Enhanced Oxygen Reduction Reaction Performance Research Articles

Improved Oxygen Reduction Reaction Activity of Graphene via Mechanochemical Activation and Halogen‐Doping

AbstractThere has been an increased effort to replace the expensive and rare platinum and platinum group metals to speed up the sluggish oxygen reduction reaction (ORR) kinetics, which limits the efficiency of fuel cells. One class of promising Pt‐alternative catalysts for ORR is metal‐free halogen‐doped carbon materials. Herein, bromine‐doped and iodine‐doped graphene were synthesized via mechanochemical activation. The synthesized samples exhibited sub‐rounded particles. Mechanical activation via ball milling increased the specific surface area of graphene by reducing particle size. Ball milling also enhanced dopant dispersibility and increased surface roughness, though it reduced surface area compared to ball‐milled graphene, likely due to the size difference between carbon and halogen atoms. Among the synthesized catalysts, iodine‐doped graphene exhibits the highest limiting current density of 1.806 mA cm−2 with the highest ORR onset potential of 0.74 V vs reversible hydrogen electrode (RHE). The iodine‐doped graphene also showed good stability after 1000 cycles of accelerated degradation test. The enhanced ORR performance of iodine‐doped graphene was reached using the optimized iodine‐to‐graphene mass ratio of 4 : 1 after 48 h ball milling time.

Optimizing Activity and Stability in AgCl‐CuO Electrocatalysts: Insights from Different Morphologies for Enhanced ORR Performance

AbstractHighly enduring and methanol‐tolerant electrocatalysts are pertinent to revolutionizing direct methanol fuel cell (DMFC) technologies. Transition‐metal‐based electrocatalysts are anticipated to rank among the remarkable electrocatalysts for boosting the sluggish oxygen reduction reaction (ORR) kinetics. In this study, we report a facile approach to optimize the activity and stability of the AgCl‐CuO systems. The tailored nanoparticle–nanorod (NP–NR) and nanoleaf (NL) like morphologies deliver remarkable bifunctional catalytic performance for oxygen reduction and hydrogen evolution reactions, sustained activity, and comparatively better stability for ORR. A meager shift of only 0.014 and 0.024 V in the half‐wave potential is observed in the NP‐NR and NL structures, respectively, for the ORR even after 2500 cycles. Furthermore, both the AgCl‐CuO composite exhibit an excellent tolerance to methanol.

Concave Grain Boundaries Stabilized by Boron Segregation for Efficient and Durable Oxygen Reduction.

The oxygen reduction reaction (ORR) is a critical process that limits the efficiency of fuel cells and metal-air batteries due to its slow kinetics, even when catalyzed by platinum (Pt). To reduce Pt usage, enhancing both the specific activity and electrochemically active surface area (ECSA) of Pt catalysts is essential. Here, ultrafine, grain boundary (GB)-rich Pt nanoparticle assemblies are proposed as efficient ORR catalysts. These nanowires offer a large ECSA and a high density of concave GB sites, which improve specific activity. Atoms at these GB sites exhibit increased coordination and lattice distortion, leading to a favorable reduction in oxygen binding energy and enhanced ORR performance. Furthermore, boron segregation stabilizes these GBs, preserving active sites during catalysis. The resulting boron-stabilized Pt nanoassemblies demonstrate ORR specific and mass activities of 9.18 mA cm-2 and 6.40 A mg-1 Pt (at 0.9 V vs. RHE), surpassing commercial Pt/C catalysts by over 35-fold, with minimal degradation after 60000 potential cycles. This approach offers a versatile platform for optimizing the catalytic performance of a wide range of nanoparticle systems.

