.23 times higher than initial BOB. Meanwhile, the unique photoswitchable protection mechanism ensures the excellent recyclability of Co-BOB. Experiments and calculations reveal the role of formation of photoswitchable Br, O binary vacancies on Co-BOB nanosheets for efficient and stable NRR performance. This work provides a new strategy for promoting sustainable NRR by combining photoswitchable facilitated reaction with the active site regeneration.

Hydrogenation of "Readily Activated Molecule" for Glycine Electrosynthesis

The hydrogenation of glyoxylate oxime is the energy-intensive step in glycine electrosynthesis. To date, there has been a lack of rational guidance for catalyst design specific to this step, and the unique characteristics of the oxime molecule have often been overlooked. In this study, we initiate a theoretical framework to elucidate the fundamental mechanisms of glycine electrosynthesis across typical transition metals. By comprehensively analyzing the competitive reactions, proton-coupled electron transfer processes, and desorption steps, we identify the unique role of the glyoxylate oxime as a "readily activated molecule". This inherent property positions Ag, featuring weak adsorption characteristics, as the "dream" catalyst for glycine electrosynthesis. Notably, a record-low onset potential of -0.09 V versus RHE and an impressive glycine production rate of 1327 µmol h-1 are achieved when using an ultralight Ag foam electrode. This process enables gram-scale glycine production within 20 h and can be widely adapted for synthesizing diverse amino acids. Our findings underscore the vital significance of considering the inherent characteristics of reaction intermediates in catalyst design.

Measurements of Local pH Gradients for Electrocatalysts in the Oxygen Evolution Reaction by Electrochemiluminescence

An accurate understanding of the mechanism of the oxygen evolution reaction (OER) is crucial for catalyst design in the hydrogen energy industry. Despite significant advancements in microscopic pH detection, selective, sensitive, speedy, and reliable detection of local pH gradients near the catalysts during the OER remains elusive. Here, we pioneer an electrochemiluminescence (ECL) method for local pH detection during the OER. For this purpose, a new class of ECL emitters based on ECL resonance energy transfer was theoretically predicted and facilely synthesized by grafting functional fluorescent dyes onto noble 2D carbon nitride. By positioning one of the as-prepared ECL emitters with pH-responsibility neighboring the OER catalysts, local pH gradient generation near the catalysts could be qualitatively measured in real-time with a subsecond resolution. It provided details of the reaction mechanism of the OER and unveiled the catalyst degrading pathway caused by proton accumulation. Besides, the average proton generation rate on the catalyst was also extractable from the local pH measurement as a quantitative descriptor of the OER reaction rate. Owing to the high designability of the grafting method, this study opens up new strategies for studying reaction mechanisms and detecting intermediates.

Insights into the halogen-induced p-band center regulation promising high-performance lithium-sulfur batteries

Sn-based halide perovskites are expected to solve the problems of the shuttle effect and sluggish redox kinetics of lithium polysulfides (LiPSs) in lithium-sulfur batteries (LSBs) due to their high conductivity and electrocatalytic activity, but their intrinsic catalytic mechanism for LiPSs remains to be explored. Herein, halide perovskites with varying halide anions, Cs2SnX6 (X = Cl, Br, I), are purposefully designed to unveil the halogen-induced regulatory mechanism. Theoretical calculations demonstrate that increasing the halogen atomic number induces the shift of the p-band center closer to the Fermi level, which results in the localized charge distribution around halide anions and rapid charge separation/transfer at Sn sites, enhancing the adsorptive and catalytic activity and redox kinetics of LiPSs. Experimental investigations exhibit that LSBs assembled with the Cs2SnI6 modified separator deliver a high initial capacity of 1000 mA h g-1 at 2C, with a minimum decay rate of 0.068% per cycle after 500 cycles. More impressively, the Cs2SnI6 battery with a high sulfur loading (6.1 mg cm-2) and a low electrolyte/sulfur ratio (5.5 μL mg-1) achieves a remarkable reversible capacity of 768.8 mA h g-1, along with robust wide-temperature-tolerant cycling performance from -20 to 50 °C. These findings underscore the critical role of p-band center regulation in rationally designing advanced LSBs.

