Challenges and Opportunities of Transition Metal Oxides as Electrocatalysts

Spin-Electrochemistry of Transition Metal Oxides for Energy Storage: Concepts, Advances and Perspectives

Developing high-capacity and cyclically stable transition metal (TM)-based electrode materials for energy storage devices, such as aqueous ion energy storage systems, is crucial for addressing the growing issue of energy scarcity. The spin state, or spin configuration of the d-electrons, plays a vital role in the electrochemical energy storage performance of these materials. However, there has been a lack of systematic descriptions regarding the role of spin configurations in electrochemical energy storage to date. This review aims to elucidate the advantages of controlling the spin states of metal centers to enhance energy storage performance and highlights recent progress in employing spin state regulation in electrochemical energy storage. Additionally, it covers the various characterization techniques used to determine spin states. Finally, we discuss the future prospects and challenges within this emerging field, with the aim of accelerating the development of spin-based electrochemical energy storage technologies. This review also seeks to provide clear and reliable directions for the design and preparation of novel energy storage materials.

Bimetallic Sulfur-Doped Nickel-Cobalt Selenides as Efficient Bifunctional Electrocatalysts for the Complete Decomposition of Water

The creation and enhancement of non-precious metal bifunctional catalysts with superior stability and stabilizing activity is necessary to achieve water splitting in alkaline media. The paper presents a method for preparing nickel-cobalt bimetallic selenides (NiCo-Sex/CF) using a combination of hydrothermal and high-temperature selenization techniques. NiCo-Sex/CF shows great potential as a catalyst for water separation. The catalyst's electronic structure and active centre can be modified by double doping with sulfur and selenium, resulting in increased selectivity and activity under varying reaction conditions. This method also offers the benefits of a simple preparation process and applicability to a wide range of catalytic reactions. Experimental results demonstrate that an overpotential of 194 mV produces a current density of 10 mA cm-2 when using this electrocatalyst as an OER catalyst. When used as a HER catalyst, the electrocatalyst required an overpotential of only 76 mV to generate a current density of 10 mA cm-2.Furthermore, a voltage of 1.5 V can drive the overall decomposition of water to achieve a current density of 10 mA cm-2. This study highlights the potential of sulfur-selenide double-doped catalysts for both scientific research and practical applications.

Screening Transition-Metal-Incorporated β-AgVO3 for Augmented Oxygen Reduction Activity

Exploring cost-effective alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) in fuel cells is crucial for their large-scale deployment in green energy applications. Silver vanadate (AgVO3) is a well-studied material for photocatalytic applications. Here, we investigate the electrocatalytic ORR activity of the thermodynamically stable β phase of AgVO3 through computational modeling based on DFT. It is found that β-AgVO3 exhibits weak catalytic activity for the ORR, with vanadium being the preferable active site. Incorporating single atoms of transition metals at surface-level vacancies in β-AgVO3 significantly modifies the ORR activity. We study the scaling of free energy changes for the ORR intermediates *OOH, *OH, and *O for various transition metals incorporated, which leads to an optimal overpotential for the system. The optimal overpotential thus obtained is remarkably lower than that of pristine β-AgVO3. For the transition metal atoms we consider here, Co-incorporated β-AgVO3 exhibits the best ORR catalytic activity due to its optimal binding of ORR species to the vanadium site. It is also observed that some of the transition metals considered like Re, Rh, Os, or Mn show weak activity, either due to strong or weak binding. Analysis of the electronic structure of the adsorbate-catalyst interface shows a strong correlation between optimal activity and evolution of midgap states in β-AgVO3, due to transition metal incorporation. Our study concludes that the ORR activity of a stable mixed transition metal oxide like β-AgVO3 can be enhanced with a minimal loading of transition metals, which could help in developing a novel series of ORR catalysts.

