Online Analysis of CO2 Production in Electroactive Biofilms by Differential Electrochemical Mass Spectrometry

Electroactive biofilms are routinely characterized in-operando by dynamic electrochemical measurement techniques such as cyclic voltammetry or electrochemical impedance spectroscopy. Since electrical signals can be recorded and processed very quickly, these techniques allow to investigate slow and fast electron transfer processes. In contrast, the dynamics of species production rates are usually not addressed because standard measurement techniques for the quantification of reaction products such as gas chromatography are slow. Instead it is often assumed that species production rates are either directly proportional to the current - under so called turnover conditions - or equal zero - under so called non-turnover conditions. To challenge this assumption, we measured species production rates of a biofilm electrode with a high time resolution by differential electrochemical mass spectrometry (DEMS). An acetate oxidizing biofilm electrode was placed just micrometers away from the mass spectrometer inlet in which enabled us to observe CO2 production directly at the electrode during cyclic voltammetry (CV) and potential steps. The measurement results showed that the CO2 production deviates significantly from the expected value calculated from the current by Faraday’s law under certain operating conditions. We analyze this effect in detail and show that it can be explained with biofilm storage capacities for charge and substrate. These capacities are quantified by deconvoluting the faradaic and non-faradaic currents. [1] Also, the onset of the complete oxidation of acetate to CO2 during CVs was determined to be just 22 mV above the standard potential for acetate oxidation. Determining this value by directly measuring CO2 instead of current is advantageous because capacitive effects can be excluded. [1] In conclusion, we demonstrate that electrical current and CO2 production can be partly decoupled in biofilm electrodes and that DEMS is a valuable technique for analyzing processes in such electrodes. [1] Kubannek, F., Schröder, U., Krewer, U. (2018). Revealing metabolic storage processes in electrode respiring bacteria by differential electrochemical mass spectrometry. Bioelectrochemistry, 121, 160–168, doi: 10.1016/j.bioelechem.2018.01.014