Bifunctional LaMn0.3Co0.7O3 Perovskite Oxide Catalyst for Oxygen Reduction and Evolution Reactions: The Optimized eg Electronic Structures by Manganese Dopant

Smart Compositional Design of B-Site Ordered Double Perovskite for Advanced Oxygen Catalysis at Ultra-High Current Densities

Perovskite oxides have been considered promising oxygen catalysts for oxygen reduction and evolution reactions (ORR and OER), owing to structural and compositional flexibility, and tailorable properties. Ingenious B-site ordered La1.5Sr0.5NiMn0.5Fe0.5O6 (LSNMF) double perovskite is strategically designed by simultaneously interposing Ni0.5Mn0.5 and Ni0.5Fe0.5 into B' and B″ sites. Controlling B-site cation systematically tailors the electronic structure of the B-site cation with a d-band center (Md) upshift close to the Fermi level, increasing the overlap of the Md center and O 2p center (OP). The strong interaction of Md and Op facilitates the adsorption of oxygen and activates the lattice oxygen to participate in the OER process, thereby enhancing the ORR and OER activity. For ORR, LSNMF exhibited an onset potential of 0.9 V along with a high limiting current of -8.05 mA cm-2. At the same time, for OER at 1 m KOH, LSNMF effectively reached a maximum current density of 3000 mA cm-2. Most importantly, the difference between EORR (at -1 mA cm-2) and EOER (at 10 mA cm-2), ΔE is 0.69 V, which stands among the best of recently reported perovskites. The as-designed LSNMF is stable, efficient, lucrative, and a promising candidate for practical application.

Perovskite Type ABO3 Oxides in Photocatalysis, Electrocatalysis, and Solid Oxide Fuel Cells: State of the Art and Future Prospects

Since photocatalytic and electrocatalytic technologies are crucial for tackling the energy and environmental challenges, significant efforts have been put into exploring advanced catalysts. Among them, perovskite type ABO3 oxides show great promising catalytic activities because of their flexible physical and chemical properties. In this review, the fundamentals and recent progress in the synthesis of perovskite type ABO3 oxides are considered. We describe the mechanisms for electrocatalytic oxygen evolution reactions (OER), oxygen reduction reactions (ORR), hydrogen evolution reactions (HER), nitrogen reduction reactions (NRR), carbon dioxide reduction reactions (CO2RR), and metal-air batteries in details. Furthermore, the photocatalytic water splitting, CO2 conversion, pollutant degradation, and nitrogen fixation are reviewed as well. We also stress the applications of perovskite type ABO3 oxides in solid oxide fuel cells (SOFs). Finally, the optimization of perovskite type ABO3 oxides for applications in various fields and an outlook on the current and future challenges are depicted. The aim of this review is to present a broad overview of the recent advancements in the development of perovskite type ABO3 oxides-based catalysts and their applications in energy conversion and environmental remediation, as well as to present a roadmap for future development in these hot research areas.

Rational Regulation of High-Entropy Perovskite Oxides through Hole Doping for Efficient Oxygen Electrocatalysis

Due to the high configuration entropy, unique atomic arrangement, and electronic structures, high-entropy materials are being actively pursued as bifunctional catalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable zinc-air batteries (ZABs). However, a relevant strategy to enhance the catalytic activity of high-entropy materials is still lacking. Herein, a hole doping strategy has been employed to enable the high-entropy perovskite La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 to effectively catalyze the ORR and OER. Hole doping experiments rely on the substitution of Sr2+ for La3+. The optimized La0.7Sr0.3(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 displays remarkable activity for the ORR and the OER, with a low potential difference of 0.880 V between the half-wave potential of the ORR and the OER potential at 10 mA cm-2, exceeding the majority of perovskite bifunctional catalysts. Further analysis of the electronic structures reveals that hole doping could regulate the eg-orbital filling of the transition-metal cations in high-entropy perovskites to an ideal position and thereby generate many highly active sites to promote the redox activity of oxygen. The assembled rechargeable ZAB with the targeted high-entropy perovskite as the cathode affords a specific capacity of 774.5 mAh gZn-1 under 10 mA cm-2 and durability for a period of 300 cycles, comparable to that of the 20%Pt/C + RuO2 ZAB. This work offers an important approach for the advancement of efficient high-entropy perovskites for ZABs.

