Interface engineering of advanced heterostructure through electrochemical synthesis for water splitting

• Main Author: Peng, Dongdong
• Other Authors: Huang Yizhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/153323
• Institution: Nanyang Technological University

Green energy has played a key role in solving the current energy crisis and environmental issues. For various green energy resources including solar energy, wind energy, tidal energy, hydropower and so on, hydrogen produced from water splitting is abundant and cost-effective clean energy upon the active electrocatalyst. In this thesis, the electrocatalyst is prepared by electrodeposition that has been proven to be an economical, easy and efficient synthesis method when compared with other fabrication methods including solvothermal method and wet chemical method. Firstly, a facile electrochemical method to grow Co9S8/Co3O4 heterostructured nanocomposite with N2 purging and subsequent annealing is carried out and reported. The gas purging is involved during the growth in order to decrease hydroxide ions (generated by the reduction of nitrate ion) concentration near electrode surface and benefit the formation of heterostructure electrocatalyst. It consists of two epitaxial phases (i.e. Co9S8 and Co3O4), that are oriented along {020} lattice plane enabling the rapid mass transport and charge transfer during HER and OER. In addition, the intrinsic high energy state interface and other factors such as small size and good conductivity contribute to the significantly improved electrocatalytic activity towards water splitting and excellent durability. As a result, to produce 10 mA·cm-2 OER current density, the composite only requires an overpotential of 250 mV with a low Tafel slope of 73.54 mV·dec-1. In fact, it offers better electrochemical redox performance than the state-of-the-art Co-based nanocomposites. Besides, this special composite is also examined to deliver high HER performance with superior durability in alkaline solution. Subsequently, an exclusive cost-effective and highly efficient OER catalyst has been successfully fabricated by simple electrodeposition followed by dealloying. The catalyst presents a core-shell structure in which the highly conductive dendritic FeCoCu alloy is the core that is covered by the FeOOH/Co(OH)2 nanosheets as the shell. The whole catalyst is supported on the copper nanotubes (Cu NTs). This design enables the formation of a hierarchical catalyst with small Rct and large ECSA which afford the enhanced intrinsic properties and high exposure of active sites. The catalyst is demonstrated to offer excellent OER performance with the current density reaching to 10 mA·cm-2 at a small overpotential of 250 mV, which is more competitive than most FeCo-based OER catalysts reported. In addition, this catalyst is also highly resistant to the corrosive attack in alkaline medium, and maintains almost the same performance after 12 h of stability test. This work provides an easy and economic design for fabricating highly efficient and durable OER catalyst, showing the potential for the large scale-up in industrial application. Lastly, a high performance hierarchical quaternary nano-porous nanostructure FeCoNiCu catalyst has been developed via electrodeposition approach followed by the electrochemical etching process. Effect of different electrodeposition time on the catalytic performance is systemically studied, and it is found that FeCoNiCu electrode at 5 min electrodeposition time demonstrates the best performance with lowest Tafel slope of 57 mV/dec and is higher than the performance of similar catalysts reported in literature. Such good performance is attributed to the nanoporous structure and the mixture of FeO(OH), Ni(OH)2 and Co(OH)2 hydroxyl-grouped phases.