(Photo)electrochemical upgrading of glycerol: from thermodynamics to device development

This thesis explores the glycerol oxidation reaction (GOR) as an alternative anodic pathway in photoelectrochemical (PEC) systems, advancing both fundamental understanding and practical implementation. GOR offers a compelling opportunity for the valorisation of glycerol, a major byproduct of biodiesel refining, while lowering the energetic cost of any electrochemical process that is coupled with it, as for example hydrogen production, CO2 electroreduction and ammonia synthesis. A rigorous thermodynamic analysis reveals that, although most GOR pathways are endergonic, they remain up to 96% less energy demanding than OER, establishing GOR as a superior anodic partner for hybrid electrolysis or the coupling with other cathodic reactions such as CO2RR and N2RR. However, this study also demonstrates that thermodynamics alone cannot dictate selectivity toward desired C₃ products such as dihydroxyacetone, glyceraldehyde, and glyceric acid. Overall, the effect of temperature on selectivity was found to be minor, as the energetics of both desired and undesired side reactions scale in a similar manner. Among all studied parameters, pH and applied potential exert the strongest influence on the product distribution. Alkaline conditions favour the formation of glycolic acid, formic acid, and CO₂, while acidic media promotes the oxidation of aldehydes to their corresponding carboxylic acids, as well as the further oxidation of formic acid to CO₂. Conversely, increasing the applied potential enhances multi-electron transfer processes, with the direct oxidation of glycerol to glyceric acid and formic acid emerging as the most favoured reaction. To overcome the limitations on selectivity, imposed by the thermodynamics of the reaction, the concept of electrolyte-driven selectivity is introduced. Here, borate buffer stops being only the supporting electrolyte that increases the conductivity of the solution but plays an active role by forming coordination complexes with glycerol. These complexes are shown to stabilize C₃ intermediates, enabling a shift in product distribution away from low value C₁–C₂ species. These finding positions electrolyte design as a powerful and underexplored lever in PEC-GOR, complementing traditional catalyst-focused approaches. At the materials level, Co doped BiVO₄ emerges as a stable and efficient photoanode, proving the feasibility of integrating complexation chemistry with PEC operation. Finally, device-scale analyses demonstrate that the physicochemical properties of glycerol, particularly its density and viscosity, can induce transport instabilities and crossover in membraneless photoelectrochemical (PEC) systems. In addition, both the device geometry and electrolyte composition strongly influence ohmic losses and the overall system performance. Through 3D simulations, this work proposes design strategies to mitigate these effects, including the use of matched electrolyte compositions, optimized flow conditions, and compact reactor architectures. Together, these contributions establish PEC-GOR as more than a laboratory curiosity: they chart a path toward a scalable, selective, and energy-efficient technology for sustainable biomass valorisation. By uniting thermodynamic insight, coordination chemistry, and device engineering, this thesis provides a framework that redefines the role of anodic reactions in photoelectrochemistry and opens new avenues for the integration of renewable energy and green chemistry.