Progress and Perspective for In Situ Studies of Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

Atomically Dispersed Metal Catalysts for Oxygen Reduction Reaction: Two-Electron vs. Four-Electron Pathways

Developing eco-friendly electrochemical devices for electrosynthesis, fuel cells (FCs), and metal-air batteries (MABs) requires precisely designing the electronic pathway in the oxygen reduction reaction (ORR) process. Understanding the principle of developing low-cost, highly active, and stable catalysts helps to reduce the usage of noble metals in ORR. Atomically dispersed metal catalysts (ADMCs) emerge as promising alternatives to replace commercial noble metals due to their high utilization of active metal atoms, high intrinsic activity, and controllable coordination environments. In this review, the research tendency and reaction mechanisms in ORR are first summarized. The basic principles concerning the geometric size and chemical coordination of two-electron ORR (2e- ORR) catalysts were then discussed, aiming to outline the evolution of material design from 2e- ORR to four-electron ORR (4e- ORR). Subsequently, recent advances in ADMCs primarily investigated for the 4e- ORR are well-documented. These advances encompass studies on M-N-C coordination, light heteroatom doping, dual-metal atoms-based coordination, and interaction between nanoparticle (NPs)/nanoclusters (NCs) and atomically dispersed metals (ADMs). Finally, the setups for 2/4e- ORR applications, key challenges, and opportunities in the future design of ADMCs for the ORR are highlighted.

Intrinsic Mechanical Effects on the Activation of Carbon Catalysts

The mechanical effects on carbon-based metal-free catalysts (C-MFCs) have rarely been explored, despite the global interest in C-MFCs as substitutes for noble metal catalysts. Stress is ubiquitous, whereas its dedicated study is severely restricted due to its frequent entanglement with other structural variables, such as dopants, defects, and interfaces in catalysis. Herein, we report a proof-of-concept study by establishing a platform to continuously apply strain to a highly oriented pyrolytic graphite (HOPG) lamina, simultaneously collecting electrochemical signals. Notably, we establish, for the first time, the correlation between the surface strain of graphitic carbon and its activation effect on the oxygen reduction reaction (ORR). Our results indicate that while in-plane and edge carbon sites in HOPG could not be further activated by applying tensile strain, a strong and repeatable dependence of catalytic activity on tensile strain was observed when the structure incorporated in-plane defects, leading to a significant ∼35.0% improvement in ORR current density with the application of ∼0.6% tensile strain. Density functional theory (DFT) simulations reveal that appropriate strain on specific defects can optimize the adsorption of reaction intermediates, and the Stone-Wales defect on graphene is correlated with the observed mechanical effect. This work elucidates fundamental principles of strain effects on the catalytic activity of graphitic carbon toward ORR and may lay the groundwork for the development of carbon-based mechano-electrocatalysis.

Molecular Understanding of the Role of Catalyst Particle Arrangement in Local Mass Transport Resistance for Fuel Cells

Platinum (Pt) catalyst performance loss caused by a high local oxygen transport resistance is an urgent problem to be solved for proton exchange membrane fuel cells (PEMFCs). Rationally arranging Pt particles on carbon support is the primary approach for reducing mass transport resistance. Herein, using a unique method coupling Hybrid Reverse Monte Carlo, molecular dynamics simulations, and experimental measurements, a Pt particle arrangement strategy is proposed to reduce local oxygen transport resistance, based on a molecular-level understanding of its impact. The densely arranged Pt particles with a small interparticle distance lead to the denser ionomer layer due to the co-attraction effect, leading to a high local oxygen transport resistance. The nonuniformly arranged Pt particles with various interparticle distances cause the heterogeneous ionomer density, inducing the heterogeneous oxygen transport. Increasing the Pt-Pt interparticle distance from 2 to 5 nm substantially reduces the local oxygen transport resistance by over 50%. The uniform arrangement of Pt particles makes the ionomer layer density more homogeneous, resulting in more uniform oxygen transport. Therefore, uniformly arranging Pt particles with an interparticle distance of >5 nm on carbon support is preferred for reducing local oxygen transport resistance and improving the homogeneity of oxygen transport.