Rapid synthesis of metastable materials for electrocatalysis

Metastable materials are considered promising electrocatalysts for clean energy conversions by virtue of their structural flexibility and tunable electronic properties. However, the exploration and synthesis of metastable electrocatalysts via traditional equilibrium methods face challenges because of the requirements of high energy and precise structural control. In this regard, the rapid synthesis method (RSM), with high energy efficiency and ultra-fast heating/cooling rates, enables the production of metastable materials under non-equilibrium conditions. However, the relationship between RSM and the properties of metastable electrocatalysts remains largely unexplored. In this review, we systematically examine the unique benefits of various RSM techniques and the mechanisms governing the formation of metastable materials. Based on these insights, we establish a framework, linking RSM with the electrocatalytic performance of metastable materials. Finally, we outline the future directions of this emerging field and highlight the importance of high-throughput approaches for the autonomous screening and synthesis of optimal electrocatalysts. This review aims to provide an in-depth understanding of metastable electrocatalysts, opening up new avenues for both fundamental research and practical applications in electrocatalysis.

Surfactant-Assisted Synthesis of Polycrystalline Binary PdBi Alloy Nanoparticles for Ethanol Electrooxidation

The morphology and compositional modulation of palladium-based nanomaterials are of paramount importance in enhancing the catalytic performance of the ethanol oxidation reaction (EOR). In this study, we employed a simple oil bath method to synthesize PdBi alloy nanoparticles (NPs), maintaining the same elemental ratio. The size of the NPs could be modulated by varying the surfactant dosage. The synthesized PdBi alloys exhibited alterations in the d-band center of Pd, which were attributed to strain and ligand effects. These alterations subsequently affected the adsorption energy of the catalyst surface for the reaction intermediates. The current density of the Pd9Bi/200 NP in the ethanol oxidation reaction was found to be as high as 1042.0 mA mg-1, which is approximately twice that of Pd/C (508.0 mA mg-1). This enhanced activity can be attributed to the modification of the electronic structure and morphology, which enables the material to possess a larger electrochemically active surface area (ESCA), thus providing a substantial number of active sites for the catalytic reaction. Furthermore, the experimental data indicated that increasing the electrocatalytic temperature, pH, and ethanol concentration could accelerate the electrooxidation rate of the EOR. This study presents an effective approach for investigating the role of surfactants in the synthesis of nanomaterials and enhancing the activity and stability of noble metal catalysts.

A Simple Descriptor toward Optimizing Electrocatalytic N2 Oxidation to HNO3 Performance over Graphene-Based Single-Atom Catalysts

Single-atom catalysts (SACs) exhibit tremendous advantages in the electrochemical N2 oxidation reaction (EN2OR) to HNO3, which is an eco-friendly alternative to the synthesis of conventional industrial nitric acid and nitrates, but methods to rationally design and rapidly screen high-efficiency EN2OR SACs are unclear. Herein, taking pyridinic nitrogen-doped graphene-supported SACs as an example, a simple descriptor has been proposed to evaluate the EN2OR performance through systematically constructing a surface reaction phase diagram. This descriptor is comprised of merely the geometric information and inherent atomic properties (occupied d electron number, electronegativity, and coordinate number) that can accurately predict the activity and selectivity of EN2OR, independent of DFT simulations. Based on this descriptor, high-throughput screening has been executed on partially N/C/O coordinated SACs, including 160 candidates; 13 candidates with the overpotential of less than 1.0 V are selected and then validated by DFT calculations with a mean absolute error (MAE) as low as 0.09 V, indicating the reliability of the descriptor. Meanwhile, the screened CoO2N2-G and RhO2N2-G SACs exhibit lower EN2OR overpotential of 0.64 and 0.68 V and more negative UL(EN2OR) - UL(OER) values of -0.34 and -0.44 V in comparison to other candidates, respectively, demonstrating the excellent activity and selectivity of EN2OR. This work offers a route to rapid discovery of high-performance SACs for EN2OR.

Electrochemical Hydrogen Evolution with Metal-Organic Framework-Derived Catalysts: Strategies for d-Band Modulation by Electronic Structure Modification

The effective use of metal-organic framework (MOF)-based materials in the electrocatalytic hydrogen evolution reaction (HER) relies on the understanding of their structural and electronic properties. While the structure and morphology of MOF-derived catalysts significantly impact HER activity, tuning the d-band structure through electronic structure modulation has emerged as a key factor in optimizing catalytic performance. Techniques such as composition tuning, heteroatom doping, surface modification, and interface engineering were found to be effective methods for manipulating the electronic configuration and, in turn, modulating the d-band. This review systematically explores the design strategies for MOF-derived catalysts by focusing on electronic structure modulation. It provides a detailed discussion of the various methods - used to modulate the electronic structure. Furthermore, the review establishes the relationship between d-band tuning, Gibbs free energy, and electronic structure modulation, supported by both spectroscopic and theoretical evidences.

