Autonomous Energy Harvesting and Prevention of Cell Reversal in MFC Stacks

Abstract

This study presents a novel method for avoiding cell reversal whilst optimising energy harvesting from stacked Microbial Fuel Cells (MFCs) by dynamically reconfiguring the electrical connections between them. The sequential changing of in-parallel and in-series electrical connections in an 8-MFC stack resulted in energy being transferred twice as fast into a super-capacitor avoiding cell reversal in MFCs as opposed to a fixed in-series configuration. This approach, allows for a lower internal resistance state within the stack compared to a fixed electrical configuration. This is critical in the initial stages of energy extraction from MFCs connected electrically in-series where the impedance of the capacitor is drawing high levels of current and cell reversals are likely to occur and hinder performance. Automation of electrical connections doubled the extracted power from the stack whilst halving the charging times without any cell reversal occurrence. The electrical reconfiguring of MFCs was performed by a USB-powered switch-box that modulated the stack's connections. This lead to the development of an energy autonomous switch-box circuitry powered solely by the MFC stack with negligible impact on the overall energy harvesting efficiency (i.e. above 90% Microbial fuel cells (MFCs) are bio-electrochemical systems comprising bacteria in the anode that convert organic matter into electricity. Stacking MFC units can increase the voltage and current output so as to meet the demands of most commercial electronic devices and allow further energising of complex and energy intensive circuitries.1-6 Nevertheless, a MFC responds to sudden chemical and biochemical changes such as substrate diffusion and concentration, pH, temperature, presence of oxygen, biofilm community structure, biofilm thickness and density.7-11 These factors can affect microbial communities and the overall redox potential in the anode which can further result in polarity reversals, a frequently observed phenomenon known to impede stacked MFCs. Reverse of polarity usually happens in an MFC when its internal resistance (R int ) increases rapidly 6,12,13 and many studies have focused on explaining and predicting this phenomenon.6,14-16 As a result, the least performing -reversed-MFC in a stack can cause large variations on the overall performance and efficiency.1 To date, reports suggest that this sudden change in R int is triggered either by a) substrate depletion in the anode, 14,15 b) a "heavy" external load -low R ext -connected to the stack that draws current at levels higher than the anodophillic biofilm can deliver, 1,17,18 or c) it is a case of slow kinetics in the anode or the cathode. 16,19 Research groups have attempted to develop power management systems (PMS) for tackling the issue. [20][21][22] In 2011, Pinto et al. 22 suggested that a maximum power point (MPP) matching algorithm could possibly prevent MFCs operating at values below the R int hence avoiding reversal. It was not until 2014 that Boghani et al. 20 developed a USB powered MPP ...

Three-dimensional graphene nanosheets as cathode catalysts in standard and supercapacitive microbial fuel cell

“…Hence, efforts have been directed towards the utilization of both amorphous as well as graphitized carbonaceous materials such as activated carbon (AC) [31] , [32] , [33] , [34] , [35] , carbon nanofibers [36] , [37] , [38] , modified carbon blacks [39] , [40] , graphene [41] , [42] , [43] and carbon nanotubes [44] , [45] , [46] , [47] as cathode catalysts for ORR in microbial fuel cells. These graphitized nanostructures have a great potential to be utilized as ORR catalysts or as support materials due to their high surface areas, long term stability in polluted conditions, mechanical strength and more importantly, highly electrical conductivities [48] , [49] .…”

From the lab to the field: Self-stratifying microbial fuel cells stacks directly powering lights

“…When the 4-module cascade was shifted towards the 5 and 6-module configurations, all the modules were connected in parallel to avoid cell reversal [30] , [32] , [39] . At this stage, the cascade was connected to a programmable DC electronic load (BK Precision 8500, B&K Precision Corp., USA) maintaining a constant voltage of 470 mV during 100 h. Once the current output stabilised, the modules of the cascade were electrically connected in series and directly connected to LED strips.…”

“…The main obstacle to overcome is the MFC voltage reversal phenomenon observed when electrically connecting units in series [30] , [31] . To avoid this, all successful studies so far have connected units in parallel, or connected stacks in series/parallel combinations [32] , [33] , and always employed power management systems with energy buffers such as capacitors [23] , [34] , [35] or batteries [20] , [36] . Therefore, the challenge for a battery-free MFC system is to reach sufficiently high voltages, through electrical series connections without any voltage reversal.…”

Microbial Fuel Cells, Concept, and Applications

“…Voltage reversal generally occurs due to the non-uniformity between stacked units, and it is inherent in the biological component of the system (Greenman et al (2011); Ledezma et al 2013a). It can be eliminated by an addition of a supporting anode in parallel (Kim et al 2015), controlling the inner current via ohmic resistance tuning (Logan et al 2018), or by the presence of parallel elements (Papaharalabos et al 2017) forming groups (modules) which then are connected in series. Multiple reactors can be fluidically isolated by an air gap (Ieropoulos et al 2008;Ledezma et al 2013a), overflow (Feng et al 2014), or baffles (Zhuang et al 2012) between modules to avoid direct hydraulic connections and limit losses.…”