La0.7Sr0.3MnO3 Perovskites for Oxygen Reduction in Zn-Air Batteries: Enhanced by In Situ Glucose Regulation

The actual ORR catalytic activity of perovskite materials is significantly lower than the theoretical value due to their inherently low specific surface area and significant segregation of inactive oxygen ions on the surface. This study reports a sol-gel synthesis approach that employs glucose as a structural regulator to fabricate La0.7Sr0.3MnO3 (LSM) perovskites. Compared with the original LSM (12.56 m2·g-1), LSM-Y2 exhibits a higher specific surface area (19.43 m2·g-1) and enhanced ORR catalytic activity. Electrochemical results show that the initial potential and half-wave potential of LSM-Y2 are positively shifted by 35 and 85 mV, respectively, with a 1.29-fold increase in intrinsic catalytic activity. Additionally, the performance of the Zn-air batteries is superior to that of the original LSM, with a peak power density of 115 mW·cm-2 and an energy density of 858 Wh·kg-1. The enhanced ORR catalytic activity of LSM-Y2 is attributed to the optimization of Mn eg orbital occupancy on the catalyst surface, facilitated by glucose introduction, and the improved adsorption of oxygen intermediates, resulting from the increased oxygen vacancy concentration. Additionally, the increased specific surface area and porosity of LSM-Y2 provided more active sites for the catalytic process, further enhancing ORR performance. This study not only elucidates the mechanism by which glucose influences the ORR catalytic activity of La0.7Sr0.3MnO3 perovskite but also presents a strategy for developing perovskite catalysts with superior ORR catalytic performance.

Sulfonated reduced graphene oxide encapsulated perovskite-type ErCoFe oxide nanoparticles for efficient electrochemical water oxidation

Perovskite oxides play a vital role as electrocatalysts in water oxidation due to their flexible and unique electronic structures. In this work, Er-based perovskites ErCo1-xFexO3-δ (x = 0.0, 0.1, 0.3, 0.5, 0.7, and 1.0) denoted as EC, ECF-0.9, ECF-0.7, ECF-0.5, ECF-0.3, and EF, respectively, are synthesized by the sol-gel method. Then, ECF-0.9 is supported on sulfonated reduced graphene oxide (S-rGO) by a hydrothermal method, with weight ratios of 1 : 1 and 3 : 1 of ECF/0.9 to S-rGO (shown as ECF-0.9/S-rGO(50%) and ECF-0.9/S-rGO(75%), respectively). The structural properties and the morphology of the synthesized materials are studied using a series of different techniques. The prepared perovskites are then used as electrode materials for electrochemical water oxidation. ECF-0.9 reveals better activity than pure EF, EC, and other perovskite oxides in terms of overpotential, Tafel slope, electrochemically active surface area (ECSA), and charge transfer resistance (Rct) values. Interestingly, when the optimized perovskite oxide catalyst ECF-0.9 is decorated on S-rGO sheets, the water oxidation activity is significantly improved. ECF-0.9/S-rGO(75%) exhibits superior activity for water oxidation with an overpotential of 290 mV@10 mA cm-2 and a Tafel slope of 41 mV dec-1. Finally, overall water splitting with ECF-0.9/S-rGO(75%) as the anode electrode shows a low electrolysis voltage of 1.60 V, alongside excellent stability for 20 h.

Strategic Design and Insights into Lanthanum and Strontium Perovskite Oxides for Oxygen Reduction and Oxygen Evolution Reactions

Perovskite oxides exhibit bifunctional activity for both oxygen reduction (ORR) and oxygen evolution reactions (OER), making them prime candidates for energy conversion in applications like fuel cells and metal-air batteries. Their intrinsic catalytic prowess, combined with low-cost, abundance, and diversity, positions them as compelling alternatives to noble metal and metal oxides catalysts. This review encapsulates the nuances of perovskite oxide structures and synthesis techniques, providing insight into pivotal active sites that underscore their bifunctional behavior. The focus centers on the breakthroughs surrounding lanthanum (La) and strontium (Sr)-based perovskite oxides, specifically their roles in zinc-air batteries (ZABs). An introduction to the mechanisms of ORR and OER is provided. Moreover, the light is shed on strategies and determinants central to optimizing the bifunctional performance of La and Sr-based perovskite oxides.