Metal-free advanced energy materials for the oxygen reduction reaction in anion-exchange membrane fuel cells

The success of the next generation of anion-exchange membrane fuel cells (AEMFCs) depends on the development of active, reliable, and economical oxygen reduction reaction (ORR) catalysts. Here, we synthesize a series of ultra-low-cost metal-free ORR catalysts by doping a common pristine graphite precursor with chemically singular-type heteroatoms, namely I, S, N, or B, using single-step planetary ball milling technique. All doped-graphites show substantially enhanced ORR performance relative to the pristine (undoped) graphite. Among all the tested catalysts, N-graphite exhibited the highest ORR onset potential of 0.87 V vs. reversible hydrogen electrode. These results are supported by density functional theory calculations. The ORR catalysts also exhibit remarkable stability as evaluated through electrochemical tests. Most importantly, the AEMFCs prepared using these ultra-low-cost doped graphites deliver notable peak power densities with impressive voltage efficiencies, which further supports their efficacy in ORR catalysis and the broad implementation of this technology.

Strengthening the oxygen reduction stability and activity of single iron active sites via a simultaneously electronic regulation and structure design strategy

Single-atom iron-nitrogen-carbon materials have demonstrated remarkable potential in catalyzing the oxygen reduction reaction (ORR). In this study, a novel approach is proposed for regulating the microenvironment of single iron sites through the coupling of neighboring Zn with an open-mesoporous carbon plane. This structural arrangement effectively modulates the density of states and d-band center of Fe atoms, resulting in moderate adsorption and desorption of oxygen intermediates and thereby reducing the energy barrier of the ORR. Moreover, the open-mesoporous structure ensures the accessibility of the active sites and facilitates the mass transfer of reactants, intermediates, and products during the ORR. Consequently, the synthesized M-Fe-ZnNC catalyst exhibits enhanced ORR performance and excellent stability. This study serves to inspire the exploration of high-performance non-precious metal catalysts for the ORR, leveraging simultaneous electronic regulation and structure design.

Molten-Salt Electrochemical-Assisted Synthesis of the CeO2-OV@GC Composite-Supported Pt Clusters with a Pt-O-Ce Structure for the Oxygen Reduction Reaction.

Highly active and robust Pt-based electrocatalysts for an oxygen reduction reaction (ORR) are of crucial significance for the development of proton exchange membrane fuel cells (PEMFCs). Herein, the high-loading and well-dispersive Pt clusters on graphitic carbon-supported CeO2 with abundant oxygen vacancies (PtAC/CeO2-OV@GC) were successfully fabricated by a molten-salt electrochemical-assisted method. The bonding of Pt with the highly electronegative O induces charge redistribution through the Pt-O-Ce structure, thus reducing the adsorption energies of oxygen-containing species. Such a PtAC/CeO2-OV@GC electrocatalyst exhibits a greatly enhanced ORR performance with a mass activity of 0.41 ± 0.02 A·mg-1Pt at 0.9 V versus a reversible hydrogen electrode, which is 2.7 times the value of a commercial Pt/C catalyst and shows negligible activity decay after 20000 cycles of accelerated degradation tests. It is anticipated that this work will provide enlightening guidance on the controllable synthesis and rational design of high-performance Pt-based electrocatalysts for PEMFCs.

Collective Effect in a Multicomponent Ensemble Combining Single Atoms and Nanoparticles for Efficient and Durable Oxygen Reduction

AbstractMetal single‐atom catalysts represent one of the most promising non‐noble metal catalysts for the oxygen reduction reaction (ORR). However, they still suffer from insufficient activity and, particularly, durability for practical applications. Leveraging density functional theory (DFT) and machine learning (ML), we unravel an unexpected collective effect between FeN4OH sites, CeN4OH motifs, Fe nanoparticles (NPs), and Fe−CeO2 NPs. The collective effect comprises differently‐weighted electronic and geometric interactions, whitch results in significantly enhanced ORR activity for FeN4OH active sites with a half‐wave potential (E1/2) of 0.948 V versus the reversible hydrogen electrode (VRHE) in alkaline, relative to a commercial Pt/C (E1/2, 0.851 VRHE). Meanwhile, this collective effect endows the shortened Fe−N bonds and the remarkable durability with negligible activity loss after 50,000 potential cycles. The ML was used to understand the intricate geometric and electronic interactions in collective effect and reveal the intrinsic descriptors to account for the enhanced ORR performance. The universality of collective effect was demonstrated effective for the Co, Ni, Cu, Cr, and Mn‐based multicomponent ensembles. These results confirm the importance of collective effect to simultaneously improve catalytic activity and durability.

Collective Effect in a Multicomponent Ensemble Combining Single Atoms and Nanoparticles for Efficient and Durable Oxygen Reduction.

Metal single-atom catalysts represent one of the most promising non-noble metal catalysts for the oxygen reduction reaction (ORR). However, they still suffer from insufficient activity and, particularly, durability for practical applications. Leveraging density functional theory (DFT) and machine learning (ML), we unravel an unexpected collective effect between FeN4OH sites, CeN4OH motifs, Fe nanoparticles (NPs), and Fe-CeO2 NPs. The collective effect comprises differently-weighted electronic and geometric interactions, whitch results in significantly enhanced ORR activity for FeN4OH active sites with a half-wave potential (E1/2) of 0.948 V versus the reversible hydrogen electrode (VRHE) in alkaline, relative to a commercial Pt/C (E1/2, 0.851 VRHE). Meanwhile, this collective effect endows the shortened Fe-N bonds and the remarkable durability with negligible activity loss after 50,000 potential cycles. The ML was used to understand the intricate geometric and electronic interactions in collective effect and reveal the intrinsic descriptors to account for the enhanced ORR performance. The universality of collective effect was demonstrated effective for the Co, Ni, Cu, Cr, and Mn-based multicomponent ensembles. These results confirm the importance of collective effect to simultaneously improve catalytic activity and durability.

Manipulation of Electronic States of Pt Sites via d-Band Center Tuning for Enhanced Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells.

Expediting the torpid kinetics of the oxygen reduction reaction (ORR) at the cathode with minimal amounts of Pt under acidic conditions plays a significant role in the development of proton exchange membrane fuel cells (PEMFCs). Herein, a novel Pt-N-C system consisting of Pt single atoms and nanoparticles anchored onto the defective carbon nanofibers is proposed as a highly active ORR catalyst (denoted as Pt-N-C). Detailed characterizations together with theoretical simulations illustrate that the strong coupling effect between different Pt sites can enrich the electron density of Pt sites, modify the d-band electronic environments, and optimize the oxygen intermediate adsorption energies, ultimately leading to significantly enhanced ORR performance. Specifically, the as-designed Pt-N-C demonstrates exceptional ORR properties with a high half-wave potential of 0.84 V. Moreover, the mass activity of Pt-N-C reaches 193.8 mA gPt-1 at 0.9 V versus RHE, which is 8-fold greater than that of Pt/C, highlighting the enormously improved electrochemical properties. More impressively, when integrated into a membrane electrode assembly as cathode in an air-fed PEMFC, Pt-N-C achieved a higher maximum power density (655.1 mW cm-2) as compared to Pt/C-based batteries (376.25 mW cm-2), hinting at the practical application of Pt-N-C in PEMFCs.

Facile Fabrication of Nickel Supported on Reduced Graphene Oxide Composite for Oxygen Reduction Reaction.

