Capacitive Anode Materials For Application In Microbial Fuel Cells

Microbial fuel cell (MFC) represents a promising technology that extracts electric energyfrom the organic compounds available in wastewaters. However, the much lower poweroutput of MFC is a significant bottleneck in hurdling its practical application as a sustainablepower source. The sluggish extracelluar electron transfer from bacteria to anode is animportant factor limiting the anode performance and thus the power output. Our first effortwas made to improve the efficiency in electron transfer from the bacteria to the anode byusing modified anode materials. The results showed that the use of capacitive materials suchas RuO2and polypyrrole/graphite oxide (PPy/GO) modified on the graphite felt anodesresulted in a substantial increase in the maximum power density, with the value of3080and1326mW/m2, increased by17times and7times in comparison with that obtained with theunmodified anode, respectively. Such increase was considered to be a consequence of theenhanced electron transfer, which was reflected by the increase in anode surface area, thedecrease in anode resistance, and the increase in the number of bacteria attached to the anode,as evidenced from the cyclic voltammetry (CV), electron impedance spectroscopy (EIS), andscanning electron microscopy (SEM) tests.In recent years, considerable attention has been paid towards the development ofmodified carbon-based anode materials in order to facilitate the anode electron transfer.However, most of the current studies did not realize that characteristics of these materials areindeed desirable for the development of electrode materials in supercapacitors. Moreover,when these capacitive materials are utilized in the anode, an overestimate of MFCperformance may occur due to the contribution of anode charging and discharging to thepower measured by the commonly-used varying circuit resistance (VCR) or linear sweepvoltammetry (LSV) method. Our second effort was made to identify the effect of anodecapacitance on transient power and stationary power of an MFC. The results showed thatnoticeable transient power was recorded when the VCR or LSV method was chosen for powermeasurement with respect to the MFCs equipped with PPy/GO modified graphite felt anodes.Calculations on the contribution of different sources to the measured maximum power densityshowed that the discharge of bio-electrons stored in the high-capacitance anode was a dominant contributor, especially when the time duration (for the VCR method) was notsufficiently long or the scan rate (for the LSV method) was not sufficiently low.By taking the advantages of the pseudo-capacitive materials which exhibit theelectron-storage capability, our third effort was made to demonstrate the concept of using theMFC with the capacitive materials such as PPy/9,10-anthraquinone-2-sulfonic acid sodiumsalt (AQS) composite films and RuO2nanoparticles as a biocapacitor, able to storebioelectrons generated from microbial oxidation of substrate and release the accumulatedcharge during the discharge experiment. The results showed that increasing anode capacitanceis responsible for the increases in the amount of electrons stored and released, and therebyleading to more energy stored and average power dissipated. Our findings suggest that theMFC anode incorporating pseudo-capacitive materials shows potential for storing energyfrom waste organic matter and releasing in a short time of high power to the electronic device.