Easy conversion perovskite fluorides KCo1-xFexF3 for efficient oxygen evolution reaction

Herein, we report an easily oxidized Co-Fe perovskite fluoride as an efficient catalyst for the oxygen evolution reaction (OER). In situ Raman spectroscopy showed that the presence of F promotes reconstruction to form highly active (Co3+Fe3+)OOH, and the current density of 10 mA cm-2 can be achieved at the overpotential of only 118 mV in 1 M KOH aqueous solution. This work helps to understand the role of fluoride during the OER.

Perovskite Oxides Toward Oxygen Evolution Reaction: Intellectual Design Strategies, Properties and Perspectives

Developing electrochemical energy storage and conversion devices (e.g., water splitting, regenerative fuel cells and rechargeable metal-air batteries) driven by intermittent renewable energy sources holds a great potential to facilitate global energy transition and alleviate the associated environmental issues. However, the involved kinetically sluggish oxygen evolution reaction (OER) severely limits the entire reaction efficiency, thus designing high-performance materials toward efficient OER is of prime significance to remove this obstacle. Among various materials, cost-effective perovskite oxides have drawn particular attention due to their desirable catalytic activity, excellent stability and large reserves. To date, substantial efforts have been dedicated with varying degrees of success to promoting OER on perovskite oxides, which have generated multiple reviews from various perspectives, e.g., electronic structure modulation and heteroatom doping and various applications. Nonetheless, the reviews that comprehensively and systematically focus on the latest intellectual design strategies of perovskite oxides toward efficient OER are quite limited. To bridge the gap, this review thus emphatically concentrates on this very topic with broader coverages, more comparative discussions and deeper insights into the synthetic modulation, doping, surface engineering, structure mutation and hybrids. More specifically, this review elucidates, in details, the underlying causality between the being-tuned physiochemical properties [e.g., electronic structure, metal-oxygen (M-O) bonding configuration, adsorption capacity of oxygenated species and electrical conductivity] of the intellectually designed perovskite oxides and the resulting OER performances, coupled with perspectives and potential challenges on future research. It is our sincere hope for this review to provide the scientific community with more insights for developing advanced perovskite oxides with high OER catalytic efficiency and further stimulate more exciting applications.

Graphical Abstract

Tailoring the d-band electronic structure of deficient LaMn0.3Co0.7O3-δ perovskite nanofibers for boosting oxygen electrocatalysis in Zn-Air batteries

The development and design of efficient bifunctional electrocatalysts towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial for rechargeable Zinc-air batteries (ZABs). Optimizing the d-band structure of active metal center in perovskite oxides is an effective method to enhance ORR/OER activity by accelerating the rate-determining step. Herein, we report a deficient method to optimize the d-band structure of Co ions in LaMn0.3Co0.7O3-δ (LMCO-2) perovskite nanofibers, which regulates the mutual effect between B-site Co ions and reactive oxygen intermediates. It is proved by experiment and theoretical calculation that the d-band center (Md) of transition metal ions in LMCO-2 is moved up and the electron filling number of eg orbital in B site is 1.01, thus leading to the reduction of Gibbs free energy required for ORR rate-determining step (OH*→H2O*) to 0.22 eV and promoting reaction proceeds. In this manner, LMCO-2 showed good bifunctional oxygen electrocatalytic activity, with a half-wave potential of 0.71 V vs. RHE. Furthermore, the high specific capacity of 811.54 mAh g-1 and power density of 326.56 mW cm-2 were obtained by using LMCO-2 as the cathode catalyst for ZABs. This study proved the feasibility of d-band structure regulation to enhance the electrocatalytic activity of perovskite oxides.

