1. Introduction
Green hydrogen has emerged as an important clean energy source for replacing the conventional energy source, such as fossil fuel which could contribute in decreasing the greenhouse effect. Electrochemical water electrolysis (EWE), the process used to generate green hydrogen, consisting of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), has been widely studied over the decades due to its advantageous merits including high hydrogen production rate and ability to generate high purity hydrogen without other byproducts.[1] However, large amount of energy is needed to split the water, although the theoretical potential for the water electrolysis is 1.23 V, additional energy is required due to the presence of overpotential arising from the electrocatalyst in each HER & OER processes which hinder energy efficient production of hydrogen. Therefore, developing advanced electrocatalyst with lower overpotential with high activity, low charge transfer resistance and excellent electrical conductivity is highly desired for energy efficient, high performing water electrolysis. [2]
Specifically, in HER process, the use of noble catalyst such as Pt/C is widely used as an electrocatalyst, owing to its low overpotential with high activity along with its great conductivity. Yet, the use of expensive, noble metal catalyst is one of the roadblocks in cost effective production of green hydrogen. [3]Therefore, many researches have been directed towards finding a new non-noble metal based electrocatalyst [4] with comparable HER activity of Pt/C catalyst. [5]Recently, various 2D Materials have been widely studied as a promising candidate for HER. Amongst the 2D materials, MoSe2,metal dichalcogenides based 2D material gained considerable interests as a promising candidate for HER due to its tunable d-spacing with lamellar structure facilitating the ion transport and tunable conductivity provided by controlling the phase of the MoSe2(metallic-semiconducting transition). [6] Amongst the phases in metal dichalcogenide based 2D materials, 2H phase (semiconducting phase) shows low HER performance compared to the 1T phase (metallic phase) due to its lower conductivity.[6-b] Therefore, careful optimization of the phase and structure of the MoSe2 is essential for high HER performances. Moreover, MoSe2 with the optimized 1T and 2H mixed phases, have shown to improve the HER activity,[6-a] Although further study is needed to investigate the origin of the high HER performance induced by mixed phase of metal dichalcogenide based electrocatalyst such as MoSe2. [6] Previous studies have explored various strategies to enhance the catalytic performance of MoSe2 for the HER. For example, Jiang et al.[7] demonstrated the synthesis of 1T-MoSe2 nanosheets with expanded interlayer spacing, which showed superior electrochemical performance compared to conventional MoSe2. They reported a low overpotential of 179 mV and a small Tafel slope of 78 mV/dec, attributing these improvements to the high conductivity and increased number of active sites in the 1T phase. Qu et al. [8] introduced MoSe2/Mo core-shell 3D hierarchical nanostructures, which significantly improved HER efficiency. Their study highlighted that the 1T-MoSe2/Mo core-shell structures exhibited a low Tafel slope of 34.7 mV/dec, suggesting enhanced electron transfer efficiency between the catalyst and electrode. Li et al.[9] investigated the effects of S-doping on MoSe2, showing that doping induces a phase transformation to 1T-MoSe2. This transformation has led to a high purity of the 1T phase, resulting in a low overpotential of 167 mV and a Tafel slope of 54 mV/dec, thereby enhancing HER performance. Additionally, Ren et al. [10]developed V-doped MoSe2 nanosheets confined on carbon black (V-MoSe2/CB) using a sol–gel process. This approach prevented agglomeration and yielded ultra-thin nanosheets with short vertical lattice arrays. The V-MoSe2/CB catalyst exhibited improved HER activity with a small overpotential of 166 mV at 10 mA/cm2 and a Tafel slope of 65 mV/dec, attributed to optimized electronic structure and enhanced electrocatalytic performance. These studies have collectively illustrated various methods to maximize the catalytic efficiency of MoSe2-based materials for HER. Furthermore, often metal-based dopants could be introduced to improve the HER performances of the metal dichalcogenide based electrocatalyst, where Pt nanoparticles/dopant has shown to be effective in enhancing the HER performances. [11]
Besides the development of the high performing electrocatalyst, HER performances can be affected by the method in which electrocatalyst deposition takes place on the current collector. Conventionally, electrocatalyst deposition is often carried out via drop-casting process, where electrocatalyst is often mixed with non-conductive binders such as Nafion or PVDF, which are then pasted onto a conductive current collector. The used of non-conductive binders often, obstructs the active sites of the electrocatalyst while increasing the resistance in the electrodes. In contrast, direct growth of electrocatalyst on the current collector is often favored due to its fast electron transfer between the electrocatalyst and the substrate, resulting in enhanced HER activity with long-term stability without the masking effect which could be more favorable method of electrocatalyst deposition compared to the conventional process. [12]
Finally, media in which HER takes place, is an important factor determining the HER performances. Generally, use of the pure water induces high overpotential of water electrolysis due to its high electrolyte resistance. Often, H2SO4 or KOH based electrolytes are used, which reduce electrolyte resistance, to increase the efficiency of water electrolysis and to decrease the overpotential. Especially, water electrolysis under acidic operation conditions has advantages such as high current density (~2 A/cm2) and low gas crossover compared to alkaline water electrolysis (AWE). Therefore, water electrolysis under acidic media is favorable for the designing of compact water electrolysis cell.[13]
Using the merits of above-mentioned materials/techniques, in this study, we synthesized mixed phase 1T/2H MoSe2 powder via hydrothermal process where phase contents in MoSe2 were controlled by the synthesis time, then the HER performances of the resultant electrocatalysts were first evaluated. The best HER performances were demonstrated by the samples with synthesis time of 36 h which exhibited overpotential of 177 mV at 10 mA/cm2under the acidic media (0.5 M H2SO4) which was chosen as optimum synthesis time. Then, we synthesized 1T/2H mixed phases of MoSe2 directly grown on a 3D porous carbon paper via hydrothermal synthesis process for 36h, where enhanced HER performances of 167 mV at 10 mA/cm2 were observed compared to the conventional deposition process using a binder. Moreover, we conducted an in-situ electrodeposition of Pt nanoparticle on MoSe2, using the chronopotentiometry technique which resulted in excellent HER performances with low overpotential of 133 mV at 10 mA/cm2, compared to the Pt-free, pristine MoSe2 powder. Finally, we conducted a thorough theoretical study (DFT calculation) on varying HER performances under various 1T/2H phase conditions to explore the origin of phase dependent HER performance in MoSe2 based electro catalyst as well as under the presence of Pt decoration on the mixed phases of MoSe2, shedding a light on atomistic insight on HER performances of 2D material based electrocatalyst such as MoSe2.