Synthesis of V-doped urchin-like NiCo2O4 with rich oxygen vacancies for electrocatalytic oxygen evolution reactions

A Nd-doped NiCo spinel dual functional catalyst for both oxygen reduction reactions and oxygen evolution reactions: Enhanced activity through surface reconstruction

The design of efficient, low-cost, highly active and thermally stable electrocatalysts is critical for both oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). While some spinel metal oxides exhibit good activities for either ORR or OER, a bifunctional spinel metal oxide that can provide decent activities for both ORR and OER would be most desirable. To date, rare earth metal-modified spinel oxides have not been well-studied, but they are thought to be able to boost both ORR and OER simultaneously. Hence, a Nd-doped NiCo2O4 catalyst was synthesized in this work to evaluate its potential for improving both ORR and OER reactions. We hypothesized that this catalyst would be a viable option, as the highly oxidized Co4+ (hydroxycobalt oxide) generated from surface reconstruction could be an active site for OER while Ni2+ is intrinsically an active site for ORR. Amazingly, our study revealed that the addition of Nd in spinel metal oxides was able to inhibit the formation of Co4+ at low potentials while the Ni species promoted the formation of Co4+ from Co2+, thus achieving a balance between Co2+ and Co4+ which resulted in a multi-step oxidation process of Co2+ → Co3+ → Co4+. In addition, by tuning the amount of Nd doped, an optimum electrocatalyst Nd0.1Ni0.9Co2O4 with excellent activities for both ORR (i.e. the half-wave potential E1/2 = 0.735 V) and OER (i.e. the overpotential at 10 mA cm-2 E10 mA·cm-2 = 302 mV) in alkaline conditions was developed. In summary, this work may have opened a new pathway for applying spinel metal oxides as bifunctional catalysts in future commercial ORR and OER processes.

Dual-Engineering Tailored Co3O4 Hollow Microspheres Assembled by Nanosheets for Boosting Oxygen Evolution Reaction

The development of efficient, low-cost electrocatalysts for the oxygen evolution reaction (OER) is crucial for advancing sustainable hydrogen production through water splitting. This study presents a dual-engineering strategy to enhance the OER performance of Co3O4 by synthesizing hollow microspheres assembled from nanosheets (HMNs) with abundant oxygen vacancies and highly active crystal facet exposure. Through a modified one-step hydrothermal process, Co3O4 HMNs with exposed (111) and (100) crystal facets were successfully fabricated, demonstrating superior OER activity compared to Co3O4 nanocubes (NCs) with only (100) facet exposure. The optimized Co3O4-5% HMNs exhibited a low overpotential of 330 mV at 10 mA cm-2 and a Tafel slope of 69 mV dec-1. The enhanced performance was attributed to the synergistic effects of crystal facet engineering and defect engineering, which optimized the Co-O bond energy, increased the number of active sites, and improved conductivity. The unique hollow structure further facilitated mass transport and prevented nanosheet stacking, exposing more edge sites for catalytic reactions. This work highlights the potential of geometric and electronic structure modulation in designing high-performance OER catalysts for sustainable energy applications.

A vanadium-containing high entropy alloy electrocatalyst with boosted electrocatalytic activity for oxygen evolution reaction

Electrocatalytic water splitting is an excellent option for green hydrogen preparation. However, the efficiency of water splitting is restricted by the sluggish reaction kinetics, especially for anodic oxygen evolution reaction (OER). High entropy alloys show good electrocatalytic activity for OER recently and the composition of high-entropy alloys can be easily adjusted to control their catalytic performance. In this work, we report a facile method to prepare a V-containing high entropy alloy catalyst using flower-like metal organic framework (MOF) as the precursor and investigated the role of V in OER. It is revealed that the introduction of V can regulate the electronic structure of ZnCoNiFeV-MOF-900 and induce the formation of high valent Ni centers, therefore reducing the reaction barrier for OER. Combined with the flower-like morphology that enables fast mass transport and exposure of adequate active sites, ZnCoNiFeV-MOF-900 exhibits a small overpotential of 284 mV at 10 mA cm-2 for OER. This work provides a new route for designing high entropy alloy electrocatalysts for energy conversion and storage.

