Temperature Effect on Co-Based Catalysts in Oxygen Evolution Reaction

Recognizing the reactive sites of SnFe2O4 for the oxygen evolution reaction: the synergistic effect of SnII and FeIII in stabilizing reaction intermediates

Among the reported spinel ferrites, the p-block metal containing SnFe2O4 is scarcely explored, but it is a promising water-splitting electrocatalyst. This study focuses on the reaction kinetics and atomic scale insight of the reaction mechanism of the oxygen evolution reaction (OER) catalyzed by SnFe2O4 and analogous Fe3O4. The replacement of FeIIOh sites with SnIIOh in SnFe2O4 improves the catalytic efficiency and various intrinsic parameters affecting the reaction kinetics. The variable temperature OER depicts a low activation energy (Ea) of 28.71 kJ mol-1 on SnFe2O4. Experimentally determined second-order dependence on [OH-] and the prominent kinetic isotope effect observed during the deuterium labelling study implies the role of hydroxide ions in the rate-determining step (RDS). Using density functional theory, the reaction mechanism on the (001) surface of SnFe2O4 and Fe3O4 is modelled. The DFT simulated free energy diagram for the reaction intermediates shows an adsorbate evolution mechanism (AEM) on both the ferrites' surfaces where the formation of *OOH is the RDS on SnFe2O4 while *O formation is the RDS on Fe3O4. In contrast to other spinel ferrites, where individual metal sites act independently, in case of SnFe2O4, a synergy between FeIIIOh and the neighbouring SnIIOh atoms is responsible for stabilizing the OER intermediates, enhancing the catalytic OER activity of SnFe2O4 as compared to isostructural Fe3O4.

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.

Structural Reconstruction of a Cobalt- and Ferrocene-Based Metal-Organic Framework during the Electrochemical Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) are increasingly being investigated as electrocatalysts for the oxygen evolution reaction (OER) due to their unique modular structures that present a hybrid between molecular and heterogeneous catalysts, featuring well-defined active sites. However, many fundamental questions remain open regarding the electrochemical stability of MOFs, structural reconstruction of coordination sites, and the role of in situ-formed species. Here, we report the structural transformation of a surface-grown MOF containing cobalt nodes and 1,1'-ferrocenedicarboxylic acid linkers (denoted as CoFc-MOF) during the OER in alkaline electrolyte. Ex situ and in situ investigations of CoFc-MOF film suggest that the MOF acts as a precatalyst and undergoes a two-step restructuring process under operating conditions to generate a metal oxyhydroxide phase. The MOF-derived metal oxyhydroxide catalyst, supported on nickel foam electrodes, displays high activity toward the OER with an overpotential of 190 mV at a current density of 10 mA cm-2. While this study demonstrates the necessity of investigating structural evolution of MOFs during electrocatalysis, it also shows the potential of using MOFs as precursors in catalyst design.

Mott-Schottky Construction Boosted Plasmon Thermal and Electronic Effects on the Ag/CoV-LDH Nanohybrids for Highly-Efficient Water Oxidation

Mott-Schottky construction and plasmon excitation represent two highly-efficient and closely-linked coping strategies to the high energy loss of oxygen evolution reaction (OER), but the combined effect has rarely been investigated. Herein, with Ag nanoparticles as electronic structure regulator and plasmon exciter, Ag/CoV-LDH@G nanohybrids (NHs) with Mott-Schottky heterojunction and notable plasmon effect are well-designed. Combining theoretical calculations with experiments, it is found that the Mott-Schottky construction modulates the Fermi level/energy band structure of CoV-LDH, which in turn leads to lowered d-band center (from -0.89 to -0.93), OER energy barrier (from 6.78 to 1.31 eV), and preeminent plasmon thermal/electronic effects. The thermal effect can offset the endothermic enthalpy change of OER, promote the deprotonation of *OOH, and accelerate electron transfer kinetics. Whereas the electronic effect can increase the density of charge carriers (from 0.70 × 1020 to 1.64 × 1020 cm-3), lower the activation energy of OER (from 30.3 to 17.7 kJ mol-1). Benefiting from these favorable factors, the Ag/CoV-LDH@G NHs show remarkable electrocatalytic performances, with an overpotential of 178 and 263 mV to afford 10 and 100 mA cm-2 for OER, respectively, and a low cell voltage of 1.42 V to drive 10 mA cm-2 for overall water splitting under near-infrared light irradiation.

