Grafting Cobalt Diselenide on Defective Graphene for Enhanced Oxygen Evolution Reaction

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Synthesis of Hollow N,P-Doped Carbon/Co2P2O7 Nanotubular Crystals as an Effective Electrocatalyst for the Oxygen Reduction Reaction

Herein, N,P-rich carbon/carbon/Co₂P₂O₇ hollow nanotubes with a multilayered wall structure were successfully fabricated for the ORR electrocatalyst. The hollow tube structure catalysts were obtained by carbonizing Co₂P₂O₇/C coated with the phytate-doped PANI. The Co₂P₂O₇/C was obtained by phosphorylating a basic cobalt carbonate with phytic acid (PA). Onset and positive half-wave potentials were measured at 0.90 and 0.84 V, respectively, with a diffusion-limited current density of 4.58 mA/cm². Effect of the thickness of polyaniline (PANI) in the electrocatalyst precursor was also investigated. The specific surface area as well as the content of graphitic N altered as the time of PANI polymerization increased, resulting in remarkably different catalytic activities. This study of hollow nanotube catalysts exhibits efficient noble-metal-free oxygen reduction reaction electrocatalysts for other chemical systems, which will provide abundant electrochemical active centers and sufficient energy.

Morphological and Electronic Dual Regulation of Cobalt-Nickel Bimetal Phosphide Heterostructures Inducing High Water-Splitting Performance

Electrocatalytic water splitting (EWS) is a key technology for generating clean and sustainable hydrogen, which can store abundant energy but is impeded by the insufficient efficiency of the anode and cathode catalyst. Designing and constructing non-noble metal composite bifunctional electrocatalysts for promoting both the cathodic hydrogen evolution (HER) and anodic oxygen evolution reactions (OER) is clearly of great importance for EWS. Thus, the chemical composition and morphology of cobalt-nickel bimetal phosphide (Ni, Co)₂P nanoparticles (NPs) encapsulated in nitrogen-doped carbon nanotube hollow microspheres (NCNHMs) can regulate the redox-active sites and enhance the electron transfer, resulting in superior splitting efficiency. Contributing to the synergistic effects between highly active Co-Ni bimetal phosphide NPs and NCNHMs, the obtained Co-Ni bimetal phosphide/NCNHMs display remarkable electrochemical performance for water splitting compared with Ni₂P/NCNHMs. Therefore, the Ni_1.4Co_0.6P/NCNHMs catalysts achieved through a nitriding-phosphidation strategy derived from a hollow Ni_1.4-Co_0.6-based metal organic framework (MOF) exhibit superior HER catalytic activity (87.9 mV at 10 mA cm^-2 tested in 0.5 M H₂SO₄ and 64.4 mV at 10 mA cm^-2 tested in 1 M KOH) and OER catalytic activity (320.0 mV at 10 mA cm^-2 tested in 1 M KOH). The Ni_1.4Co_0.6P/NCNHMs deliver excellent water-splitting catalytic activity (1.55 V at 10 mA cm^-2 tested in 1 M KOH), which is competitive with that of current non-noble metal electrocatalysts. Density functional theory (DFT) simulations and related experimental results suggest that the electron transfer from Co doping and coating with NCNHMs improves the electronic states, which would enhance the binding strength with H-bonds and then promote the electrocatalytic activity.

A Ni/Fe-based heterometallic phthalocyanine conjugated polymer for the oxygen evolution reaction

A Ni/Fe-based heterometallic phthalocyanine polymer Fe_0.5Ni_0.5Pc-CP has been prepared, exhibiting high catalytic activity towards the oxygen evolution reaction due to the electronic interactions between the neighboring Fe and Ni atoms.

A Co-Doped Nanorod-like RuO2 Electrocatalyst with Abundant Oxygen Vacancies for Acidic Water Oxidation

Active and highly stable electrocatalysts for oxygen evolution reaction (OER) in acidic media are currently in high demand as a cleaner alternative to the combustion of fossil fuels. Herein, we report a Co-doped nanorod-like RuO₂ electrocatalyst with an abundance of oxygen vacancies achieved through the facile, one-step annealing of a Ru-exchanged ZIF-67 derivative. The compound exhibits ultra-high OER performance in acidic media, with a low overpotential of 169 mV at 10 mA cm⁻² while maintaining excellent activity, even when exposed to a 50-h galvanostatic stability test at a constant current of 10 mA cm⁻². The dramatic enhancement in OER performance is mainly attributed to the abundance of oxygen vacancies and modulated electronic structure of the Co-doped RuO₂ that rely on a vacancy-related lattice oxygen oxidation mechanism (LOM) rather than adsorbate evolution reaction mechanism (AEM), as revealed and supported by experimental characterizations as well as density functional theory (DFT) calculations.