Due to the depletion of fossil fuels, the demand for renewable energy has increased, thus stimulating the development of novel materials for energy conversion devices such as fuel cells. In this work, nickel nanoparticles loaded on reduced graphene oxide (Ni/rGO) with small size and good dispersibility were successfully prepared by controlling the pyrolysis temperature of the precursor at 450 °C, assisted by a microwave-assisted hydrothermal method, and exhibited enhanced electrocatalytic activity towards oxygen reduction reaction (ORR). Additionally, the electron enrichment on Ni NPs was due to charge transfer from the rGO support to metal nickel, as evidenced by both experimental and theoretical studies. Metal-support interactions between nickel and the rGO support also facilitated charge transfer, contributing to the enhanced ORR performance of the composite material. DFT calculations revealed that the first step (from O2 to HOO*) was the rate-determining step with an RDS energy barrier lower than that of the Pt(111), indicating favorable ORR kinetics. The HOO* intermediates can be transferred onto rGO by the solid-phase spillover effect, which reduces the chemical adsorption on the nickel surface, thereby allowing continuous regeneration of active nickel sites. The HO2- intermediates generated on the surface of rGO by 2e- reduction can also efficiently diffuse towards the nearby Ni surface or the interface of Ni/rGO, where they can be further rapidly reduced to OH-. This mechanism acts as the pseudo-four-electron path on the RRDE. Furthermore, Ni/rGO-450 demonstrated superior stability, methanol tolerance, and durability compared to a 20 wt% Pt/C catalyst, making it a cost-effective alternative to conventional noble metal ORR catalysts for fuel cells or metal-air batteries.

Boosting Oxygen Reduction through Microenvironment Modulation to Enhance Mass Transportation

Electroreduction of oxygen driven by renewable electricity holds significant promise for the sustainable production of value‐added hydrogen peroxide (H2O2). While water is a desirable source of protons and electrons for this reaction, its low gas solubility often limits the transportation of gas molecules and consequently leads to large concentration overpotential, resulting in unsatisfactory energy efficiencies. Herein, a facile and effective strategy to promote the 2e− oxygen reduction reaction (ORR) through microenvironment modulation is presented. In this work, it is specifically aimed to facilitate oxygen transportation at the reaction interface, particularly at high rates. To achieve this, hydrophobic polytetrafluoroethylene (PTFE) particles are introduced into a catalyst layer containing amino‐group‐functionalized carbon nanotube (CNT‐NH2) as the ORR catalyst for H2O2 production. As a result, the PTFE‐modified CNT‐NH2‐based gas‐diffusion electrode (GDE) substantially improves ORR activity. At 100 mA cm−2, the PTFE‐modified CNT‐NH2 achieves a high cathodic energy efficiency of 92%, 1.5 times higher than the pristine CNT‐NH2‐based GDE (63%). Detailed kinetic analysis reveals that this enhanced ORR performance is indeed due to the enhanced oxygen transportation induced by the persistent hydrophobic microenvironment created by the PTFE‐modified catalyst layer, reducing the concentration overpotential during ORR.

Facile graphene quantum dot-anchoring strategy synthesis of single-atom iron-nitrogen electrocatalyst with enhanced ORR performance

Single-atom catalysts (SACs), especially atomically dispersed Fe–N[Formula: see text]–C based SACs, hold great promise to replace Pt-based electrocatalysts for oxygen-reduction reaction (ORR). Currently, synthesizing high-activity ORR electrocatalysts with atomically dispersed Fe–N[Formula: see text] site structures is still challenging due to their high surface free energy, which leads to easy migration and serious aggregation. Herein, we have designed a general graphene quantum dots (GQDs)-anchoring strategy to synthesize a single-iron-atom electrocatalyst (Fe–N-GQDs/PC) applied to ORR through calcining of N-GQDs-Fe[Formula: see text] modified porous carbon (PC) and melamine. Experiments demonstrate the N-GQDs consist of abundant oxygenated groups, which could lead to complexing metal ions and thus facilitating the formation of SACs. Furthermore, the Fe–N-GQDs/PC electrocatalyst exhibits outstanding electrocatalytic ORR activity in 0.1 M KOH media with half-wave potentials of 0.84 versus 0.85 V for Pt/C. This strategy has opened up new feasible ideas to produce SACs for electrochemical energy devices.

