Morphology and composition regulation of Prussian blue analogues to boost electrocatalytic oxygen evolution reaction

Prussian blue analogues (PBAs) have well-dispersed active sites and porous nanostructure. Reasonable design and construction of the nanostructure of PBAs is a promising option to obtain cost-effective electrocatalysts for high-efficiency oxygen evolution reaction (OER). Nevertheless, current structural engineering is costly and complex due to the inevitable involvement of additional etching agents or procedures. In this paper, a simple temperature-control strategy without additional etchant, is presented for the preparation of hollow CoFe-PBA precursors with porous nanobox structure, and then the hollow CoFe-PBA@NiFeRu-LDH nanoboxes (named as CoFe-PBA@NiFeRu-LDH NBs) heterogeneous catalyst is obtained. The preferable composition and structure provide more accessible active sites and better electronic structure for OER. As a result, the optimized CoFe-PBA@NiFeRu-LDH NBs demonstrates an impressive ability to accelerate OER in alkaline electrolyte with a minimal overpotential (219 mV at 10 mA cm-2) and excellent stability. More importantly, by combining CoFe-PBA@NiFeRu-LDH NBs with Pt/C for overall water electrolysis, an ultra-low voltage (1.52 V at 10 mA cm-2) is required. This study offers a facile and effective idea for chemical and morphological control in the manufacture of efficient electrocatalysts.

Surface Engineering of Perovskite Oxide LaCo0.67Cu0.33O3 for Improved Overall Water Splitting Activity

Perovskite oxides continue demonstrating suboptimal electrocatalytic performance for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) due to their inherently low activity, inadequate electronic conductivity, and restricted availability of active sites. Defect engineering has attracted significant attention as a promising approach to enhancing reaction kinetics. In this study, a LaCo0.67Cu0.33O3 (LCCO) composite perovskite electrocatalyst was synthesized using a sol-gel method followed by acid etching for defect engineering (LCCO-x, where x = 6, 12, 24, and 30, indicating the treatment time in hours). Particularly, LCCO-24 exhibited high activity and improved reaction kinetics for both the OER and HER under alkaline conditions. When employed for overall water splitting, it achieved a low full-cell voltage of 1.49 V at a current density of 10 mA·cm-2, comparable to leading noble metal catalysts. Analysis confirmed that the enhancement in bifunctional electrocatalytic activity was attributed to the increased presence of oxygen defects and the increase in surface area. This study demonstrates that defect engineering is an effective approach for investigating perovskite oxides in the context of electrochemical water splitting.

Monophasic Titanate-Based Photocatalyst with Heteroatom Mixed Iso-Aliovalency Enabling Water Oxidation

Rhodium-doped SrTiO3 perovskite as a monophasic titanate-based catalyst (SrTi1-xRhxO3) showed photocatalytic activity for oxygen evolution reaction (OER) from water under solar light irradiation with an instant induction period, although Rh4+ in SrTiO3 introduces deep trap states thereby diminishing the efficiency of the hydrogen evolution reaction (HER). Despite its potential, the exact crystal structure of Rh:SrTiO3 has not been yet completely investigated. Overcoming these challenges, here, we synthesized a monophasic SrTi0.95Rh0.05O3 (RSTO) perovskite oxide with a precisely determined crystal structure and highlighted an unconsidered pivotal role of the Rh iso-aliovalency reversibility that enables excellent photocatalytic water oxidation. With structural, morphological, optical, and electronic insights from XRD, FE-SEM, HR-TEM, XPS, and advanced XAS measurements in both total electron yield (TEY) and fluorescence yield (TFY), the oxygen evolution reaction (OER) process is attributed to the redox dynamics of Rh4+ ↔ Rh3+ synergistic interplay.

