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

Optimizing the Mass Transport and Atomic Fe Intrinsic Activity to Achieve High-Performing Fuel Cells

Due to the insufficient three-phase interfaces and high oxygen transport resistance, the high intrinsic activity cannot be sufficiently utilized in practical proton-exchange membrane fuel cells (PEMFCs). The efficient transport of protons and reactants within the catalyst layers (CL) is largely influenced by the pore structure of the carbon support, hosting both metal sites and ionomers. Herein, we constructed a porous nanosheet Pt-free catalyst (FeAC-N-SC) by selecting a highly nitrogen-rich GT-18 MOF via salt template to realize the improvement of PEMFC performance. The simulation and experimental results illustrate that the microstructure can benefit the homogeneous dispersion of ionomers and facilitate oxygen mass transport in the cathode CL, ultimately achieving efficient utilization of catalytic activities. The PEMFC assembled from the FeAC-N-SC catalyst exhibited an outstanding peak power density of 1.1 W cm-2 and durability (61% power density retention after AST-30k cycles and 92% voltage retention after 100 h OCV test). DFT results demonstrated that the introduction of Fe atomic clusters can boost the intrinsic activity of ORR by regulating the electron distribution of single-atomic Fe-N4 sites. This study reveals the relationship between CL design, mass transport, and electrode microstructure, which successfully exploits the intrinsic activity of cathode catalysts and enhances the power generation capacity.

Atomic-Level Insights into Cation-Mediated Mechanism in Electrochemical Nitrogen Reduction

The electrochemical nitrogen reduction reaction (NRR) provides a sustainable alternative to green ammonia synthesis. However, challenges persist due to limited accessibility of N2 molecules at the electrode interface and competition from abundant protons at catalytic active sites, resulting in low N2 coverage and compromised selectivity. In this work, we investigate the critical role of potassium cations (K+) in modulating the interfacial environment, particularly focusing on how varying K+ concentrations influence N2 adsorption, *NH3 desorption, and hydrogen transfer (HT) mechanisms under operating electrochemical conditions. Our results demonstrate that a highly concentrated K+ electrode interface significantly enhances N2 adsorption and *NH3 desorption, collectively leading to improved NRR selectivity, in alignment with the experimental observations. We further uncover insights into HT kinetics, identifying two key steps: protonation (HT1) and diffusion (HT2). Among these, diffusion (HT2) is the rate-limiting step, driven by hydrogen bond connectivity and proton shuttling strength within the cation-induced microenvironments. Specifically, at a low applied potential, a highly concentrated K+ interface exhibits weak connectivity and sluggish proton shuttling, therefore limiting NRR efficiency. However, microkinetic modeling (MKM) analysis indicates that optimizing electrode potential and electrolyte compositions can overcome these limitations by promoting proton shuttling. Last but not least, we also provide a detailed map of the interplay among K+ molarity, electrode potential, and NH3 selectivity. Our work offers critical insights to guide the improvement of NRR efficiency through electrolyte and microenvironmental modulation.

Rational Dual-Atom Design to Boost Oxygen Reduction Reaction on Iron-Based Electrocatalysts

The oxygen reduction reaction (ORR) is critical for energy conversion technologies like fuel cells and metal-air batteries. However, advancing efficient and stable ORR catalysts remains a significant challenge. Iron-based single-atom catalysts (Fe SACs) have emerged as promising alternatives to precious metals. However, their catalytic performance and stability remain constrained. Introducing a second metal (M) to construct Fe─M dual-atom catalysts (Fe─M DACs) is an effective strategy to enhance the performance of Fe SACs. This review provides a comprehensive overview of the recent advancements in Fe-based DACs for ORR. It begins by examining the structural advantages of Fe─M DACs from the perspectives of electronic structure and reaction pathways. Next, the precise synthetic strategies for DACs are discussed, and the structure-performance relationships are explored, highlighting the role of the second metal in improving catalytic activity and stability. The review also covers in situ characterization techniques for real-time observation of catalytic dynamics and reaction intermediates. Finally, future directions for Fe─M DACs are proposed, emphasizing the integration of advanced experimental strategies with theoretical simulations as well as artificial intelligence/machine learning to design highly active and stable ORR catalysts, aiming to expand the application of Fe─M DACs in energy conversion and storage technologies.

Asymmetric Manganese Sites in Covalent Organic Frameworks for Efficient Nitrate-to-Ammonia Electrocatalysis

The electrocatalytic nitrate reduction reaction (NO3 -RR) holds tremendous potential for remediating NO3 - pollution while enabling clean ammonia (NH3) production. However, most catalysts achieve high conversion efficiency relying on high NO3 - concentrations. How to catalyze the NO3 -RR with a low concentration of NO3 - is still a challenge due to the competing hydrogen evolution reaction (HER). Herein, we constructed a novel asymmetric isolated Mn atom based on N-coordination covalent organic framework (COF) (ImPy-COF-Mn), for efficient NO3 -RR at a low NO3 - concentration of 2 mg mL-1. This bidentate-coordinated COF featured a robust and chemically stable framework, while the synergistic interaction between asymmetric imine N and pyridine N modified the charge distribution of Mn atoms to optimize catalytic efficiency. ImPy-COF-Mn demonstrated remarkable catalytic performance, with 95.64% NH3 selectivity and a maximum NH3 yield rate of 1927 mmol h-1 gcat. -1, exceeding the corresponding parameters of symmetric Mn sites by factors of 1.27 and 1.41, respectively. In situ ATR-FTIR measurements and theoretical calculations revealed that the asymmetric isolated Mn facilitated a reduction in the energy barrier for *NO-to-*NOH conversion, and thus contributed to higher activity and selectivity.

Hydrophilic Single-Atom Interface Empowered Pure Formic Acid Fuel Cells

Single-atom catalysts (SACs), offering high mass activity and enhanced resistance to poisoning, are regarded as superior alternatives to traditional Pt/Pd nanocatalysts for direct formic acid fuel cells (DFAFCs). However, failure toward operation in concentrated formic acid (FA), which is critical for portable electronics, challenges their antipoisoning advantage and highlights a missing part in the understanding of the reaction. We herein demonstrate that the interfacial hydrophilicity of SACs is pivotal for high-performance DFAFCs, enabling, for the first time, stable operation with pure FA (>99%). By incorporating transition metal single atoms (Co, Fe, Ni, Ru) into Ir/NC catalysts, we engineered highly hydrophilic interfaces, as validated by molecular dynamics simulations and experimental studies. The optimized IrCo/NC anode exhibited a mass activity 342 times higher than that of nanoparticle-based catalysts and represented as the first SAC to achieve a higher peak power density (107.7 mW cm-2). A new reaction mechanism is revealed, where CO acts as a reactive intermediate rather than a poison. Further, in situ spectroscopy and isotope kinetic analyses identified water intermediate involvement in the rate-determining step, underscoring the critical role of hydrophilic interface engineering in DFAFC.

