A simple approach to tailor OER activity of SrxCo0.8Fe0.2O3 perovskite catalysts

Correlation of Structure and Electrocatalytic Performance of Bulk Oxides for Water Electrolysis

Hydrogen-centered electrochemical technologies play a vital role in sustainable energy conversion and storage. One of the challenges in achieving cheap hydrogen is to bridge the gap between advanced electrocatalysts and highly effective electrodes. The key lies in designing electrocatalysts with high intrinsic activity and understanding the structure-activity relationship in water electrolysis. Being proposed as promising electrocatalysts, bulk oxides, with their compositional and crystal structure flexibility, provide a good platform for studying the correlation between intrinsic activity and electronic structure and also for screening superior catalysts for water electrolysis. In this review, we discuss the recent developments of oxide electrocatalysts in understanding the structure-activity relationship. Firstly, we present a thorough overview of recent advances from both theoretical and experimental aspects. Subsequently, we highlight the design principles to provide guidance for promoting performance. Finally, the remaining challenges and perspectives about this field are presented. This review aims to provide guidance for the design of highly advanced oxide electrocatalysts for water electrolysis and large-scale green energy supply.

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.

Unraveling the O2 Reduction Reaction on 2D Monolayer LaNiO3 Perovskite

The O2 reduction reaction (ORR) occurring at cathodes is a critical reaction in many electrochemical energy-converting devices such as fuel cells. The reaction kinematics of the ORR is generally very slow with high overpotentials and needs to be enhanced by using an efficient electrocatalyst. The highly recognized Pt-based electrocatalyst needs to be replaced with a low-cost non-noble metal-based electrocatalyst for catalyzing the ORR. We theoretically studied the structural and electronic properties of 3D bulk LaNiO3 perovskite. We have cleaved the (0 0 1) surface from 3D LaNiO3, which has a zero band gap (E g), to create 2D monolayer LaNiO3 computationally and studied its electronic properties. Our study demonstrates that the 2D monolayer LaNiO3 is a suitable candidate for catalyzing the ORR because of its high catalytic activity with a tiny electronic band gap of 0.25 eV. We explored the ORR mechanism on the 2D monolayer LaNiO3 perovskite by inspecting each intermediate. Our present findings show that the 2D monolayer LaNiO3 can efficiently catalyze the ORR through a four-electron (4e-) reduction reaction due to the excellent catalytic activity of its basal plane, which accords with the experimental findings. The change in Gibbs free energy (ΔG) calculations of various intermediate steps of the ORR demonstrates that all reaction steps are spontaneous and thermodynamically favorable. The 2D monolayer LaNiO3 perovskite can be a potential candidate for catalyzing the ORR efficiently. This study helps to enable the development of high-activity, stable 2D perovskites for use in future solid oxide fuel cells and related applications in green energy technologies.

Application of response surface methodology for optimization of the test condition of oxygen evolution reaction over La0.8Ba0.2CoO3 perovskite-active carbon composite

The Experimental Design was applied to optimize the electrocatalytic activity of La0.8Ba0.2CoO3 perovskite oxide/Active Carbon composite material in the alkaline solution for the Oxygen Evolution Reaction. After the preparation of La0.8Ba0.2CoO3, and structural characterizations, the experimental design was utilized to determine the optimal amount of the composite material and testing conditions. The overpotential was defined as the response variable, and the mass ratio of perovskite/active carbon, Potassium hydroxide (KOH) concentration, and Poly(vinylidene fluoride) (PVDF) amount were considered effective parameters. The significance of model terms is demonstrated by P-values less than 0.0500. The proposed prediction model determined the optimal amounts of 0.665 mg of PVDF, a KOH concentration of 0.609 M, and A perovskite/Active Carbon mass ratio of 2.81 with 308.22 mV overpotential (2.27% greater than the actual overpotential). The stability test of the optimized electrode material over 24 h suggests that it could be a good candidate electrocatalyst for OER with reusability potential.

Redox-active ligands enhance oxygen evolution reaction activity: Regulating the spin state of ferric ions and accelerating electron transfer

Metal-organic frameworks (MOFs) are considered as one of the most promising catalysts for oxygen evolution reaction (OER). However, only a few have introduced redox-active ligands into MOFs and explored their role in the OER process. In this work, we synthesized FeNi DHBQ/NF using the redox-active ligand 2,5-dihydroxy-1,4-benzoquinone (DHBQ), which exhibited excellent redox activity and required only 207 and 242 mV overpotentials to achieve current densities of 10 and 100 mA cm-2. Our research confirms that (i) the doping of Fe leads to the formation of Ni → O → Fe electron transfer channels in the MOFs and stronger electron transfer, attributed to the stronger d-π conjugation between the metal center and the ligand and reduced the d-orbital crystal field splitting energy of Fe3+; (ii) the rate determination step (RDS) in the OER process of the catalyst is the formation of O*, while Fe and redox-active ligands effectively regulate the adsorption energy of oxygen-containing intermediates, reducing the energy barrier of the RDS; (iii) the redox-active ligands can act as "electron reservoirs" in the electrochemical process, making Ni more readily oxidized to Ni3+ or even Ni4+ at low potentials, which is beneficial to the subsequent OER process.

