Electrochemical Production of Formic Acid / Formate By CO2 Electrolysis Using High Performance Gas Diffusion Electrodes

Abstract

Electrochemical low-temperature conversion of CO2 is a promising way to reduce GHG emissions and to valorize CO2 as a carbon-source to produce basic chemicals and/or to store excess electricity from renewable resources. The most widely studied products for these purposes are formic acid/formate, carbon monoxide and methane. In contrast to carbon monoxide and methane, aqueous formic acid/formate solutions show advantages in terms of handling, storage, toxicity and safety. The stored energy can thereby be recovered either in direct formate or formic acid fuel cells (DFFC or DFAFC), or by upstream hydrogen production from dehydrogenation and subsequent use in a H2-fuel cell. Besides the energetic use, formic acid may also serve as precursor for the decentralized production of synthesis gas as it can be decomposed to hydrogen or carbon monoxide. Formatotrophic bacteria might also use formic acid as substrate in fermentation towards higher hydrocarbons like isobutene or propane.[1] CO2 + H2O + 2e- --> HCOO- + OH- φ0=-0.665 V (vs. SHE, pH=10) Two of the biggest problems to overcome in electrochemical CO2 reduction reaction (CO2RR) is its low energetic efficiency and the sluggish reaction kinetics, which strongly limits the achievable current density. The energetic efficiency not only suffers from a high overpotential, which might be greatly reduced by state-of-the-art electrocatalysts, but also by several sources of ohmic losses throughout the whole electrolysis cell. Thus, for industrial application these resistances are to minimize by choosing an appropriate reactor design, but also by optimizing the used electrodes and reaction conditions. In order to overcome the slow reaction rates caused by the low solubility of CO2 in aqueous media, gas diffusion electrodes (GDEs) have become the number one solution. Their highly porous structure provides a huge three-phase boundary between gaseous CO2, electrolyte and catalyst. As most groups use layered GDEs with a supporting gas diffusion layer as backbone and the active component bound on top by a hydrophobic binding agent (mostly PTFE or Nafion®), we want to emphasize single-layer GDEs (SLGDE) that comprise all of these three components in one homogenous layer. SLGDEs in CO2RR have already been proven to show one of the highest current densities in literature of 400 mA cm-2 while maintaining a faradaic efficiency of at least 75 %.[2] Optimizing such SLGDEs is one of our current subject of research. It includes multiple aspects, like the catalyst’s nature, dispersion and loading [3], electrode composition, including type of carbon, porosity and hydrophobicity, as well as the extent of electrolyte intrusion during the reaction. Besides the electrodes properties, reaction conditions are of upmost importance. This includes temperature, type of electrolyte and concentration, but also controlling the electrochemical stress, especially during start-up phase. The results shown in figure 1 demonstrate the importance of these parameters. Acetylene black based gas diffusion electrodes, loaded with SnOx nanoparticles were screened for their maximum current density (defined as minimum 80 % formate faradaic efficiency for 45 min), showing a temperature depended maximum in performance at 1000 mA cm-2. This maximum results from slow intrinsic reaction kinetics and little mass transport coefficients at lower temperatures and decreasing CO2 solubility at higher temperatures. Optimizing the used electrolyte and carefully starting-up the GDE helps to further raise the performance up to 1200 mA cm-2. A comparison with values from literature (figure 1, right side) illustrates the high performance of such SLGDEs. Further investigations need to focus on transferring these results into continuous mode of operation and stabilization of the three-phase boundary for industrial relevant lifetimes.[4] [1] www.eforfuel.eu [2] D. Kopljar, N. Wagner, E. Klemm, Chem. Eng. Technol., 39 (11), 2016, p. 2042-2050. [3] D. Kopljar, A. Inan, P. Vindayer, N. Wagner, E. Klemm, J. Appl. Electrochem., 44 (10), 2014, p. 1107-1116 [4] D. Kopljar, A. Inan, P. Vindayer, R. Scholz, N. Frangos, N. Wagner, E. Klemm, Chem. Ing. Techn., 87 (6), 2015, p. 855-859 The authors would like to thank the German BMWi (Bundesministerium für Wirtschaft und Energie - support code: 03ET1037B) for the financial support. Figure 1