Preparation and Performance of Nickel-Doped LaSrCoO3-SrCO3 Composite Materials for Alkaline Oxygen Evolution in Water Splitting

Perovskites exhibit catalytic properties on the oxygen evolution reaction (OER) in water electrolysis. Elemental doping by specific preparation methods is a good strategy to obtain highly catalytical active perovskite catalysts. In this work, La0.5Sr0.5Co1-xNixO3-δ perovskite materials doped with different ratios of nickel were successfully synthesized by the sol-gel method. The electrochemical measurement results show that for OER in 1 M KOH solution, La0.5Sr0.5Co0.8Ni0.2O3-δ prepared by the sol-gel method requires only a low overpotential of 213 mV to reach 10 mA cm-2, which is significantly lower than that of La0.5Sr0.5Co0.8Ni0.2O3-δ prepared by the hydrothermal method for the increasing about 45.24% (389 mV at 10 mA cm-2). In addition, La0.5Sr0.5Co0.8Ni0.2O3-δ by the sol-gel method can be kept stable in an alkaline medium tested for 30 h without degradation. This indicates that the prepared La0.5Sr0.5Co0.8Ni0.2O3-δ has better OER performance. The X-ray diffraction (XRD) results show that SrCO3 is the main phase formed, which is a disadvantage of this method. The performance improvement may be affected by the carbonate phase. The scanning electron microscopy (SEM) results show that layer structured La0.5Sr0.5Co0.8Ni0.2O3-δ by the sol-gel method has more surface pores with a pore diameter of about 0.362 μm than spherical granular structured La0.5Sr0.5Co0.8Ni0.2O3-δ by the hydrothermal method. X-ray photoelectronic spectroscopy (XPS) results reveal that the crystal lattice of La0.5Sr0.5Co0.8Ni0.2O3-δ by nickel doping is lengthened, and the electronic configuration of Co is also changed by the sol-gel preparation process. The improved electrocatalytic performance of La0.5Sr0.5Co0.8Ni0.2O3-δ may be attributed to the pore structure formed providing more active sites during the sol-gel process and the improved oxygen mobility with Ni doping by the sol-gel method. The doping strategy using the sol-gel method provides valuable insights for optimizing perovskite catalytic properties.

Comparative Study on the Electrocatalytic Performance of ABO3-Based Hexagonal Perovskite Oxides with Different [AO3] Layers

To systematically investigate the influence of the number of [AO3] layers in the unit cell of hexagonal perovskite oxide on the oxygen evolution reaction performance, we successfully synthesized the three new hexagonal perovskite oxides 2H-BaCo0.9Ru0.1O3-δ, 6H-BaCo0.9Ru0.1O3-δ, and 10H-BaCo0.9Ru0.1O3-δ with the same element composition but different [BaO3] layers via the sol-gel method. Here, 2H, 6H, and 10H refer to the number of [BaO3] layers contained in the unit cell of the BaCo0.9Ru0.1O3-δ system. Experimentally, 10H-BaCo0.9Ru0.1O3-δ, featuring ten layers of [BaO3], exhibits optimal electrochemical activity among the three oxide catalysts, and in situ Raman results under various bias voltages confirm its ability to maintain a high surface crystal structural stability. Notably, as the number of [BaO3] layers increases, the effective magnetic moments and the valence state of surface Co ions in these three catalysts also increase, with the spin configuration of the surface Co ions being in a high-spin state. More importantly, DFT calculations provide the evolution rules of the p-band center (εp) with the number of [BaO3] layers, predicting the electrochemical performance of the BaCo0.9Ru0.1O3-δ system with different [BaO3] layers. Our experimental results offer a distinctive perspective for the future design, synthesis, and application of hexagonal perovskite oxides in electrocatalysis.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Metal-organic frameworks (MOFs) for hybrid water electrolysis: structure-property-performance correlation

