Decarbonizing Industrial Biodiesel Wastewater Treatment Through Electrified Glycerol Oxidation Coupled to Waste CO₂ Conversion into Methanol

<b>Introduction</b><br/>Biodiesel has huge potential to displace petrochemical fuels but its production generates a large amount of wastewater that challenges the economic viability of the whole process. Glycerol is the major component of biodiesel wastewater and can be electrochemically oxidized at low redox potentials, enabling plants to efficiently co-valorize abundant wastewater and CO₂ into useful products. We explored the catalytic activity of non-noble metallic Ni as an anodic material and elucidated interfacial fouling during short- and long-term operations for sustainable waste glycerol valorization. This study will provide insight into fit-for-purpose modification strategies of Ni for glycerol oxidation based on a detailed understanding of the anodic behavior of Ni.<br/><b>Materials and Methods</b><br/>Synthetic crude glycerol as an anolyte was constructed to simulate a real crude glycerol matrix from biodiesel production. All electrochemical experiments and analyses were performed using a potentiostat in a flow cell with two chambers separated by an anion exchange membrane. Commercial Ni foam, Pt gauze, and Hg/HgO electrode were used as an anode, cathode, and a reference electrode, respectively. Liquid products were analyzed with HPLC and IC. FTIR was adopted to characterize the anodic surface.<br/><b>Results and Discussion</b><br/><i>Catalytic activity of Ni in an alkaline medium - </i>Cyclic voltammetry (CV) of Ni foams was performed in different solutions. In 0.3 M KOH, NiOOH formation followed by oxygen evolution reaction (OER) was observed in the forward scan while NiOOH reduction to NiOH₂ was found in the reverse scan. When 0.1 M glycerol was co-present, a surge of current due to glycerol oxidation reaction (GOR) was found in the forward scan. The absence of the NiOOH reduction peak in the reverse scan confirmed the characteristic catalytic behavior of Ni for GOR in an alkaline condition. Importantly, GOR was found to be a promising alternative to OER saving approx. 300 mV in the potential window of CV.<br/><i>Ni with high selectivity toward formate -</i> Anodic valorization of glycerol was performed for 30 min at varying anodic potentials ranging from 500 ~ 900 mV in a solution of 0.3 M KOH and 0.1 M glycerol. High Faradaic efficiency (73 - 77%) and selectivity (~ 90%) toward formate at 500 - 700 mV emphasized that Ni may not need further modification when formate is a target value-added product.<br/><i>Sign of surface accumulation during GOR -</i> After short-term GOR operation at different anodic potentials, Ni foams were recovered, rinsed with deionized water, and analyzed for CV in 0.3 M KOH (i.e., no external glycerol addition). All used Ni generated oxidation currents larger than Ni oxidized only in 0.3 M KOH without glycerol, indicating surface coverage initiation by glycerol and/or intermediates.<br/><i>GOR efficiency worsened by surface deposits - </i>GOR operation was extended to 24 hrs to examine the impact of surface accumulation. Glycerol concentration continuously declined but charge consumption estimated based on it substantially surpassed actual charge consumption, suggesting a non-electrochemical glycerol loss. FTIR spectrum progression suggested surface inactivation initially driven by intermediates followed by glycerol.<br/><b>Conclusion</b><br/>Non-noble metallic Ni without any modifications can be used as an anode with a high formate selectivity for glycerol oxidation. However, surface deposition may originate from the rate-limiting intermediate oxidation such as glyceraldehyde and glycerate. Ni surface covered by these intermediates can significantly hinder catalytic GOR by promoting glycerol adsorption on the surface irrelevant to GOR. Therefore, Ni modification to alleviate bottle-necking intermediate reaction steps may be a key for stable glycerol valorization.