Electronic and vacancy engineering of ruthenium doped hollow-structured NiO/Co3O4 nanoreactors for low-barrier electrochemical urea-assisted energy-saving hydrogen production

Discovering a valid approach to achieve a novel and efficient water splitting catalyst is essential for the development of hydrogen energy technology. Herein, unique hollow-structured ruthenium (Ru)-doped nickel-cobalt oxide (Ru-NiO/Co3O4/NF) nanocube arrays are fabricated as high-efficiency bifunctional electrocatalysts for hydrogen evolution reaction (HER)/urea oxidation reaction (UOR) through combined electronic and vacancy engineering. The structural characterization and experimental results indicate that the doping of Ru can not only effectively modulate the electronic structure of Ru-NiO/Co3O4/NF, but also increase the content of oxygen vacancies in the structure of Ru-NiO/Co3O4/NF to stabilize the existence of oxygen vacancies during the catalytic process. This can optimize the adsorption and desorption of the reactive intermediates on the surface of Ru-NiO/Co3O4/NF and dramatically accelerate the HER and UOR kinetics. As a result, the Ru-NiO/Co3O4/NF hollow structure nanocube arrays exhibit overpotentials of 21 and 60 mV for HER, as well as potentials of 1.36 and 1.42 V for UOR at 10 and 100 mA cm-2, respectively. Furthermore, the coupled HER and UOR system requires only 1.59 V of cell voltage to drive a current density of 100 mA cm-2, which is approximately 240 mV lower than conventional water electrolysis. This work provides a tremendous promise for the development of novel and high-activity electrocatalysts in future energy conversion applications.

L-lysine and surfactant-assisted synthesis of NiCo bimetal oxides for electrochemical water splitting

In the present study, bimetallic oxides comprising nickel (Ni) and cobalt (Co) were synthesized using a facile hydrothermal method in the presence of CTAB and L-lysine. Their efficacy in catalyzing hydrogen production under alkaline conditions was assessed. Structural, vibrational, and morphological characteristics were analyzed utilizing X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) techniques. The SEM images revealed a needle-like shape which is due to the surfactant addition. The NiCo oxides exhibited the lowest onset potential of 83 mV for HER and 130 mV for OER under standard conditions. The catalysts needed a potential of 286 and 450 mV to attain a current density of 50 mA/cm2 along with Tafel slope values of 119 and 332 mV/dec for HER and OER, respectively. These results suggested that L-lysine as a surfactant is highly effective in the fabrication of NiCo bimetal oxides for electrolytic water splitting applications.

Tuning electronic structure of hedgehog-like nickel cobaltite via molybdenum-doping for enhanced electrocatalytic oxygen evolution catalysis

As a typical spinel oxide, nickel cobaltite (NiCo2O4) is considered to be a promising and reliable oxygen evolution reaction (OER) catalyst due to its abundant oxidation states and the synergistic effect of multiple metal species. However, the electrocatalytic OER performance of NiCo2O4 has always been limited by the low specific surface area and poor intrinsic conductivity of spinels. Herein, the hedgehog-like molybdenum-doped NiCo2O4 (Mo-NiCo2O4) catalyst was prepared as an efficient OER electrocatalyst via a facile hydrothermal method followed with high-temperature annealing. The Mo-NiCo2O4-0.075 with Mo doping concentration of ∼ 1.95 wt% exhibits excellent OER performance with a low overpotential of 265 mV at a current density of 10 mA·cm-2and a Tafel slope of 126.63 mV·dec-1, as well as excellent cyclingstability.The results demonstrated that the hedgehog-like structure provides Mo-NiCo2O4 with the high surface area and mesopores that enhance electrolyte diffusion and optimal active site exposure. The in-situ Raman spectra and density functional theory calculations show that the Mo cations doping improve the intrinsic conductivity of the NiCo2O4 while modulating the chemisorption of intermediates. Meanwhile, the energy barriers of *OH and O* formation decrease significantly after Mo doping, effectively facilitating water dissociation and optimizing the reaction kinetics.

Synergistic engineering of heteroatom doping and heterointerface construction in V-doped Ni(OH)2/FeOOH to boost both oxygen evolution and urea oxidation reactions