Digital Descriptors in Predicting Catalysis Reaction Efficiency and Selectivity

Accurately controlling the interactions and dynamic changes between multiple active sites (e.g., metals, vacancies, and lone pairs of heteroatoms) to achieve efficient catalytic performance is a key issue and challenge in the design of complex catalytic reactions involving 2D metal-supported catalysts, metal-zeolites, metal-organic catalysts, and metalloenzymes. With the aid of machine learning (ML), descriptors play a central role in optimizing the electrochemical performance of catalysts, elucidating the essence of catalytic activity, and predicting more efficient catalysts, thereby avoiding time-consuming trial-and-error processes. Three kinds of descriptors─active center descriptors, interfacial descriptors, and reaction pathway descriptors─are crucial for understanding and designing metal-supported catalysts. Specifically, vacancies, as active sites, synergize with metals to significantly promote the reduction reactions of energy-relevant small molecules. By combining some physical descriptors, interpretable descriptors can be constructed to evaluate catalytic performance. Future development of descriptors and ML models faces the challenge of constructing descriptors for vacancies in multicatalysis systems to rationally design the activity, selectivity, and stability of catalysts. Utilization of generative artificial intelligence and multimodal ML to automatically extract descriptors would accelerate the exploration of dynamic reaction mechanisms. The transferable descriptors from metal-supported catalysts to artificial metalloenzymes provide innovative solutions for energy conversion and environmental protection.

Ordered Pt3Mn Intermetallic Setting the Maximum Threshold Activity of Disordered Variants for Glycerol Electrolysis

Glycerol electrolysis is a promising strategy for generating hydrogen at the cathode and value-added products at the anode. However, the effect of the atomic distribution within catalysts on their catalytic performance remains largely unexplored, primarily because of the inherent complexity of the glycerol oxidation reaction (GOR). Herein, an ordered Pt3Mn (O-Pt3Mn) intermetallic compound and a disordered Pt3Mn (D-Pt3Mn) alloy are used as model catalysts, and their performance in the GOR and hydrogen evolution reaction (HER) is studied. O-Pt3Mn consistently outperforms D-Pt3Mn and commercial Pt/C catalysts. It can generate high-value glycerate at a notable production rate of 17 mM h-1 while achieving an impressively low cell voltage of 0.76 V for glycerol electrolysis, which is ∼0.98 V lower than that required for water electrolysis. Statistical analysis using theoretical calculations reveals that Pt-Pt-Pt hollow sites are crucial for the catalytic GOR and HER. The averaged adsorption energies of key intermediates (simplified as C*, O*, and H*) on diverse catalysts closely correlate with their experimentally observed activity. Our proposed linear models accurately predict these adsorption energies, exhibiting high correlation coefficients ranging from 0.97 to 0.99 and highlighting the significance of the distribution of the topmost and subsurface-corner Mn atoms in determining these adsorption energies. By sampling all possible Mn configurations within the fitted linear models, we confirm that O-Pt3Mn establishes the maximum activity threshold for the GOR and HER compared with any disordered variant. This study presents an innovative framework for exploring the effect of the atomic distribution within catalysts on their catalytic performance and designing high-performance catalysts for complex reactions.

Beyond conventional structures: emerging complex metal oxides for efficient oxygen and hydrogen electrocatalysis

The core of clean energy technologies such as fuel cells, water electrolyzers, and metal-air batteries depends on a series of oxygen and hydrogen-based electrocatalysis reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which necessitate cost-effective electrocatalysts to improve their energy efficiency. In the recent decade, complex metal oxides (beyond simple transition metal oxides, spinel oxides and ABO3 perovskite oxides) have emerged as promising candidate materials with unexpected electrocatalytic activities for oxygen and hydrogen electrocatalysis owing to their special crystal structures and unique physicochemical properties. In this review, the current progress in complex metal oxides for ORR, OER, and HER electrocatalysis is comprehensively presented. Initially, we present a brief description of some fundamental concepts of the ORR, OER, and HER and a detailed description of complex metal oxides, including their physicochemical characteristics, synthesis methods, and structural characterization. Subsequently, we present a thorough overview of various complex metal oxides reported for ORR, OER, and HER electrocatalysis thus far, such as double/triple/quadruple perovskites, perovskite hydroxides, brownmillerites, Ruddlesden-Popper oxides, Aurivillius oxides, lithium/sodium transition metal oxides, pyrochlores, metal phosphates, polyoxometalates and other specially structured oxides, with emphasis on the designed strategies for promoting their performance and structure-property-performance relationships. Moreover, the practical device applications of complex metal oxides in fuel cells, water electrolyzers, and metal-air batteries are discussed. Finally, some concluding remarks summarizing the challenges, perspectives, and research trends of this topic are presented. We hope that this review provides a clear overview of the current status of this emerging field and stimulate future efforts to design more advanced electrocatalysts.