Bubble-water/catalyst triphase interface microenvironment accelerates photocatalytic OER via optimizing semi-hydrophobic OH radical

Photocatalytic water splitting (PWS) as the holy grail reaction for solar-to-chemical energy conversion is challenged by sluggish oxygen evolution reaction (OER) at water/catalyst interface. Experimental evidence interestingly shows that temperature can significantly accelerate OER, but the atomic-level mechanism remains elusive in both experiment and theory. In contrast to the traditional Arrhenius-type temperature dependence, we quantitatively prove for the first time that the temperature-induced interface microenvironment variation, particularly the formation of bubble-water/TiO2(110) triphase interface, has a drastic influence on optimizing the OER kinetics. We demonstrate that liquid-vapor coexistence state creates a disordered and loose hydrogen-bond network while preserving the proton transfer channel, which greatly facilitates the formation of semi-hydrophobic •OH radical and O-O coupling, thereby accelerating OER. Furthermore, we propose that adding a hydrophobic substance onto TiO2(110) can manipulate the local microenvironment to enhance OER without additional thermal energy input. This result could open new possibilities for PWS catalyst design.

Anchoring Polydopamine on ZnCo2 O4 Nanowire To Facilitate Urea Water Electrolysis

To overcome the sluggishness of the oxygen evolution reaction (OER), the urea oxidation reaction was developed. In the case of OER application studies ZnCo2 O4 is an excellent electrocatalyst, towards the UOR has been performed with surface-grown polydopamine (PDA) with surface-grown polydopamine (PDA). ZnCo2 O4 @PDA is produced over the surface of nickel foam by a hydrothermal method followed by self-polymerization of dopamine hydrochloride. Dopamine hydrochloride was varied in solution to study the optimal growth of PDA necessary to enhance the electrochemical activity. Prepared ZnCo2 O4 @PDA was characterized by X-ray diffraction, electronic structural, and morphology/microstructure studies. With successful confirmation, the developed electrode material was applied to UOR and ZnCo2 O4 @PDA-1.5, delivering an excellent low overpotential of 80 mV at 20 mA cm-2 in the electrolyte mixture of 1 M potassium hydroxide+0.33 M urea. To support the excellent UOR activity, other electrochemical properties such as the Tafel slope, electrochemical surface active sites, and electrochemical impedance spectroscopy were also studied. Furthermore, a schematic illustration explaining the UOR mechanism is shown to allow a clear understanding of the obtained electrochemical activity. Finally, urea water electrolysis was carried out in a two-electrode symmetrical cell and compared with water electrolysis. This clearly showed the potential of the developed material for efficient electrochemical hydrogen production.

Solar-Light-Responsive Zinc-Air Battery with Self-Regulated Charge-Discharge Performance based on Photothermal Effect

It is extremely challenging to significantly increase the voltaic efficiency, power density, and cycle stability of a Zn-air battery by just adjusting the catalytic performance of the cathode with nanometers/atomistic engineering because of the restriction of thermodynamic equilibrium potential. Herein, inspired by solar batteries, the S-atom-bridged FeNi particles and N-doped hollow carbon nanosphere composite configuration (FeNi-S,N-HCS) is presented as a prototype of muti-functional air electrode material (intrinsic electrocatalytic function and additional photothermal function) for designing photoresponsive all-solid-state Zn-air batteries (PR-ZABs) based on the photothermal effect. The local temperature of the FeNi-S,N-HCS electrode can well respond to the stimuli of sunlight irradiation because of their superior photothermal effect. As expected, under illumination, the power density of the as-fabricated PR-ZABs based on the FeNi-S,N-HCS electrode can be improved from 77 mW cm^-2 to 126 mW cm^-2. Simultaneously, charge voltage can be dramatically reduced, and cycle lifetime is also prolonged under illumination, because of the expedited electrocatalytic kinetics, the increased electrical conductivity, and the accelerated desorption rate of O₂ bubbles from the electrode. By exerting the intrinsic electrocatalytic and photothermal efficiency of the electrode materials, this research paves new ways to improve battery performance from kinetic and thermodynamic perspectives.

Mapping the electrocatalytic water splitting activity of VO2 across its insulator-to-metal phase transition

The electrocatalytic water splitting activity of V-based oxides has been rarely investigated, even though several polymorphs in VO₂ are expected to exhibit different electrocatalytic activities depending on their crystal and electronic structures. The rutile structure of VO₂(R), showing metallic character, is a good candidate for a new electrocatalyst since it undergoes insulator-to-metal transition (IMT) from the insulating VO₂(M1) at a low temperature of 68 °C, and involves a substantially increased electrical conductivity by three orders of magnitude. The extensive improvements in the electrocatalytic activity for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) are confirmed when the IMT is induced where the overpotential (η₁₀) is reduced from 1056 mV to 598 mV in the OER and 411 mV to 136 mV in the HER, respectively. This improvement is attributed to the increased electrochemically active surface area (ECSA), reduced charge transfer resistance, and increased electron density, driven by the IMT to the metallic VO₂(R) phase.

Understanding the Effects of Ultrasound (408 kHz) on the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER) on Raney-Ni in Alkaline Media

The hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) occurring at the Raney-Ni mesh electrode in 30 wt.-% aqueous KOH solution were studied in the absence (silent) and presence of ultrasound (408 kHz, ∼54 W, 100% acoustic amplitude) at different electrolyte temperatures (T = 25, 40 and 60 °C). Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) experiments were performed to analyse the electrochemical behaviour of the Raney-Ni electrode under these conditions. Under silent conditions, it was found that the electrocatalytic activity of Raney-Ni towards the HER and the OER depends upon the electrolyte temperature, and higher current densities at lower overpotentials were achieved at elevated temperatures. It was also observed that the HER activity of Raney-Ni under ultrasonic conditions increased at low temperatures (e.g., 25 °C) while the ultrasonic effect on the OER was found to be insignificant. In addition, it was observed that the ultrasonic effect on both the HER and OER decreases by elevating the temperature. In our conditions, it is suggested that ultrasound enhances the electrocatalytic performance of Raney-Ni towards the HER due to principally the efficient gas bubble removal from the electrode surface and the dispersion of gas bubbles into the electrolyte, and this effect depends upon the behaviour of the hydrogen and oxygen gas bubbles in alkaline media.

Principles of Water Electrolysis and Recent Progress in Cobalt-, Nickel-, and Iron-Based Oxides for the Oxygen Evolution Reaction

Water electrolysis that results in green hydrogen is the key process towards a circular economy. The supply of sustainable electricity and availability of oxygen evolution reaction (OER) electrocatalysts are the main bottlenecks of the process for large-scale production of green hydrogen. A broad range of OER electrocatalysts have been explored to decrease the overpotential and boost the kinetics of this sluggish half-reaction. Co-, Ni-, and Fe-based catalysts have been considered to be potential candidates to replace noble metals due to their tunable 3d electron configuration and spin state, versatility in terms of crystal and electronic structures, as well as abundance in nature. This Review provides some basic principles of water electrolysis, key aspects of OER, and significant criteria for the development of the catalysts. It provides also some insights on recent advances of Co-, Ni-, and Fe-based oxides and a brief perspective on green hydrogen production and the challenges of water electrolysis.

Recent advances in electron manipulation of nanomaterials for photoelectrochemical biosensors

This feature article discusses the recent advances and strategies of building photoelectrochemical (PEC) biosensors from the perspective of regulating the electron transfer of nanomaterials.

Spinel-Oxide-Integrated BiVO4 Photoanodes with Photothermal Effect for Efficient Solar Water Oxidation

Spinel oxide materials have been widely used as oxygen evolution catalysts to enhance the photoelectrochemical (PEC) performance of photoelectrodes. Herein, we demonstrate that the water splitting efficiency of a photoanode can be further enhanced by introducing its photothermal effect. Under near-infrared radiation, the temperature of the NiCo₂O₄/BiVO₄ photoanode increases moderately, leading to improved water oxidation kinetics and charge transport simultaneously. With the assistance of the photothermal effect, the obtained photoanode reaches a photocurrent density of 6.20 mA cm^-2 at 1.23 V vs reversible hydrogen electrode. A series of spinel-type MCo₂O₄ oxides (M = Mn, Zn, Cu, and Fe) are deposited on the surface of the BiVO₄ photoanode to show similar photothermally enhanced PEC performance. The research discovery provides a way for improving the catalytic activity of photoanode materials with a photothermal effect, which may be applied to various fields of energy conversion, including CO₂ reduction, N₂ fixation, and pollutant degradation.

