Electrochemically Deposited Amorphous Cobalt-Nickel-Doped Copper Oxide as an Efficient Electrocatalyst toward Water Oxidation Reaction

Greenly Synthesized Conducting Polymer Nanotunnels with Metal-Hydroxide Nanobundles in Single Dais for Unmitigated Water Oxidation

Electrochemical water splitting required efficient electrocatalysts to produce clean hydrogen fuel. Here, we adopted greenway coprecipitation (GC) method to synthesize conducting polymer (CP) nanotunnel network affixed with luminal-abluminal CoNi hydroxides (GC-CoNiCP), namely, GC-Co1Ni2CP, GC-Co1.5Ni1.5CP, and GC-Co2Ni1CP. The active catalyst, GC-Co2Ni1CP/GC, has low oxygen evolution reaction (OER) overpotential (307 mV) and a smaller Tafel slope (47 mV dec-1) than IrO2 (125 mV dec-1). The electrochemical active surface area (EASA) normalized linear sweep voltammetry (LSV) curve exhibited outstanding intrinsic activity of GC-Co2Ni1CP, which required 285 mV to attain 10 mA cm-2. At 1.54 V, the estimated turnover frequency (TOF) of GC-Co2Ni1CP/GC (0.017337 s-1) was found to be 3-fold higher than that of IrO2 (0.0014 s-1). Furthermore, the GC-Co2Ni1CP/NF consumed a very low overpotential (281 mV) with a small Tafel slope of 121 mV dec-1. The ultrastability of GC-Co2Ni1CP for industrial application was confirmed by durability at 10 and 100 mA cm-2 for the OER (GC/NF-8 h, 2.0%/100 h, 2.2%) and overall water splitting (100 h, 3.8%), which implies that GC-Co2Ni1CP had adequate kinetics to address the elevated rates of water oxidation. The effect of pH and addition of tetramethylammonium cation (TMA+) reveal that GC-Co2Ni1CP follows the lattice oxygen mechanism (LOM). The solar-powered water electrolysis at 1.55 V supports the efficacy of GC-Co2Ni1CP in the solar-to-hydrogen conversion. The environmental impact studies and solar-driven water electrolysis proved that GC-CoNiCP has excellent greenness and efficiency, respectively.

Electrochemically Assisted Single Crystal Growth of Reduced Preyssler Polyoxometalates Decorated with M2+ (M = Co, Ni) and Cubane-Like Ni4O4 Units

Polyoxometalates (POMs) are of great interest to the scientific community, and their reduction and nucleation have been well-established by multi-step techniques. The present study develops an electrochemical approach for simultaneous reduction and nucleation of polyoxometalate-containing solids. Herein we report crystal growth of reduced Preyssler polyoxotungstate-based (anionic formula [NaP5W30O110]14-) new crystalline solids made of Preyssler anions interlinked by Co2+ and Ni2+ ions. Crystal nucleation and in situ reduction were achieved at room temperature using a two silver wire electrode setup in various aqueous solutions under constant applied potentials. The POM material was deposited on the cathode, and its structure was characterized by X-ray diffraction techniques. The primary structure type observed involves POMs decorated by disordered Co2+/Ni2+ octahedra and fused into 1-D pillars by additional Co2+/Ni2+ octahedra. A secondary phase was observed in the Ni-based reactions, where reduced Preyssler anions are decorated by Ni4O4 cubane-like units. To understand the electrochemical process, polarization curves of the electrolyte solutions are presented, suggesting an applied potential best suited for crystal growth. The work highlights the effectiveness of an electrochemical pathway where nucleation and simultaneous reduction of POMs can make novel reduced POM solids.