Cationic Defect Engineering in Perovskite La2CoMnO6 for Enhanced Electrocatalytic Oxygen Evolution

The urgent need to promote the development of sustainable energy conversion requires exploration of highly efficient oxygen evolution reaction (OER) electrocatalysts. Defect engineering is a promising approach to address the inherent low electrical conductivity of metal oxides and limited reaction sites, for use in clean air applications and as electrochemical energy-storage electrocatalysts. In this article, oxygen defects are introduced into La₂CoMnO_6-δ perovskite oxides through the A-site cation defect strategy. By tuning the content of the A-site cation, oxygen defect concentration and corresponding electrochemical OER performance have been greatly improved. As a result, the defective La_1.8CoMnO_6-δ (L_1.8CMO) catalyst exhibits exceptional OER activity with an overpotential of 350 mV at 10 mA cm^-2, approximately 120 mV lower than that of the pristine perovskite. This enhancement can be attributed to the increase in surface oxygen vacancies, optimized e_g occupation of transition metal at the B-site, and enlarged Brunauer-Emmett-Teller surface area. The reported strategy facilitates the development of novel defect-mediated perovskites in electrocatalysis.

Activating Octahedral Center in Co-Doped NiFe2 O4 via Bridging Amorphous MoSx for Electrocatalytic Water Oxidation: A Case for eg Orbital Regulation in Spinel Oxide

Moderate e_g filling for octahedral metal cations (M_Oh ) is strongly correlated with the electrocatalytic water oxidation performance in the oxides system. Here, the e_g fillings of Ni_Oh and Fe_Oh in NiFe₂ O₄ -based spinel are controllably regulated by introducing an external radical of catalytically inactive MoS_x as an electron acceptor via a novel ultrasonic anchored pyrolysis strategy. The electron occupied in e_g orbit of M_Oh emigrates with the amount of MoS hanging on the apical of octahedral sites, and results in a salutary transition from high to medium e_g occupancy state, as confirmed by the X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. In addition, benefiting from the abundant unsaturated S atoms in amorphous MoS_x , the M_Oh at the surface furthest activates and consequently shows a superior water oxidation performance. Density functional theory also reveals that the e_g fillings of Ni and Fe decrease to 1.4 and 1.2 after MoS_x modification, which can effectively reduce the free energy of the OOH* intermediates in the oxygen evolution reaction process. This work opens an avenue for further releasing the electrocatalytic activity of octahedral sites through bridging external phases with rational electron-capturing/donating capability.

Nitrogen-doped Fe2O3/NiTe2 as an excellent bifunctional electrocatalyst for overall water splitting

The development of inexpensive, high efficiency electrocatalysts is a major challenge for electrolytic water to hydrogen production. Herein, an efficient porous nanoblock catalyst N-doped Fe₂O₃/NiTe₂ heterojunction for overall water splitting is reported. Notably, the obtained 3D self-supported catalysts exhibit good hydrogen evolution. reaction (HER) activity and oxygen evolution reaction (OER) properties in alkaline solution (only 70 mV and 253 mV of overpotential are needed to provide 10 mA cm^-2 current density, respectively). This is mainly due to the N-doped electronic structure optimized, the strong electronic interaction between Fe₂O₃ and NiTe₂ that facilitates rapid electron transfer, the porous structure which allows the catalyst to have large surface area for effective gas release, and their synergistic effect. When used as a dual function catalyst with overall water splitting, it achieved a current density of 10 mA cm^-2 under 1.54 V with good durability (at least 42 h). The present work provides a new methodology for the study of high-performance, low-cost, and corrosion-resistant bifunctional electrocatalysts.

Vacancy effect on the electrocatalytic activity of LaMn1/2Co1/2O3-δ for hydrogen and oxygen evolution reactions

Development of efficient electrocatalysts for water splitting can be a significant step toward green hydrogen generation. In this work, a remarkable enhancement of electrocatalytic properties is achieved through the incorporation of oxygen-vacancies in a perovskite oxide, while maintaining the same structural framework. The oxygen-deficient material La₂MnCoO_6-δ (LaMn_0.5Co_0.5O_3-δ) is isostructural to the parent stoichiometric material, but shows drastically enhanced electrocatalytic properties for both half-reactions of water-splitting, namely hydrogen-evolution and oxygen-evolution reactions, due to the oxygen-vacancies.

