High-performance AEM unitized regenerative fuel cell using Pt-pyrochlore as bifunctional oxygen electrocatalyst

Sol-Gel-Synthesized Pt, Ni and Co-Based Electrocatalyst Effects of the Support Type, Characterization, and Possible Application in AEM-URFC

This study explores the synthesis and characterization of platinum (Pt), nickel (Ni), and cobalt (Co)-based electrocatalysts using the sol-gel method. The focus is on the effect of different support materials on the catalytic performance in alkaline media. The sol-gel technique enables the production of highly uniform electrocatalysts, supported on carbon-based substrates, metal oxides, and conductive polymers. Various characterization techniques, including X-ray diffraction (XRD) and scanning electron microscopy (SEM), were used to analyze the structure of the synthesized materials, while their electrochemical properties, which are relevant to their application in unitized regenerative fuel cells (URFCs), were investigated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). This hydrogen energy-converting device integrates water electrolyzers and fuel cells into a single system, reducing weight, volume, and cost. However, their performance is constrained by the electrocatalyst's oxygen bifunctional activity. To improve URFC efficiency, an ideal electrocatalyst should exhibit high oxygen evolution (OER) and oxygen reduction (ORR) activity with a low bifunctionality index (BI). The present study evaluated the prepared electrocatalysts in an alkaline medium, finding that Pt25-Co75/XC72R and Pt75-Co25/N82 demonstrated promising bifunctional activity. The results suggest that these electrocatalysts are well-suited for both electrolysis and fuel cell operation in anion exchange membrane-unitized regenerative fuel cells (AEM-URFCs), contributing to improved round-trip efficiency.

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

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

Modulating the Ruthenium-Cobalt Active Pair with Moderate Spacing for Enhanced Acidic Water Oxidation

Ruthenium (Ru)-based catalysts have emerged as promising alternatives to Iridium (Ir) catalysts in proton exchange membrane water electrolysis cells due to their lower price and excellent oxygen evolution reaction (OER) activity. However, their stability is compromised by generation of unstable high-valence Ru sites and oxygen vacancy in a lattice oxygen-mediated (LOM) pathway. Here, a low-load Ru site on a Barium (Ba)-doped Co3O4 (RuBaxCo3-xO4) catalyst is developed with abundant Ruthenium─Cobalt (Ru─Co) pairs for enhanced acidic OER activity. The incorporation of Ba can efficiently modulate the lattice of Co3O4, creating Ru─Co active pairs with optimized spacing through compression stress. In situ characterizations exhibit contractive Ru─Co pairs that promote the rapid and direct coupling of *O─O* radicals, bypassing the sluggish *OOH species and avoiding the oxygen vacancies, which can trigger the oxide path mechanism (OPM) for an efficient and stable OER process. As a result, the designed catalyst delivers a low overpotential of 219 mV to achieve a current density of 10 mA cm-2, and also demonstrates excellent stability, maintaining performance over 50 h of continuous operation at a larger current density of 50 mA cm-2. These findings highlight the potential of the RuBaxCo3-xO4 catalysts for durable and efficient OER applications.

Designing bifunctional perovskite catalysts for the oxygen reduction and evolution reactions

The development of unified regenerative fuel cells (URFCs) necessitates an active and stable bifunctional oxygen electrocatalyst. The unique challenge of possessing high activity for both the oxygen reduction (ORR) and oxygen evolution (OER) reactions, while maintaining stability over a wide potential window impedes the design of bifunctional oxygen electrocatalysts. Herein, two design strategies are explored to optimize their performance. The first incorporates active sites for the ORR and OER, Mn and Co, into a single perovskite structure, which is achieved with the perovskites Ba0.5Sr0.5Co0.8Mn0.2O3-δ (BSCM) and La0.5Ba0.25Sr0.25Co0.5Mn0.5O3-δ (LBSCM). The second combines an active ORR perovskite catalyst (La0.4Sr0.6MnO3-δ (LSM)) with an OER active perovskite catalyst (Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)) in a physical mixed composite (BSCF/LSM). The success of the two strategies is investigated by measuring the catalysts' catalytic performance and response to alternating reducing and oxidizing potentials to mimic the dynamic conditions experienced during the operation of URFCs. Additionally, the continuous, potentiodynamic change in Mn, Co, and Fe oxidation states during the ORR and OER is elucidated with operando X-ray absorption spectroscopy (XAS) measurements, revealing key insights into the nature of the active sites. The results reveal important catalyst physiochemical properties and provide a guide for future research and design principles for bifunctional oxygen electrocatalysts.

