In-Situ Electrodeposition of Nickel on a Biocathode to Enhance Methane Production from Carbon Dioxide in a Microbial Electrosynthesis System

Abstract

Power-to-gas (P2G) technologies convert electrical energy into renewable gaseous fuels or value-added molecules to be used for transportation, heat production, or as a feedstock for the chemical industry, respectively. Microbial Electrosynthesis Systems (MES) use electricity and microorganisms to produce such energy carriers, such as hydrogen (H2) or methane (CH4), Figure 1a. Also known as Bioelectrochemical power-to-gas (BEP2G), MES combines CO2 sequestration, renewable energy carrier production, and energy storage. The BEP2G technology presents several advantages such as i) single-step CO2 conversion into CH4, so that no separate process is needed to generate H2 for the reduction of CO2, ii) bioelectrochemical reactions take place at ambient conditions, which translates into lower costs of materials and of energy consumption, and iii) higher CH4 yields can be achieved, when compared to conventional (electro)catalytic reactions.In this study, a continuous conversion of CO2 to CH4 in a laboratory scale membraneless Microbial Electrosynthesis System (MES) is achieved. The MES cell (Figure 1b) for CH4 production was built using Plexiglass plates assembled to create two compartments divided by two layers of electrically insulating Nylon cloth, with a total area of 96 cm2 (anode-cathode interface area). The cathode compartment consisted of a stainless steel mesh (current collector) and it was filled with 45 g of carbon felt while the anode consisted of a Ti/IrO2 mesh current collector and it was filled with 20 g of carbon felt. The cathode compartment was maintained at a constant temperature of 30°C and continuously supplied with CO2 and a solution of nutrients. The cathode compartment was initially inoculated with 20 mL of homogenized anaerobic sludge. To further increase CH4 production, nickel was (in-situ) electrodeposited on the cathode carbon felt.The introduction of Ni at 0.2 g L-1 into the influent exhibited a remarkable improvement in CO2 conversion. At an applied voltage of 2.8 V, the MES with the Ni-modified carbon-felt cathode accomplished a CH4 production rate of 2.3 L d-1 which was 30% higher than that of this system before Ni was introduced to the influent stream (1.6 L d-1), Figure 1c. Similar improvements were noted with other performance parameters, such as the Coulombic efficiency and the energy consumption, which considerably improved from 56% to 73% and 11.5 Wh LH 2 −1 to 7.4 Wh LH 2 −1, respectively. Furthermore, once dissolved Ni was removed from the influent solution, the performance remained unchanged suggesting its successful in-situ electrodeposition on carbon felt. Electrochemical characterization using EIS and cyclic voltammetry techniques confirmed Ni electrodeposition. These results demonstrated a great potential of using in-situ Ni electrodeposition to improve catalytic properties of the MES biocathode. Also, with respect to the energy storage capacity of the system, a progressive increase of the MES internal capacitance from 2.6 F to 3.7 F was observed, suggesting that electroactive biofilms can be used to develop bioelectrochemical supercapacitors.It can be concluded that bioelectrochemical conversion of CO2 to CH4 can be exploited as a potential technology for long-term (power-to-gas conversion) and short term (bio-capacitor) energy storage combined with CO2 sequestration. This approach could be seen as a pathway for MES integration with the existing industrial processes producing significant amounts of CO2 as well as with new technologies based on the biorefinery concept.Figure 1: a) The concept of Microbial Electrosynthesis System (MES) for CH4 production; b) schematic representation of a MES cell for CH4 production from CO2, and c) summary of the results obtained before and after the in situ electrodeposition of nickel at the cathode. Figure 1