The Impact of Nife-Based Metal Alloy on CO2 Conversion to CH4 and Carboxylic Acids in a Microbial Electrosynthesis Cell

Abstract

The goal of this study was to evaluate the impact of NiFe-based metal alloys on the CO2 conversion to carboxylic acids and methane (CH4) in CO2-fed Microbial Electrosynthesis (MES) cells. First, the impact of transition metal alloys on CO2 conversion and product specificity was studied in a MES cell with a conductive polymer cathode with electrodeposited NiFeBi alloy. It was found that the presence of the NiFeBi alloy significantly decreased production of CH4 from 0.5 to 0.1 L (Lc d) – 1, suggesting that methanogenic activity was suppressed in the presence of NiFeBi. On the other hand, the production of carboxylic acids consisting mainly of acetate, propionate and butyrate increased. Specifically, acetate production, which represented 80% of the total carboxylic acids in the cathode liquid increased by 70% to 1.0 g(Lc d)-1. This initial study demonstrated that product selectivity can be influenced by electrodeposition of transition metal alloys such as NiFeBi. It might be preferable to achieve selective production of high value long chain carboxylic acids such as valerate and caproate instead of acetate.Accordingly, in the following experiments NiFeMn and NiFeSn alloys were electrodeposited on carbon felt cathodes and evaluated for enhanced CO2 conversion in MES cells. Both alloys enhanced CH4 production, which reached 0.8 L (Lc d)-1. However, there was no observable improvement in acetate production (0.2 – 0.5 g (Lc d)-1) or other higher chain carboxylic acids in comparison to NiFeBi – coated MES. It was suggested that by using a more biocompatible carbon-based support for alloy electrodeposition, it is possible to improve acetate, butyrate and caproate production. In addition, it is important to facilitate nutrient transport through the three-dimensional (3D) cathode. Limited transport of CO2, H2 and nutrients was identified as a potential rate-limiting factor when using carbon felt electrode. 3D-printed conductive polymer lattices were manufactured and electrodeposited with NiFeSn and NiFeMn alloys. The NiFeMn-coated lattice showed insignificant improvement in higher chain carbon production, however caproate concentration was increased five folds on NiFeSn-coated cathode. Also, electrochemical characterization demonstrated increased hydrogen evolution reaction rate over time. Nevertheless, the throughput of this product remained low at 0.1 g (Lc d)-1).These results indicate that the presence of the NiFe-based metal alloys significantly influenced the electron transfer efficiency from the cathodes to microbial electroactive biofilms leading to increased formation of carboxylic acids in the cathode liquid. Therefore, the next step is to design more efficient 3D conductive polymer lattice electrodeposited with NiFeSn metal alloy to increase CO2 conversion to longer chain carboxylic acids.