Boosting catalytic activity of SrCoO2.52 perovskite by Mn atom implantation for advanced peroxymonosulfate activation

Material-enhanced heterogeneous peroxymonosulfate (PMS) activation for degradation of antibiotic in water has attracted intensive attention. However, one challenge is the electron transfer efficiency from the material to PMS for reactive oxygen species (ROS) production. Considering that the B-sites of perovskite oxides are closely associated with the catalytic performance, partial substitution of the B-sites of perovskite oxides can enhance the redox cycle of metals. Consequently, adjusting the ratio of each element at the B site can introduce oxygen vacancies on the surface of perovskite. Herein, a method was developed in which manganese (Mn) partially substitutes B-sites to modify surface properties of SrCoO_2.52 perovskite oxides, resulting in the enhancement of catalytic activity. In degradation kinetics studies using SrCoMnO_3-δ-0.5/PMS (SrCoMnO_3-δ-0.5 denotes that the molar substitution of Mn at the B site of SrCoO_2.52 perovskite oxide is 0.5) reaction system and sulfamethoxazole (SMX) as the target pollutant, it was found that the reaction rate constant (k_obs) is 0.287 min^-1 which is 2.4 times that of SrCoO_2.52/PMS system. Experimental and theoretical analyses revealed that Mn-O covalent bonding governs the intrinsic catalytic activity of SrCoMnO_3-δ-0.5 perovskite oxides. The Mn sites exhibits stronger adsorption energy with PMS than the Co sites, facilitating the breaking of O-O bond. Simultaneously, oxygen vacancies and surface adsorbed oxygen species have a synergistic effect for PMS adsorption. This work can provide a potential route in developing advanced catalysts based on manipulation of the B-sites of perovskite oxides for PMS activation.

Transition metal oxides with perovskite and spinel structures for electrochemical energy production applications

Transition metal oxide-based materials are an interesting alternative to substitute noble-metal based catalyst in energy conversion devices designed for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution reactions (HER). Perovskite (ABO₃) and spinel (AB₂O₄) oxides stand out against other structures due to the possibility of tailoring their chemical composition and, consequently, their properties. Particularly, the electrocatalytic performance of these materials depends on features such as chemical composition, crystal structure, nanostructure, cation substitution level, e_g orbital filling or oxygen vacancies. However, they suffer from low electrical conductivity and surface area, which affects the catalytic response. To mitigate these drawbacks, they have been combined with carbon materials (e.g. carbon black, carbon nanotubes, activated carbon, and graphene) that positively influence the overall catalytic activity. This review provides an overview on tunable perovskites (mainly lanthanum-based) and spinels featuring 3d metal cations such as Mn, Fe, Co, Ni and Cu on octahedral sites, which are known to be active for the electrochemical energy conversion.

Yttrium-based Double Perovskite Nanorods for Electrocatalysis

Herein, we investigate the effect of the chemical composition of double perovskite nanorods on their versatile electrocatalytic activity not only as supports for the oxidation of small organic molecules but also as catalysts for the oxygen evolution reaction. Specifically, Y₂CoMnO₆ and Y₂NiMnO₆ nanorods with average diameters of 300 nm were prepared by a two-step hydrothermal method, in which the individual effects of synthetic parameters, such as the pH, annealing temperature, and precursor ratios on both the composition and morphology, were systematically investigated. When used as supports for Pt nanoparticles, Y₂CoMnO₆/Pt catalysts exhibited an electrocatalytic activity for the methanol oxidation reaction, which is 2.1 and 1.3 times higher than that measured for commercial Pt/C and Y₂NiMnO₆/Pt, respectively. Similarly, the Co-based catalyst support material displayed an ethanol oxidation activity, which is 2.3 times higher than both Pt/C and Y₂NiMnO₆/Pt. This clear enhancement in the activity for Y₂CoMnO₆ can largely be attributed to strong metal-support interactions, as evidenced by a downshift in the binding energy of the Pt 4f bands, measured by X-ray photoelectron spectroscopy (XPS), which is often correlated not only with a downshift in the d-band center but also to a decreased adsorption of poisoning adsorbates. Moreover, when used as catalysts for the oxygen evolution reaction, Y₂CoMnO₆ displayed a much greater activity as compared with Y₂NiMnO₆. This behavior can largely be attributed not only to a preponderance of comparatively more favorable oxidation states and electronic configurations but also to the formation of an active layer on the surface of the Y₂CoMnO₆ catalyst, which collectively gives rise to improved performance metrics and greater stability as compared with both IrO₂ and Y₂NiMnO₆. Overall, these results highlight the importance of both the chemical composition and the electronic structure of double perovskites, especially when utilized in multifunctional roles as either supports or catalysts.