Highly Defective Ultrafine Carbon Nanoreactors Enriched with Edge-Type Zn-N3P1 Moiety Boosting Oxygen Electrocatalysis

High-active nonplatinum group metal oxygen reduction reaction (ORR) catalysts have great potential to improve fuel cell and metal-air battery performance due to their efficiency and cost-effectiveness. However, a fundamental understanding of their size-dependent structure-performance relationships remain elusive. Here a mesoporous-dominant carbon nanoreactor with dimensions in the range of 15-43 nm with edge-rich defective atomic Zn sites is designed. The crystal size and pore diameter of this carbon nanoreactors can be precisely adjusted to enable tunable mass diffusion pathways and porosities. Importantly, the hydrophobic nature of 25 nm nanoreactors maximizes the nonkinetic advantages of active site exposure and rapid O2 mass transfer at the triple-phase interface. The developed Zn-N-P/NPC catalysts delivers outstanding alkaline and acidic ORR performance with half-wave potentials of 0.92 and 0.80 V, respectively, as well as excellent zinc-air battery performance with charge/discharge over 400 h under 20 mA cm-2. X-ray absorption spectroscopy and theoretical calculations indicate that the enhanced ORR catalytic activity of Zn-N-P/NPC stems from the introduction of P atoms and edge carbon defects effectively exciting the localized electronic asymmetric distribution of Zn species. The findings provide new perspectives on the size effect of porous carbon supports for the development of efficient cathodes catalysts with multifunctionality.

Recent Progress in Oxygen Reduction Reaction Toward Hydrogen Peroxide Electrosynthesis and Cooperative Coupling of Anodic Reactions

Electrosynthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction reaction (2e- ORR) is a promising alternative to the anthraquinone oxidation process. To improve the overall energy efficiency and economic viability of this catalytic process, one pathway is to develop advanced catalysts to decrease the overpotential at the cathode, and the other is to couple 2e- ORR with certain anodic reactions to decrease the full cell voltage while producing valuable chemicals on both electrodes. The catalytic performance of a 2e- ORR catalyst depends not only on the material itself but also on the environmental factors. Developing promising electrocatalysts with high 2e- ORR selectivity and activity is a prerequisite for efficient H2O2 electrosynthesis, while coupling appropriate anodic reactions with 2e- ORR would further enhance the overall reaction efficiency. Considering this, here a comprehensive review is presented on the latest progress of the state-of-the-art catalysts of 2e- ORR in different media, the microenvironmental modulation mechanisms beyond catalyst design, as well as electrocatalytic system coupling 2e- ORR with various anodic oxidation reactions. This review also presents new insights regarding the existing challenges and opportunities within this rapidly advancing field, along with viewpoints on the future development of H2O2 electrosynthesis and the construction of green energy roadmaps.

Electric-Double-Layer Mechanism of Surface Oxophilicity in Regulating the Alkaline Hydrogen Electrocatalytic Kinetics

Regulating the surface oxophilicity of the electrocatalyst is known as an efficient strategy to mitigate the order-of-magnitude kinetic slowdown of hydrogen electrocatalysis in a base, which is of great scientific and technological significance. So far, its mechanistic origin remains mainly ascribed to the bifunctional or electronic effects that revolve around the catalyst-intermediate interactions and is under extensive debate. In addition, the understanding from the perspective of interfacial electric-double-layer (EDL) structures, which should also strongly depend on the electrode property, is still lacking. Here, by decorating a Pt electrode with Mo, Ru, Rh, and Au metal atoms to tune surface oxophilicity and systematically combining electrochemical activity tests, in situ surface-enhanced infrared absorption spectroscopy, density functional theory calculation, and ab initio molecular dynamics simulation, we found that there exist consistent volcano-type relationships between *OH adsorption strength and alkaline hydrogen evolution activity, the stretching/bending vibration information on interfacial water, and the potential of zero charge (PZC) of the electrode. This demonstrates that the origin of surface oxophilicity in impacting the alkaline hydrogen electrocatalytic activity lies in its modification toward the electrode PZC, which thereby dictates the electric field strength, rigidity, and hydrogen bonding network structure in EDL and ultimately governs the interfacial proton transfer kinetics. These findings emphasize the importance of focusing on electrocatalytic interface structures to understand electrode property-dependent reaction kinetics.

Water in Electrocatalysis

Renewable electricity-powered electrocatalysis technologies occupy a central position in clean energy conversion and the pursuit of a net-zero carbon emission future. Water can serve multiple roles in electrocatalytic reactions, for instance, as a reaction medium, reactant, modifier, promoter, etc. This significantly influences the mass transport, active site, intermediate adsorption and reaction kinetics, ultimately determining the electrocatalytic performance (e.g., activity, selectivity, and stability) as well as device efficiency. As the heart location where electrocatalytic reactions occur, the typical electrical-double layer is established at a water-electrode interface. Therefore, the comprehension and regulation of water are crucial topics in electrocatalysis, which encourages us to organize this review. We begin with the fundamental understanding on structure of water and its behavior under electrochemical conditions. Subsequently, we delve into the "water effect" by elucidating specific functions of water in electrocatalysis. Recent advances in manipulating water to enhance electrocatalytic efficiency of representative reactions such as hydrogen evolution/oxidation, oxygen evolution/reduction, CO2 reduction, N2 reduction and organic electrosynthesis, are also highlighted. We finally discuss the remaining challenges and future opportunities in this field.

Selective two-electron and four-electron oxygen reduction reactions using Co-based electrocatalysts

The oxygen reduction reaction (ORR) can take place via both four-electron (4e-) and two-electron (2e-) pathways. The 4e- ORR, which produces water (H2O) as the only product, is the key reaction at the cathode of fuel cells and metal-air batteries. On the other hand, the 2e- ORR can be used to electrocatalytically synthesize hydrogen peroxide (H2O2). For the practical applications of the ORR, it is very important to precisely control the selectivity. Understanding structural effects on the ORR provides the basis to control the selectivity. Co-based electrocatalysts have been extensively studied for the ORR due to their high activity, low cost, and relative ease of synthesis. More importantly, by appropriately designing their structures, Co-based electrocatalysts can become highly selective for either the 2e- or the 4e- ORR. Therefore, Co-based electrocatalysts are ideal models for studying fundamental structure-selectivity relationships of the ORR. This review starts by introducing the reaction mechanism and selectivity evaluation of the ORR. Next, Co-based electrocatalysts, especially Co porphyrins, used for the ORR with both 2e- and 4e- selectivity are summarized and discussed, which leads to the conclusion of several key structural factors for ORR selectivity regulation. On the basis of this understanding, future works on the use of Co-based electrocatalysts for the ORR are suggested. This review is valuable for the rational design of molecular catalysts and material catalysts with high selectivity for 4e- and 2e- ORRs. The structural regulation of Co-based electrocatalysts also provides insights into the design and development of ORR electrocatalysts based on other metal elements.

Metal-organic double layer to stabilize selective multi-carbon electrosynthesis

Stable operation of the gas diffusion electrodes is key for industrial-scale electrochemical CO2 reduction (eCO2R). To enhance the electrolytic stability, we shield the Cu-coated gas diffusion electrode with a polycationic sheath via electrospinning and propose a Metal-Organic Double Layer (MODL) scheme to depict the triphasic interface. The as-fabricated electrode exhibits a high multi-carbon Faradaic efficiency of 91.2 ± 3.8%, along with operational stability for over 300 h at 300 mA cm-2 in an alkaline flow cell. In a membrane electrode assembly with pure water as the anolyte, it further achieves an ethylene Faradaic efficiency over 50% at 200 mA cm-2. Mechanistic investigations unveil that replacing hydrated cationic counter ions in the conventional double layer with hydrogen bond-woven polycationic groups in the MODL allows simultaneously tailoring the local electric field and interfacial water structure. This study introduces a molecular-level redesign of the electric double layer in eCO2R systems, achieving precisely tunable electrostatic characteristics and tailored chemical microenvironments while leveraging sustainable electrolysis systems to enable highly efficient and stable multi-carbon production.

