Annual Report 2016 – Electrochemistry Laboratory
Electrocatalysis & Interfaces
CO₂ electroreduction to renewable fuels on Cu based thin films A.A. Permyakova, A. Pătru, J. Herranz, T.J. Schmidt phone: +41 56 310 2084, e-mail: anastasia.permyakova@psi.ch
In order to effectively address global warming issues, carbon dioxide emissions into the atmosphere must be reduced. Electrochemical reduction of carbon dioxide (CO₂RR) is an attractive way to utilise this greenhouse gas by converting it into useful carbon neutral fuel using renewable energy sources (wind and/or solar). That would allow not only to store excess energy, but also to eventually close the global carbon cycle [1, 2]. CO₂ combined with H₂O can be used as a feed in a co-electrolyser system and tuned to produce valuable chemical products, such as methanol and ethanol that can be used as a liquid fuel in direct methanol fuel cells (DMFCs) or in modified diesel engines. Offering promising advantages, this process has been widely studied in recent years. Despite the effort, the overall energy efficiencies and product selectivities still require improvement in order to make this process economically valuable [3]. One of the key challenges is the slow reaction kinetics that involves multiple proton-coupled electron transfer steps and is directly connected to the catalyst development. Most heavily investigated catalyst is Cu, since it can produce hydrocarbons with reasonable Faradaic efficiencies; however it is also relatively unselective and yields more than 16 different products. On the other hand, recent experimental studies on Cu electrodes have shown that higher CO₂ reduction efficiencies can be achieved by modifying these metallic electrodes [5, 6]. For example, Cu thin films prepared by electrochemical reduction of thermally grown Cu oxide (Cu₂O) layers exhibit dramatically improved selectivity and up to 50 % efficiency towards ethanol at –0.35 V [6]. The present contribution further examines the behaviour of modified Cu based thin film electrodes fabricated by reactive sputter deposition. Here, thin films are subsequently modified by heat treatment varying the temperature and exposure time. Electrochemical reduction of CO₂ is performed in 0.05 M Cs₂CO₃ using a custom made parallel plate electrochemical plate cell configuration [4]. Reaction products are analysed by gas and high-performance liquid chromatography. The crystal structure of pristine and modified electrodes is studied via grazing angle X-ray diffraction, whilst morphology by scanning electron microscopy.
(platinum foil) parallel to the working electrode. The geometric surface area of both electrodes was 1.00 cm². To separate the anodic and cathodic compartments and to prevent the oxidation of reduced CO₂ products an anion exchange membrane (Selemion® AMV, AGC Inc.) was placed in between. Both anodic and cathodic compartments contained small electrolyte volumes (2 mL and 1.8 mL, respectively) to ensure high concentration of liquid products and, therefore, decrease the detection limits. A Ag/AgCl electrode (HARVARD APPARATUS GmbH) was used as the reference. Data were converted to a Reversible Hydrogen Electrode (RHE) scale by equation: E vs. RHE = E vs. Ag/AgCl + 0.197 V + 0.0591 x pH (6.8). Before every electrolysis experiment, the solution resistance was first determined using electrochemical impedance spectroscopy by scanning from 1 MHz to 10 Hz. Before the cyclic voltammetry and chronoamperometry measurements Ohmic resistance was corrected by potentiostat set to compensate for 85 % of the measured IR drop. The gas products analysis was carried out with a gas chromatograph (GC, SRI instruments) equipped with a packed HaySep D column and a packed MolSieve 5A column. A flame ionization detector (FID) with a methanizer was used to detect hydrocarbons with He as the carrier gas. A thermal conductivity detector (TCD) was used to detect hydrogen with nitrogen as the carrier gas. An online gaseous product analysis was carried every 20 min by the GC, after CO₂ passed through the cell directly into the gas sampling loop. For all experiments gas analysis was performed at 10, 30, 50, 70 and 90 min, during the 1.5 h of continuous electrolysis. Afterwards, obtained data were averaged to obtain the faradaic efficiencies of the various products. The liquid products were analyzed by high-performance liquid chromatography (HPLC) with an UltiMate 3000 instrument from Thermo Scientific after 1.5 h of electrolysis. An Aminex HPX 87-H column (Bio-Rad) and diluted sulfuric acid (1 Mm) as the eluent were used. Both the cathode and anode electrolytes were analyzed.
Experimental Caesium carbonate (≥ 99.999 % metals basis) from Sigma Aldrich was used without further purification. Electrolyte solution was prepared with 18.2 MΩ.cm deionized water from a Millipore system. 0.05 M Cs₂CO₃ solution was bubbled with CO₂ gas until 0.1 M CsHCO₃ electrolyte solution was prepared (a pH of 6.8 was achieved). Electrochemical measurements were carried out using a Biologic SP-300 potentiostat. CO₂ electrolysis was performed at ambient pressure in a gas-tight custom-made cell made of PEEK and fitted with O-rings. The cell configuration is designed to ensure a uniform potential distribution across the surface of working electrode by placing counter electrode
Figure 1. X-ray diffraction on pristine (red) and heat treated at 300 °C for 30 min (green) and 2 h (blue) Cu thin films.
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