Engineering nickel based layered double hydroxides and their derivatives for water electrolysis

• Main Author: Dinh, Ngoc Khang
• Other Authors: Alex Yan Qingyu
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/137864
• Institution: Nanyang Technological University

The world demands a reliable and renewable energy to replace the soon-to-be-depleted fossil fuel. Solar power has arisen as the best solution so far, in which storage energy in the form of chemical bonds is the key. Among all candidates, storing energy via hydrogen produced by solar-light-coupled electrochemical water splitting is the most viable one. However, the catalysts employed to facilitate the electrolysis of water are still based on precious metals (Pt at the cathode and Ir/Ru at the anode), which hinders their large-scale implementation. Recent studies have demonstrated the promise of Ni-based layered double hydroxides (LDHs) as alternatives to noble metals towards alkaline electrolysis due to the low overpotentials and decent stability. Notwithstanding, the poor electronic conductivity and limited specific surface area are the main reasons holding them back from being applied. Therefore, this dissertation focus on the scalable facile fabrication and engineering of low-cost Ni-based LDHs and their phosphide/sulfide derivatives materials to address the aforementioned issues and further optimizing water electrolysis. In the very first part of the research work, the electronic reconfiguration of LDH-derived sulfides NiS2 by cation doping is proposed. The semiconductor-to-conductor transition of NiS2 after Cu incorporation has been validated by both DFT calculations and temperature-dependent resistivity measurements. Such transition can facilitate reaction kinetics, reduce the loss of electron transport during electrocatalysis, increase the density of active sites, and give rise to intrinsic activities. Further calculations also confirm the advantages of Cu incorporation, which does not only significantly reduce Gibbs free energy of H adsorption of Ni site from 1.33 eV to 0.07 eV but also activate the inert S sites. Besides, Cu-doped NiS2 surface offers even more favorable water dissociation than that of Pt (111), making the former perform well in alkaline conditions. Thus, the modulated Cu-NiS2 illustrates remarkable improvement in electrocatalytic activity toward HER and OER. Specifically, it needs overpotentials of only 240 mV and 139 mV to drive OER and HER current densities of ± 10 mA cm−2. In the second part, a combined-strategy of cation tuning and surface engineering for the fabrication of highly active, earth-abundant, and robust LDHs-derived Ni2P is reported. Density functional theory (DFT) calculations suggest the effectiveness of vanadium doping and oxygen plasma, which do not only enhance the density-of-state at Fermi level, but also make the Ni sites more susceptible to OH− adsorption. The O2 plasma treatment increase the wettability of the catalyst toward KOH solution, improving the contact angle from 44.95o to 16.8o, and also induce a higher BET surface area; hence, more active sites and lower charge transfer resistance are obtained. As a result, the catalyst requires small overpotentials of 257 mV and 108 mV to drive ±10 mA cm−2 alongside with modest Tafel slope of 43.5 mV dec−1 and 72.3 mV dec−1 for OER and HER in 1.0 M KOH solution, respectively. When employed for overall water splitting, the catalyst demonstrates a low voltage of 1.56 V to achieve 10 mA cm−2 with good stability and durability, outperforming the state-of-the-art IrO2 || Pt/C which needs 1.69 V. Last but not least, the incorporation of vanadium into the hydrothermally growth of NiFe-LDHs nanosheets on nickel foam (NF) substrate to form NiFeV-LDHs/NF is reported as a highly efficient electrode toward overall water splitting in alkaline media. The lateral size of the nanosheets is about few hundreds of nanometers with the thickness of ~10 nanometers. Among all molar ratio investigated, the Ni0.75Fe0.125V0.125-LDHs/NF electrode depicts the optimized performance. It displays an excellent catalytic activity with a modest overpotential of 231 mV for OER and 125 mV for HER in 1.0 M KOH. Its exceptional activity is further shown in its small Tafel slope of 39.4 mV dec−1 and 62.0 mV dec−1 for OER and HER, respectively. More importantly, remarkable durability and stability are also observed. When used for overall water splitting, the Ni0.75Fe0.125V0.125-LDHs/NF electrodes require a voltage of only 1.591 V to reach 10 mA cm−2 in alkaline solution. These outstanding performances are mainly attributed to the synergistic effect of the ternary metal system that boosts the intrinsic catalytic activity and active surface area while lower the charge transfer resistance.