Oxygen Radical Coupling on Short-Range Ordered Ru Atom Arrays Enables Exceptional Activity and Stability for Acidic Water Oxidation

The discovery of efficient and stable electrocatalysts for oxygen evolution reaction (OER) in acid is vital for the commercialization of the proton-exchange membrane water electrolyzer. In this work, we demonstrate that short-range Ru atom arrays with near-ideal Ru-Ru interatomic distances and a unique Ru-O hybridization state can trigger direct O*-O* radical coupling to form an intermediate O*-O*-Ru configuration during acidic OER without generating OOH* species. Further, the Ru atom arrays suppress the participation of lattice oxygen in the OER and the dissolution of active Ru. Benefiting from these advantages, the as-designed Ru array-Co3O4 electrocatalyst breaks the activity/stability trade-off that plagues RuO2-based electrocatalysts, delivering an excellent OER overpotential of only 160 mV at 10 mA cm-2 in 0.5 M H2SO4 and outstanding durability during 1500 h operation, representing one of the best acid-stable OER electrocatalysts reported to date. 18O-labeled operando spectroscopic measurements together with theoretical investigations revealed that the short-range Ru atom arrays switched on an oxide path mechanism (OPM) during the OER. Our work not only guides the design of improved acidic OER catalysts but also encourages the pursuit of short-range metal atom array-based electrocatalysts for other electrocatalytic reactions.

Dual-axial engineering on atomically dispersed catalysts for ultrastable oxygen reduction in acidic and alkaline solutions

Atomically dispersed catalysts are a promising alternative to platinum group metal catalysts for catalyzing the oxygen reduction reaction (ORR), while limited durability during the electrocatalytic process severely restricts their practical application. Here, we report an atomically dispersed Co-doped carbon-nitrogen bilayer catalyst with unique dual-axial Co-C bonds (denoted as Co/DACN) by a smart phenyl-carbon-induced strategy, realizing highly efficient electrocatalytic ORR in both alkaline and acidic media. The corresponding half-wave potential for ORR is up to 0.85 and 0.77 V (vs. reversible hydrogen electrode (RHE)) in 0.5 M H2SO4 and 0.1 M KOH, respectively, representing the best ORR activity among all non-noble metal catalysts reported to date. Impressively, the Zn-air battery (ZAB) equipped with Co/DACN cathode achieves outstanding durability after 1,688 h operation at 10 mA cm-2 with a high current density (154.2 mA cm-2) and a peak power density (210.1 mW cm-2). Density functional theory calculations reveal that the unique dual-axial cross-linking Co-C bonds of Co/DACN significantly enhance the stability during ORR and also facilitate the 4e- ORR pathway by forming a joint electron pool due to the improved interlayer electron mobility. We believe that axial engineering opens a broad avenue to develop high-performance heterogeneous electrocatalysts for advanced energy conversion and storage.