Coupling LaNiO3 Nanorods with FeOOH Nanosheets for Oxygen Evolution Reaction

Perovskite-based electrocatalysts with compositional flexibility and tunable electronic structures have emerged as one of the promising non-noble metal candidates for oxygen evolution reaction (OER). Here, we propose a heterostructure comprising perovskite oxide (LaNiO3) nanorods and iron oxide hydroxide (FeOOH) nanosheets as an effective electrochemical catalyst for OER. The optimized 0.25Fe-LNO catalyst with an interesting 1D-2D hierarchical structure shows a low overpotential of 284 mV at 10 mA cm−2 and a small Tafel slope of 69 mV dec−1. The enhanced performance can be explained by the synergistic effect between LaNiO3 and FeOOH, resulting in an improved electrochemically active surface area, facilitated charge transfer and the optimized adsorption of OH intermediates.

The Spin Modulation Stimulated Efficient Electrocatalytic Oxygen Evolution Reaction over LaCoO3 Perovskite

Perovskite is a promising non-noble catalyst and has been widely investigated for the electrochemical oxygen evolution reaction (OER). However, there is still serious lack of valid approaches to further enhance their catalytic performance. Herein, we propose a spin state modulation strategy to improve the OER electrocatalytic activity of typical perovskite material of LaCoO₃ . Specifically, the electronic configuration transition was realized by a simple high temperature thermal reduction process. M-H hysteresis loop results reveal that the reduction treatment can produce more unpaired electrons in 3d orbit by promoting the electron transitions of Co from low spin state to high spin state, and thus lead to the increase of the spin polarization. Electrochemical measurements show that the catalytic performance of LaCoO₃ is strongly dependent on its electronic configuration. With the optimized reduction treatment, the overpotential for the OER process in 0.5 M KOH electrolyte solution at 10 mA cm^-2 current density was 396 mV, significantly lower than that of the original state. Furthermore, it can mediate efficient OER with an overpotential of 383 mV under an external magnetic field, which is attributed to the appropriate electron filling. Our results show that electron spin state regulation is a new way to boost the OER electrocatalytic activity.

Surface Defect Engineering on Perovskite Oxides as Efficient Bifunctional Electrocatalysts for Water Splitting

The design of high-performance and cost-effective electrocatalysts for water splitting is of prime importance for efficient and sustainable hydrogen production. In this work, a surface defect engineering method is developed for optimizing the electrocatalytic activity of perovskite oxides for water electrolysis. A typical ferrite-based perovskite oxide material La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) is used and regulated by selective acid etching. The optimal parameters for the surface treatment are identified. An efficient bifunctional perovskite oxide, denoted LSCF-30, is prepared by selectively corroding the A-site Sr element in the surface region, which is found to not only increase the exposure and decrease the coordination of B-site metals but also effectively modulate the electronic structure of these metals. The crystal lattice of the perovskite bulk is kept constant during surface engineering, which ensures the structural stability of the perovskite catalyst. The findings demonstrate an effective strategy of surface defect engineering in enhancing the performance of perovskite oxide electrocatalysts for water splitting.

Recent progress of electrocatalysts for oxygen reduction in fuel cells

Oxygen reduction reaction (ORR) has gradually been in the limelight in recent years because of its great application potential for fuel cells and rechargeable metal-air batteries. Therefore, significant issues are increasingly focused on developing effective and economical ORR electrocatalysts. This review begins with the reaction mechanisms and theoretical calculations of ORR in acidic and alkaline media. The latest reports and challenges in ORR electrocatalysis are traced. Most importantly, the latest advances in the development of ORR electrocatalysts are presented in detail, including platinum group metal (PGM), transition metal, and carbon-based electrocatalysts with various nanostructures. Furthermore, the development prospects and challenges of ORR electrocatalysts are speculated and discussed. These insights would help to formulate the design guidelines for highly-active ORR electrocatalysts and affect future research to obtain new knowledge for ORR mechanisms.

