Coupling amorphous cobalt hydroxide nanoflakes on Sr2Fe1.5Mo0.5O5+δ perovskite nanofibers to induce bifunctionality for water splitting

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

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

Probing Dynamic Self-Reconstruction on Perovskite Fluorides toward Ultrafast Oxygen Evolution

Exploring low cost, highly active, and durable electrocatalysts for oxygen evolution reaction (OER) is of prime importance to boost energy conversion efficiency. Perovskite fluorides are emerging as alternative electrocatalysts for OER, however, their intrinsically active sites during real operation are still elusive. Herein, the self-reconstruction on newly designed NiFe coupled perovskite fluorides during OER process is demonstrated. In situ Raman spectroscopy, ex situ X-ray absorption spectroscopy, and theoretical calculation reveal that Fe incorporation can significantly activate the self-reconstruction of perovskite fluorides and efficiently lower the energy barrier of OER. Benefiting from self-reconstruction and low energy barrier, the KNi_0.8 Fe_0.2 F₃ @nickel foam (KNFF2@NF) electrocatalyst delivers an ultralow overpotential of 258 mV to afford 100 mA cm^-2 and an excellent durability for 100 h, favorably rivaling most the state-of-the-art OER electrocatalysts. This protocol provides the fundamental understanding on OER mechanism associated with surface reconstruction for perovskite fluorides.

Metal organic framework derived perovskite/spinel heterojunction as efficient bifunctional oxygen electrocatalyst for rechargeable and flexible Zn-air batteries

Interface engineering strategy has been developed to design efficient catalysts for boosting electrocatalytic performance in past few decades. Herein, heterojunctions of PrCoO₃/Co₃O₄ nanocages (PCO/Co₃O₄ NCs) with atomic-level engineered interfaces and rich oxygen vacancies are proposed for Zn-air batteries. The synthesized product shows exceptional bifunctional activity and robust stability towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The enhanced catalytic capacity is primary attributed to the synergistic effect of PCO/Co₃O₄, evidenced by the experimental results and theoretical calculations. More importantly, the PCO/Co₃O₄ NCs assembled liquid Zn-air battery exhibits a power density of 182 mW cm^-2 and a long-term operation of 185 h. When assembled into solid-state cable type battery, this newly designed catalyst also reaches a stable open circuit voltage (1.359 V) and a peak power density of 85 mW cm^-3. Our findings provide essential guidelines of engineering heterostructured electrocatalysts for future wearable electronic devices.

Recent Advances in Perovskite Catalysts for Efficient Overall Water Splitting

Hydrogen is considered a promising clean energy vector with the features of high energy capacity and zero-carbon emission. Water splitting is an environment-friendly and effective route for producing high-purity hydrogen, which contains two important half-cell reactions, namely, the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). At the heart of water splitting is high-performance electrocatalysts that efficiently improve the rate and selectivity of key chemical reactions. Recently, perovskite oxides have emerged as promising candidates for efficient water splitting electrocatalysts owing to their low cost, high electrochemical stability, and compositional and structural flexibility allowing for the achievement of high intrinsic electrocatalytic activity. In this review, we summarize the present research progress in the design, development, and application of perovskite oxides for electrocatalytic water splitting. The emphasis is on the innovative synthesis strategies and a deeper understanding of structure–activity relationships through a combination of systematic characterization and theoretical research. Finally, the main challenges and prospects for the further development of more efficient electrocatalysts based on perovskite oxides are proposed. It is expected to give guidance for the development of novel non-noble metal catalysts in electrochemical water splitting.

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.

Overdoping strategy for preparing of two-phase oxide electrocatalyst to boost oxygen evolution reaction

The oxygen evolution reaction is of great significance to the production of hydrogen from high efficiency electrolytic water, hydrogen oxygen fuel cell and other energy conversion devices, but there are many challenges such as high cost, low efficiency and poor stability of catalysts. Among non-precious metal catalysts, oxide has its unique advantages. We used overdoping strategy to prepare two-phase oxide electrocatalyst SrCo 0.9 Fe 0.05 Mo 0.35 O x (SCFM 0.35 ) containing double perovskite and Co3O4 with excellent OER electrocatalytic activity and stability in alkaline solution. It required an overpotential of 361.7 mV to reach a 10 mA cm -2 current density and its performance only degrades by 3.48% after 1000 CV cycles accelerated stability tests, whose electrochemical performance is superior to that of single-phase double perovskites and undoped perovskites. SrCo 0.9 Fe 0.1 O 3 (SCF) ordinary perovskite is doped with slightly molybdenum (Mo), and then the ordinary perovskite turns into double perovskite because of the polyvalence characteristics of Mo. When Mo is overdoped, Co 3 O 4 phase was precipitated while Mo entered perovskite phase. This process causes lattice distortion and makes the surface electronic structure benign changes. Furthermore, the microscopic morphology of the material surface and the valence state of cobalt element are changed, thereby improving the microenvironment of the electrochemical process.

Perovskite Oxides as Electrocatalysts for Hydrogen Evolution Reaction

Hydrogen generation through electrocatalytic splitting of water, i.e., hydrogen evolution reaction (HER), is an attractive method of converting the electricity generated from renewable sources into chemical energy stored in hydrogen molecules. A wide variety of materials have been studied in an effort to develop efficient and cost-effective electrocatalysts that can replace the traditional platinum/carbon catalyst. One family of functional materials that holds promise for this application is perovskite oxides. This mini-review discusses some of the progress made in the development of HER electrocatalysts based on perovskite oxides in the past decade. Given the diverse range of possible compositions of perovskite oxides, various studies have focused on compositional modifications to develop single-phase catalysts, whereas others have investigated heterostructures and composites that take advantage of synergistic interactions of different compounds with perovskite oxides. The recent advances indicate that this family of materials have great potential for utilization in HER electrocatalysis.

Amorphous Yolk-Shelled ZIF-67@Co3(PO4)2 as Nonprecious Bifunctional Catalysts for Boosting Overall Water Splitting

It is challenging to generate inexpensive and noble metal-free catalysts for efficient overall water splitting (OWS). To achieve this goal, suitable tuning of the structure and composition of electrocatalytic materials is a promising approach that has attracted much attention in recent years. Herein, novel hybrid amorphous ZIF-67@Co₃(PO₄)₂ electrocatalysts with yolk-shell structures were prepared using a reflux method. It is demonstrated that yolk-shelled ZIF-67@Co₃(PO₄)₂ is not only an active catalyst for the hydrogen evolution reaction (HER) but also an efficient catalyst for the oxygen evolution reaction (OER). The optimized composite electrode showed superior performance with low overpotentials of 73 and 334 mV @ 10 mA·cm^-2 toward HER and OER, respectively, and a low potential of 1.62 V @ 10 mA·cm^-2 and 1.66 V @ 30 mA·cm^-2 in a practical OWS test under alkaline conditions. N-O bonds were formed to connect the two components of ZIF-67 and Co₃(PO₄)₂ in the composite ZIF-67@Co₃(PO₄)₂, which indicates that the two components are synergistic but not isolated, and this synergistic effect may be one of the important reasons to boost the oxygen and hydrogen evolution performances of the hybrid. Based on experimental data, the high electrocatalytic performance was inferred to be related to the unique structure of ZIF-67, tuning the ability of Co₃(PO₄)₂ and synergism between ZIF-67 and Co₃(PO₄)₂. The preparation strategy reported herein can be extended for the rational design and synthesis of cheap, active, and long-lasting bifunctional electrocatalysts for OWS and other renewable energy devices.

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.