Hybrid water electrolysis (HWE) is a promising pathway for the simultaneous production of high-value chemicals and clean H2 fuel. Unlike conventional electrochemical water splitting, which relies on the oxygen evolution reaction (OER), HWE involves the anodic oxidation reaction (AOR). The AORs facilitate the conversion of organic or inorganic compounds at the anode into valuable chemicals, while the cathode carries out the hydrogen evolution reaction (HER) to produce H2. Recent literature has witnessed a surge in papers investigating various AORs with organic and inorganic substrates using a series of transition metal-based catalysts. Over the past two decades, metal-organic frameworks (MOFs) have garnered significant attention for their exceptional performance in electrochemical water splitting. These catalysts possess distinct attributes such as highly porous architectures, customizable morphologies, open facets, high electrochemical surface areas, improved electron transport, and accessible catalytic sites. While MOFs have demonstrated efficiency in electrochemical water splitting, their application in hybrid water electrolysis has only recently been explored. In recent years, a series of articles have been published; yet there is no comprehensive article summarizing MOFs for hybrid water electrolysis. This article aims to fill this gap by delving into the recent progress in MOFs specifically tailored for hybrid water electrolysis. In this article, we systematically discuss the structure-property-performance relationships of various MOFs utilized in hybrid water electrolysis, supported by pioneering examples. We explore how the structure, morphology, and electronic properties of MOFs impact their performance in hybrid water electrolysis, with particular emphasis on value-added chemical generation, H2 production, potential improvement, conversion efficiency, selectivity, faradaic efficiency, and their potential for industrial-scale applications. Furthermore, we address future advancements and challenges in this field, providing insights into the prospects and challenges associated with the continued development and deployment of MOFs for hybrid water electrolysis.

Electrocatalytic Properties of Quasi-2D Oxides LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, and Zn) for Hydrogen and Oxygen Evolution Reactions

Designing cost-effective and highly efficient electrocatalysts for water splitting is a significant challenge. We have systematically investigated a series of quasi-2D oxides, LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, Zn), to enhance the electrocatalytic properties of the two half-reactions of water-splitting, namely oxygen and hydrogen evolution reactions (OER and HER). The four materials are isostructural, as confirmed by Rietveld refinements with X-ray diffraction. The oxygen contents and metal valence states were determined by iodometric titrations and X-ray photoelectron spectroscopy. Electrical conductivity measurements in a wide range of temperatures revealed semiconducting behavior for all four materials. Electrocatalytic properties were studied for both half-reactions of water-splitting, namely, oxygen-evolution and hydrogen-evolution reactions (OER and HER). For the four materials, the trends in both OER and HER were the same, which also matched the trend in electrical conductivities. Among them, LaSrMn0.5Co0.5O4 showed the best bifunctional electrocatalytic activity for both OER and HER, which may be attributed to its higher electrical conductivity and favorable electron configuration.

Study on Water Splitting of the 214-Type Perovskite Oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga)

We present a study on the electrocatalysis of 214-type perovskite oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga) with semiconducting-like behavior synthesized using the sol-gel method. Among these five catalysts, PrSrCoO4 exhibits the optimal electrochemical performance in both the oxygen evolution reaction and the hydrogen evolution reaction, mainly due to its larger electrical conductivity, mass activity, and turnover frequency. Importantly, the weak dependency of LSV curves in a KOH solution with different pH values, revealing the adsorbate evolving mechanism in PrSrCoO4, and the density functional theory (DFT) calculations indicate that PrSrCoO4 has a smaller Gibbs free energy and a higher density of states near the Fermi level, which accelerates the electrochemical water splitting. The mutual substitution of different rare-earth elements will change the unit-cell parameters, regulate the electronic states of catalytic active site Co ions, and further affect their catalytic performance. Furthermore, the magnetic results indicate strong spin-orbit coupling in the electroactive sites of Co ions in SmSrCoO4 and EuSrCoO4, whereas the magnetic moments of Co ions in the other three catalysts mainly arise from the spin itself. Our experimental results expand the electrochemical applications of 214-type perovskite oxides and provide a good platform for a deeper understanding of their catalytic mechanisms.

One-Dimensional La0.2Sr0.8Cu0.4Co0.6O3-δ Nanostructures for Efficient Oxygen Evolution Reaction

Producing oxygen and hydrogen via the electrolysis of water has the advantages of a simple operation, high efficiency, and environmental friendliness, making it the most promising hydrogen production method. In this study, La0.2Sr0.8Cu0.4Co0.6O3-δ (LSCC) nanofibers were prepared by electrospinning to utilize non-noble perovskite oxides instead of noble metal catalysts for the oxygen evolution reaction, and the performance and electrochemical properties of LSCC nanofibers synthesized at different firing temperatures were evaluated. In an alkaline environment (pH = 14, 6 M KOH), the nanofibers calcined at 650 °C showed an overpotential of 209 mV at a current density of 10 mA cm-2 as well as good long-term stability. Therefore, the prepared LSCC-650 NF catalyst shows excellent potential for electrocatalytic oxygen evolution.