Application of Oxygen-Group-Based Amorphous Nanomaterials in Electrocatalytic Water Splitting

Environmentally friendly energy sources (e.g., hydrogen) require an urgent development targeting to address the problem of energy scarcity. Electrocatalytic water splitting is being explored as a convenient catalytic reaction in this context, and promising amorphous nanomaterials (ANMs) are receiving increasing attention due to their excellent catalytic properties.Oxygen group-based amorphous nanomaterials (O-ANMs) are an important component of the broad family of ANMs due to their unique amorphous structure, large number of defects, and abundant randomly oriented bonds, O-ANMs induce the generation of a larger number of active sites, which favors a better catalytic activity. Meanwhile, amorphous materials can disrupt the inherent features of conventional crystalline materials regarding electron transfer paths, resulting in higher flexibility. O-ANMs mainly include VIA elements such as oxygen, sulfur, selenium, tellurium, and other transition metals, most of which are reported to be free of noble metals and have comparable performance to commercial catalysts Pt/C or IrO2 and RuO2 in electrocatalysis. This review covers the features and reaction mechanism of O-ANMs, the synthesis strategies to prepare O-ANMs, as well as the application of O-ANMs in electrocatalytic water splitting. Last, the challenges and prospective remarks for future development in O-ANMs for electrocatalytic water splitting are concluded.

Surface Reconstruction of Cobalt-Based Polyoxometalate and CNT Fiber Composite for Efficient Oxygen Evolution Reaction

Polyoxometalates (POMs), as carbon-free metal-oxo-clusters with unique structural properties, are emerging water-splitting electrocatalysts. Herein, we explore the development of cobalt-containing polyoxometalate immobilized over the carbon nanotube fiber (CNTF) (Co4POM@CNTF) towards efficient electrochemical oxygen evolution reaction (OER). CNTF serves as an excellent electron mediator and highly conductive support, while the self-activation of the part of Co4POM through restructuring in basic media generates cobalt oxides and/or hydroxides that serve as catalytic sites for OER. A modified electrode fabricated through the drop-casting method followed by thermal treatment showed higher OER activity and enhanced stability in alkaline media. Furthermore, advanced physical characterization and electrochemical results demonstrate efficient charge transfer kinetics and high OER performance in terms of low overpotential, small Tafel slope, and good stability over an extended reaction time. The significantly high activity and stability achieved can be ascribed to the efficient electron transfer and highly electrochemically active surface area (ECSA) of the self-activated electrocatalyst immobilized over the highly conductive CNTF. This research is expected to pave the way for developing POM-based electrocatalysts for oxygen electrocatalysis.

Binder-Free Fabrication of Prussian Blue Analogues Based Electrocatalyst for Enhanced Electrocatalytic Water Oxidation

Developing a cost-effective, efficient, and stable oxygen evolution reaction (OER) catalyst is of great importance for sustainable energy conversion and storage. In this study, we report a facile one-step fabrication of cationic surfactant-assisted Prussian blue analogues (PBAs) M_x[Fe(CN)₅CH₃C₆H₄NH₂]∙yC₁₉H₃₄NBr abbreviated as SF[Fe-Tol-M] (where SF = N-tridecyl-3-methylpyridinium bromide and M = Mn, Co and Ni) as efficient heterogeneous OER electrocatalysts. The electrocatalysts have been characterized by Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS). In the presence of cationic surfactant (SF), PBAs-based electrodes showed enhanced redox current, high surface area and robust stability compared to the recently reported PBAs. SF[Fe-Tol-Co] hybrid catalyst shows superior electrochemical OER activity with a much lower over-potential (610 mV) to attain the current density of 10 mA cm⁻² with the Tafel slope value of 103 mV·dec⁻¹ than that for SF[Fe-Tol-Ni] and SF[Fe-Tol-Mn]. Moreover, the electrochemical impedance spectroscopy (EIS) unveiled that SF[Fe-Tol-Co] exhibits smaller charge transfer resistance, which results in a faster kinetics towards OER. Furthermore, SF[Fe-Tol-Co] offered excellent stability for continues oxygen production over extended reaction time. This work provides a surface assisted facile electrode fabrication approach for developing binder-free OER electrocatalysts for efficient water oxidation.