Ionic Covalent Organic Frameworks-Derived Cobalt Single Atoms and Nanoparticles for Efficient Oxygen Electrocatalysis

Metal single atoms show outstanding electrocatalytic activity owing to the abundant atomic reactive sites and superior stability. However, the preparation of single atoms suffers from inexorable metal aggregation which is harmful to electrocatalytic activity. Here, ionic covalent organic frameworks (iCOFs) are employed as the sacrificial precursor to mitigate the metal aggregation and subsequent formation of bulky particles. Molecular dynamics simulation shows that iCOFs can trap and confine more Co ions as compared to neutral COFs, resulting in the formation of a catalyst composed of Co single atoms and uniformly distributed Co nanoparticles (Co_SA &Co_NP-10 ). However, the neutral COFs derive a catalyst composed of Co atomic clusters and large Co nanoparticles (Co_AC &Co_NP-25 ). The Co_SA &Co_NP-10 catalyst exhibits higher oxygen bifunctional electrocatalytic activities than Co_AC &Co_NP-25 , coinciding with the density functional theory results. Taking the Co_SA &Co_NP-10 as the air cathode in Zn-air batteries (ZABs), the aqueous ZAB presents a high power density of 181 mW cm^-2 , a specific capacity of 811 mAh g^-1 as well as a long cycle life of 407 h at a current density of 10 mA cm^-2 , while the quasi-solid state ZAB displays a power density of 179 mW cm^-2 and the cycle life of 30 h.

Pyrochlores for Advanced Oxygen Electrocatalysis

Fuel cells (FCs), water electrolyzers (WEs), unitized regenerative fuel cells (URFCs), and metal-air batteries (MABs) are among the emerging electrochemical technologies for energy storage, fuel (H₂), oxidant (O₂), and clean energy production. Their commercial applications are hindered by the low oxygen reduction reaction/oxygen evolution reaction (ORR/OER) bifunctional activity (for URFCs and MABs), OER selectivity (brine electrolysis in seawater and Martian environments), and high cost of the benchmark electrocatalysts (OER: RuO₂, IrO₂ and ORR: Pt/C) which affects the performance and affordability of the devices. Low-cost electrocatalysts with highly symmetric ORR/OER bifunctional activity and high OER selectivity are crucial for large-scale FC, WE, URFC, and MAB application. Recent studies have revealed that tuning the structure of pyrochlore oxides provides a pathway to enhancing OER and ORR activity over a wide range of pH. Pyrochlore oxides commonly contain a cubic A₂B₂O_7-x structure with two types of tetrahedrally coordinated O atoms containing (1) A-O-A and (2) A-O-B types with a cationic radii mismatch of r_A/r_B > 1.5 and propensity toward oxygen vacancy formation. The variety of pyrochlore oxides and their tunable properties make them attractive for a wide spectrum of applications. Among all the metal oxides, Ru-based pyrochlores (e.g., Pb₂Ru₂O_7-x) exhibit the best bifunctional oxygen electrocatalytic activity, i.e., low bifunctionality index (BI), in alkaline medium. Furthermore, pyrochlores exhibit high OER selectivity in brine electrolytes due to the presence of surface oxygen vacancies, making them suitable for space applications (brine electrolysis on Mars) and coastal hydrogen generation. Their bifunctional activity and selectivity can be further amplified by (1) substituting "A" and "B" sites of pyrochlores (AA'BB'O_7-x), (2) tuning metal oxidation states of A and B by varying synthesis conditions, and (3) modulating oxygen vacancy concentration, each of which yield favorable structural and electronic variations. In recent years, research on the synthesis and understanding of pyrochlores has significantly enhanced their viability, offering a new horizon in the quest for economical and active electrocatalysts. However, an account that focuses on critical developments in this field is still lacking.In this Account, we focus on the recent development of a variety of pyrochlore electrocatalysts to understand intrinsic structure-activity-selectivity-stability relationships in these materials. Recent developments and applications of pyrochlore-based electrocatalysts are discussed under the following headings: (1) modulation of crystal and electronic structure of pyrochlores, (2) structure-activity-stability relationships of different pyrochlores for OER and ORR, (3) development of OER-selective pyrochlores for brine electrolysis, and (4) the application of pyrochlores in electrochemical devices. Finally, we highlight some unaddressed issues such as the precise identification of active sites, which can be addressed in the future through advanced in situ and ex situ characterization techniques coupled with the density functional theory-based analyses. This Account provides foundational understanding to guide the comprehensive development of highly active, selective, stable and low-cost structurally engineered pyrochlores for high performance electrochemical devices.