Coupling atomically ordered PtCo catalysts with ultrathin nitrogen-doped carbon shell for enhanced oxygen reduction

Atomically ordered intermetallic catalysts have great potential for catalyzing efficient oxygen reduction reaction (ORR), while the coupling of metal and support are not emphasized enough. Herein, we report an atomically ordered PtCo bimetallic catalyst coupled with N-doped ultrathin carbon shells (PtCo/NCS) for ORR by a facile natural ligand annealing strategy. The formed NCS prevents the high-temperature sintering of catalysts by physical confinement, and the coupling effect results in a stronger affinity between PtCo and NCS for PtCo/NCS. The optimal PtCo/NCS-5 exhibits more exceptional activity and durability, outperforming commercial Pt/C and Pt3Co/C catalysts (C-Pt3Co/C). It is proved that PtCo and NCS synergically account for the enhanced ORR performance for PtCo/NCS-5. More importantly, the compressive lattice strain effectively promotes electron redistribution and optimizes the electronic structure for PtCo/NCS-5. This work further deepens the understanding of catalytic mechanisms and provides new insight into the rational design of advanced electrocatalysts toward ORR.

A general strategy for direct growth of yolk-shell MOF-on-MOF hybrids

Construction of metal–organic framework (MOF) hybrids with yolk-shell nanostructures via MOF-on-MOF growth is significant for MOF design toward elaborate properties and enhanced performances, while remains challenging. Here, we report the direct growth of yolk-shell MOF-on-MOF hybrids via an interfacial growth-reorganization strategy. The synthesis relies on the precise growth of guest CoZn-ZIF (ZIF = zeolite imidazole framework) with appropriate crystal diameter and arrangement on host MOFs, inducing an in-situ transformation of core–shell intermediate into yolk-shell product. This strategy can be generally applied to the growth of CoZn-ZIF onto various host MOFs with different shapes, crystal structures and compositions including rod-like and octahedral Fe-based NH2-MIL-88B (MIL = Material Institute Lavoisier) and cake-like Ti-based NH2-MIL-125 toward three kinds of yolk-shell MOF-on-MOF hybrids. Moreover, functional carbon materials with bridged yolk-shell architecture can be prepared by pyrolysis of MOF-on-MOF hybrids as precursors, exhibiting enhanced oxygen reduction reaction performance than their monomeric counterparts. Our work paves the way for the design and application of MOF based yolk-shell nanostructures.

Enhanced Oxygen Reduction Reaction Performance by Adsorbed Water on Edge Sites.

Pt-based alloy nanoparticles have broad application prospects as cathode catalyst materials for proton-exchange membrane fuel cells (PEMFCs). Optimization of the oxygen adsorption energy is crucial to boost the performance of oxygen reduction catalysis. We successfully synthesized well-dispersed Pt1.2Ni tetrahedra and obtained the Pt1.2Ni/C catalyst adopting the one-pot synthetic protocol, which exhibits superb activity and good long-term stability for oxygen reduction reaction (ORR), achieving a mass activity of 1.53 A/mgPt at 0.90 VRHE, which is 12 times higher than that of commercial Pt/C. On combining X-ray photoelectron spectroscopy and density functional theory calculations, abundant water is adsorbed stably on the Pt1.2Ni alloy surface. We find that the intense interaction between the adsorbed O atom and adsorbed water can weaken the adsorption of oxygen, contributing to the ORR performance.

