The influence of Cu2-xS nanoparticles on the electroreduction of CO2

Abstract

With CO2 playing a profound role in climate change, valorizing CO2 is receiving increasing attention. A promising route is to use CO2 as a feedstock to produce hydrocarbons by CO2 Reduction Reaction (CO2RR). Previous research showed the formation of various C1 and C2 hydrocarbons through CO2RR using Cu catalysts, albeit with relatively low activity and low selectivity. A potential strategy to achieve higher selectivity is to introduce p-block elements into the Cu crystal since these may participate in bond formation at the surface, thus circumventing scaling relations. In this work CuS and Cu2S nanoparticles were scrutinized for CO2RR. These nanoparticles were synthesized by incipient wetness impregnation and heating up synthesis. This produced graphite nanoplatelets (GNP) supported Cu2-xS nanoparticles and colloidal Cu2-xS nanoparticles respectively. X-ray diffraction showed monophasic CuS/GNP and Cu2S/GNP nanoparticles. Transmission electron microscopy revealed for both colloidal and impregnated samples that Cu2S nanoparticles favored spherical morphology. The diameter of Cu2S/GNP nanoparticles was 14.5±2.5 nm and the diameter of colloidal Cu2S nanoparticles was 10.2±1.0 nm. CuS favoured the formation of nanoplatelets. The CuS/GNP nanoparticles were 43.7±8.9 nm long and 15.2±8.9nm wide. The colloidal CuS nanoparticles were 20.7±2.7 nm long and 8.7±1.4 nm wide. After synthesis, the nanoparticles were deposited on carbon paper substrates. Scanning electron microscopy verified the homogeneous dispersion of individual Cu2S and Cu2S carbon supported nanoparticles. These Cu2-xS loaded electrodes were then electrochemically evaluated for CO2RR. Cyclic voltammetry revealed the immediate reduction of both CuS/GNP and colloidal CuS nanoparticles to presumably another Cu2-xS phase under production of H2S(g). For both Cu2S/GNP and colloidal Cu2S this reduction was less apparent. However, in-situ x-ray absorption fine structure spectroscopy suggested the partial reduction of CuS/GNP to Cu2S/GNP and metallic Cu(s) at -1.2V vs RHE. Cu2S/GNP seemed to partially reduce to Cu/GNP. Gas chromatography and high-performance liquid chromatography were employed to analyze gaseous and liquid product selectivity respectively. Both Cu2-xS/GNP catalysts showed the production of formate with approximately 12% faradaic efficiency at -0.9V vs RHE with 4.28 μmol/h formate production for Cu2S/GNP and 2.72 μmol/h formate production for CuS/GNP. The remainder of faradaic efficiency was attributed to hydrogen production. It could not be established to what extent the Cu2-xS nanoparticles were active for HER due to the high activity of GNP for HER. No HPLC for CO2RR on colloidal Cu2-xS nanoparticles was performed at -0.9V vs RHE to observe formate production, but the selectivity for HER with both CuS and Cu2S was minor.