Construction of Fe3O4@Au catalysts via the surface functional group effect of ferric oxide for efficient electrocatalytic nitrite reduction

Surface modification is one of the effective strategies to control the morphology and electrocatalytic performance of noble metal/transition metal oxide matrix composite catalysts. In this work, we successfully introduced modification groups such as -NH2, -COOH, and -SH on the surface of Fe3O4 using the hydrothermal method. It was found that when the modification group -COOH was introduced, the regular spherical morphology of Fe3O4 was still maintained in Fe3O4-COOH, while Fe3O4-COOH had a relatively smaller spherical particle size (≈155.9 nm). Due to its smaller particle size, Fe3O4-COOH has a larger active area than Fe3O4, exposing more active sites. The abundant active sites in Fe3O4-COOH provide more nucleation and growth sites for Au particles, which is beneficial for the recombination between Fe3O4-COOH and Au. In addition, the experimental results of exploring the effect of Au precursor dosage on the synthesis of the Fe3O4-COOH@Au structure and performance show that the synthesized Fe3O4-COOH@Au1.0 catalyst has higher electrocatalytic activity. Due to the larger electrochemically active surface area of the Fe3O4-COOH@Au1.0 catalyst compared to those of Fe3O4-COOH@Au0.5 and Fe3O4-COOH@Au1.5 catalysts, the adsorption and activation of NO2- reactants were accelerated, thereby improving the electrocatalytic performance. Therefore, owing to the morphological and structural characteristics of Fe3O4-COOH combined with the high activity of Au nanoparticles, the synthesized Fe3O4-COOH@Au exhibits effective electrocatalytic activity in the electrocatalytic NO2-RR synthesis of ammonia. At a voltage of -0.8 V (vs. RHE), the ammonia yield reached 2092.8 μg h-1 mgcat-1 and Faraday efficiency reached 81.2%. The findings of this work will enrich our understanding of the construction of efficient Fe3O4@Au catalysts based on surface functionalization and help to design efficient electrocatalytic NO2-RR catalysts for practical applications.

Tailoring surface morphology on anatase TiO2 supported Au nanoclusters: implications for O2 activation

Strong interaction between the support surface and metal clusters activates the adsorbed molecules at the metal cluster-support interface. Using plane-wave DFT calculations, we precisely model the interface between anatase TiO2 and small Au nanoclusters. Our study focusses on the adsorption and activation of oxygen molecules on anatase TiO2, considering the influence of oxygen vacancies and steps on the surface. We find that the plane (101) and the stepped (103) surfaces do not support O2 activation, but the presence of oxygen vacancies results in strong adsorption and O-O bond length elongation. Modifying the TiO2 surface with supported small Au n nanoclusters (n = 3-5) also significantly enhances O2 adsorption and stretches the O-O bond. We observe that manipulating the cluster orientation through discrete rotations results in improved O2 adsorption and promotes charge transfer from the surface to the molecule. We propose that the orientation of the supported cluster may be manipulated by making the cluster adsorb at the step-edge of (103) TiO2. This results in activated O2 at the cluster-support interface, with a peroxide-range bond length and a low barrier for dissociation. Our modeling demonstrates a straightforward means of exploiting the interface morphology for O2 activation under low precious metal loading, which has important implications for electrocatalytic oxidation reactions and the rational design of supported catalysts.

MOF-Derived Ni/NiO-C Nanocomposites as Bifunctional Electrocatalysts Capable of Driving Both ORR and OER

Bifunctional electrocatalysts, capable of efficiently driving both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), are crucial for advancing electrochemical processes. While noble-metal-based catalysts are widely recognized for their role in oxygen processes, current state-of-the-art designs are limited to either ORR or OER activity, presenting a notable research gap. In addressing this challenge, we have developed a novel Ni/NiO-C nanocomposite catalyst derived from a nickel-based metal-organic framework (Ni-SKU-5). For the ORR, the Ni/NiO-C catalyst exhibits an onset potential of 0.95 V vs RHE in a 1.0 M KOH solution, coupled with a Tafel slope of -99 mV dec-1 at 1600 rpm. Moreover, the catalyst displays excellent stability, maintaining a performance of over 90% after 10 h of continuous reaction. Furthermore, the catalyst proves effective in the OER, boasting an overpotential of 370 mV (at 10 mA cm-2) and a Tafel slope of 114 mV dec-1, highlighting its bifunctionality. The bifunctional overpotential of the Ni/NiO-C composite is measured at 820 mV, surpassing that of the 20% Pt/C electrocatalyst (860 mV), highlighting its potential for practical applications. Comparative experiments establish the origin of the robust bifunctional reactivity toward the conformal hybrid structure, porous framework, and the synergistic effect operating among the constituents of the nanocomposite.