Ink formulation and electrolytes affect electrochemical oxygen reduction into H2O2: a kinetic study

The electrocatalytic production of hydrogen peroxide (H2O2) through the 2-electron oxygen reduction reaction (2e ORR) has garnered significant research interest as an appealing alternative to the conventional anthraquinone process. To advance industrial applications, carbon materials emerge as one of the most promising catalysts for the 2e ORR. The evaluation of their electrochemical activity and selectivity is the most fundamental of research in this field. However, tremendous studies have been reported by using various ink formulations and electrolytes, neglecting their underlying influences on apparent ORR kinetics. By employing carbon black as the model catalyst, this study investigates the impact of ink formulations, catalyst loading, and electrolytes on the efficiency and selectivity of the 2e ORR. Results indicated that an optimal ink formulation containing 10 μL of Nafion and 1000 μL of alcohol exhibited superior efficiency for H2O2 generation. Moreover, the optimal H2O2 generation over a gas diffusion electrode can be acquired with a current density of 0.8 mA cm-2 and an electron transfer number of 2 under a catalyst loading of 0.1 mg cm-2. Besides, the linear correlation between transferred electron numbers and electrolyte concentrations indicates that the ideal 2e ORR can be achieved in 0.98 mol L-1 KOH. This study describes the effects of carbon-based electrode preparation methodologies and offers basic insights into the electrochemical synthesis of H2O2.

Multichannel Photon Stimulated C-C Coupling for CO2 Reduction in a Mixed Water/Acetonitrile Solvent

Designing effective photocatalysts for carbon dioxide reduction reaction (CO2RR) requires a precise understanding of the dynamically photocatalytic mechanism under real conditions (e.g. solvent, light field). Herein, we coupled ab-initio non-adiabatic molecular dynamics (NAMD) simulation and density functional theory (DFT) calculation to theoretically reveal the detailed dynamic process for C-C Coupling under light field on Co-supported monolayer black phosphorus (BP) catalyst Co@BP. Specially, Co@BP features excellent property for photocatalyst: high stability, long-lived photogenerated carriers and stronger reducing ability. Thermodynamically, it shown that a dramatic difference in catalytic properties with 99.99 % selectivity for HCOOH in solvent-free condition (Ea=0.29 eV) and 99.34 % selectivity for CH2CH2 in solution (Ea=0.33 eV). Slow-growth based MD simulation results uncover that mixed solvent of water/acetonitrile (H2O/ACN) is beneficial for the formation of C2+ product and the optimal ratio H2O/ACN (1 : 9) mixture solvent for the conversion of CO2 to CH2CH2. Under light irradiation, we found that multichannel photons enable adsorbed CHO* to couple with CHO species diffusing from nearby active sites, further forming C2 intermediates in solution. This work highlights the importance of the reaction medium on the photogenerative carrier dynamics and offers a strategy to regulate product selectivity in photocatalytic CO2 conversion.

Precisely tuning the electronic states of organic polymer electrocatalysts via thiophene-based moieties for enhanced oxygen reduction reaction

Optimizing molecular structures in oxygen reduction reaction (ORR) is crucial for enhancing catalytic efficiency and stability, particularly with respect to the effective adsorption and conversion of reaction intermediates. Sulfur-containing heterocyclic compound thiophene can precisely modulate the electronic states and local charge densities, thereby fine-tuning the adsorption and reactivity of microporous polymers, yet, it remains a largely unexplored area. Herein, thiophene-based building blocks featuring diversified linkers into a phenyl-containing model Ph-CMP are developed, affording the thiophene-fused BPT-CMP and the thiophene-linked BCT-CMP. The electron density and adsorption capacity of the frameworks are well regulated through condensation and connecting modification, showing excellent half-wave potentials compared to the reversible hydrogen electrode, surpassing even most metal-free polymer electrocatalysts. Through theoretical calculations and experimental results, we have validated that the thiophene-fused skeleton (BPT-CMP) triggers the activation of thiophene units, with the exposed pentatomic heterocyclic carbon atom (site-3) serving as the catalytic active site.

High-throughput Design of Single-atom Catalysts with Nonplanar and Triple Pyrrole-N Coordination for Highly Efficient H2O2 Electrosynthesis

Single-atom catalysts (SACs) with nonplanar configurations possess unique capabilities for tailoring the oxygen reduction reaction (ORR) catalytic performance compared with the ones with planar configurations, owing to the additional orbital rearrangement arising from the asymmetric coordination atoms. However, the systematic investigation of these nonplanar SACs has long been hindered by the difficulty in screening feasible nonplanar configurations and precisely controlling the coordination structures. Herein, we demonstrate a combined high-throughput screening and experimental verification of nonplanar SACs (ppy-MN3) with metal atoms triple-coordinated by pyrrole-N, for highly active and selective 2e- ORR electrocatalysis. With the additional p-orbital rearrangement of N-ligands for ppy-MN3 during catalysis, a new descriptor on the energy difference between d-band center of metal sites and p-band centers of N-ligands (Δϵd-p) is proposed to accurately identify the relationship between their catalytic activities and electronic structures, on top of the conventional d-band center theory. Consequently, ppy-ZnN3 is identified with excellent 2e- ORR activity (η=0.08 eV) and selectivity, as well as a low 2e- ORR kinetic barrier under alkaline condition owing to a strong hydrogen bonding between OOH* intermediate and interfacial water, which is then experimentally verified by its high electrocatalytic H2O2 yield (43 mol g-1 h-1) and selectivity (92 %) under alkaline condition. This study thus presents a proof-of-concept demonstration of the performance-oriented and precise coordination design of nonplanar SACs for efficient H2O2 electrosynthesis, and, more importantly, provides an essential complement to the d-band theory for more accurately predicting the catalytic activities of catalysts with nonplanar configurations for series potential electrochemical processes.

Enhanced Activity and Stability for Electrocatalytic Nitrate Reduction to Ammonia over Low-Coordinated Cobalt

It is still challenging to develop an effective strategy to simultaneously enhance the activity and stability of electrocatalysts for electrocatalytic nitrate reduction reaction (eNO3RR). Herein, taking metallic cobalt as an example, it is demonstrated that the construction of low-coordinated cobalt nanosheets (L-Co NSs) by H2 plasma etching of electrodeposited cobalt nanosheets (Co NSs) can greatly enhance the activity and stability of metallic cobalt for eNO3RR. Compared with Co NSs, at -0.4 V versus RHE, the nitrate removal rate, ammonia partial current density, and ammonia yield are increased by L-Co NSs from 82.14% to 98.57%, from 476 to 683 mA cm-2, and from 2.11 to 2.54 mmol h-1 cm-2, respectively. In addition, L-Co NSs demonstrate negligible activity decay after 30 cycles of stability test, while the Co NSs show significant activity decline. In situ electrochemical characterizations and theoretical calculations verify that the abundance of Co vacancies in L-Co NSs not only contribute to the optimized electronic structure and enhanced desorption of key intermediate to boost the activity but also facilitate the transformation of Co(OH)2 to Co0 to promote the stability. Furthermore, L-Co NSs exhibit favorable performance in removing nitrate from simulated wastewater and air plasma discharge-electrocatalytic reduction cascade system to produce ammonia.

