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Herein, bimetal (Mn, Co) codoping on a CuO host is aimed at enhancing oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity. Codoping of Mn and Co on CuO to enhance bifunctional action in electrochemical water splitting has not yet been investigated to the best of our knowledge. Literatures are focused on unary Mn-doped CuO or Co-doped CuO nanostructures. Mn, Co codoped CuO through an easy chemical coprecipitation method has been successfully attempted and is more beneficial which is the novelty of the present work. Defect-enriched ample active sites (Mn2+/Mn3+ and Co2+/Co3+) along with Cu2+ in the host CuO achieved high current density (100 mA/cm2) in OER and HER with low overpotential such as 468 mV and 271 mV, respectively. Faster charge transfer and diffusion ability was stimulated by the bimetal codoping CuO. Reasonable Tafel plot values (OER: 199 mV/dec, and HER: 21 mV/dec) with improved water-splitting reaction kinetics were achieved for the Mn, Co codoped CuO nanoflakes. The double-layer capacitance ( C dl ) value of 27.5 mF/cm2 for Mn, Co codoped CuO nanoflakes was achieved. Similarly, the increasing order of an electrochemically active surface area (EASA) was exhibited by the consequent addition of bimetal doping on CuO, denoted as Mn , Co / CuO > Co / CuO > Mn / CuO > CuO. The evidence shows that the codoping strategy could facilitate rapid reaction kinetics to develop overall water splitting. The charge transfer resistances ( R ct ) of 3.6 Ω and 1.2 Ω for the Mn, Co codoped CuO nanostructure corresponding to the OER and HER, respectively, were reported. The long-term stability over 16 h with negligible loss was reported for both the OER and the HER performance. Thus, this work contributes to better insight and analysis of the successful codoping of bimetal elements in earth-abundant electrocatalysts to enhance and make practical the electrocatalytic water-splitting activity. [ABSTRACT FROM AUTHOR]

Highly proficient, long-lasting non-noble-metal-supported electrocatalysts for hydrogen evolution reaction are important for hydrogen energy production through alkaline electrolyte. In transition metal dichalcogenides, tungsten sulfide is one of the popular materials and is used in a wide variety of applications. In this work, pristine and cationic surfactant assisted tungsten sulfide were successfully synthesized via simple hydrothermal method in alkaline electrolyte for hydrogen evolution reaction. X-ray diffraction confirmed tungsten sulfide rhombohedral structure formation with high index plane (003). Scanning electron microscopy explored tungsten sulfide nanosheet-assembled mesoporous sphere morphology. Tungsten sulfide and cationic surfactant assisted tungsten sulfide overpotential values were found to be 141 and 102 mV at 10 mA/cm2 from linear sweep voltammetry curves. The obtained Tafel slope of tungsten sulfide and cationic surfactant assisted tungsten sulfide were 26.9 and 21.4 mV/dec, respectively. The solution and charge transfer resistance of the prepared materials were obtained from Nyquist plot and found to be 1.7 and 1.4 Ω, and 53.6 and 32 Ω, respectively. The specific surface area, pore diameter, and volume were analyzed by Brunauer–Emmett–Teller method and obtained values for surface area, pore size, and volume of cetyltrimethylammonium bromide assisted tungsten sulfide were 65.05 m2/g, 3.502 nm, and 0.067 cc/g, respectively. The obtained result suggested that 0.1 M cationic surfactant assisted tungsten sulfide showed high catalytic activity with minimum overpotential. Long-term durability studies also suggested that the optimized surfactant-assisted tungsten sulfide had excellent stability over a prolong time of 16 h without any decay. However, the stability of the material exhibited only 90% retention which will improve in our near future works by using carbon-based materials. Hence, carbon-based materials exhibited outstanding chemical stability and offer large surface area, which significantly improves the hydrogen evolution reaction activity and suggests to be used in large-scale applications. [Display omitted] • Tungsten sulfide (WS 2) revealed rhombohedral structure with high index plane (003). • WS 2 surface area, pore size and volume was 65.05 m2/g, 3.502 nm and 0.067 cc/g. • WS 2 explored low 102 mV overpotential with Tafel slope of 21.4 mV/dec. • Cetyltrimethylammonium bromide assisted WS 2 exhibited 90% retention in 16 h CA test. • The optimum concentration of cationic surfactant was fixed as 0.1 M. [ABSTRACT FROM AUTHOR]

[Display omitted] • CW1, CW2 and CW3 electrodes overpotential were 123, 115 and 102 mV. • Tafel slope values of prepared electrodes were 245, 244 and 238 mV/dec. • CW3 electrode exhibited high C dl value of 32.8 mF/cm2 and enhanced HER. • CW3 electrode exhibited 75% retention in 16 h durability test. Hexamethylenetetramine (HMT) assisted calcium tungstate nanostructures were successfully synthesized through co-precipitation method. The phase and material confirmation of scheelite type calcium tungstate nanostructures with tetragonal unit cell (a = 5.193 and c = 11.255) were done by employing X-ray diffraction (XRD). All the prepared electrocatalysts such as CW1, CW2 and CW3 crystallite sizes were 29.01, 24.86 and 17.40 nm respectively. The calculated band gap values were found to be 3.31, 3.31 and 3.27 eV for CW1, CW2 and CW3 materials respectively. Different morphology such as ball like and dumbbell shaped calcium tungstate were revealed by scanning electron microscopy (SEM). Surfactant largely plays a significant role to reduce nanostructures agglomeration. The specific surface area, pore volume and diameter of 0.1 M hexamethylenetetramine (HMT) assisted calcium tungstate nanostructure was about 24.7 m2/g, 0.141 cc/g and 3.365 nm. The surface area of the calcium tungstate nanostructures drastically enhanced hydrogen evolution reaction (HER) performance as recognized by linear sweep voltammetry. The overpotential values of CW1, CW2 and CW3 electrodes were 123, 115 and 102 mV respectively. Solution resistance (R s) and charge transfer resistance (R ct) of synthesized materials was finding out by electrochemical impedance spectroscopy and all prepared electrocatalyst estimated values were 1.07, 1.25, 1.34 Ω and 4.06, 1.62, 0.76 Ω respectively. The stability of 0.1 M hexamethylenetetramine (HMT) assisted CaWO 4 was analyzed by 16 h and it exhibited excellent robustness without any decay. The synthesized CaWO 4 material exhibited superior hydrogen evolution reaction (HER) activity and the prepared electrodes was capable electro catalysts in water splitting applications. [ABSTRACT FROM AUTHOR]