The exploitation of cost-effective and abundant non-noble metal electrocatalysts holds great significances in enhancing the efficiency of oxygen evolution reaction (OER) and/or urea oxidation reaction (UOR). Herein, we report an electrocatalyst with co-existing V-dopants and Ni(OH)2/FeOOH interfaces (referred to as A-NiFeV/NF, with "A" indicating "activated"). The electron coupling between Ni, Fe and V, analyzed through X-ray photoelectron spectroscopy, indicates that Ni and Fe both receive electrons from the V. Additionally, the Fe can also lead to a bias toward a lower valence of the Ni centers in Ni(OH)2. Further in situ Raman spectroscopy reveals that Ni2+(OH)2 inevitably undergoes transformation into amorphous Ni3+OOH during the activation process, however, the synergistic effects of V-dopants and Ni(OH)2/FeOOH interfaces keep the Ni centers mostly in a lower oxidation state of +2 even at high potential ranges. These low-valence Ni centers are proposed to be positively correlated with the optimized OER activity of the Ni-based electrocatalysts. As a result, the designed A-NiFeV/NF electrocatalyst exhibits low overpotentials of 234 and 313 mV to propel current densities of 10 and 100 mA/cm2, and a small Tafel slope of 37.8 mV/dec for OER in 1.0 M KOH. The catalyst demonstrates a stable OER activity for over 100 h at 100 mA/cm2. Additionally, it can be integrated with a solar cell to construct a solar-driven electrolytic OER device without additional electric input. Similarly, for the small molecule oxidation, UOR, only ∼1.33 and ∼1.39 V vs. RHE (RHE: reversible hydrogen electrode) are required to achieve 10 and 100 mA/cm2, respectively, in an electrolyte composed of 1.0 M KOH with 0.33 M urea.

MOF-derived S-doped NiCo2O4 hollow cubic nanocage for highly efficient electrocatalytic oxygen evolution

Inducing the surface reconstruction of spinels is critical for improving the electrocatalytic oxygen evolution reaction (OER) activity. Herein, S-doped NiCo2O4 hollow cubic nanocage was synthesized by anion etching Metal-Organic Frameworks (MOFs) template and air annealing strategies. The hollow structure possesses a large specific surface area and pore size, facilitating active site exposure and mass transport. S2- doping regulates the electronic structure, reducing the oxidation potential of Ni sites during the OER process, thus promoting the surface reconstruction into γ-NiOOH active species. Meanwhile, S2- doping enhances conductivity, accelerating interfacial charge transfer. As a result, S-NiCo2O4-6 exhibits superior OER activity (262 mV overpotential @ 10 mA cm-2) and stability in 1.0 M KOH solution. Furthermore, 20 % Pt/C‖S-NiCo2O4-6 only needs 1.832 V to achieve 50 mA (the electrochemical active area is 4 cm2) in a homemade anion exchange membrane (AEM) electrolyzer. This work proposes a novel approach for preparing efficient anion-doped spinel-based OER electrocatalysts.

A well-dispersed OV-NiCo2O4 nanosphere modified separator for Li-S batteries

The commercialization of lithium-sulfur batteries is facing great challenges, such as the "shuttle effect" and the poor conductivity of sulfur and Li2S2/Li2S, so it is extremely important to design new separator-modified materials with fast charge transfer capability and effective immobilization of polysulfides (LiPSs) to facilitate their conversion to address these challenges. In this paper, we propose a simple way to synthesize NiCo2O4 nanospheres containing oxygen vacancies (OV-NiCo2O4 NSs) and thus modify the separator. The synthesized OV-NiCo2O4 NSs accelerated the conversion of LiPSs through strong chemical interactions. In addition, the introduction of oxygen vacancies provided more active sites for LiPSs, which improved the electron conduction rate and accelerated the ion transport. Based on the above advantages, the battery with an OV-NiCo2O4 modified separator showed excellent electrochemical performance (the initial capacity of the battery was 801 mA h g-1 at 0.5 C, the specific capacity of discharge was maintained at 695 mA h g-1 after 500 cycles, and the capacity retention rate was as high as 87%).

Constructing an efficient electrocatalyst for water oxidation: an Fe-doped CoO/Co catalyst enabled by in situ MOF growth and a solvent-free strategy

The development of non-precious metal electrocatalysts with high activity for the oxygen evolution reaction (OER) is a crucial and challenging task. In this work, we proposed a solvent-free in situ metal-organic framework (MOF) growth strategy for the fabrication of an Fe-doped CoO/Co electrocatalyst. This approach not only partially granted the MOF's porous structure to the catalyst but also resulted in a tighter combination between the Co metal and CoO, thereby enhancing its electrical conductivity. Furthermore, this method enabled the Fe species to be more uniformly dispersed on CoO/Co, which significantly exposed more active sites for efficient electrocatalysis. The entire synthesis process was solvent-free, except for a small amount of water and ethanol used during catalyst washing. The as-synthesized Fe-CoO/Co electrocatalyst exhibited superior OER activity on a glass carbon electrode, with η = 276 mV at a current density of 10 mA cm-2, even higher than that of the commercial precious IrO2/C catalyst. Additionally, it was also extended to prepare a Ni-doped CoO/Co electrocatalyst by the same procedure with satisfactory OER performance. This work presents a new preparation approach for MOF-derived catalysts with potential applications in energy conversion and beyond.