Breaking the N-limitation with N-enriched porous submicron carbon spheres anchored Fe single-atom catalyst for superior oxygen reduction reaction and Zn-air batteries

Nitrogen-doped carbon supported metal single-atom catalysts (M-N-C SACs), especially Fe-N-C SACs appear as very promising catalysts for oxygen reduction reaction (ORR). However, precisely modulating of the Fe-Nx configuration and geometric microenvironment in Fe-N-C SACs to achieve the highest level of catalytic activity remains grand challenges. Herein, we describe a N-rich heterocycle regulated supramolecular coordination self-assembly strategy to fabricate Fe single atoms anchored on N-enriched porous submicron carbon spheres (FeSA/N-PSCS), with ultra-high N-dopant content (14.81 at.%) for facilitating the formation of atomically dispersed Fe-N4. Density functional theory calculations validate that N-doping at the periphery of the Fe-N4 active sites optimizes the adsorption of oxygen-containing intermediates and significantly reduces the ORR overpotential. Benefitting from the localized N-enriched atomic configuration, highly microporous, and regular submicron-spherical structure, FeSA/N-PSCS exhibit enhanced ORR performance. More importantly, FeSA/N-PSCS catalyzed Zn-air battery (ZAB) outperforms Pt/C+RuO2-based ZAB in the aspects of maximum power density, specific capacity and cycling stability.

Spin-Selective Coupling in Mott-Schottky Er2 O3 -Co Boosts Electrocatalytic Oxygen Reduction.

Alkaline oxygen reduction reaction (ORR) is critical to electrochemical energy conversion technology, yet the rational breaking of thermodynamic inhibition for ORR through spin regulation remains a challenge. Herein, a Mott-Schottky catalyst consisting of Er2 O3 -Co particles uniformly implanted into carbon nanofibers (Er2 O3 -Co/CNF) is designed for enhancing ORR via spin-selective coupling. The optimized Er2 O3 -Co/CNF affords a high half-wave potential (0.835V vs reversible hydrogen electrode, RHE) and onset potential (0.989 VRHE ) for the ORR surpassing individual Co/CNF and Er2 O3 /CNF. Theoretical calculations reveal the introduction of Er2 O3 optimizes the electronic structure of Co through Er(4f)-O(2p)-Co(3d) gradient orbital coupling, resulting in significantly enhanced ORR performance. Through gradient orbital coupling, the induced spin-up hole in Co 3d states endows the Er-O-Co unit active site with a spin-selective coupling channel for electron transition. This favors the decrease of the energy gap in the potential-limiting step, thus achieving a high theoretical limiting potential of 0.77 VRHE for the Er2 O3 -Co. Moreover, the potential practicability of Er2 O3 -Co/CNF as an air-cathode is also demonstrated in Zn-air batteries. This work is believed to provide, new perspectives for the design of efficient ORR electrocatalysts by engineering spin-selective coupling induced by rare-earth oxides.

Defects in Carbon-Based Materials for Electrocatalysis: Synthesis, Recognition, and Advances.

ConspectusOwing to climate change and over-reliance on fossil fuels, the study and development of sustainable energy is of essential importance in the next few decades. In recent years, rapid advances have been witnessed in various power to gas electrocatalysis technologies including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) for realizing the target of blue planet with carbon neutrality. Nevertheless, practical applications with superior performance and affordable cost are largely limited by the electrode materials because the reactions are regularly driven by precious metals such as platinum (Pt) or iridium (Ir) based catalysts. Therefore, it is of significance to develop novel electrocatalysts with high electroactivity and limited cost for boosting the commercialization of green hydrogen technology.Since nitrogen-doped carbon nanotubes were first reported for enhanced ORR performance in 2009, the exploitation of carbon-based metal-free catalysts (CMFCs) as potential replacements for the precious metal electrocatalysts has become an attractive research field. To date, great progress has been made in developing new dopant strategies for CMFCs; however, the details of the catalytic mechanism and identification of active sites remain unclear, owing to the complexity in controlling the dopants and their homogeneity in carbon-based materials. To tackle this issue, our group has presented a series of works on defects catalyzing electrochemical reactions and proposed a defect catalysis mechanism since 2015. This theory is now widely accepted by the research community and has become a very important area in electrocatalysis worldwide.In this Account, we first present the defect theory for the reasonable design of defective carbon-based materials (DCMs) and subsequently summarize our previous works on the state-of-the-art defect engineering strategies to design DCMs possessing high activity, with the particular emphasis on the conjunction between defect structures and electrochemical performances. We also categorize recent defect modulation approaches on active sites in DCMs as well as showcase the advanced characterization techniques to confirm the types and densities of defects in DCMs. Finally, several perspectives on the challenges and future research opportunities of this exciting field are proposed. Remarkably, rapid advances of DCMs possessing both high electrochemical activities and low cost as a new generation of electrode materials may greatly facilitate the deployment of sustainable energy infrastructures.