Acidic and Alkaline pH Controlled Oxygen Reduction Reaction Pathway over Co-N4C Catalyst

Enhanced oxygen reduction reaction (ORR) kinetics and selectivity are crucial to advance energy technologies like fuel cells and metal-air batteries. Single-atom catalysts (SACs) with M-N4/C structure have been recognized to be highly effective for ORR. However, the lack of a comprehensive understanding of the mechanistic differences in the activity under acidic and alkaline environments is limiting the full potential of the energy devices. Here, a porous SAC is synthesized where a cobalt atom is coordinated with doped nitrogen in a graphene framework (pCo-N4C). The resulting pCo-N4C catalyst demonstrates a direct 4e- ORR process and exhibits kinetics comparable to the state-of-the-art (Pt/C) catalyst. Its higher activity in an acidic electrolyte is attributed to the tuned porosity-induced hydrophobicity. However, the pCo-N4C catalyst displays a difference in ORR activity in 0.1 m HClO4 and 0.1 m KOH, with onset potentials of 0.82 V and 0.91 V versus RHE, respectively. This notable activity difference in acidic and alkaline media is due to the protonation of coordinated nitrogen, restricted proton coupled electron transfer (PCET) at the electrode/electrolyte interface. The effect of pH over the catalytic activity is further verified by Ab-initio molecular dynamics (AIMD) simulations using density functional theory (DFT) calculations.

Engineering the Lewis Acidity of Fe Single-Atom Sites via Atomic-Level Tuning of Spatial Coordination Configuration for Enhanced Oxygen Reduction

Nitrogen-doped carbon-supported Fe catalysts (Fe-N-C) with Fe-N4 active sites hold great promise for the oxygen reduction reaction (ORR). However, fine-tuning the structure of Fe-N4 active sites to enhance their performance remains a grand challenge. Herein, we report an innovative design strategy to promote the ORR activity and kinetics of Fe-N4 sites by engineering their Lewis acidity, which is achieved by tuning the spatial Fe coordination geometry. Theoretical calculations indicated that Fe1-N4SO2 sites (with an axial -SO2 group bonded to Fe) offered favorable Lewis acidity for the ORR, leading to optimized adsorption energies for the key ORR intermediates. To implement this strategy, we developed a molecular-cage-encapsulated coordination strategy to synthesize a Fe single-atom site catalyst (SAC) with Fe1-N4SO2 sites. In agreement with theory, the Fe1-N4SO2/NC catalyst demonstrated outstanding ORR performance in both alkaline (E1/2 = 0.910 V in 0.1 M KOH) and acidic media (E1/2 = 0.772 V in 0.1 M HClO4), surpassing commercial Pt/C and traditional Fe SACs with Fe1-N4 sites or planar S-coordinated Fe1-N4-S sites. Moreover, this newly developed catalyst showed great application potential in quasi-solid-state Zn-air batteries, delivering superior performance across a wide temperature range.

Proton Relay in Hydrogen-Bond Networks Promotes Alkaline Hydrogen Evolution Electrocatalysis

Common O-/H-down orientation of H2O molecules on electrocatalysts brings favorable OH/H delivery; however, adverse H/OH delivery in their dissociation process hampers the H2O dissociation kinetics of the alkaline hydrogen evolution reaction (HER). To overcome this challenge, we raised a synergetic H2O dissociation concept of metal-supported electrocatalysts involving efficient OH delivery from O-down H2O to the metal, timely proton relay from O-down H2O on the metal to H-down H2O on the support through the hydrogen-bond network, and prompt H delivery from H-down H2O to the support. After theoretically profiling that a high work function difference between the metal and the support (ΔΦ) induces a strong electric field at the metal-support interface that increases hydrogen-bond connectivity to promote proton relay, we practiced this concept over cobalt phosphide-supported ruthenium (Ru/CoP) catalysts with a high ΔΦ = 0.4 eV, achieving a record-high Ru utilization HER activity of 66.1 A mgRu-1 at -0.1 V vs RHE. The insights into this synergetic H2O dissociation mechanism provide opportunity for the design of bicomponent alkaline HER electrocatalysts.

Converting Fe-N-C Single-atom Catalyst to a New FeNxSey Cluster Catalyst for Proton-exchange Membrane Fuel Cells

Iron-nitrogen-carbon (Fe-N-C) single-atom catalyst is the most promising alternative to platinum catalyst for proton-exchange membrane fuel cells (PEMFCs), however its high performance cannot be maintained for a long enough time in device operation. The construction of a new Fe coordination environment that is completely different from the square-planar Fe-N4 configuration in classic Fe-N-C catalyst is expected to break the current stability limits of Pt-free catalysts, which however remains unexplored. Here, we report, for the first time, the conversion of Fe-N-C catalyst to a new FeNxSey cluster catalyst, where the active Fe sites are three-dimensionally (3D) co-coordinated by N and Se atoms. Due to this unique Fe coordination configuration, the FeNxSey catalyst exhibits much better 4e- ORR activity and selectivity than the state-of-the-art Fe-N-C catalyst. Specifically, the yields of hydrogen peroxide (H2O2) and ⋅OH radicals on the FeNxSey catalyst are only one-quarter and one-third of that on the Fe-N-C counterpart, respectively. Therefore, the FeNxSey catalyst exhibits outstanding cyclic stability, losing only 10 mV in half-wave potential E1/2 after 10,000 potential cycles, much smaller than that of the Fe-N-C catalyst (56 mV), representing the most stable Pt-free catalysts ever reported for PEMFCs. More significantly, the 3D co-coordination structure effectively inhibits the Fe demetallization of the FeNxSey catalyst in the presence of H2O2. As a result, the FeNxSey based PEMFC shows excellent durability, with the current density attenuation significantly lower than that of the Fe-N-C based device after accelerated durability testing. Our work provides guidance for the development of next-generation Pt-free catalysts for PEMFCs.

Kinetic cation effect in alkaline hydrogen electrocatalysis and double layer proton transfer

Unveiling the so far ambiguous mechanism of the significant dependence on the identity of alkali metal cation would prompt opportunities to solve the more than two orders of magnitude slowdown of hydrogen electrocatalytic kinetics in base relative to acid, which has hampered the effort to reduce the precious metal usage in fuel cells by using the hydroxide exchange membrane. Herein, we present atomic-scale evidences from ab-initio molecular dynamics simulation and in-situ surface-enhanced infrared absorption spectroscopy which show that it is the apparent discrepancies in the electric double-layer structures induced by differently sized cations that lead to largely different interfacial proton transfer barriers and therefore hydrogen electrocatalytic kinetics in base. Concretely, severe accumulation of larger cation in electric double-layer causes more discontinuous interfacial water distribution and H-bond network, thus rendering the proton transfer from bulk to interface more obstructed. Such notion is strikingly different from the previously envisioned impact of cation-intermediate interactions on the energetics of surface steps, providing a unique interfacial perspective for understanding the ubiquitous cation specificity in electrocatalysis.

Dynamics and kinetics exploration of the oxygen reduction reaction at the Fe-N4/C-water interface accelerated by a machine learning force field

Understanding the oxygen reduction reaction (ORR) mechanism and accurately characterizing the reaction interface are essential for improving fuel cell efficiency. We developed an active learning framework combining machine learning force fields and enhanced sampling to explore the dynamics and kinetics of the ORR on Fe-N4/C using a fully explicit solvent model. Different possible reaction paths have been explored and the O2 adsorption process is confirmed as the rate-determining step of the ORR at the Fe-N4/C-water interface, which needs to overcome a free energy barrier of 0.39 eV. By statistical analysis of solvent configurations for proton-coupled electron transfer (PCET) processes, it is revealed that the configurations of interface water remarkably influence the reaction efficiency. More hydrogen bonds and longer lifetimes facilitate the PCET reactions and even make them barrierless. Our theoretical framework highlights the significance of solvent configurations in determining free energy barriers, and offers new insights into the reaction mechanism of the ORR on Fe-N4/C catalysts.

