Indirect electrochemical reduction of carbon dioxide to carbon nanopowders in molten alkali carbonates: Process variables and product properties Happiness V. Ijije, Chenggong Sun, George Z. Chen * Department of Chemical and Environmental Engineering, and Energy and Sustainability Research Division, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UK ARTICLE INFO Article history: Received 15 October 2013 Accepted 15 February 2014 Available online 20 February 2014 ABSTRACT Carbon was deposited on a mild steel cathode during electrolysis in the molten mixture of Li 2 CO 3 and K 2 CO 3 (mole ratio: 62:38) under CO 2 or mixed N 2 and CO 2 atmospheres at 3.0–5.0 V and 540–700 °C. In a three-electrode cell, cyclic voltammetry was applied on a plat- inum working electrode to study the reduction and deposition processes. A two-electrode cell helped correlate electrolysis variables, e.g. temperature and voltage, with the deposi- tion rate, current efficiency, and properties of the deposited carbon powders. High current efficiency (>90%) and deposition rate (>0.11 g cm 2 h 1 ) were achieved in the study. Elemen- tal analysis of the electro-deposits, following washing with HCl solutions (2.3–7.8 mol L 1 ), showed carbon as the dominant element (75–95 wt.%) plus oxygen (5–10 wt.%) and small amounts of other elements related to materials of the electrolytic cell. Thermogravimetry detected fairly low onset combustion temperatures (310–430 °C), depending on the electrol- ysis and acid washing conditions. Amorphous and various nanostructures (sheet, rings and quasi-spheres) were revealed by electron microscopy in carbon samples deposited under different process conditions. The specific surface area of the carbon deposited at 5.0 V and 540 °C was as high as 585 m 2 g 1 . An analysis of the energy consumption suggests several ways for efficiency improvement so that the electrolytic carbon from CO 2 will become commercially attractive. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction High demands for energy and materials have undoubtedly resulted in numerous concerns, primary of which is the increasing amount of released carbon dioxide (CO 2 ) and its ad- verse effect on atmospheric conditions. There have been dif- ferent approaches on tackling the increase in CO 2 emissions. Nevertheless, fossil fuels are still being used and the produc- tion of CO 2 will continue. The switch to using renewable sources of energy (e.g. wind, solar, tidal, biomass and hydro- power) have gained universal acceptance, but the develop- ment is slow, largely due to the significant cost, unsecured availability and intermittency issues. Due to these concerns, it is predicted that for decades to come, fossil fuels will still play a dominant role in the energy industry. This therefore emphasises the need to capture the CO 2 produced where pos- sible and store it safely in for example appropriate geological formations, or transform it into a useful material. Such carbon http://dx.doi.org/10.1016/j.carbon.2014.02.052 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved. * Corresponding author: Fax: +441159514115. E-mail address: george.chen@nottingham.ac.uk (G.Z. Chen). CARBON 73 (2014) 163 – 174 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon