Tunable Syngas Formation at Industrially Relevant Current Densities via CO2 Electroreduction and Hydrogen Evolution over Ni and Fe-derived Catalysts obtained via One-Step Pyrolysis of Polybenzoxazine Based Composites.

Simultaneous electroreduction of CO2 and H2O to syngas can provide a sustainable feed for established processes used to synthesize carbon-based chemicals. The synthesis of MOx/M-N-Cs (M = Ni, Fe) electrocatalysts reported via one-step pyrolysis that shows increased performance during syngas electrosynthesis at high current densities with adaptable H2/CO ratios, e.g., for the Fischer-Tropsch process. When embedded in gas diffusion electrodes (GDEs) with optimized hydrophobicity, the NiOx/Ni-N-C catalyst produces syngas (H2/CO = 0.67) at -200 mA cm-2 while for the FeOx/Fe-N-C syngas production occurs at ≈-150 mA cm-2. By tuning the electrocatalyst's microenvironment, stable operation for >3 h at -200 mA cm-2 is achieved with the NiOx/Ni-N-C GDE. Post-electrolysis characterization revealed that the restructuring of the catalyst via reduction of NiOx to metallic Ni NPs still enables stable operation of the electrode at -200 mA cm-2, when embedded in an optimized microenvironment. The ionomer and additives used in the catalyst layer are important for the observed stable operation. Operando Raman measurements confirm the presence of NiOx during CO formation and indicate weak adsorption of CO on the catalyst surface.

Intrinsically Conductive and Cu-Functionalized Polymer-Composite Membranes as Gas Diffusion Electrodes for CO2 Electroreduction.

We introduced a new class of gas diffusion electrodes (GDEs) with adjustable pore morphology. We fabricated intrinsically conductive polymer-composite membranes containing carbon filler, enabling a pore structure variation through film casting cum phase separation protocols. We further selectively functionalized specific pore regions of the membranes with Cu by a NaBH4-facilitated coating strategy. The as-obtained GDEs can facilitate the electrochemical CO2 reduction reaction (CO2RR) at Cu active sites that are presented inside a defined and electrically conductive pore system. When employing them as free-standing cathodes in a CO2 flow electrolyzer, we achieved >70% Faradaic efficiencies for CO2RR products at up to 200 mA/cm2. We further demonstrated that deposition of a dense Cu layer on top of the membrane leads to obstruction of the underlying pore openings, inhibiting an excessive wetting of the pore pathways that transport gaseous CO2. However, the presentation of Cu inside the pore system of our novel membrane electrodes increased the C2H4/CO selectivity by a factor of up to 3 compared to Cu presented in the dense layer on top of the membrane. Additionally, we found that gaseous CO2 could still access Cu in macropores after wetting with electrolyte, while CO2RR was completely suppressed in wetted nm-scale pores.