Optimized design and protocols eliminate power density gap between microbial fuel cells at different scales

In addition to offering a promising approach for niche applications in environmental sensing and portable power sources, microfluidic microbial fuel cells (MFCs) can also accelerate the development of mainstream energy applications through studies into fundamental mechanisms and optimization, without complications from nutrient cycling, membrane fouling, or uncontrollable concentration gradients. However, the main hurdle in leveraging microfluidic MFCs for discovery and optimization is their underperformance compared to macrosystems on certain key metrics, notably area-normalized power. To bridge this gap, we showcase a strategy that focuses on (i) technology improvements, (ii) establishment of new performance benchmarks, and (iii) presentation of a universally applicable normalization method for direct comparisons across all MFC scales and that complements areal power densities. Using a pure-culture Geobacter sulfurreducens electroactive biofilm (EAB) applied to a new system that adheres to the strategy above, we observed optimal anode colonization, resulting in the highest recorded power density for a microfluidic MFC of 3.88 W m-2 (24.37 kW m-3) and a normalized energy recovery (0.21 kWh m-3) that nearly matches the average value observed in macrosystems. With these results, the performance gap between micro- and macroscale MFCs is closed, and a road map to move forward is presented

Microbial fuel cell power overshoot studied with microfluidics: from quantification to elimination

Power overshoot can hinder determination of maximum power densities in microbial fuel cells (MFCs). In this work, a microfluidic approach was used to study overshoot in an MFC containing a pure culture of electroactive biofilms (EAB) containing Geobacter sulfurreducens. After 1-month operation under constant flow of an ideal nutrient medium, the MFC health began to degrade, marked by voltage loss and the appearance of anomalies in the power density curves. One such anomaly was a chronic power overshoot, accompanying a loss of both measured power and current density on the high-current side of the power density curve. The degree of power overshoot was quantified while certain flow-based interventions were applied, notably the shear erosion of the EAB outer layer. Next, two approaches to acclimation were demonstrated to treat the remaining overshoot. The standard approach, which acclimates the MFC to high currents before a standard polarization test, eliminated the remaining overshoot and returned maximum power densities to initial levels, but maximum current density remained lower than the initial level. A microfluidic-assisted “long-hold polarization test” enabled efficient in situ acclimation of each external resistor during the measurement. Despite the health-compromised MFC, this method provided long-term stability during the polarization test, resulting in power and current density measurements that exceeded those made on the healthy MFC using the standard polarization test. We conclude that slower electron transfer kinetics in unhealthy MFCs can provoke overshoot by prolonging the time to reach steady state during the polarization test, but a properly designed measurement overcomes this problem

A high performance membraneless microfluidic microbial fuel cell for stable, long-term benchtop operation under strong flow

Strong control over experimental conditions in microfluidic channels provides a unique opportunity to study and optimize membraneless microbial fuel cells (MFCs), particularly with respect to the role of flow. However, improved performance and transferability of results to the wider MFC community require improvements to device stability under all applied operational conditions. To address these challenges, we present an easy-to-fabricate membraneless MFC that combines i) O2 protection via a gas diffusion barrier, ii) integrated graphite electrodes, and iii) optimized electrode placement to avoid cross-contamination under all applied flow rates. Attention to all of these design features in the same platform resulted in the operation of a MFC with a pure-culture anaerobic Geobacter sulfurreducens biofilm for half a year, that is, six times longer than previously reported, without the use of an oxygen scavenger. As a result of higher device stability under high flow rates, power densities were four times higher than reported previously for microfluidic MFCs with the same biofilm