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.

Study on the Effect of A/B Site Co-Doping on the Oxygen Evolution Reaction Performance of Strontium Cobaltite

The perovskite oxide SrCoO3−x is a promising oxygen electrocatalyst for renewable energy storage and conversion technologies. Here, A, B-site Co-doped perovskite Sr0.5Ba0.5Co0.95Mn0.05O3−x nanoparticles were rationally designed and synthesized by the sol-gel method with an average size of 30–40 nm. It has a remarkable intrinsical activity and stability in 1 M KOH solution. Compared with other A-site (SraA1−aCoO3−x A=Ba, Ca) and B-site doped perovskite (SrCobR1−bO3−x R=Mn, Fe, Ni, B) catalysts, Sr0.5Ba0.5Co0.95Mn0.05O3−x exhibits superior oxygen evolution reaction (OER) performance, smaller Tafel slope, and lower overpotential. The high electrochemical performance of Sr0.5Ba0.5Co0.95Mn0.05O3−x is attributed to its optimized crystal structure and the increase in the content of Co3+. This study demonstrates that highly symmetrical cubic perovskite structure catalytic displays better OER performance.

Sm0.5Sr0.5Co1−xNixO3−δ—A Novel Bifunctional Electrocatalyst for Oxygen Reduction/Evolution Reactions

The development of non-precious metal catalysts with excellent bifunctional activities is significant for air–metal batteries. ABO3-type perovskite oxides can improve their catalytic activity and electronic conductivity by doping transition metal elements at B sites. Here, we develop a novel Sm0.5Sr0.5Co1−xNixO3−δ (SSCN) nanofiber-structured electrocatalyst. In 0.1 M KOH electrolyte solution, Sm0.5Sr0.5Co0.8Ni0.2O3−δ (SSCN82) with the optimal Co: Ni molar ratio exhibits good electrocatalytic activity for OER/ORR, affording a low onset potential of 1.39 V, a slight Tafel slope of 123.8 mV dec−1, and a current density of 6.01 mA cm−2 at 1.8 V, and the ORR reaction process was four-electron reaction pathway. Combining the morphological characteristic of SSCN nanofibers with the synergistic effect of cobalt and nickel with a suitable molar ratio is beneficial to improving the catalytic activity of SSCN perovskite oxides. SSCN82 exhibits good bi-functional catalytic performance and electrochemical double-layer capacitance.

Perovskite With Tunable Active-Sites Oxidation State by High-Valence W for Enhanced Oxygen Evolution Reaction

Perovskite oxides have been established as a promising kind of catalyst for alkaline oxygen evolution reactions (OER), because of their regulated non-precious metal components. However, the surface lattice is amorphous during the reaction, which gradually decreases the intrinsic activity and stability of catalysts. Herein, the precisely control tungsten atoms substituted perovskite oxides (Pr_0.5Ba_0.5Co_1-xW_xO₃-_δ) nanowires were developed by electrostatic spinning. The activity and Tafel slope were both dependent on the W content in a volcano-like fashion, and the optimized Pr_0.5Ba_0.5Co_0.8W_0.2O₃-_δ exhibits both excellent activity and superior stability compared with other reported perovskite oxides. Due to the outermost vacant orbitals of W⁶⁺, the electronic structure of cobalt sites could be efficiently optimized. Meanwhile, the stronger W-O bond could also significantly improve the stability of latticed oxide atoms to impede the generation of surface amorphous layers, which shows good application value in alkaline water splitting.

Construction of bimetallic FeCo–SA/DABCO nanosheets by modulating the electronic structure for improved electrocatalytic oxygen evolution

For energy conversion and storage, the electrochemical oxygen evolution process (OER) is the crucial half-reaction process.

Development of Perovskite Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Perovskite oxides are studied as electrocatalysts for oxygen evolution reactions (OER) because of their low cost, tunable structure, high stability, and good catalytic activity. However, there are two main challenges for most perovskite oxides to be efficient in OER, namely less active sites and low electrical conductivity, leading to limited catalytic performance. To overcome these intrinsic obstacles, various strategies are developed to enhance their catalytic activities in OER. In this review, the recent developments of these strategies is comprehensively summarized and systematically discussed, including composition engineering, crystal facet control, morphology modulation, defect engineering, and hybridization. Finally, perspectives on the design of perovskite oxide-based electrocatalysts for practical applications in OER are given.