Confined Water for Catalysis: Thermodynamic Properties and Reaction Kinetics

Water is a salient component in catalytic systems and acts as a reactant, product and/or spectator species in the reaction. Confined water in distinct local environments can display significantly different behaviors from that of bulk water. Therefore, the wide-ranging chemistry of confined water can provide tremendous opportunities to tune the reaction kinetics. In this review, we focus on drawing the connection between confined water properties and reaction kinetics for heterogeneous (electro)catalysis. First, the properties of confined water are presented, where the enthalpy, entropy, and dielectric properties of water can be regulated by tuning the geometry and hydrophobicity of the cavities. Second, experimental and computational studies that investigate the interactions between water and inorganic materials, such as carbon nanotubes (1D confinement), charged metal or metal oxide surfaces (2D), zeolites and metal-organic frameworks (3D) and ions/solvent molecules (0D), are reviewed to demonstrate the opportunity to create confined water structures with unique H-bonding network properties. Third, the role of H-bonding structure and dynamics in governing the activation free energy, reorganization energy and pre-exponential factor for (electro)catalysis are discussed. We highlight emerging opportunities to enhance proton-coupled electron transfer by optimizing interfacial H-bond networks to regulate reaction kinetics for the decarbonization of chemicals and fuels.

Innovative strategies for designing and constructing efficient fuel cell electrocatalysts

Polymer electrolyte membrane fuel cells (PEMFCs) are one of the most promising energy conversion devices due to their high efficiency and zero emission; however, two major challenges, high cost and short lifetime, have been hindering the commercialization of fuel cells. Achieving low-Pt or non-precious metal oxygen reduction reaction (ORR) electrocatalysts is one of the main research ideas in this field. In this review, the degradation mechanism of Pt-based catalysts is firstly explained and elucidated, and then five strategies are suggested for the reduction of Pt usage without loss of activity and durability: modulation of metal-support interactions, optimization of local ionomers and mass transport, modulation of composition, modulation of structure, and multi-site synergistic effects. For carbon-based non-precious metal catalysts, the problems and challenges faced by heteroatom/transition-metal doped carbon-based catalysts are discussed, and several strategies to improve the activity of heteroatom/transition-metal doped carbon catalysts are suggested. Particularly, an innovative quantum well catalyst structure reported quite recently is presented which may open up new prospects for the development of fuel cell technology. Finally, this review concludes with a brief conclusion and prospects for future development of low-Pt and non-precious metal fuel cell electrocatalysts.

TiN Boosting the Oxygen Reduction Performance of Fe-N-C through the Relay-Catalyzing Mechanism for Metal-Air Batteries

Metal-air batteries desire highly active, durable, and low-cost oxygen reduction catalysts to replace expensive platinum (Pt). The Fe-N-C catalyst is recognized as the most promising candidate for Pt; however, its durability is hindered by carbon corrosion, while activity is restricted due to limited oxygen for the reaction. Herein, TiN is creatively designed to be hybridized with Fe-N-C (TiN/Fe-N-C) to relieve carbon corrosion and absorb more oxygen when catalyzing oxygen reduction. The half-wave potential of TiN/Fe-N-C is 0.915 V vs reverse hydrogen electrode with 15 mV lost after 30,000 cycles accelerated durability test, higher than 0.893 V and 26 mV of Pt/C. The solid zinc-air battery of TiN/Fe-N-C achieves a peak power density of 238 mW/cm2, 2100 cycle stability at 30 °C, and long-term durability of 1100 h under -20 °C, superior to 150 mW/cm2 and 500 h (-20 °C) of Pt/C. Both calculations and experiments indicate that TiN has dual functions which not only relay abundant oxygen for the reaction but also strengthen the adsorption force for intermediates of carbon corrosion reaction, thus, enhancing the activity and durability of Fe-N-C. Therefore, the proposed relay catalytic strategy by TiN offers an efficient Fe-N-C catalyst for energy conversion devices.

Transforming Adsorbate Surface Dynamics in Aqueous Electrocatalysis: Pathways to Unconstrained Performance

Developing highly efficient catalysts to accelerate sluggish electrode reactions is critical for the deployment of sustainable aqueous electrochemical technologies, yet remains a great challenge. Rationally integrating functional components to tailor surface adsorption behaviors and adsorbate dynamics would divert reaction pathways and alleviate energy barriers, eliminating conventional thermodynamic constraints and ultimately optimizing energy flow within electrochemical systems. This approach has, therefore, garnered significant interest, presenting substantial potential for developing highly efficient catalysts that simultaneously enhance activity, selectivity, and stability. The immense promise and rapid evolution of this design strategy, however, do not overshadow the substantial challenges and ambiguities that persist, impeding the realization of significant breakthroughs in electrocatalyst development. This review explores the latest insights into the principles guiding the design of catalytic surfaces that enable favorable adsorbate dynamics within the contexts of hydrogen and oxygen electrochemistry. Innovative approaches for tailoring adsorbate-surface interactions are discussed, delving into underlying principles that govern these dynamics. Additionally, perspectives on the prevailing challenges are presented and future research directions are proposed. By evaluating the core principles and identifying critical research gaps, this review seeks to inspire rational electrocatalyst design, the discovery of novel reaction mechanisms and concepts, and ultimately, advance the large-scale implementation of electroconversion technologies.

Interfacial Water Orientation in Neutral Oxygen Catalysis for Reversible Ampere-Scale Zinc-Air Batteries

The neutral oxygen catalysis is an electrochemical reaction of the utmost importance in energy generation, storage application, and chemical synthesis. However, the restricted availability of protons poses a challenge to achieving kinetically favorable oxygen catalytic reactions. Here, we alter the interfacial water orientation by adjusting the Brønsted acidity at the catalyst surface, to break the proton transfer limitation of neutral oxygen electrocatalysis. An unexpected role of water molecules in improving the activity of neutral oxygen catalysis is revealed, namely, increasing the H-down configuration water in electric double layers rather than merely affecting the energy barriers for reaction limiting steps. The proposed porous nanofibers with atomically dispersed MnN3 exhibit record-breaking activity (EORR@1/2/EOER@10 mA = 0.85/1.65 V vs. RHE) and reversibility (2500 h), outperforming all previously reported neutral catalysts and rivaling conventional alkaline systems. In particular, practical ampere-scale zinc-air batteries (ZABs) stack are constructed with a capacity of 5.93 Ah and can stably operate under 1.0 A and 1.0 Ah conditions, demonstrating broad application prospects. This work provides a novel and feasible perspective for designing neutral oxygen electrocatalysts and reveals the future commercial potential in mobile power supply and large-scale energy storage.

Importing Atomic Rare-Earth Sites to Activate Lattice Oxygen of Spinel Oxides for Electrocatalytic Oxygen Evolution

Spinel oxides have emerged as highly active catalysts for the oxygen evolution reaction (OER). Owing to covalency competition, the OER process on spinel oxides often follows an arduous adsorbate evolution mechanism (AEM) pathway. Herein, we propose a novel rare-earth sites substitution strategy to tune the lattice oxygen redox of spinel oxides and bypass the AEM scaling relationship limitation. Taking NiCo2O4 as a model, the incorporation of Ce into the octahedral site induces the formation of Ce-O-M (M=Ni, Co) bridge, which triggers charge redistribution within NiCo2O4. The developed Ce-NiCo2O4 exhibits remarkable OER activity with a low overpotential, satisfactory electrochemical stability, and good practicability in anion-exchange membrane water electrolyzer. Theoretical analyses reveal that OER on Ce-NiCo2O4 surface follows a more favorable lattice oxygen mechanism (LOM) pathway and non-concerted proton-electron transfers compared to pure NiCo2O4, as also verified by pH-dependent behavior and in situ Raman analysis. The 18O-labeled electrochemical mass spectrometry provides direct evidence that the oxygen released during the OER originates from the lattice oxygen of Ce-NiCo2O4. We discover that electron delocalization of Ce 4f states triggers charge redistribution in NiCo2O4 through the Ce-O-M bridge, favoring antibonding state occupation of Ni-O bonding in [Ce-O-Ni] unit site, thereby activating lattice oxygen redox of NiCo2O4 in OER. This work provides a new perspective for designing highly active spinel oxides for OER and offers significant insights into the rare-earth-enhanced LOM mechanism.