Designing of Highly Efficient Oxygen Evolution Reaction Electrocatalysts Utilizing A Correlation Factor: Theory and Experiment

The theoretical prediction of the catalytic activity is very beneficial for the design of highly efficient catalysts. At present, most theoretical descriptors focus on estimating the catalytic activity and understanding the enhancement mechanism of catalysts, while it is also quite important to find a factor to correlate the descriptors with preparation methods. In this work, a correlation factor, the d electron density of transition metal ions, was developed to correlate the d band center values of transition metal ions with the preparation methods of amorphization and Al introduction. According to the results of theoretical simulations, the correlation factor not only exhibited favorable linear relationships with the theoretical overpotentials of (CoFeAl_x)₃O₄ and (CoFeAl_x)₃O₄ + (CoFeAl_x)OOH systems but also correlated with two preparation methods by altering the volume of systems. Based on theoretical guidance, the electrocatalytic activities of the prepared (CoFeAl_x)₃O₄ specimens were gradually improved by the preparation methods of amorphization and Al introduction, and the Am-CoFeAl-2-10h specimen exhibited a low kinetic barrier of 268 mV, fast charge transfer rate, and stable electrocatalytic activity. This strategy could be applied to design highly efficient catalysts by adjusting the correlation factor of the active site with suitable preparation methods.

Cerium substitution in LaCoO3 perovskite oxide as bifunctional electrocatalysts for hydrogen and oxygen evolution reactions

Perovskite oxides have attracted great attention in electrochemistry due to their compositional and structural flexibility. Herein, microwave/ultrasound assisted hydrothermal procedures were developed to synthesize Ce-doped LaCoO₃ perovskite oxide as bifunctional electrocatalysts for OER and HER application, achieving highly efficient bifunctional catalytic performance. The obtained LCC4 exhibited excellent electrocatalytic activity with an overpotential of 380 mV and 305 mV at 10 mA cm^-2 toward OER and HER, respectively. The lower Tafel slopes of 80 mV per decade and 144 mV per decade for OER and HER, respectively, indicated the faster reaction kinetics for the improved inherent electrocatalytic activity. The outstanding long-term durability of LCC4 in alkaline conditions was also vital to the practical applications of water electrolysis. The improved bifunctional electrocatalytic activity was attributed to the synergistic effects of excellent conductivity and enriched active sites arising from A-site substitution. This work not only provides an efficient strategy for the development of perovskite oxide-based electrocatalysts but also puts forward a new insight on bifunctional electrocatalysts for overall water splitting.

Metal-organic framework derived ultralow-loading platinum-copper catalyst: a highly active and durable bifunctional electrocatalyst for oxygen-reduction and evolution reactions

Electrocatalysts with high active oxygen reduction (ORR) and oxygen evolution reaction (OER) activities are key factors in renewable energy technologies. Unlike common strategies for adjusting the proportion of metal centers in a multi-metal organic framework (MOF), herein, we designed and synthesized bifunctional electrocatalysts using cetyltrimethylammonium bromide (CTAB)-capped ultra-low content platinum (Pt) (≤0.5 wt.% Pt) and copper (Cu) nanoparticles and doped on the surface of zinc-based MOF (Zn-MOF-74) and calcinated at 900 °C. According to the electrochemical activity, the Pt/Cu/NPC-900 exhibits superior catalytic activities towards both the ORR with the onset (E₀) and half-wave (E_1/2) potentials were 1.0 V and 0.89 V versus RHE, respectively, and OER (E_o = 1.48 V versus RHE and overpotential (η) = 0.265 V versus RHE) in an alkaline electrolyte at ambient temperature. Also, Pt/Cu/NPC-900 catalyzes through a 4^-electron process and exhibits superior stability. Such insightful findings, as well as a newly developed approach, provides rational design and synthesis of an economical and efficient strategy for bifunctional electrocatalyst development.

