Effect of Carbon Support on Selectivity of CO2 Electrochemical Reduction to C2H4 on Copper Nanoparticles

Abstract

The properties of supports influence the activities of carbon-supported Cu nanoparticle catalysts towards the CO2 electroreduction (CO2 RR) to hydrocarbon fuels and the reaction selectivity [1, 2]. Activity of Cu nanoparticles towards the CO2 electroreduction (CO2 RR) to hydrocarbon fuels and reaction selectivity is affected by carbon support [1, 2]. Recently, we compared electrocatalytic activities of Cu nanoparticles supported on reduced graphene oxide (RGO), single walled carbon nanotubes (SWNT) and onion-like carbon (OLC) towards CO2 reduction to CH4 and C2H4[3]. These nanostructured carbon supports share some common features, such as chemical and thermal stabilities and high specific surface areas. However, geometrical configurations for the three supports are completely different. While RGO can be represented by large, flat two-dimensional (2D) sheets, SWNTs are one-dimensional (1D) cylinders with an extremely high length-to-diameter ratio, and OLC consists of multiple zero-dimensional (0D) concentric stacked graphitic spheres. Among the three supported catalysts, Cu/OLC was found to be superior to the other two in terms of stability, activity and selectivity towards C2H4 generation. Herein, we examine the mechanism of the effect of nanostructured support on catalysts’ activity and selectivity. In addition, we determine whether supported Cu nanoparticles are durable enough to withstand potential excursions to open circuit potential (OCP), which are commonly encountered during CO2 electrolysis. Cu nanoparticles of 15-25 nm diameters on different carbon supports were synthesized by the reduction of CuCl2 using NaBH4 in suspensions of carbon supports in aqueous ethylene glycol solutions (20% v/v). X-Ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-Ray photoelectron spectroscopy (XPS) were used for ex-situ analysis of the supported nanoparticles. The catalytic activities of supported Cu nanoparticles and their selectivities towards hydrocarbon generation were evaluated using (1) a sealed rotating disk electrode (RDE) setup connected to a gas chromatograph (GC) and (2) a scanning flow cell (SFC) coupled to differential electrochemical mass-spectrometer (DEMS) [4]. Durability of supported Cu nanoparticles was evaluated using SFC connected to inductively coupled plasma mass spectrometer (ICP-MS) setup [5]. Fig. 1 a-d displays Faradaic efficiencies (FEs) as a function of applied potential for the three catalysts and Cu nanoparticles supported on vulcan carbon (VC) used as a reference. At -1.8 V, the FE of Cu/OLC catalyst towards C2H­4 production (Fig. 1 c) reaches 60%, which is one of the highest FEs reported to date. We propose that enhanced selectivity of the Cu/OLC catalyst towards the C2H4 production is due to the ability of the OLC support to reduce CO2 to CO more efficiently than the other two supports. The larger quantity of CO molecules generated on surfaces of OLCs dimerize on Cu NP (100) planes, and, subsequently, yield additional C2H4 molecules. An important metrics of an electrocatalyst catalyst is its stability under exposure to OCP. Fig. 2 demonstrates Cu susceptibility to dissolution under constant potential hold and its excursion to anodic potentials within the OCP range. At a reductive potential of -0.6 V, insignificant Cu dissolution is observed for the four catalysts triggered by reduction of Cu oxides. When potential becomes more positive, but not greater than 0.45 V, Cu is stabilized. At ~ 0.45 V, Cu dissolution current increases abruptly, demonstrating Cu anodic dissolution. Dissolution is similar for all four catalysts, which may be due to a local saturation by the products of dissolution. This implies that exposure of Cu-based cathodes of CO2 electrolyzers to OCP will lead to Cu dissolution.