Understanding the catalytic performances of metal-doped Ta2O5 catalysts for acidic oxygen evolution reaction with computations

The design of stable and active alternative catalysts to iridium oxide for the anodic oxygen evolution reaction (OER) has been a long pursuit in acidic water splitting. Tantalum pentoxide (Ta2O5) has the merit of great acidic stability but poor OER performance, yet strategies to improve its intrinsic OER activity are highly desirable. Herein, by using density functional theory (DFT) calculations combined with aqueous stability assessment from surface Pourbaix diagrams, we systematically evaluated the OER activity and acidic stability of 14 different metal-doped Ta2O5 catalysts. Apart from the experimentally reported Ir-doped Ta2O5, we computationally identified Ru- and Nb-doped Ta2O5 catalysts as another two candidates with reasonably high stability and activity in acidic OER. Our study also underscores the essence of considering stable surface states of catalysts under working conditions before a reasonable activity trend can be computationally achieved.

Transition metal-based electrocatalysts for alkaline overall water splitting: advancements, challenges, and perspectives

Water electrolysis is a promising method for efficiently producing hydrogen and oxygen, crucial for renewable energy conversion and fuel cell technologies. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two key electrocatalytic reactions occurring during water splitting, necessitating the development of active, stable, and low-cost electrocatalysts. Transition metal (TM)-based electrocatalysts, spanning noble metals and TM oxides, phosphides, nitrides, carbides, borides, chalcogenides, and dichalcogenides, have garnered significant attention due to their outstanding characteristics, including high electronic conductivity, tunable valence electron configuration, high stability, and cost-effectiveness. This timely review discusses developments in TM-based electrocatalysts for the HER and OER in alkaline media in the last 10 years, revealing that the exposure of more accessible surface-active sites, specific electronic effects, and string effects are essential for the development of efficient electrocatalysts towards electrochemical water splitting application. This comprehensive review serves as a guide for designing and constructing state-of-the-art, high-performance bifunctional electrocatalysts based on TMs, particularly for applications in water splitting.

Silica-Derived Nanostructured Electrode Materials for ORR, OER, HER, CO2RR Electrocatalysis, and Energy Storage Applications: A Review

Silica-derived nanostructured catalysts (SDNCs) are a class of materials synthesized using nanocasting and templating techniques, which involve the sacrificial removal of a silica template to generate highly porous nanostructured materials. The surface of these nanostructures is functionalized with a variety of electrocatalytically active metal and non-metal atoms. SDNCs have attracted considerable attention due to their unique physicochemical properties, tunable electronic configuration, and microstructure. These properties make them highly efficient catalysts and promising electrode materials for next generation electrocatalysis, energy conversion, and energy storage technologies. The continued development of SDNCs is likely to lead to new and improved electrocatalysts and electrode materials. This review article provides a comprehensive overview of the recent advances in the development of SDNCs for electrocatalysis and energy storage applications. It analyzes 337,061 research articles published in the Web of Science (WoS) database up to December 2022 using the keywords "silica", "electrocatalysts", "ORR", "OER", "HER", "HOR", "CO2RR", "batteries", and "supercapacitors". The review discusses the application of SDNCs for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO2RR), supercapacitors, lithium-ion batteries, and thermal energy storage applications. It concludes by discussing the advantages and limitations of SDNCs for energy applications.