Proton Exchange Membrane (PEM) Fuel Cells with Platinum Group Metal (PGM)-Free Cathode

Proton exchange membrane (PEM) fuel cells have gained increasing interest from academia and industry, due to its remarkable advantages including high efficiency, high energy density, high power density, and fast refueling, also because of the urgent demand for clean and renewable energy. One of the biggest challenges for PEM fuel cell technology is the high cost, attributed to the use of precious platinum group metals (PGM), e.g., Pt, particularly at cathodes where sluggish oxygen reduction reaction takes place. Two primary ways have been paved to address this cost challenge: one named low-loading PGM-based catalysts and another one is non-precious metal-based or PGM-free catalysts. Particularly for the PGM-free catalysts, tremendous efforts have been made to improve the performance and durability-milestones have been achieved in the corresponding PEM fuel cells. Even though the current status is still far from meeting the expectations. More efforts are thus required to further research and develop the desired PGM-free catalysts for cathodes in PEM fuel cells. Herein, this paper discusses the most recent progress of PGM-free catalysts and their applications in the practical membrane electrolyte assembly and PEM fuel cells. The most promising directions for future research and development are pointed out in terms of enhancing the intrinsic activity, reducing the degradation, as well as the study at the level of fuel cell stacks.

Phase-Segregated SrCo0.8Fe0.5-xO3-δ/FexOy Heterostructured Catalyst Promotes Alkaline Oxygen Evolution Reaction

Perovskite oxide is a promising alternative to noble metal electrocatalysts for the oxygen evolution reaction (OER). However, as one of the most active oxide catalysts, cubic SrCoO₃ presents poor OER performance relative to the theoretically predicted activity. Appropriate introduction of a guest component in the lattice and surface could largely promote the OER activity. Herein, we present a thermal-induced phase-segregation strategy to synthesize a heterostructured SrCo_0.8Fe_0.5-xO_3-δ/Fe_xO_y (SC8F5) catalyst for OER. This novel perovskite/Fe₃O₄ heterostructure allows us to enhance the electrical conductivity ability, increase the Co oxidation state, and activate the surface oxygen to active oxygen species (O₂^2-/O^-) for efficient OER. In contrast to the poor stability of SrCo_0.8Fe_0.2O_3-δ, we found that the SC8F5 heterostructure with segregated Fe₃O₄ on the surface can mitigate surface reconstruction and stabilize the catalyst structure, thereby increasing catalytic stability.

A Universal Process: Self-Templated and Orientated Fabrication of XMoO4 (X: Ni, Co, or Fe) Nanosheets on MoO2 Nanoplates as Electrocatalysts for Efficient Water Splitting

Fabrication of superior nonprecious electrocatalysts is essential for water electrolysis. Herein, the epitaxial growth of the XMoO₄ (X = Ni, Co, Fe) nanosheets on the hexagonal MoO₂ nanoplates are carried out. The preoxidation of MoO₂ nanoplate is fatal to the epitaxial growth of a nanosheets array on MoO₂ nanoplates. The hierarchical heterostructure of the vertically aligned NiMo nanosheets on MoO₂ nanoplate (NiMo/MoO₂) is well-maintained in the process of in situ topotactic reduction transformation from NiMoO₄·xH₂O/MoO₂. Attributing it to the rich electroactive sites from nanosheets array, together with the intrinsic electrocatalytic performance of NiMo alloy, the as-engineered NiMo/MoO₂ as electrocatalyst exhibits admirable hydrogen evolution reaction (HER) activity with a small onset potential of -12 mV vs RHE (1 mA cm^-2) and a tafel value of 43.6 mV dec^-1 at alkaline media. Furthermore, the obtained CoMoO₄/MoO₂ possesses excellent oxygen evolution performance, which is verified by an ultralow overpotential of 230 mV@10 mA cm^-2, small Tafel slope (51 mV dec^-1), and robust durability. The developed NiMo/MoO₂ and CoMoO₄/MoO₂ electrocatalysts are assembled into an alkaline electrolyzer, which affords a cell potential of 1.51 V at 10 mA cm^-2, as well as outstanding operational durability, which is superior to the typically constructed 20 wt % Pt/C-RuO₂ system (1.59 V at 10 mA cm^-2). Hence, the universal strategy using MoO₂ nanoplates as Mo source and epitaxial substrate may be extended to explore and construct economical and superior Mo-based electrocatalysts for water electrolysis.