Bifunctional and Self-Supported NiFeP-Layer-Coated NiP Rods for Electrochemical Water Splitting in Alkaline Solution

Designing efficient and robust nonprecious metal-based electrocatalysts for overall water electrolysis, which is mainly limited by the oxygen evolution reaction (OER), for hydrogen production remains a major challenge for the hydrogen economy. In this work, a bimetallic NiFeP catalyst is coated on nickel phosphide rods grown on nickel foam (NiFeP@NiP@NF). This self-supported and interfacially connected electrode structure is favorable for mass transfer and reducing electrical resistance during electrocatalysis. The preparation of NiFeP@NiP@NF is optimized in terms of (i) the coprecipitation time of the NiFe Prussian blue analogue layer that serves as phosphides precursor and (ii) the phosphidation temperature. The optimized sample exhibits excellent OER performance delivering current densities of 10 and 100 mA cm^-2 at low overpotentials of 227 and 252 mV in 1.0 M KOH, respectively, and maintaining 10 mA cm^-2 for more than 120 h without obvious degradation. Moreover, it can also be operated as a hydrogen evolution electrocatalyst, requiring an overpotential of 105 mV at 10 mA cm^-2 in the same medium. Thus, the as-prepared material was tentatively utilized as a bifunctional electrocatalyst in a symmetric electrolyzer, requiring a voltage bias of 1.57 V to afford 10 mA cm^-2 in 1.0 M KOH, while exhibiting outstanding stability.

The Gas Diffusion Electrode Setup as Straightforward Testing Device for Proton Exchange Membrane Water Electrolyzer Catalysts

Hydrogen production from renewable resources and its reconversion into electricity are two important pillars toward a more sustainable energy use. The efficiency and viability of these technologies heavily rely on active and stable electrocatalysts. Basic research to develop superior electrocatalysts is commonly performed in conventional electrochemical setups such as a rotating disk electrode (RDE) configuration or H-type electrochemical cells. These experiments are easy to set up; however, there is a large gap to real electrochemical conversion devices such as fuel cells or electrolyzers. To close this gap, gas diffusion electrode (GDE) setups were recently presented as a straightforward technique for testing fuel cell catalysts under more realistic conditions. Here, we demonstrate for the first time a GDE setup for measuring the oxygen evolution reaction (OER) of catalysts for proton exchange membrane water electrolyzers (PEMWEs). Using a commercially available benchmark IrO₂ catalyst deposited on a carbon gas diffusion layer (GDL), it is shown that key parameters such as the OER mass activity, the activation energy, and even reasonable estimates of the exchange current density can be extracted in a realistic range of catalyst loadings for PEMWEs. It is furthermore shown that the carbon-based GDL is not only suitable for activity determination but also short-term stability testing. Alternatively, the GDL can be replaced by Ti-based porous transport layers (PTLs) typically used in commercial PEMWEs. Here a simple preparation is shown involving the hot-pressing of a Nafion membrane onto a drop-cast glycerol-based ink on a Ti-PTL.

Photocatalytic Oxygen Evolution from Water Splitting

Photocatalytic water splitting has attracted a lot of attention in recent years, and O2 evolution is the decisive step owing to the complex four-electrons reaction process. Though many studies have been conducted, it is necessary to systematically summarize and introduce the research on photocatalytic O2 evolution, and thus a systematic review is needed. First, the corresponding principles about O2 evolution and some urgently encountered issues based on the fundamentals of photocatalytic water splitting are introduced. Then, several types of classical water oxidation photocatalysts, including TiO2, BiVO4, WO3, α-Fe2O3, and some newly developed ones, such as Sillén-Aurivillius perovskites, porphyrins, metal-organic frameworks, etc., are highlighted in detail, in terms of their crystal structures, synthetic approaches, and morphologies. Third, diverse strategies for O2 evolution activity improvement via enhancing photoabsorption and charge separation are presented, including the cocatalysts loading, heterojunction construction, doping and vacancy formation, and other strategies. Finally, the key challenges and future prospects with regard to photocatalytic O2 evolution are proposed. The purpose of this review is to provide a timely summary and guideline for the future research works for O2 evolution.

Efficient tri-metallic oxides NiCo2O4/CuO for the oxygen evolution reaction

In this study, a simple approach was used to produce nonprecious, earth abundant, stable and environmentally friendly NiCo2O4/CuO composites for the oxygen evolution reaction (OER) in alkaline media. The nanocomposites were prepared by a low temperature aqueous chemical growth method. The morphology of the nanostructures was changed from nanowires to porous structures with the addition of CuO. The NiCo2O4/CuO composite was loaded onto a glassy carbon electrode by the drop casting method. The addition of CuO into NiCo2O4 led to reduction in the onset potential of the OER. Among the composites, 0.5 grams of CuO anchored with NiCo2O4 (sample 2) demonstrated a low onset potential of 1.46 V vs. a reversible hydrogen electrode (RHE). A current density of 10 mA cm-2 was achieved at an over-potential of 230 mV and sample 2 was found to be durable for 35 hours in alkaline media. Electrochemical impedance spectroscopy (EIS) indicated a small charge transfer resistance of 77.46 ohms for sample 2, which further strengthened the OER polarization curves and indicates the favorable OER kinetics. All of the obtained results could encourage the application of sample 2 in water splitting batteries and other energy related applications.