Boosting electrocatalytic performance and durability of Pt nanoparticles by conductive MO2−x (M = Ti, Zr) supports

Metal oxides, especially TiO2, have been studied as an alternative support to replace the carbon in the conventional Pt/C catalysts for their high electrochemical stability at high electrode potentials. The low conductivity of metal oxides has been a big hurdle. In this work, we successfully overcome this issue by forming conductive MO2−x (M = Ti and Zr) through solid state reduction with NaBH4. The temperature of the reaction has turned out to be a crucial parameter to obtain highly conductive MO2−x. Pt/MO2−x catalysts were prepared by depositing Pt nanoparticles (NPs) on MO2−x supports whose analysis data, show that the Pt NPs are uniformly deposited on the surface of MO2−x supports and that there is a strong electronic interaction between Pt NPs and MO2−x supports. The electrocatalysis of Pt/MO2−x catalysts for oxygen reduction reaction (ORR) has been studied. Pt/MO2−x catalysts show significantly enhanced mass activity (MA) and specific activity (SA) from those of Pt/C catalyst. More importantly, Pt/MO2−x catalysts show a superior long-term durability. After 50,000 cycles of durability test, Pt/T370 catalyst retains 75%/84% of initial MA/SA, and Pt/Z438 catalyst retains 81%/88% of initial MA/SA, while Pt/C catalyst keeps only 36%/56% of initial MA/SA after 30,000 cycles. The significantly enhanced ORR performance of Pt/MO2−x catalysts is attributed to the strong metal-support interaction (SMSI) effect between MO2−x and Pt NPs as well as the high conductivity of MO2−x supports. We believe Pt/MO2−x catalysts are a promising form of electrocatalysts that can replace the presently dominating but not quite satisfactory Pt/C in fuel cell applications.

In Situ Structure of a Mo-Doped Pt-Ni Catalyst during Electrochemical Oxygen Reduction Resolved from Machine Learning-Based Grand Canonical Global Optimization.

Pt-Ni alloy is by far the most active cathode material for oxygen reduction reaction (ORR) in the proton-exchange membrane fuel cell, and the addition of a tiny amount of a third-metal Mo can significantly improve the catalyst durability and activity. Here, by developing machine learning-based grand canonical global optimization, we are able to resolve the in situ structures of this important three-element alloy system under ORR conditions and identify their correlations with the enhanced ORR performance. We disclose the bulk phase diagram of Pt-Ni-Mo alloys and determine the surface structures under the ORR reaction conditions by exploring millions of likely structure candidates. The pristine Pt-Ni-Mo alloy surfaces are shown to undergo significant structure reconstruction under ORR reaction conditions, where a surface-adsorbed MoO4 monomer or Mo2O x dimers cover the Pt-skin surface above 0.9 V vs RHE and protect the surface from Ni leaching. The physical origins are revealed by analyzing the electronic structure of O atoms in MoO4 and on the Pt surface. In viewing the role of high-valence transition metal oxide clusters, we propose a set of quantitative measures for designing better catalysts and predict that six elements in the periodic table, namely, Mo, Tc, Os, Ta, Re, and W, can be good candidates for alloying with PtNi to improve the ORR catalytic performance. We demonstrate that machine learning-based grand canonical global optimization is a powerful and generic tool to reveal the catalyst dynamics behavior in contact with a complex reaction environment.