Nanoflower-like FeVNi3S2-xas efficient electrocatalyst for alkaline oxygen evolution reaction

The development of low cost efficient catalysts for oxygen evolution reaction (OER) is still a obstacle to realize the commercialization of electrocatalytic water splitting. Herein, interface engineering and heteroatom doping is adopted to synthesize iron and vanadium doped nickel sulfide on nickel foam via hydrothermal method followed by hydrogen treatment to create sulfur defects. The optimized nanoflower-like FeVNi3S2-x/NF is an efficient OER electrocatalyst that outperforms many of the reported transition metals catalysts. Benefiting from abundant sulfur defects and the synergistic effect of heteroatom doping, FeVNi3S2-x/NF exhibits an ultralow overpotential of 230 mV to reach a current density of 100 mA cm-2, a rapid reaction kinetics with a small Tafel slope of 46.6 mV dec-1, and a stable long-term durability in 1 M KOH. Experimental results and characterizations confirm that sulfur vacancies together with the synergistic effect from multiple heteroatom doping can effectively regulate the electronic structure, resulting in increased electrical conductivity and electrochemically active surface area, thus enhancing OER performance. Furthermore,in situRaman spectroscopy reveals that, the reconstitution amorphous nickel oxyhydroxide (NiOOH) on the catalyst surface is responsible for catalyzing the OER reaction. This work represents a promising methodology to synthesize low-cost and highly active OER electrocatalysts.

Promoting the electrocatalytic oxygen evolution reaction on NiCo2O4 with infrared-thermal effect: A strategy to utilize the infrared solar energy to reduce activation energy during water splitting

Utilization of the infrared (IR) solar energy remains a challenging task for traditional photo(electro)catalysis. Taking advantage of the IR-thermal effect to facilitate sluggish electrocatalytic reactions emerges as a promising way to utilize the IR band of the solar spectrum. In this work, nickel foam (NF) supported NiCo₂O₄ nanoneedles (NF/NiCo₂O₄ NNs) were prepared to promote the oxygen evolution reaction (OER) via the IR-thermal effect, with the NF/NiCo₂O₄ NNs acting as both the IR absorbing antennae and the OER active anode. The potential required to deliver a current density of 200 mA cm^-2 is negatively shifted from 1.618 V in the dark to 1.578 V under IR irradiation, and the Tafel slope is also decreased from 106 to 89 mV dec^-1. We demonstrate that the enhancement of OER activity is due to the localized temperature rise under IR irradiation. We measured the electrochemical activation energy of OER on NF/NiCo₂O₄ with and without IR irradiation, and the results reveal that IR irradiation reduces the kinetic energy barrier of the OER by IR-thermal effect and then facilitates OER kinetics. This work highlights a new approach to utilizing the IR portion of the sunlight to produce renewable hydrogen energy via water splitting.

Perovskites with Enriched Oxygen Vacancies as a Family of Electrocatalysts for Efficient Nitrate Reduction to Ammonia

Electrochemical nitrate reduction to ammonia (NRA) provides an efficient, sustainable approach to convert the nitrate pollutants into value-added products, which is regarded as a promising alternative to the industrial Haber-Bosch process. Recent studies have shown that oxygen vacancies of oxide catalysts can adjust the adsorption energies of intermediates and affect their catalytic performance. Compared with other metal oxides, perovskite oxides can allow their metal cations to exist in abnormal or mixed valence states, thereby resulting in enriched oxygen vacancies in their crystal structures. Here, the catalytic activities of perovskite oxides toward NRA catalysis with respect to the amount of oxygen vacancies are explored, where four perovskite oxides with different crystal structures (including cubic LaCrO₃ , orthorhombic LaMnO₃ and LaFeO₃ , hexagonal LaCoO₃ ) are chosen and investigated. By combining X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and electrochemical measurements, it is found that the amount of oxygen vacancies in these perovskite oxides surprisingly follow the same order as their activities toward NRA catalysis (LaCrO₃  < LaMnO₃  < LaFeO₃  < LaCoO₃ ). Further theoretical studies reveal that the existence of oxygen vacancies in LaCoO₃ perovskite can decrease the energy barriers for reduction of *HNO₃ to *NO₂ , leading to its superior NRA performance.

Ultra-small RuO2/NHC nanocrystal electrocatalysts with efficient water oxidation activities in acidic media

RuO2/NHC3 with ultra-small and abundant electrochemically active sites requires a low overpotential of 186 mV at 10 mA cm−2 for acidic OER and maintains wonderful long-term stability within 27 h in 0.5 M H2SO4.