Designer topological-single-atom catalysts with site-specific selectivity

Designing catalysts with well-defined, identical sites that achieve site-specific selectivity, and activity remains a significant challenge. In this work, we introduce a design principle of topological-single-atom catalysts (T-SACs) guided by density functional theory (DFT) and Ab initio molecular dynamics (AIMD) calculations, where metal single atoms are arranged in asymmetric configurations that electronic shield topologically misorients d orbitals, minimizing unwanted interactions between reactants and the support surface. Mn1/CeO2 catalysts, synthesized via a charge-transfer-driven approach, demonstrate superior catalytic activity and selectivity for NOx removal. A life-cycle assessment (LCA) reveals that Mn1/CeO2 significantly reduces environmental impact compared to traditional V-W-Ti catalysts. Through in-situ spectroscopic characterizations combined with DFT calculations, we elucidate detailed reaction mechanisms. This study establishes T-SACs as a promising class of catalysts, offering a systematic framework to address catalytic challenges by defining site characteristics. The concept highlights their potential for advancing selective catalytic processes and promoting sustainable technologies.

Achievements and Challenges in Surfactants-Assisted Synthesis of MOFs-Derived Transition Metal-Nitrogen-Carbon as a Highly Efficient Electrocatalyst for ORR, OER, and HER

Metal-organic frameworks (MOFs) are excellent precursors for preparing transition metal and nitrogen co-doped carbon catalysts, which have been widely utilized in the field of electrocatalysis since their initial development. However, the original MOFs derived catalysts have been greatly limited in their development and application due to their disadvantages such as metal atom aggregation, structural collapse, and narrow pore channels. Recently, surfactants-assisted MOFs derived catalysts have attracted much attention from researchers due to their advantages such as hierarchical porous structure, increased specific surface area, and many exposed active sites. This review mainly focuses on the synthesis methods of surfactants-assisted MOFs derived catalysts and comprehensively introduces the action of surfactants in MOFs derived materials and the structure-activity relationship between the catalysts and the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction performance. Apparently, the aims of this review not only introduce the status of surfactants-assisted MOFs derived catalysts in the field of electrocatalysis but also contribute to the rational design and synthesis of MOFs derived catalysts for fuel cells, metal-air cells, and electrolysis of water toward hydrogen production.

Optimizing the Ratio of Metallic and Single-Atom Co in CoNC via Annealing Temperature Modulation for Enhanced Bifunctional Oxygen Evolution Reaction/Oxygen Reduction Reaction Activity

Developing low-cost, efficient alternatives to catalysts for bifunctional oxygen electrode catalysis in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical for advancing the practical applications of alkaline fuel cells. In this study, Co particles and single atoms co-loaded on nitrogen-doped carbon (CoNC) were synthesized via pyrolysis of a C3N4 and cobalt nitrate mixture at varying temperatures (900, 950, and 1000 °C). The pyrolysis temperature and precursor ratios were found to significantly influence the ORR/OER performance of the resulting catalysts. The optimized CoNC-950 catalyst demonstrated exceptional ORR (E1/2 = 0.85 V) and OER (Ej10 = 320 mV) activities, surpassing commercial Pt/C + RuO2-based devices when used in a rechargeable zinc-air battery. This work presents an effective strategy for designing high-performance non-precious metal bifunctional electrocatalysts for alkaline environments.

Understanding the interfacial water structure in electrocatalysis

The structure of interfacial water molecules plays a crucial role in modulating the electrochemical surface kinetics. This article provides an in-depth understanding of the water molecule structure inside the double layer and its main influencing factors at the molecular scale.

Recent advances in microenvironment regulation for electrocatalysis

High-efficiency electrocatalysis could serve as the bridge that connects renewable energy technologies, hydrogen economy and carbon capture/utilization, promising a sustainable future for humankind. It is therefore of paramount significance to explore feasible strategies to modulate the relevant electrocatalytic reactions and optimize device performances so as to promote their large-scale practical applications. Microenvironment regulation at the catalytic interface has been demonstrated to be capable of effectively enhancing the reaction rates and improving the selectivities for specific products. In this review we summarize the latest advances in microenvironment regulation in typical electrocatalytic processes (including water electrolysis, hydrogen-oxygen fuel cells, and carbon dioxide reduction) and the related in situ/operando characterization techniques and theoretical simulation methods. At the end of this article, we present an outlook on development trends and possible future directions.

Evolution of Nitrogen-Coordinated Metal Single Atoms Toward Single-Atom Alloys on MgH₂ as Efficient and Stable Hydrogen Spillover Catalysts

M-N-C catalysts with nitrogen-coordinated metal single-atom active sites have demonstrated high activity for hydrogen storage materials, but their stability in this application remains uncertain. This study addresses this issue by using nickel phthalocyanine (NiPc) molecules on MgH₂ particles as a model system. It is found that the N-coordinated high-valence Ni single atoms in the NiN₄ active site are unstable in the reducing environment of hydrogen storage, spontaneously evolving into zero-valence Ni, forming a Ni₁-Mg single-atom alloy (SAA). The Ni₁-Mg SAA exhibits remarkable stability in catalyzing Mg hydrogen storage reactions. Furthermore, it demonstrates comprehensive catalytic activity for each step of hydrogen absorption and desorption from Mg, surpassing the efficiency of the NiN₄ active site, especially in the critical steps of hydrogenation and dehydrogenation. Overall, the catalytic performance of Ni₁-Mg SAA is superior to most known nickel-based catalysts. This evolutionary process is also observed in FePc, CoPc, and tetraphenylporphyrin nickel (Ni-TPP), suggesting that this reducing transformation is a universal phenomenon for MN₄-type active sites in hydrogen storage catalysis.

Manipulating the Electronic Properties of an Fe Single Atom Catalyst via Secondary Coordination Sphere Engineering to Provide Enhanced Oxygen Electrocatalytic Activity in Zinc-Air Batteries

Oxygen reduction and evolution reactions are two key processes in electrochemical energy conversion technologies. Synthesis of nonprecious metal, carbon-based electrocatalysts with high oxygen bifunctional activity and stability is a crucial, yet challenging step to achieving electrochemical energy conversion. Here, an approach to address this issue: synthesis of an atomically dispersed Fe electrocatalyst (Fe1/NCP) over a porous, defect-containing nitrogen-doped carbon support, is described. Through incorporation of a phosphorus atom into the second coordination sphere of iron, the activity and durability boundaries of this catalyst are pushed to an unprecedented level in alkaline environments, such as those found in a zinc-air battery. The rationale is to delicately incorporate P heteroatoms and defects close to the central metal sites (FeN4P1-OH) in order to break the local symmetry of the electronic distribution. This enables suitable binding strength with oxygenated intermediates. In situ characterizations and theoretical studies demonstrate that these synergetic interactions are responsible for high bifunctional activity and stability. These intrinsic advantages of Fe1/NCP enable a potential gap of a mere 0.65 V and a high power density of 263.8 mW cm-2 when incorporated into a zinc-air battery. These findings underscore the importance of design principles to access high-performance electrocatalysts for green energy technologies.

