Exploring Glycerol Electrooxidation as an Alternative to Oxygen Evolution in Electrochemical Energy Conversion

As global awareness of the energy crisis, the environmental damage caused by fossil fuels use, and the high carbon intensity of manufacturing continues to grow, the demand for sustainable development becomes increasingly urgent. At the core of this shift is the adoption of cleaner, renewable energy sources. Numerous efforts have been made to facilitate this transition, aiming to reduce pollution via producing important chemicals without consuming fossil fuels, using clean energy (e.g., green hydrogen produced from water and renewable electricity), and converting pollutants (e.g., CO2) to valuable products. However, green hydrogen production remains limited because of its high production cost, which arises from multiple factors (i.e., engineering requirements, energy inputs, and additional transport, storage, and usage expenses). Similarly, the carbon dioxide reduction reaction (CO2RR) is still immature. Despite extensive efforts have been made to improve both water and CO2 electrolyzers efficiency, water oxidation at the anode remains one of the least addressed bottlenecks in electrolytic processes. Advanced catalysts and technologies can only help reduce the overpotential associated with the anodic reaction, but cannot fundamentally change its intrinsically energy-intensive nature (i.e., its thermodynamic reduction/oxidation potential). Therefore, replacing the oxygen evolution reaction (OER) at the anode is emerging as a compelling strategy to lower the overall energy demand while simultaneously producing valuable chemical products. The target of this dissertation is to develop and demonstrate a new approach to address the aforementioned challenges in current electrochemical systems for CO2RR and green hydrogen production by replacing OER with Glycerol ElectroOxidation Reaction (GEOR). It is noteworthy that glycerol is a biofuel industrial waste; therefore, converting glycerol to value-added chemicals can enhance both energy and economic efficiency of hydrogen production, CO2RR, and the biofuel industry. However, the development of low-cost, efficient, and selective catalysts for GEOR remains a challenge, and a significant gap still exists between laboratory research and industrial application. Pairing GEOR with different cathodic reactions (i.e., hydrogen evolution reaction or CO2RR) presents bottlenecks today but opportunities for tomorrow. This research aims to address intertwined challenges of energy, environment, and economy- a single approach to benefit all three. This thesis develops various cost-effective and stable nickel-based catalysts for GEOR and explores the feasibility of replacing OER by GEOR in GEOR-CO2RR and GEOR-HER electrolyzers for industrial applications. Besides, this work provides a thorough understanding of the in-situ transformation of the materials, interface interaction, and GEOR reaction pathway. In the first study, we designed CuO@NiBiOx (CNBO) for highly selective and efficient GEOR. This catalyst achieved nearly 100% GEOR Faradaic efficiency (FEGEOR), 80%-90% of which is conveyed into formate (FA). The incorporation of bismuth modified the electronic structure of the mixed oxide, increasing the surface concentration of Ni(III) species and enhancing the GEOR activity. In-situ studies revealed the formation of NiOOH, which is identified as the active site for GEOR and suggests an indirect GEOR mechanism. This study highlights the potential of GEOR to replace OER in CO2RR electrolyzers. Depending on the selected CO2RR catalyst (Ag or Sn), the system can produce either an easy-to-separate mixture of high-added value products (carbon monoxide, CO, and FA) or a single product (FA) with FEFA > 85% at both electrodes. In addition, the replacement of OER with GEOR was shown to reduce the overall energy input for electrolysis by up to 25%. The simultaneous generation of formate at both the anode and cathode effectively reduces the energy requirement per mole of formate production by half. Therefore, this research work successfully demonstrates the feasibility of the GEOR-CO2RR approach. In the second study, Ni-Xy catalysts (X=O, S, P, Se) were synthesized, and the effect of the heteroatoms on the active site formation and GEOR selectivity was investigated. Among all, NiSe2 exhibited superior GEOR activity, delivering 100 mA cm-2 at 1.45 VRHE, which is 150 mV lower than that required for OER at the same reaction rate. GEOR on NiSe2 achieved nearly 100% Faradaic efficiency (FEGEOR), with 95% of the charge directed towards formate production. The exceptional performance is attributed to surface reconstruction of the catalysts, including the rapid formation of γ-NiOOH, the large electrochemically active surface area, and an optimized glycerol adsorption strength. The GEOR pathway and active sites were elucidated through a combination of complementary spectroscopic and diffraction in-situ techniques. The practical feasibility of green hydrogen production in a HER/GEOR electrolyzer was demonstrated under industrially relevant conditions, i.e., zero-gap configuration anion exchange membrane electrolysis at 400 mA cm-2 and 60°C, achieving a cell voltage (Ecell) of 1.7 V with ≈ 95% FEGEOR and without parasitic OER. This research provides design principles for Ni-based catalysts for efficient and stable GEOR oxidation and underscores the potential of GEOR- hydrogen evolution strategy to simultaneously enhance both energy efficiency and economic viability of green hydrogen production via electrolysis. Overall, these findings establish GEOR as a viable replacement for OER in both GEOR-CO2RR and GEOR-HER electrolyzer cells, offering the advantage of safer and more energy-efficient chemical production. This work represents an important step toward bridging the gap between laboratory-scale GEOR studies and industrial deployment in sustainable production.