Influence of the Amount of Carbon during the Synthesis of LaFe0.8Co0.2O3/Carbon Hybrid Material in Oxygen Evolution Reaction

The oxygen evolution reaction (OER) and the hydrogen evolution reaction occurred at the anode and cathode, which depends on the electronic structure, morphology, electrochemically active surface area, and charge-transfer resistance of the electrocatalyst. Transition metals like cobalt, nickel, and iron have better OER and oxygen reduction reaction activities. At the same time, transition-metal oxide/carbon hybrid has several applications in electrochemical energy conversion reactions. The rich catalytic site of transition metals and the excellent conductivity of carbon material make these materials as a hopeful electrocatalyst in OER. Carbon-incorporated LaFe_0.8Co_0.2O₃ was prepared by a simple solution combustion method for the development of the best performance of the electrocatalyst. The catalyst can deliver 10 mA/cm² current density at an overpotential of 410 mV with better catalytic stability. The introduction of carbon material improves the dispersion ability of the catalyst and the electrical conductivity. The Tafel slope and onset potential of the best catalyst are 49.1 mV/dec and 1.55 V, respectively.

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.

Surface Reconstruction of La0.8Sr0.2Co0.8Fe0.2O3-δ for Superimposed OER Performance

Perovskites have become important OER electrocatalysts. Herein, as-prepared La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ (LSCF-0) is chosen as a sample to exhibit the superimposed effect of surface reconstruction accompanied by reduction of Co³⁺ to Co²⁺ on the further improvement of its activity and stability. As-synthesized LSCF-0 perovskite is chemically treated by simply immersing in an aqueous solution of NaBH₄ for 1.0 h at room temperature. The optimized LSCF (LSCF-2) owns an amorphous layer consisting of nanosized particles of ∼20 nm (vs smooth bulk crystalline surface for untreated LSCF), which exhibits superior OER performance to LSCF-0. LSCF-2 has an overpotential of 248 mV (10 mA cm^-2) and a Tafel slope of 51 mV dec^-1 (vs 355 mV and 76 mV dec^-1 for LSCF-0 and 381 mV and 91 mV dec^-1 for LCO) and an excellent cycle stability for 20 h running. This work supplies a new strategy to enhance OER performance through surface reconstruction of as-prepared perovskites.

Rose-like Nanocomposite of Fe-Ni Phosphides/Iron Oxide as Efficient Catalyst for Oxygen Evolution Reaction

In order to accelerate the reaction rate of water splitting, it is of immense importance to develop low-cost, stable and efficient catalysts. In this study, the facile synthesis of a novel rose-like nanocomposite catalyst (Ni₂ P/Fe₂ P/Fe₃ O₄ ) is reported. The synthesis process includes a solvothermal step and a phosphatization step to combine iron oxides and iron-nickel phosphides. Ni₂ P/Fe₂ P/Fe₃ O₄ performs well in catalyzing oxygen evolution reaction, with a very low overpotential of 365 mV to reach 10 mA cm^-2 current density. The Tafel slope is as low as 59 mV dec^-1 . Ni₂ P/Fe₂ P/Fe₃ O₄ has a large double-layer capacitance that contributes to a high electrochemically active area. Moreover, this catalyst is very stable for long-term use. Therefore, the Ni₂ P/Fe₂ P/Fe₃ O₄ catalyst has a high potential for use in oxygen evolution reactions.

Enhancing oxygen and hydrogen evolution activities of perovskite oxide LaCoO3via effective doping of platinum

In this study, a series of perovskite oxides LaCo_1−xPt_xO_3−δ (x = 0, 0.02, 0.04, 0.06, and 0.08) were prepared by the citric acid–ethylenediaminetetraacetic acid (CA–EDTA) complexing sol–gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Then, the samples were investigated as OER and HER bifunctional electrocatalysts in alkaline media. Compared with other catalysts, LaCo_0.94Pt_0.06O_3−δ had good stability and presented more activity at a lower overpotential of 454 mV (at 10 mA cm⁻²), a lower Tafel slope value of 86 mV dec⁻¹ and a higher mass activity of 44.4 A g⁻¹ for OER; it displayed a lower overpotential of 294 mV (at −10 mA cm⁻²), a lower Tafel slope value of 148 mV dec⁻¹ and a higher mass activity of −34.5 A g⁻¹ for HER. The improved performance might depend on a larger ECSA, faster charge transfer rate and higher ratio of the highly oxidative oxygen species (O₂²⁻/O⁻). Furthermore, the e_g orbital filling of Co approaching 1.2 in the B site might play a leading role.

Among the perovskite LaCo_1−xPt_xO_3−δ catalysts, LaCo_0.94Pt_0.06O_3−δ proved best for catalyzing OER/HER, with η = 454/294 mV, which might be attributed to LCP6 having the e_g orbital filling of Co closest to 1.2.