Potential-Regulated Design for Direct Recycling of Degraded LiFePO4 Cathode

The rapid proliferation of power sources equipped with lithium-ion batteries poses significant challenges in terms of post-scrap recycling and environmental impacts, necessitating urgent attention to the development of sustainable solutions. The cathode direct regeneration technologies present an optimal solution for the disposal of degraded cathodes, aiming to non-destructively re-lithiate and straightforwardly reuse degraded cathode materials with reasonable profits and excellent efficiency. Herein, a potential-regulated strategy is proposed for the direct recycling of degraded LiFePO4 cathodes, utilizing low-cost Na2SO3 as a reductant with lower redox potential in the alkaline systems. The aqueous re-lithiation approach, as a viable alternative, not only enables the re-lithiation of degraded cathode while ignoring variation in Li loss among different feedstocks but also utilizes the rapid sintering process to restore the cathode microstructure with desirable stoichiometry and crystallinity. The regenerated LiFePO4 exhibits enhanced electrochemical performance with a capacity of 144 mA h g-1 at 1 C and a high retention of 98% after 500 cycles at 5 C. Furthermore, this present work offers considerable prospects for the industrial implementation of directly recycled materials from lithium-ion batteries, resulting in improved economic benefits compared to conventional leaching methods.

Construction of an Axial Charge Transfer Channel Between Single-Atom Fe Sites and Nitrogen-Doped Carbon Supports for Boosting Oxygen Reduction

The introduction of axial-coordinated heteroatoms in Fe─N─C single-atom catalysts enables the significant enhancement of their oxygen reduction reaction (ORR) performance. However, the interaction relationship between the axial-coordinated heteroatoms and their carbon supports is still unclear. In this work, a gas phase surface treatment method is proposed to prepare a series of X─Fe─N─C (X = O, P, and S) single-atom catalysts with axial X-coordination on graphitic-N-rich carbon supports. Synchrotron-based X-ray absorption near-edge structure spectra and X-ray photoelectron spectroscopy indicate the formation of an axial charge transfer channel between the graphitic-N-rich carbon supports and single-atom Fe sites by axial O atoms in O─Fe─N─C. As a result, the O─Fe─N─C exhibits excellent ORR performance with a half-wave potential of 0.905 V versus RHE and a high specific capacity of 884 mAh g-1 for zinc-air battery, which is superior to other X─Fe─N─C catalysts without axial charge transfer and the commercial Pt/C catalyst. This work not only demonstrates a general synthesis strategy for the preparation of single-atom catalysts with axial-coordinated heteroatoms, but also presents insights into the interaction between single-atom active sites and doped carbon supports.

Fe-N co-doped carbon nanofibers with Fe3C decoration for water activation induced oxygen reduction reaction

Proton activity at the electrified interface is central to the kinetics of proton-coupled electron transfer (PCET) reactions in electrocatalytic oxygen reduction reaction (ORR). Here, we construct an efficient Fe3C water activation site in Fe-N co-doped carbon nanofibers (Fe3C-Fe1/CNT) using an electrospinning-pyrolysis-etching strategy to improve interfacial hydrogen bonding interactions with oxygen intermediates during ORR. In situ Fourier transform infrared spectroscopy and density functional theory studies identified delocalized electrons as key to water activation kinetics. Specifically, the strong electronic perturbation of the Fe-N4 sites by Fe3C disrupts the symmetric electron density distribution, allowing more free electrons to activate the dissociation of interfacial water, thereby promoting hydrogen bond formation. This process ultimately controls the PCET kinetics for enhanced ORR. The Fe3C-Fe1/CNT catalyst demonstrates a half-wave potential of 0.83 V in acidic media and 0.91 V in alkaline media, along with strong performance in H2-O2 fuel cells and Al-air batteries.

General In Situ Engineering of Carbon-Based Materials on Carbon Fiber for In Vivo Neurochemical Sensing

Developing real-time, dynamic, and in situ analytical methods with high spatial and temporal resolutions is crucial for exploring biochemical processes in the brain. Although in vivo electrochemical methods based on carbon fiber (CF) microelectrodes are effective in monitoring neurochemical dynamics during physiological and pathological processes, complex post modification hinders large-scale productions and widespread neuroscience applications. Herein, we develop a general strategy for the in situ engineering of carbon-based materials to mass-produce functional CFs by introducing polydopamine to anchor zeolitic imidazolate frameworks as precursors, followed by one-step pyrolysis. This strategy demonstrates exceptional universality and design flexibility, overcoming complex post-modification procedures and avoiding the delamination of the modification layer. This simplifies the fabrication and integration of functional CF-based microelectrodes. Moreover, we design highly stable and selective H+, O2, and ascorbate microsensors and monitor the influence of CO2 exposure on the O2 content of the cerebral tissue during physiological and ischemia-reperfusion pathological processes.

Anionic Surfactant-Modulated Electrode-Electrolyte Interface Promotes H2O2 Electrosynthesis

Conventional strategies for highly selective and active hydrogen peroxide (H2O2) electrosynthesis primarily focus on catalyst design. Electrocatalytic reactions take place at the electrified electrode-electrolyte interface. Well-designed electrolytes, when combined with commercial catalysts, can be directly applied to high-efficiency H2O2 electrosynthesis. However, the role of electrolyte components is equally crucial but is significantly under-researched. In this study, anionic surfactant n-tetradecylphosphonic acid (TDPA) and its analogs are used as electrolyte additives to enhance the selectivity of the two-electron oxygen reduction reaction. Mechanistic studies reveal that TDPA assembled over the electrode-electrolyte interface modulates the electrical double-layer structure, which repels interfacial water and weakens the hydrogen-bond network for proton transfer. Additionally, the hydrophilic phosphonate moiety affects the coordination of water molecules in the solvation shell, thereby directly influencing the proton-coupled kinetics at the interface. The TDPA-containing catalytic system achieves a Faradaic efficiency of H2O2 production close to 100% at a current density of 200 mA cm-2 using commercial carbon black catalysts. This research provides a simple strategy to enhance H2O2 electrosynthesis by adjusting the interfacial microenvironment through electrolyte design.

Oxygen Functional Groups Regulate Cobalt-Porphyrin Molecular Electrocatalyst for Acidic H2O2 Electrosynthesis at Industrial-Level Current

Electrosynthesis of hydrogen peroxide (H2O2) based on proton exchange membrane (PEM) reactor represents a promising approach to industrial-level H2O2 production, while it is hampered by the lack of high-efficiency electrocatalysts in acidic medium. Herein, we present a strategy for the specific oxygen functional group (OFG) regulation to promote the H2O2 selectivity up to 92 % in acid on cobalt-porphyrin molecular assembled with reduced graphene oxide. In situ X-ray adsorption spectroscopy, in situ Raman spectroscopy and Kelvin probe force microscopy combined with theoretical calculation unravel that different OFGs exert distinctive regulation effects on the electronic structure of Co center through either remote (carboxyl and epoxy) or vicinal (hydroxyl) interaction manners, thus leading to the opposite influences on the promotion in 2e- ORR selectivity. As a consequence, the PEM electrolyzer integrated with the optimized catalyst can continuously and stably produce the high-concentration of ca. 7 wt % pure H2O2 aqueous solution at 400 mA cm-2 over 200 h with a cell voltage as low as ca. 2.1 V, suggesting the application potential in industrial-scale H2O2 electrosynthesis.