Strain engineering of CoSANC catalyst toward enhancing the oxygen reduction reaction activity

Developing efficient and cost-effective platinum-group metal-free (PGMF) catalysts for the oxygen reduction reaction (ORR) is crucial for energy conversion and storage devices. Among these catalysts, metal-nitrogen-carbon (MNC) materials, particularly cobalt single-atom catalysts (CoSANC), show promise as ORR electrocatalysts. However, their ORR activity is often hindered by strong hydroxyl (OH) adsorption on the Co sites. While the impact of strain engineering on MNC electrocatalysts has been minimally explored, recent studies suggest its potential to enhance catalytic performance and optimize intrinsic activity in traditional bulk catalysts. In this context, we investigate the effect of surface strain on CoSANC for ORR activity and correlate substrate-strain-induced geometric distortions with catalytic activity using experimental and theoretical methods. The findings suggest that the d-band center gap of spin states (Δεd) may be a preferred descriptor for predicting strain-dependent ORR performance in MNC catalysts. Leveraging CoSANC moiety placed on a substrate with an average size of 1.0 μm, we achieve performance comparable to that of commercial Pt/C catalysts when used as a cathode catalyst in zinc-air batteries. This investigation unveils the structure-function relationship of MNC electrocatalysts regarding strain engineering and provides valuable insights for future ORR activity design and enhancement.

"Straw in the Clay Soil" Strategy: Anticarbon Corrosive Fluorine-Decorated Graphene Nanoribbons@CNT Composite for Long-Term PEMFC

Carbon corrosion poses a significant challenge in polymer electrolyte membrane fuel cells (PEMFCs), leading to reduced cell performance due to catalyst layer degradation and catalyst detachment from electrodes. A promising approach to address this issue involves incorporating an anticorrosive carbon material into the oxygen reduction reaction (ORR) electrode, even in small quantities (≈3 wt% in electrode). Herein, the successful synthesis of fluorine-doped graphene nanoribbons (F-GNR) incorporated with graphitic carbon nanotubes (F-GNR@CNT), demonstrating robust resistance to carbon corrosion is reported. By controlling the synthesis conditions using an exfoliation method, the properties of the composite are tailored. Electronic structural studies, employing density functional theory (DFT) calculations, to elucidate the roles of fluorine dopants and graphitic carbon nanotubes (CNTs) in mitigating carbon corrosion are conducted. Physicochemical and electrochemical characterization of F-GNR@CNT reveal its effectiveness as a cathode additive at the single-cell scale. The addition of F-GNR@CNT to the Pt/C cathode improves durability by enhancing carbon corrosion resistance and water management, thus mitigating the flooding effect through tailored surface properties. Furthermore, advanced impedance analysis using a transmission line model is performed to gain insights into the internal resistance and capacitive properties of electrode structure.

Structure-Activity Relationships in Oxygen Electrocatalysis

Oxygen electrocatalysis, as the pivotal circle of many green energy technologies, sets off a worldwide research boom in full swing, while its large kinetic obstacles require remarkable catalysts to break through. Here, based on summarizing reaction mechanisms and in situ characterizations, the structure-activity relationships of oxygen electrocatalysts are emphatically overviewed, including the influence of geometric morphology and chemical structures on the electrocatalytic performances. Subsequently, experimental/theoretical research is combined with device applications to comprehensively summarize the cutting-edge oxygen electrocatalysts according to various material categories. Finally, future challenges are forecasted from the perspective of catalyst development and device applications, favoring researchers to promote the industrialization of oxygen electrocatalysis at an early date.

In Situ Construction of Highly Dispersed Pd on Cobalt Nanoparticle on Hollow Functional Cubic Graphene by Double Framework for ORR

Developing advanced functional carbon materials is essential for electrocatalysis, caused by their vast merits for boosting many key energy conversion reactions. Herein, the covalent organic frameworks (COFs) is utilized on metal-organic frameworks (MOFs) as the template, under the controllable metal atoms thermal migration process successfully in situ constructs Pd-Co alloy nanoparticles on hollow cubic graphene. The electrocatalytic oxygen reduction reaction (ORR) evaluation showed excellent performances with a half-wave potential of 0.866 V, and a limited current density of 4.975 mA cm-2, that superior to the commercial Pt/C and Co nanoparticles. The contrast experiments and X-ray absorption spectrum demonstrated the aggregated electrons at highly dispersed Pd atoms on Co nanoparticle that promoted the main activities. This work not only enlightens the novel carbon materials designing strategies but also suggests heterogeneous electrocatalysis.

Monitoring the Morphological Changes of Skeleton-PtCo Electrocatalyst during PEMFC Start-Up/Shut-Down probed by in situ WAXS and SAXS

Advanced in situ analyses are indispensable for comprehending the catalyst aging mechanisms of Pt-based PEM fuel cell cathode materials, particularly during accelerated stress tests (ASTs). In this study, a combination of in situ small-angle and wide-angle X-ray scattering (SAXS & WAXS) techniques were employed to establish correlations between structural parameters (crystal phase, quantity, and size) of a highly active skeleton-PtCo (sk-PtCo) catalyst and their degradation cycles within the potential range of the start-up/shut-down (SUSD) conditions. Despite the complex case of the sk-PtCo catalyst comprising two distinct fcc alloy phases, our complementary techniques enabled in situ monitoring of structural changes in each crystal phase in detail. Remarkably, the in situ WAXS measurements uncover two primary catalyst aging processes, namely the cobalt depletion (regime I) followed by the crystallite growth via Ostwald ripening and/or particle coalescence (regime II). Additionally, in situ SAXS data reveal a continuous size growth over the AST. The Pt-enriched shell thickening based on the Co depletion within the first 100 SUSD cycles and particle growth induced by additional potential cycles were also collaborated by ex situ STEM-EELS. Overall, our work shows a comprehensive aging model for the sk-PtCo catalyst probed by complementary in situ WAXS and SAXS techniques.