Promoting Oxygen Evolution Reaction of Co-Based Catalysts (Co3 O4 , CoS, CoP, and CoN) through Photothermal Effect

The increase of reaction temperature of electrocatalysts is regarded as an efficient method to improve the oxygen evolution reaction (OER) activity. Herein, it is reported that the electrocatalytic performance of dual functional (i.e., electrocatalytic and photothermal functions) Co₃ O₄ can be dramatically improved via its photothermal effect. The operating temperature of the Co₃ O₄ electrode is elevated in situ under near infrared (NIR) light irradiation, resulting in enhanced oxygen evolution activity due to its accelerated electrical conductivity, reaction kinetics, and desorption rate of O₂ bubbles from the electrode. In addition, photothermal effect can also enhance the electrocatalytic reaction rates of metal-doped Co₃ O₄ electrodes, indicating that it is able to significantly improve the OER activities of electrodes together with other modification strategies. With the assistance of the photothermal effect, the obtained Ni-doped Co₃ O₄ catalyst requires an extremely low overpotential of 208 mV to achieve a benchmark of 10 mA cm^-2 with a small Tafel slope, superior to most reported Co-based catalysts. Significantly, the electrocatalytic performance of other electrodes with photothermal effect, such as CoN, CoP, and CoS, are also boosted under NIR light irradiation, indicating opportunities for implementing photothermal enhancement in electrocatalytic water splitting.

Highly Active Core-Shell Carbon/NiCo2 O4 Double Microtubes for Efficient Oxygen Evolution Reaction: Ultralow Overpotential and Superior Cycling Stability

Developing highly efficient electrocatalysts with earth abundant elements for oxygen evolution reaction (OER) is a promising way to store light or electrical energy in the form of chemical energy. Here, a new type of electrocatalyst with core-shell carbon/NiCo₂ O₄ double microtubes architecture is successfully synthesized through a hydrothermal method combined with the calcination process with wet tissues as the template and carbon resource. The outer NiCo₂ O₄ nanosheet arrays contain abundant defects, which come from reduction of the carbon in wet tissues. This indicates that carbon is a very excellent defect inducer. The inner carbon microtubes can act as the robust structure skeleton and these core-shell double microtubes provide abundant diffusion channels for oxygen and electrolyte, both of which contribute to improving the stability by avoiding damage to the electrode from produced O₂ bubbles and the collapse of the outer NiCo₂ O₄ microtubes. Electrochemical results show that the electrode, core-shell carbon/NiCo₂ O₄ double microtubes loaded on carbon cloth, exhibits prominent electrocatalytic activity with an overpotential of only 168 mV at 10 mA cm^-2 and a Tafel slope as low as 57.6 mV dec^-1 in 1.0 mol L^-1 KOH. This new type of electrocatalyst possesses great potential in water electrolyzers and rechargeable metal-air batteries.

3D Self-Supported Porous NiO@NiMoO4 Core-Shell Nanosheets for Highly Efficient Oxygen Evolution Reaction

Novel 3D self-supported porous NiO@NiMoO₄ core-shell nanosheets are grown on nickel foam through a facile stepwise hydrothermal method. Ultrathin NiO nanosheets on the nickel foam cross-linked to each other are used as the core, and tiny NiMoO₄ nanosheets are further engineered to be immobilized uniformly on the NiO nanosheets to form the shell. This step-by-step construction of the architecture composed of ultrathin primary and secondary nanosheets efficiently avoids the agglomeration problems of individual ultrathin nanosheets. The ingenious architecture possesses the advantages of numerous diffusion channels for electrolyte ions, ideal pathways for electrons, and a large interfacial area for electrochemical reaction. The introduction of the NiMoO₄ secondary nanosheets on the NiO primary nanosheets not only endows the heterostructure with high electrical conductivity and a large active area but also promotes an increase in oxygen vacancy content, which favors the improvement of electrocatalytic properties for the oxygen evolution reaction. The Tafel plot for the NiO@NiMoO₄ core-shell architecture is as low as 32 mV dec^-1, and the overpotential needed to reach 10 mA·cm^-2 for NiO@NiMoO₄ nanosheets is only 0.28 V.

Syntheses, structures, magnetism and electrocatalytic oxygen evolution for four cobalt, manganese and copper complexes with dinuclear, 1D and 3D structures

Structures, magnetism and electrocatalytic oxygen evolution for four dinuclear, 1D and 3D complexes based on transition metals were reported.