ABSTRACT
The electrochemical hydrogen production from water splitting is a promising strategy for obtaining new energy sources and replacing fossil fuels. In this study, nickel nanocones were first deposited on a nickel foam substrate using a direct current method. Then, a nickel-cobalt-manganese ternary alloy with a nanosheet morphology was deposited on the nanocones using a cyclic voltammetry method with different cycles and sweep rates. The results show that the sample synthesized in 3 cycles with a sweep rate of 10 mV s-1 exhibits the best electrocatalytic activity and requires -81, -121, and -214 mV overpotentials to reach 10, 20 and 100 mA cm-2 current densities, respectively. Electrochemical impedance spectroscopy studies also improved the HER performance with the lowest charge transfer resistance among all of the synthesized electrodes. This study introduces an effective and facile method for the fabrication of highly active and stable electrocatalysts.
ABSTRACT
This study aims to establish easy-to-fabricate and novel structures for the synthesis of highly active and enduring electrocatalysts for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Gradient electrodeposition and four different time regimes were utilized to synthesize Ni-S 3D patterns with the optimization of electrodeposition time. Pulse electrodeposition was employed for the synthesis of Ni-Fe-S nanosheets at three different frequencies and duty cycles to optimize the pulse electrodeposition parameters. The sample synthesized at 13 min of gradient electrodeposition with a 1 Hz frequency and 0.7 duty cycle for pulse electrodeposition demonstrated the best electrocatalytic performance. The optimized electrode further showed remarkable performance for HER and UOR reactions, requiring only 54 mV and 1.25 V to deliver 10 mA cm-2 for HER and UOR, respectively. Moreover, the overall cell voltage of the two-electrode system in 1 M KOH and 0.5 M urea was measured at 1.313 V, delivering 10 mA cm-2. Constructing Ni-Fe-S nanosheets on 3D Ni-S significantly increased the electrochemical surface area from 51 to 278 for the Ni-S and Ni-Fe-S layers. Tafel slopes were measured as 138 and 182 mV dec-1 for the HER and UOR for the Ni-S coating layer and 97 mV dec-1 for the HER and 131 mV dec-1 for the UOR for the optimal Ni-Fe-S nanosheets on Ni-S. Minimal changes in the potential were observed at 100 mA cm-2 in 50 h regarding the HER and UOR, signifying exceptional electrocatalytic stability. This study provides economically viable, highly active, and long-lasting electrocatalysts suitable for HER and UOR applications.