Constructing core-shell structured Co3O4-MnWO4 composite photoelectrode with superior PEC water purification performance

Semiconductor photoelectrocatalytic (PEC) technology is one of the most effective methods for removing organic pollutants from wastewater in advanced oxidation processes(AOPs). The selection of suitable semiconductor materials as photoanodes is a crucial factor for achieving superior PEC performance. Here, a core-shell structured Co3O4-MnWO4 architecture is created by enveloping MnWO4 nanoparticles onto the surface of Co3O4 nanowires through a two-step hydrothermal process. The optimized Co3O4-MnWO4-5 photoelectrode showed superior PEC degradation efficiency for KN-R (∼91.2% in 2 h) and durable stability (the accelerated lifetime reached ∼9100 s at a current density of 50 mA cm-2). Three actual wastewaters were also collected to verify the practical applicability of the photoelectrode.The energy consumption was measured at 4.48 kWhm-3, with a COD removal efficiency of 83% and a decolorization rate of 98%. These results demonstrate the excellent performance and promising application of the photoelectrode. The enhancement of PEC performance for the core-shell structured Co3O4-MnWO4 architecture can be attributed to the suitable energy band structure of the Co3O4-MnWO4 composite, higher OEP, larger electrochemical active surface area, accelerated transport of interface carriers, and lower charge transfer resistance. The energy band structure of the Co3O4-MnWO4 composite showed a strong redox ability to induce electrons/holes (e-/h+), which enhances the generation of intermediate active species (hydroxyl radical ·OH and superoxide radicals ·O2-). Therefore, the rationally designed core-shell structured Co3O4-MnWO4 architecture exhibited excellent practical applicability in the degradation of organic pollutants.

Lewis Acid Mediated Alteration of Electron Density on a Copper Site via pπ-dπ Mixing Enhances the Electrochemical Nitrogen Reduction Reaction

Strategic modulation of the electronic structure of the catalyst to foster the electrochemical nitrogen reduction reaction (eNRR) to the ammonia process significantly is still an area that needs to be explored. Herein, we report the incorporation of the Lewis acid into an electron-rich copper site regulating the electron density of the metal, which has been experimentally proved from the d-band center position to have a direct influence on the adsorption of N2 compared to the protons. The catalyst boron doped copper-cuprous oxide hybrid system (B-Cu/Cu2O) has shown promising Faradaic efficiency of 32% at -0.2 V vs reversible hydrogen electrode (RHE) compared to the pristine cuprous oxide (Cu2O(N)) system. The in situ Fourier transform infrared study confirms the presence of intermediates evolved during the electroreduction process. This study demonstrates the design of the active center with a specific push-pull interaction via the pπ-dπ bonding-antibonding approach and can shed light on the electrochemical activation and reduction of dinitrogen to produce ammonia.

Identifying Stable Electrocatalysts Initialized by Data Mining: Sb2 WO6 for Oxygen Reduction

Data mining from computational materials database has become a popular strategy to identify unexplored catalysts. Herein, the opportunities and challenges of this strategy are analyzed by investigating a discrepancy between data mining and experiments in identifying low-cost metal oxide (MO) electrocatalysts. Based on a search engine capable of identifying stable MOs at the pH and potentials of interest, a series of MO electrocatalysts is identified as potential candidates for various reactions. Sb2 WO6 attracted the attention among the identified stable MOs in acid. Based on the aqueous stability diagram, Sb2 WO6 is stable under oxygen reduction reaction (ORR) in acidic media but rather unstable under high-pH ORR conditions. However, this contradicts to the subsequent experimental observation in alkaline ORR conditions. Based on the post-catalysis characterizations, surface state analysis, and an advanced pH-field coupled microkinetic modeling, it is found that the Sb2 WO6 surface will undergo electrochemical passivation under ORR potentials and form a stable and 4e-ORR active surface. The results presented here suggest that though data mining is promising for exploring electrocatalysts, a refined strategy needs to be further developed by considering the electrochemistry-induced surface stability and activity.