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A Co-doped RuO₂ electrocatalyst with an abundance of oxygen vacancies was synthesized

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The compound exhibits ultra-high OER performance in acidic media

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The oxygen vacancies contribute to the high OER performance

Bifunctional Electrocatalyst of Low-Symmetry Mesoporous Titanium Dioxide Modified with Cobalt Oxide for Oxygen Evolution and Reduction Reactions

Hybrids of low-symmetry (disordered) mesoporous titanium dioxide modified with different weight ratios of cobalt oxide nanoparticles (Co3O4(x)/lsm-TiO2) are prepared using a one-pot self-assembly surfactant template. The physicochemical characterization of Co3O4(x)/lsm-TiO2 hybrids by scanning and transmission electron microscopy, X-ray diffraction, N2 adsorption–desorption isotherms, and X-ray photoelectron spectroscopy confirm the successful incorporation of cobalt oxide nanoparticles (2–3 nm in diameter) with preservation of the highly mesoporous structure of titanium dioxide substrate. Among these mesoporous hybrids, the ~3.0 wt.% Co3O4/lsm-TiO2 exhibits the best performance toward both the oxygen evolution (OER) and reduction (ORR) reactions in alkaline solution. For the OER, the hybrid shows oxidation overpotential of 348 mV at 10 mA cm−2, a turnover frequency (TOF) of 0.034 s−1, a Tafel slope of 54 mV dec−1, and mass activity of 42.0 A g−1 at 370 mV. While for ORR, an onset potential of 0.84 V vs. RHE and OER/ORR overpotential gap (ΔE) of 0.92 V are achieved which is significantly lower than that of commercial Pt/C, hexagonal mesoporous, and bulk titanium dioxide analogous. The Co3O4/lsm-TiO2 hybrid demonstrates significantly higher long-term durability than IrO2. Apparently, such catalytic activity performance originates from the synergetic effect between Co3O4 and TiO2 substrate, in addition to higher charge carrier density and the presence of disordered mesopores which provide short ions diffusion path during the electrocatalytic process.

Amorphous FeNi-bimetallic infinite coordination polymers as advanced electrocatalysts for the oxygen evolution reaction

Amorphous bimetallic coordination polymers have been prepared by a mild room temperature solution phase method and utilized as an OER electrocatalyst. Their excellent performance with an overpotential of 228 mV at 10 mA cm^-2 and a Tafel slope of 30.3 mV dec^-1 exhibits their great potential in the field of the OER.

Controlled Self-Assembled NiFe Layered Double Hydroxides/Reduced Graphene Oxide Nanohybrids Based on the Solid-Phase Exfoliation Strategy as an Excellent Electrocatalyst for the Oxygen Evolution Reaction

Layered double hydroxides (LDHs), as an effective oxygen evolution reaction (OER) electrocatalyst, face many challenges in practical applications. The main obstacle is that bulk materials limit the exposure of active sites. At the same time, the poor conductivity of LDHs is also an important factor. Exfoliation is one of the most direct and effective strategies to increase the electrocatalytic properties of LDHs, leading to exposure of many active sites. However, developing an efficient exfoliation strategy to exfoliate LDHs into stable monolayer nanosheets is still challenging. Therefore, we report a new and efficient solid-phase exfoliation strategy to exfoliate NiFe LDH and graphene oxide (GO) into monolayer nanosheets and the exfoliating ratios of NiFe LDH and GO can reach up to 10 and 5 wt %, respectively. Based on the solid-phase exfoliation strategy, we accidentally discovered that there is a dynamic evolution process between NiFe-LDH nanosheets (NiFe-LDH-NS) and GO nanosheets (GO-NS) to assemble new NiFe-LDH/GO nanohybrids, i.e., NiFe-LDH-NS could be horizontal bespreading on GO-NS or well-organized standing on GO-NS, or both simultaneously. The electrocatalytic OER property test results show that NiFe-LDH/RGO-3 (NFRG-3) nanohybrids obtained by the reduction treatment of NiFe-LDH/GO-3 (NFGO-3) nanohybrids, in which NiFe-LDH-NS are well-organized standing on GO-NS, have excellent electrocatalytic properties for OER in an alkaline solution (with a small overpotential of 273 mV and a Tafel slope of 49 mV dec^-1 at the current density of 30 mA cm^-2). The excellent electrocatalytic properties for OER of NFRG-3 nanohybrids could be attributed to the unique three-dimensional arraylike structure with many active sites. At the same time, reduced graphene oxide (RGO) with excellent conductivity can improve the charge-transfer efficiency and synergistically improve OER properties of nanohybrids.

pH-Universal Water Splitting Catalyst: Ru-Ni Nanosheet Assemblies

Although electrochemical water splitting is an effective and green approach to produce oxygen and hydrogen, the realization of efficient bifunctional catalysts that are stable in variable electrolytes is still a significant challenge. Herein, we report a three-dimensional hierarchical assembly structure composed of an ultrathin Ru shell and a Ru-Ni alloy core as a catalyst functioning under universal pH conditions. Compared with the typical Ir/C-Pt/C system, superior catalytic performances and excellent durability of the overall water splitting under universal pH have been demonstrated. The introduction of Ni downshifts the d-band center of the Ru-Ni electrocatalysts, modulating the surface electronic environment. Density functional theory results reveal that the mutually restrictive d-band interaction lowers the binding of (Ru, Ni) and (H, O) for easier O-O and H-H formation. The structure-induced eg-dz² misalignment leads to minimization of surface Coulomb repulsion to achieve a barrier-free water-splitting process.

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A facile method for the synthesis of 3D hierarchical assembly Ru-Ni NA catalyst

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Superior catalytic reactivity and stability of water splitting in universal pH

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The introduction of Ni can modify the d-band and surface electronic environment

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Minimization of the surface Coulomb repulsion leads to barrier-free catalysis process