Study of Copper and Copper Supported on N-Doped Graphene Nanoparticles for Enhanced Electrochemical Reduction of CO2 to Ethanol

Abstract

Excessive carbon dioxide (CO2) emissions due to combustion of fossil fuels create serious impact on our atmosphere and will result in unwanted climate change and species extinction. In addition to the recent attempts to establish environmentally sustainable energy sources and to advance CO2 capture and sequestration technology, the conversion of CO2 into useful chemicals is a pragmatic approach for reducing its concentration in the atmosphere [1]. In this respect, sustainable electricity-powered electrochemical CO2 reduction (ECR) offers an exciting way because it not only transforms CO2 into useful fuels and chemicals, but also provides a solution for the long-term storage of volatile renewable energy [2,3]. Therefore, the design of efficient electrocatalysts with high selectivity (Faradaic efficiency (FE)), low overpotential, and good stability/reusability is key consideration for the development of ECR [4]. To date, copper is the only electrocatalyst that can reduce CO2 into hydrocarbons and alcohols which can be further used as fuel [1,4].For this, we have prepared metallic copper nanoparticles (Cu NPs) with and without N-doped graphene support and characterized by Scanning Electron Microscopy (SEM), High-resolution Transmission Electron Microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and X-ray Diffraction (XRD). The electrochemical experiments were performed in a customized H-type cell using 0.1 M KHCO3 electrolyte solution. Proton nuclear magnetic resonance (1H NMR) and High performance liquid chromatography (HPLC) was employed to analyze and quantify the liquid products. In the case of Cu NPs, we found ~46 % Faradaic efficiency (FE) and ~2.3 mA cm-2 current density (CD) towards Formic acid at -0.8 V vs. RHE, which would suggest that these particular Cu NPs are an ineffective catalyst for multicarbon production. While, Cu/NGN exhibit the highest selectivity for multicarbon products in the studied potential range. It gives a total 26 % FE and ~7.5 mA cm-2 CD at -1.0 V (vs. RHE) for the ethanol products. The study reveals that the structural and electronic properties of the electrode were enhanced by the addition of Cu NPs on NGN surface. In addition, reusability and long term stability is also studied. Acknowledgements The authors would like to thank Prof. P. K. Bajpai for his comments. This work was supported by Science and Engineering Research Board (SERB), Department of Science and Technology (DST), India (Grant No. EMR/2016/007437, dated March 13, 2018).