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

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

Modulating the electronic structures of cobalt-organic frameworks for efficient electrocatalytic oxygen evolution

The oxygen evolution reaction (OER) is a key process in various energy storage/generation technologies. Tuning the electronic structures of catalysts is an effective approach to improve the catalyst's activity. In this work, we synthesized Ce-doped cobalt-organic frameworks with benzene-1, 4-dicarboxylic acid (BDC) as the ligand as efficient OER electrocatalysts (denoted as Co3Ce1 BDC) with excellent stability and improved catalytic performance. The introduced Ce in Co3Ce1 BDC changes the surface configuration and tunes electronic structures of the active Co site, leading to enhanced interaction between intermediates and catalysts. Besides, the specific surface area, reaction kinetics, charge transfer efficiency, and turnover frequency are also improved in the presence of Ce. As a result, the Co3Ce1 BDC demonstrated excellent performance with a low overpotential of 285 mV at a current of 10 mA·cm-2, a preferable Tafel slope of 56.1 mV·dec-1, and an excellent durability in 1 M KOH, indicating the potential for practical applications in water splitting and other energy storage technologies wherein the OER plays a critical role. Comprehensive theoretical calculations and modeling further identified the key role of Ce in modulating the electronic structure and OER activity of cobalt-organic frameworks. Most importantly, this work provides a new strategy to the development of efficient cobalt-organic framework catalysts in OER-related applications.

Interfacial built-in electric-field for boosting energy conversion electrocatalysis

The formation of a built-in electric field (BIEF) can induce electron-rich and electron-poor counterparts to synergistically modify electronic configurations and optimize the binding strengths with intermediates, thereby leading to outstanding electrocatalytic performance. Herein, a critical review regarding the concept, modulation strategies, and applications of BIEFs is comprehensively summarized, which begins with the fundamental concepts, together with the advantages of BIEF for boosting electrocatalytic reactions. Then, a systematic summary of the advanced strategies for the modulation of BIEF along with the in-detail mechanisms in its formation are also added. Finally, the applications of BIEF in driving electrocatalytic reactions and some cascade systems for illustrating the conclusive role from the induced BIEF are also systematically discussed, followed by perspectives on the future deployment and opportunity of the BIEF design.

Hollow Heterostructured Nanocatalysts for Boosting Electrocatalytic Water Splitting

The implementation of electrochemical water splitting demands the development and application of electrocatalysts to overcome sluggish reaction kinetics of hydrogen/oxygen evolution reaction (HER/OER). Hollow nanostructures, particularly for hollow heterostructured nanomaterials can provide multiple solutions to accelerate the HER/OER kinetics owing to their advantageous merit. Herein, the recent advances of hollow heterostructured nanocatalysts and their excellent performance for water splitting are systematically summarized. Starting by illustrating the intrinsically advantageous features of hollow heterostructures, achievements in engineering hollow heterostructured electrocatalysts are also highlighted with the focus on structural design, interfacial engineering, composition regulation, and catalytic evaluation. Finally, some perspective insights and future challenges of hollow heterostructured nanocatalysts for electrocatalytic water splitting are also discussed.

Puffing ultrathin oxides with nonlayered structures

Two-dimensional (2D) oxides have unique electrical, optical, magnetic, and catalytic properties, which are promising for a wide range of applications in different fields. However, it is difficult to fabricate most oxides as 2D materials unless they have a layered structure. Here, we present a facile strategy for the synthesis of ultrathin oxide nanosheets using a self-formed sacrificial template of carbon layers by taking advantage of the Maillard reaction and violent redox reaction between glucose and ammonium nitrate. To date, 36 large-area ultrathin oxides (with thickness ranging from ~1.5 to ~4 nm) have been fabricated using this method, including rare-earth oxides, transition metal oxides, III-main group oxides, II-main group oxides, complex perovskite oxides, and high-entropy oxides. In particular, the as-obtained perovskite oxides exhibit great electrocatalytic activity for oxygen evolution reaction in an alkaline solution. This facile, universal, and scalable strategy provides opportunities to study the properties and applications of atomically thin oxide nanomaterials.

Ru doping induces the construction of a unique core-shell microflower self-supporting electrocatalyst for highly efficient overall water splitting

Since the large reaction energy barrier caused by multi-step electron transfer processes of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) gravely restricts the practical application of electrocatalytic water splitting, it is urgent to develop a dual-functional electrocatalyst which can effectively reduce the reaction energy barrier and actually speed up the reaction. Herein, the Ru species are doped into the complex of magnetite and FeNi-layered double hydroxide by a one-step oil bath method, and a self-supporting binder-free bifunctional electrocatalyst was synthesized on the surface of iron foam (named Ru-Fe₃O₄@FeNi-LDH/IF). The unique 3D core-shell microflower structure of Ru-Fe₃O₄@FeNi-LDH/IF, the combination of active ingredient and conductive substrate, together with the doping of Ru may immensely provide a large number of active sites, adjust the electronic structure, accelerate electron transfer, and thus greatly improve the electrocatalytic activity and durability. It is worth mentioning that when Ru-Fe₃O₄@FeNi-LDH/IF is used as the anode and cathode for overall water splitting, only 1.52 V battery voltage can generate a current density of 10 mA cm^-2, and also maintain a prominent stability for at least 36 hours. This work provides a feasible strategy for heteroatom-doping LDH as a bifunctional electrocatalyst.

Interface Engineering of Heterogeneous CeO2-CoO Nanofibers with Rich Oxygen Vacancies for Enhanced Electrocatalytic Oxygen Evolution Performance

The development of highly efficient and cheap electrocatalysts for the oxygen evolution reaction (OER) is highly desirable in typical water-splitting electrolyzers to achieve renewable energy production, yet it still remains a huge challenge. Herein, we have presented a simple procedure to construct a new nanofibrous hybrid structure with the interface connecting the surface of CeO₂ and CoO as a high-performance electrocatalyst toward the OER through an electrospinning-calcination-reduction process. The resultant CeO₂-CoO nanofibers exhibit excellent electrocatalytic properties with a small overpotential of 296 mV at 10 mA cm^-2 for the OER, which is superior to many previously reported nonprecious metal-based and commercial RuO₂ catalysts. Furthermore, the prepared CeO₂-CoO nanofibers display remarkable long-term stability, which can be maintained for 130 h with nearly no attenuation of OER activity in an alkaline electrolyte. A combined experimental and theoretical investigation reveals that the excellent OER properties of CeO₂-CoO nanofibers are due to the unique interfacial architecture between CeO₂ and CoO, where abundant oxygen vacancies can be generated due to the incomplete matching of atomic positions of two parts, leading to the formation of many low-coordinated Co sites with high OER catalytic activity. This research provides a practical and promising opportunity for the application of heterostructured nonprecious metal oxide catalysts for high-efficiency electrochemical water oxidation.

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.