A simple hydrothermal synthesis of an oxygen vacancy-rich MnMoO4 rod-like material and its highly efficient electrocatalytic nitrogen reduction

Electrocatalytic nitrogen reduction (NRR) for artificial ammonia synthesis under ambient conditions is considered a promising alternative to the traditional Haber-Bosch process. However, it still faces multiple challenges such as the difficulty of N2 adsorption and activation and limited Faraday efficiency. In this work, a bimetallic oxide MnMoO4 was prepared by a hydrothermal method and low temperature calcination. The influence of the sintering temperature on the microstructure (crystallinity and oxygen vacancies) of the oxide and its NRR properties were systematically explored. The results showed that MnMoO4 sintered at 500 °C had the highest concentration of OVs and showed excellent NRR performance, with the highest NH3 yield (up to 12.28 μg h-1 mgcat-1), high Faraday efficiency (23.04% at -0.30 V vs. RHE), and good stability at -0.40 V vs. RHE, and the catalytic performance was about two times higher than that of Mn2O3 and MoO3. It is also superior to other bimetallic oxide NRR electrocatalysts reported in some cases. In addition, we also explored the ratio between Mn and Mo metals, and the catalytic effect was the best when Mn : Mo = 1 : 1. Due to the synergistic effect between Mn and Mo metals and the large number of OVs present internally, the catalytic activity for the NRR was largely improved. This study suggests that the bimetallic oxide MnMoO4 may be a promising NRR electrocatalyst.

Double-single-atom MoCu-embedded porous carbons boost the electrocatalytic N2 reduction reaction

NH3 is an essential ingredient of chemical, fertilizer, and energy storage products. Industrial nitrogen fixation consumes an enormous amount of energy, which is counter to the concept of carbon neutrality, hence eNRR ought to be implemented as a clean alternative. Herein, we propose a double-single-atom MoCu-embedded porous carbon material derived from uio-66 (MoCu@C) by plasma-enhanced chemical vapor deposition (PECVD) to boost eNRR capabilities, with an NH3 yield rate of 52.4 μg h-1 gcat.-1 and a faradaic efficiency (FE) of 27.4%. Advanced XANES shows that the Mo active site receives electrons from Cu, modifies the electronic structure of the Mo active site and enhances N2 adsorption activation. The invention of rational MoCu double-single-atom materials and the utilization of effective eNRR approaches furnish the necessary building blocks for the fundamental study and practical application of Mo-based materials.

Origin of the superior oxygen reduction activity of zirconium nitride in alkaline media

The anion exchange membrane fuel cell (AEMFC), which can operate in alkaline media, paves a promising avenue for the broad application of earth-abundant element based catalysts. Recent pioneering studies found that zirconium nitride (ZrN) with low upfront capital cost can exhibit high activity, even surpassing that of Pt in alkaline oxygen reduction reaction (ORR). However, the origin of its superior ORR activity was not well understood. Herein, we propose a new theoretical framework to uncover the ORR mechanism of ZrN by integrating surface state analysis, electric field effect simulations, and pH-dependent microkinetic modelling. The ZrN surface was found to be covered by ∼1 monolayer (ML) HO* under ORR operating conditions, which can accommodate the adsorbates in a bridge-site configuration for the ORR. Electric field effect simulations demonstrate that O* adsorption on a 1 ML HO* covered surface only induces a consistently small dipole moment change, resulting in a moderate bonding strength that can account for the superior activity. Based on the identified surface state of ZrN and electric field simulations, pH-dependent microkinetic modelling found that ZrN reaches the Sabatier optimum of the kinetic ORR volcano model in alkaline media, with the simulated polarization curves being in excellent agreement with the experimental data of ZrN and Pt/C. Finally, we show that this theoretical framework can lead to a good explanation for the alkaline oxygen electrocatalysis of other transition metal nitrites such as Fe3N, TiN, and HfN. In summary, this study proposes a new framework to rationalize and design transition metal nitrides for alkaline ORR.

Anchoring ultra-small molybdenum oxide species on covalent triazine frameworks for efficient electrochemical nitrogen fixation

We report a smart ion-exchange strategy to anchor molybdenum oxide particles on charge-modulated conjugated triazine frameworks (Mo/CTF-I) for electrochemically fixing nitrogen. The strong interaction between MoO_x and CTF-I is conducive to the activation of the inert N₂ molecule in the electro-chemical process. As a result, 5% Mo/CTF-I exhibited an excellent faradaic efficiency of 27.3% and an NH₃ yield rate of 7.23 μg h^-1 mg_cat.^-1 at -0.405 V vs. RHE in 0.1 M KOH, surpassing most previous reports.