Protons buffer at cathode via Al3+(H2O)6 hydrolysis for enhancing electrochemical reduction : examples of bismuth-oxychloride storage battery and carbon dioxide conversion

Abstract

Hydrogen evolution reaction (HER) is an undesirable cathodic reaction in aqueous electrochemical technologies such as carbon dioxide reduction (ERCO2) and bismuth-based electrode discharge in storage batteries. Most aqueous electrochemical systems are in either acidic or alkaline media which promote the unwanted HER. This thesis aims at deploying Al3+(H2O)6 buffered aqueous electrolyte to facilitate the reduction of BiOCl electrode and ERCO2 with improvement in performance. Proton transfer is critical during the reduction step at the BiOCl/Bi electrode in storage battery and Bi in ERCO2. When the bismuth-based electrode is negatively charged in a storage battery or for the reduction reaction for CO2, the electrostatically attracted Al cations at the electrode surface undergo hydrolysis and provide indirect mass transport of protons towards the electrode. 1H NMR analysis reveals the proton environment around Al ions species and its correlation to pH and HER. The reversible hydrolysis of Al3+(H2O)6, Al2+(OH)(H2O)5 and Al+(OH)2(H2O)4 species can maintain a steady supply of protons at the outer Helmholtz plane (OHP) of the electrode at different pH, but with minimum level of HER. This is in contrast to other buffers which are mostly anionic such as the phosphate buffer with pKa = 2.1, 7.2, 12.7. The BiOCl/Bi electrode has been reported as a promising electrode for rechargeable aqueous batteries. Compared with other counterparts of HCl, NaCl, and MgCl2, the AlCl3 electrolyte greatly improves the reduction activity of BiOCl because of the regulated proton supply at the electrode surface. Moreover, the high loading BiOCl nanosheets are directly synthesized on carbon cloth (BiOCl/CC) by the facile solution combustion synthesis method. The binder-free approach and high surface area of nanosheet morphology significantly improve the charge transfer. As a result, the prepared BiOCl/CC in Al3+(H2O)6 buffered electrolyte demonstrated good rate performance. It can deliver 221 mAh/g at ~1.1 A/g and 181 mAh/g at ~ 4.4 A/g with 89% and 94% current efficiency respectively. These values are much higher than those of other electrodes. Al3+(H2O)6 buffered electrolyte can maintain a mildly acidic environment near the Bi-based electrode surface, thus increasing the current efficiency of ERCO2 > 91% at -0.34 V (vs RHE) producing formic acid using bismuth-based catalysts compared to literature data. Metal-organic-framework (MOF)-derived bismuth/reduced graphene oxide (rGO) on carbon cloth is developed. The carbon cloth is pre-deposited with rGO, followed by hydrothermal synthesis of CAU-17 MOF. The MOF-derived Bi nanoparticles synthesized onto rGO/CC show reduced electrochemical impedance and higher current density in ERCO2.