Status : Verified
Personal Name Acedera, Rose Anne E.
Resource Title Cobalt (II, III) oxide-based electrocatalysts for oxygen evolution reaction in an alkaline anion exchange membrane water electrolyser
Date Issued 15 September 2021
Abstract Water electrolysis presents numerous advantages over other technologies, such as steam reforming of natural gas, and gasification of coal, to produce clean and renewable energy in the form of high purity hydrogen gas (H2), an ideal energy carrier in place of fossil fuels. However, due to the high overpotential and slow reaction kinetics of the anodic oxygen evolution reaction (OER), catalysts are needed to improve the efficiency of the electrolyzer system. Over the recent years, cobalt (II, III) oxide (Co3O4) has been used as an OER electrocatalyst due to its abundance, cost-effectiveness, environmental sensitivity, good stability, and comparable electrocatalytic activity to noble metal-based catalysts such as ruthenium (Ru), and iridium (Ir). Herein, spinel Co3O4-based electrocatalysts were synthesized through solution combustion and ethanol-assisted hydrothermal route.
Initially, porous spinel Co3O4 nanoparticles were synthesized through solution combustion. The effects of calcination on the morphology, phase composition, and electrocatalytic behavior towards oxygen evolution reaction (OER) of the synthesized oxides were investigated. The as-synthesized powder prepared at stoichiometric conditions (fuel-to-oxidizer ratio, φ = 1) and pH = 3 was a mixture of spinel Co3O4 and monoclinic CoO. After calcination at 300 and 500 °C, the product was transformed to pure Co3O4. Additionally, the powders became more compact and dense. . The mixed-phase oxide exhibited excellent electrocatalytic performance in 1 M KOH with onset overpotential and Tafel slope values as low as 361 mV and 87.54 mV·dec-1, respectively. Its enhanced properties compared to the calcined samples could be ascribed to its high specific surface area, lower crystallinity, and excellent porosity. Following such findings, uncalcined samples were produced with different φ and pH values. The sample produced at φ = 0.5 and pH = 3 exhibited the best OER catalytic activity with an onset overpotential and Tafel slope as low as 334 mV and 61. 77 mV dec-1. Catalytic activity enhancement is possibly due to better control of phase composition and morphology achieved by employing fuel-lean (φ < 1) and acidic conditions.
For the second part of this research, hierarchical copper (Cu)-doped Co3O4 (CuxCo3-xO4) catalysts were. prepared through an ethanol-assisted hydrothermal method to utilize the synergistic effect of a hierarchical morphology and improved electrical conductivity on the OER activity of CuxCo3-xO4.The effects of increasing copper (Cu) concentration (x = 0, 0.25, 0.5, 0.75, and 1) on the morphology, structure, and OER performance of spinel Co3O4 were studied. The synthesized CuxCo3-xO4 catalysts exhibited a micro-urchin morphology. With increasing Cu concentration, the nanowires forming the urchin-like structures became thinner and finer. The crystallite size based on XRD results also decreased with increasing Cu-content. These findings agree well with the specific surface area measured by BET. In general, Cu-doped Co3O4 samples performed better than the undoped spinel catalyst prepared by solution combustion and hydrothermal methods. The observed enhancement in the OER activity after Cu-doping is possibly due to the higher specific surface area of the catalysts, which provided more active sites for the reaction. The sample synthesized with Cu-doping at x = 0.75 (CCO–0.75) performed the best among the catalysts tested. It recorded an overpotential of 385 mV at 10 mA cm-2, which is 24% lower than the value for the undoped Co3O4 sample. CCO–0.75 exhibited fair stability even after subjecting it to 2000 cycles of polarization from 0.2 to 0.55 V (vs. Ag/AgCl) and recorded only a 9% increase in the overpotential required to reach 10 mA cm-2. An alkaline anion exchange water electrolyzer was assembled using CCO0.75 on Ti GDL, Pt/C on carbon GDL, and LDPE-VBC-TMA as the anode, cathode, and alkaline anion exchange membrane, respectively. The CCO–0.75 || Pt/C only required 1.74 and 1.65 V cell voltage to reach 100 mA cm-2 at 20, and 60 °C. This is 23% lower than those recorded for copper-cobalt mixed oxide thin films previously used as OER catalysts for alkaline water electrolysis.
Degree Course MS Energy Engineering
Language English
Keyword alkaline water electrolysis; Co3O4; electrocatalysis; oxygen evolution reaction
Material Type Thesis/Dissertation
Preliminary Pages
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