Theory of Ion and Electron Transport Coupled with Biochemical Conversions in an Electroactive Biofilm

TL;DR

This paper develops and validates a dynamic model of ion, electron, and biochemical transport in biofilms of microbial fuel cells, revealing insights into limiting factors and pH effects on current production.

Contribution

It introduces a physics-based model of coupled transport and biochemical reactions in biofilms, enhancing understanding of bioelectrochemical system performance.

Findings

Ion transport is not limiting at typical wastewater conditions.

Deep biofilm pH drops significantly, potentially inhibiting current.

Biofilm electronic conductivity does not significantly limit charge transport.

Abstract

Bioelectrochemical systems are electrochemical cells that rely on conductive biofilms covering an electrode. We consider the example of a microbial fuel cell, and we derive a dynamic model of ion transport, biochemical reactions and electron transport inside such a biofilm. After validating the model against data, we evaluate model output to obtain an understanding of the transport of ions and electrons through a current-producing biofilm. For a system fed with a typical wastewater stream containing organic molecules and producing 5 A/m$^{2}$, our model predicts that transport of the organic molecules is not a limiting factor. However, the pH deep within the biofilm drops significantly, which can inhibit current production of such biofilms. Our results suggest that the electronic conductivity of the biofilm does not limit charge transport significantly, even for a biofilm as thick as 100 $\mu\mathrm{m}$. Our study provides an example of how physics-based modelling helps to understand complex coupled processes in bioelectrochemical systems.