Advancing H2O2 electrosynthesis: enhancing electrochemical systems, unveiling emerging applications, and seizing opportunities

Hydrogen peroxide (H2O2) is a highly desired chemical with a wide range of applications. Recent advancements in H2O2 synthesis center on the electrochemical reduction of oxygen, an environmentally friendly approach that facilitates on-site production. To successfully implement practical-scale, highly efficient electrosynthesis of H2O2, it is critical to meticulously explore both the design of catalytic materials and the engineering of other components of the electrochemical system, as they hold equal importance in this process. Development of promising electrocatalysts with outstanding selectivity and activity is a prerequisite for efficient H2O2 electrosynthesis, while well-configured electrolyzers determine the practical implementation of large-scale H2O2 production. In this review, we systematically summarize fundamental mechanisms and recent achievements in H2O2 electrosynthesis, including electrocatalyst design, electrode optimization, electrolyte engineering, reactor exploration, potential applications, and integrated systems, with an emphasis on active site identification and microenvironment regulation. This review also proposes new insights into the existing challenges and opportunities within this rapidly evolving field, together with perspectives on future development of H2O2 electrosynthesis and its industrial-scale applications.

Effect of Different N/C Coordination Electronic Structures on the Activity of Bifunctional Rare-Earth Ytterbium Electrocatalysts for Oxygen Electrodes

The research and development of bifunctional electrocatalysts for the oxygen electrode is of great significance to solve the problem of electrochemical energy. Herein, the effect of different structure-activity relationships on the performance of YbNxCy-gra catalysts was explored. The bifunctional activity of graphene with a vacancy defect supported by single-atom rare-earth ytterbium was studied by density functional theory (DFT) calculations. We systematically analyzed the stability, electronic properties, and catalytic performance of potential bifunctional catalysts. The results showed that all catalysts were thermodynamically and kinetically stable. Under acidic conditions, YbN2C2-oppo-gra and YbN2C2-pen-gra showed good ORR activity, and their overpotentials were 0.53 and 0.65 V, respectively. In an alkaline environment, most of the Yb(OH)NxCy-gra catalysts showed excellent ORR and OER bifunctional catalytic activity. Their overpotentials were all below 0.6 V. In particular, the ηORR and ηOER of the Yb(OH)N4C0-gra electrocatalyst were as low as 0.33 and 0.42 V. This verified the practicability and feasibility of hydroxyl-modified catalysts to enhance activity. This research provides theoretical insights into the further design and development of high-efficiency rare-earth-supported bifunctional catalysts.

Ultralow-Loading Ruthenium-Iridium Fuel Cell Catalysts Dispersed on Zn-N Species-Doped Carbon

Developing low-loading platinum-group-metal (PGM) catalysts is one of the key challenges in commercializing anion-exchange-membrane-fuel-cells (AEMFCs), especially for hydrogen oxidation reaction (HOR). Here, ruthenium-iridium nanoparticles being deposited on a Zn-N species-doped carbon carrier (Ru6Ir/Zn-N-C) are synthesized and used as an anodic catalyst for AEMFCs. Ru6Ir/Zn-N-C shows extremely high mass activity (5.87 A mgPGM -1) and exchange current density (0.92 mA cm-2), which is 15.1 and 3.9 times that of commercial Pt/C, respectively. Based on the Ru6Ir/Zn-N-C AEMFCs achieve a peak power density of 1.50 W cm-2, surpassing the state-of-the-art commercial PtRu catalysts and the power ratio of the normalized loading is 14.01 W mgPGM anode -1 or 5.89 W mgPGM -1 after decreasing the anode loading (87.49 µg cm-2) or the total PGM loading (0.111 mg cm-2), satisfying the US Department of Energy's PGM loading target. Moreover, the solvent and solute isotope separation method is used for the first time to reveal the kinetic process of HOR, which shows the reaction is influenced by the adsorption of H2O and OH-. The improvement of the hydrogen bond network connectivity of the electric double layer by adjusting the interfacial H2O structure together with the optimized HBE and OHBE is proposed to be responsible for the high HOR activity of Ru6Ir/Zn-N-C.

Controlling the Selectivity of Electrocatalytic NO Reduction through pH and Potential Regulation on Single-Atom Catalysts

Electrocatalytic nitrogen oxide reduction (NOxRR) emerges as an effective way to bring the disrupted nitrogen cycle back into balance. However, efficient and selective NOxRR is still challenging partly due to the complex reaction mechanism, which is influenced by experimental conditions such as pH and electrode potential. Here, we have studied the enzyme-inspired iron single-atom catalysts (Fe-N4-C) and identified that the selectivity roots in the first step of the nitric oxide reduction. Combining the constrained molecular dynamics (MD) simulations with the quasi-equilibrium approximation, the effects of electrode potential and pH on the reaction free energy were considered explicitly and predicted quantitatively. Systematic heat maps for selectivity between single-N and N-N-coupled products in a wide pH-potential space are further developed, which have reproduced the experimental observations of NOxRR. The approach presented in this study allows for a realistic simulation of the electrocatalytic interfaces and a quantitative evaluation of interfacial effects. Our results in this study provide valuable and straightforward guidance for selective NOx reduction toward desired products by precisely designing the experimental conditions.

Tip-like Fe-N4 Sites Induced Surface Microenvironments Regulation Boosts the Oxygen Reduction Reaction

Single atom catalysts with defined local structures and favorable surface microenvironments are significant for overcoming slow kinetics and accelerating O2 electroreduction. Here, enriched tip-like FeN4 sites (T-Fe SAC) on spherical carbon surfaces were developed to investigate the change in surface microenvironments and catalysis behavior. Finite element method (FEM) simulations, together with experiments, indicate the strong local electric field of the tip-like FeN4 and the more denser interfacial water layer, thereby enhancing the kinetics of the proton-coupled electron transfer process. In situ spectroelectrochemical studies and the density functional theory (DFT) calculation results indicate the pathway transition on the tip-like FeN4 sites, promoting the dissociation of O-O bond via side-on adsorption model. The adsorbed OH* can be facilely released on the curved surface and accelerate the oxygen reduction reaction (ORR) kinetics. The obtained T-Fe SAC nanoreactor exhibits excellent ORR activities (E1/2 =0.91 V vs. RHE) and remarkable stability, exceeding those of flat FeN4 and Pt/C. This work clarified the in-depth insights into the origin of catalytic activity of tip-like FeN4 sites and held great promise in industrial catalysis, electrochemical energy storage, and many other fields.

Cation-Induced Interfacial Hydrophobic Microenvironment Promotes the C-C Coupling in Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO2RR) toward C2 products is a promising way for the clean energy economy. Modulating the structure of the electric double layer (EDL), especially the interfacial water and cation type, is a useful strategy to promote C-C coupling, but atomic understanding lags far behind the experimental observations. Herein, we investigate the combined effect of interfacial water and alkali metal cations on the C-C coupling at the Cu(100) electrode/electrolyte interface using ab initio molecular dynamics (AIMD) simulations with a constrained MD and slow-growth approach. We observe a linear correlation between the water-adsorbate stabilization effect, which manifests as hydrogen bonds, and the corresponding alleviation in the C-C coupling free energy. The role of a larger cation, compared to a smaller cation (e.g., K+ vs Li+), lies in its ability to approach the interface through desolvation and coordinates with the *CO+*CO moiety, partially substituting the hydrogen-bonding stabilizing effect of interfacial water. Although this only results in a marginal reduction of the energy barrier for C-C coupling, it creates a local hydrophobic environment with a scarcity of hydrogen bonds owing to its great ionic radius, impeding the hydrogen of surrounding interfacial water to approach the oxygen of the adsorbed *CO. This skillfully circumvents the further hydrogenation of *CO toward the C1 pathway, serving as the predominant factor through which a larger cation facilitates C-C coupling. This study unveils a comprehensive atomic mechanism of the cation-water-adsorbate interactions that can facilitate the further optimization of the electrolyte and EDL for efficient C-C coupling in CO2RR.