A paired alkaline electrolyzer for furfural oxidation and hydrogen evolution over noble metal-free NiFe/Ni and Co/MXene catalysts†

Producing green hydrogen via water electrolysis using renewable energy sources holds promise for a sustainable future. However, current challenges arise from the energy-intensive oxygen evolution reaction (OER) and the potential risks associated with the mixing of H₂ and O₂. To address these challenges, there has been significant emphasis on replacing the OER with more thermodynamically favorable aldehyde oxidation for the production of carboxylic acids. In this work, we combined a novel two-dimensional (2D) early transition metal carbide (MXene) supported cobalt catalyst (Co/Mo₂TiC₂-700) for the hydrogen evolution reaction (HER) and a NiFe/Ni foam fabricated by an electrodeposition method for the furfural oxidation reaction (FOR) to design a paired flow electrolyzer. In H-type half-cell tests, the NiFe/Ni foam anode catalyst exhibited a faradaic efficiency (FE) of 47% towards 2-furoic acid (2-FA) and a conversion of 95% with 50 mM furfural at 65 mA cm⁻². This FE rose to 97% at a conversion of 67% when the furfural concentration was increased to 150 mM. The optimized Co/Mo₂TiC₂-700 cathode catalyst exhibited outstanding HER performances of 100% FE towards H₂, and low overpotentials of 244 mV and 321 mV at 100 mA cm⁻² and 400 mA cm⁻², respectively. A two-electrode flow cell with 2 × 2 cm² electrodes was then assembled for simultaneous electrochemical furfural oxidation and hydrogen evolution. Remarkably, Co/Mo₂TiC₂-700 outperformed a commercial Pt/C electrode with the same loading of 0.5 mg-metal per cm² by reducing the cell voltage by 150 mV at a high current density of 300 mA cm⁻², while maintaining the FE-to-H₂ conversion at >90%. The overall FE increased from 120% to 151% at a high current density of 200 mA cm⁻², and the cell voltage dropped to 2.688 V for the electrolyzer with the Co/Mo₂TiC₂-700 catalyst, compared to 3.185 V when using Pt/C at the cathode.