Magnetic Array-Aided Visualizing PEMFC Degradation Heterogeneity

Analyzing degradation heterogeneity of proton exchange membrane fuel cell (PEMFC) while maintaining high practicality is consistently challenging, primarily due to the destructive and costly nature of existing techniques relying on material characterization. In this work, a designed magnetic array integrating 16 sensors within 25 cm2 space is used for direct scanning and imaging of PEMFC performance heterogeneity during its degradation. Results are validated through degradation mechanism analysis and material characterization, confirming its potential in guiding the development of durable materials.

Selective oxygen reduction reaction: mechanism understanding, catalyst design and practical application

The oxygen reduction reaction (ORR) is a key component for many clean energy technologies and other industrial processes. However, the low selectivity and the sluggish reaction kinetics of ORR catalysts have hampered the energy conversion efficiency and real application of these new technologies mentioned before. Recently, tremendous efforts have been made in mechanism understanding, electrocatalyst development and system design. Here, a comprehensive and critical review is provided to present the recent advances in the field of the electrocatalytic ORR. The two-electron and four-electron transfer catalytic mechanisms and key evaluation parameters of the ORR are discussed first. Then, the up-to-date synthetic strategies and in situ characterization techniques for ORR electrocatalysts are systematically summarized. Lastly, a brief overview of various renewable energy conversion devices and systems involving the ORR, including fuel cells, metal-air batteries, production of hydrogen peroxide and other chemical synthesis processes, along with some challenges and opportunities, is presented.

PGM-free single atom catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells

The progress of proton exchange membrane fuel cells (PEMFCs) in the clean energy sector is notable for its efficiency and eco-friendliness, although challenges remain in terms of durability, cost and power density. The oxygen reduction reaction (ORR) is a key sluggish process and although current platinum-based catalysts are effective, their high cost and instability is a significant barrier. Single-atom catalysts (SACs) offer an economically viable alternative with comparable catalytic activity for ORR. The primary concern regarding SACs is their operational stability under PEMFCs conditions. In this article, we review current strategies for increasing the catalytic activity of SACs, including increasing active site density, optimizing metal center coordination through heteroatom doping, and engineering porous substrates. To enhance durability, we discuss methods to stabilize metal centers, mitigate the effects of the Fenton reaction, and improve graphitization of the carbon matrix. Future research should apply computational chemistry to predict catalyst properties, develop in situ characterization for real-time active site analysis, explore novel catalysts without the use of platinum-based catalysts to reduce dependence on rare and noble metal, and investigate the long-term stability of catalyst under operating conditions. The aim is to engineer SACs that meet and surpass the performance benchmarks of PEMFCs, contributing to a sustainable energy future.

Salt Effect Engineering Single Fe-N2P2-Cl Sites on Interlinked Porous Carbon Nanosheets for Superior Oxygen Reduction Reaction and Zn-Air Batteries

Developing efficient metal-nitrogen-carbon (M-N-C) single-atom catalysts for oxygen reduction reaction (ORR) is significant for the widespread implementation of Zn-air batteries, while the synergic design of the matrix microstructure and coordination environment of metal centers remains challenges. Herein, a novel salt effect-induced strategy is proposed to engineer N and P coordinated atomically dispersed Fe atoms with extra-axial Cl on interlinked porous carbon nanosheets, achieving a superior single-atom Fe catalyst (denoted as Fe-NP-Cl-C) for ORR and Zn-air batteries. The hierarchical porous nanosheet architecture can provide rapid mass/electron transfer channels and facilitate the exposure of active sites. Experiments and density functional theory (DFT) calculations reveal the distinctive Fe-N2P2-Cl active sites afford significantly reduced energy barriers and promoted reaction kinetics for ORR. Consequently, the Fe-NP-Cl-C catalyst exhibits distinguished ORR performance with a half-wave potential (E1/2) of 0.92 V and excellent stability. Remarkably, the assembled Zn-air battery based on Fe-NP-Cl-C delivers an extremely high peak power density of 260 mW cm-2 and a large specific capacity of 812 mA h g-1, outperforming the commercial Pt/C and most reported congeneric catalysts. This study offers a new perspective on structural optimization and coordination engineering of single-atom catalysts for efficient oxygen electrocatalysis and energy conversion devices.