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Cobalt hydroxide Co(OH)2 is considered to be a potential material for electrocatalyst, especially for oxygen evolution reaction (OER), owing to the earth's abundance, environmentally benign nature, and redox-active properties of cobalt. Herein, we report the fabrication of pristine Co(OH)2 and AgNP-integrated Co(OH)2 (Ag–Co(OH)2) by simple wet chemical methods and explore the electrocatalytic OER activity in the alkaline medium. High-resolution transmission electro-microscopic (HR-TEM) analysis revealed featureless nanostructures for Co(OH)2 with coexisting amorphous and crystalline phases, as well as the inclusion of crystalline AgNPs in Ag–Co(OH)2. X-ray photoelectron spectroscopic (XPS) analysis confirmed the inclusion of metallic AgNPs and the presence of Co at a mixed oxidation state. Electrocatalytic OER studies indicated that pristine Co(OH)2 required the overpotential of 299 mV to achieve a geometric current density of 10 mA cm−2. The integration of AgNPs in Co(OH)2 (Ag–Co(OH)2) showed a gradual improvement in the OER activity. The optimized sample, Ag–Co(OH)2-5, required the overpotential of 253 mV to produce 10 mA cm−2 current density. Tafel slope analysis revealed a lower value upon AgNP integration and electrochemical impedance showed lower charge transfer resistance. The lower Tafel value and charge transfer resistance of Ag–Co(OH)2-5 indicated good chemical coupling and faster reaction kinetics at the electrode surface. AgNPs incorporation with Co(OH)2 also showed enhanced turn-over frequency, electrochemical active surface area, and double-layer capacitance. The fabricated hybrid Ag–Co(OH)2-5 catalyst also exhibited good stability over 60 h. Thus, the electrocatalytic activity of low-cost Co(OH)2 was improved by fabricating coexisting amorphous and crystalline phases and integrating noble silver nanoparticles. [ABSTRACT FROM AUTHOR]

Developing an efficient transition metal-based non-noble electrocatalyst is important for the industrial water splitting process and producing green sustainable hydrogen (H2) fuel. Particularly, the direct fabrication of electrocatalysts on an electrode surface can exhibit strong electronic coupling that results in increased catalytic activity. Herein, copper (Cu) and cuprous oxide (Cu2O) nanoparticles were directly deposited on a nickel foam (NF) substrate and the controlled calcination of the Cu produced mixed copper–copper oxides (CuO, Cu2O–CuO and Cu–Cu2O–CuO) that exhibited excellent composition-controlled bifunctional electrocatalytic (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) activity in an alkaline medium with good stability. Powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopic (XPS) studies confirmed the formation of pure and mixed copper–copper oxide nanomaterials on NF. Mixed copper oxide (Cu2O–CuO@NF) and pure CuO@NF exhibited excellent electrocatalytic OER and HER activity, respectively, in alkaline media. To the best of our knowledge, Cu2O–CuO@NF required an overpotential of 262 mV to achieve the lowest geometric current density (10 mA cm−2) among the pure copper-based electrocatalysts reported. Cu2O–CuO@NF also produced a high current density (130.9 mA cm−2) at a relatively low applied potential (1.56 V) compared to other catalysts. The low Tafel slope (45.8 mV dec−1) and charge-transfer resistance indicated that good electronic coupling and fast charge transport led to the higher activity. In the HER, pure CuO@NF required a much lower overpotential of 165 mV to produce geometric current density. It is worth noting that CuO@NF showed strong HER activity in the alkaline medium. The Tafel slope of 121.81 mV dec−1 for CuO@NF suggests that the desorption process could be the rate-determining step in the HER process. Cu2O–CuO@NF and CuO@NF exhibited good stability in the alkaline medium. The highly enhanced current density and good stability of Cu2O–CuO@NF in 6.0 M KOH at 60 °C highlights its potential for practical applications. Overall, an earth-abundant copper-based electrocatalyst with excellent OER and HER activity in alkaline media has been developed by directly fabricating on a conducting substrate and controlling the copper oxides composition. [ABSTRACT FROM AUTHOR]

Metal organic framework (MOF) based electrocatalysts have received increased attention in recent years because of their tailorable structure, functionality and porosity. Herein, cobalt-terephthalic acid (CoTPA) based microcrystalline MOFs particles are synthesized in the presence of four different organic amines (DABCO (D), butylamine (B), hexylamine (H) and triethylamine (T)) and the electrocatalytic oxygen evolution reaction was explored. Powder X-ray diffraction (PXRD) analysis confirmed the formation of crystalline CoTPA MOFs with a similar structure but with different faceted orientation. Further, morphological studies revealed the formation of CoTPA MOFs microparticles with different morphologies depending on the amine utilized in the reaction. BET analysis confirmed the porosity with a different surface area and pore diameter. Thermogravimetric analysis indicated the water coordination in all four MOFs. X-ray photoelectron spectroscopy (XPS) analysis indicated the coexistence of Co2+ and Co3+ in the MOFs. All four MOFs exhibited strong OER activity in alkaline medium. CoTPA-D exhibited relatively higher activity and required an overpotential of 273 mV to produce the benchmark current density of 10 mA cm−2 compared to CoTPA-B (281 mV), CoTPA-H (287 mV) and CoTPA-T (278 mV). The low Tafel slope and charge transfer resistance indicated the faster reaction kinetics whereas the higher electrochemical surface area and double layer capacitance (Cdl) suggested the availability of more active sites in CoTPA-D MOFs. The post catalytic analysis suggested the conversion of CoMOFs into CoOOH during electrocatalysis, which resulted in the enhanced OER reaction. Thus, the present work reports the facile preparation of CoMOFs with varied porosity and functionality that resulted in different electrocatalytic OER activity. [ABSTRACT FROM AUTHOR]