The replacement of the traditionally applied anode half reaction, i.e., water oxidation (OER) with alternative oxidation reactions could significantly decrease the cell voltages in CO₂ electrolyzer cells in parallel with generating valuable platform chemicals. Besides the criteria typically well-addressed in (the not that numerous) studies (the reaction can be driven at considerably lower potentials compared to the OER in parallel with generating products that are more valuable than the substrate molecule) one aspect is almost always overlooked: the carbon-neutral/negative operation (from the cradle to the gate) of the complete process is ensured. Even though this is one of the most important requirements for the industrial deployment of CO₂ electrolysis technology. Quite a few reactions have been already considered from which the selective electrocatalytic oxidation of glycerol (GOR) seems to be a particularly promising direction. Unpurified (i.e., crude) glycerol (glycerol content is in between 45 to 80 wt%) can be accessed in large quantities from a relatively pure source (biodiesel/soap industry). Most of the studies in the literature use pure glycerol as the substrate even though purification of the reactant/product stream can significantly increase the carbon footprint potentially tipping out the carbon balance of the whole system.

In this study, CO2RR to CO was driven at the cathode (Ag nanoparticles), and the GOR was performed at the anode of a small (1 cm² active surface area) microfluidic electrolyzer cell utilizing crude glycerol (at least 80 wt% glycerol content) as the reactant. Due to the several impurities in crude glycerol (methanol, various (metal) ions, fatty acid residues and other organics) that can negatively affect the activity, selectivity and most importantly, the stability of the electrocatalysts, both noble metal (Pt and its bimetallic alloys), and non-noble metal based (Fe, and Ni-based single atom catalysts) electrocatalysts were screened. When pristine glycerol was replaced by crude glycerol, achievable current densities rapidly dropped for the noble metal samples along with a change in selectivity: still, a mixture of C1-C3 products formed (glycerate, glycolate, tartronate, oxalate, lactate and formate), but only 50% of the passed charge was consumed by their formation when Pt was used as the anode catalyst. Contrastingly, the major GOR product was formate (around 90% FE) in the case of all single atom catalysts regardless of the purity of the used glycerol source. Under optimal conditions, the paired CO2RR/GOR electrolyzer cell was operated for five hours at industrially relevant current densities (≈ 100 mA cm^-2) with stable CO2RR and GOR selectivity using a mixed Fe-Ni single atom catalyst as the anode and crude glycerol as the reactant.