Challenges and Constraints of Using Oxygen Cathodes in Microbial Fuel Cells

Abstract

The performance of oxygen reduction catalysts (platinum, pyrolyzed iron(ll) phthalocyanine (pyr-FePc) and cobalt tetramethoxyphenylporphyrin (pyr-CoTMPP)) is discussed in light of their application in microbial fuel cells. It is demonstrated that the physical and chemical environment in microbial fuel cells severely affects the thermodynamics and the kinetics of the electrocatalytic oxygen reduction. The neutral pH in combination with low buffer capacities and low ionic concentrations strongly affect the cathode performance and limit the fuel cell power output. Thus, the limiting current density in galvanodyanamic polarization experiments decreases from 1.5 mA cm(-2) to 0.6 mA cm(-2) (pH 3.3, E(cathode) = 0 V) when the buffer concentration is decreased from 500 to 50 mM. The cathode limitations are superposed by the increasing internal resistance of the MFC that substantially contributes to the decrease of power output. For example, the maximum power output of a model MFC decreased by 35%, from 2.3 to 1.5 mW, whereas the difference between the electrode potentials (deltaE = E(anode) - E(cathode)) decreased only by 10%. The increase of the catalyst load of pyr-FePc from 0.25 to 2 mg cm(-2) increased the cathodic current density from 0.4 to 0.97 mA cm(-2) (pH 7, 50 mM phosphate buffer). The increase of the load of such inexpensive catalyst thus represents a suitable means to improve the cathode performance in microbial fuel cells. Due to the low concentration of protons in MFCs in comparison to relatively high alkali cation levels (ratio C(Na+,K+)/C(H+) = 5 x E5 in pH 7, 50 mM phosphate buffer) the transfer of alkali ions through the proton exchange membrane plays a major role in the charge-balancing ion flux from the anodic into the cathodic compartment. This leads to the formation of pH gradients between the anode and the cathode compartment.

Impact of Fe(III) as an effective electron-shuttle mediator for enhanced Cr(VI) reduction in microbial fuel cells: Reduction of diffusional resistances and cathode overpotentials

“…At β < 0.5 the activation energy to bring the electron acceptor to an activated state, required for exchanging electrons with the cathode, is very high [36]. Such low value of β observed in the absence of Fe(III), demonstrates an equivalent difficulty, as for the reduction of oxygen [36], in performing Cr(VI) reduction, despite the higher standard redox potential of 1.33 V for Cr(VI) reduction and 1.23 V for oxygen reduction [38]. The highest β of 0.37 together with an improvement of 70% in the exchange current density was obtained at the highest Fe(III) dosage of 150 mg/L (Table 2).…”

Effects of current generation and electrolyte pH on reverse salt flux across thin film composite membrane in osmotic microbial fuel cells

“…2A). A higher catholyte pH would create more potential loss (0.059 V/pH), resulting in lower electricity generation (Zhao et al, 2006). The lower electricity generation would have less inhibition of RSF.…”

Sequential anode–cathode configuration improves cathodic oxygen reduction and effluent quality of microbial fuel cells

“…Nitrate can be reduced to N 2 in biocatalysed cathodes (Clauwaert et al, 2007), which enable removal of this nutrient aside from the anode-associated organic carbon removal. For energy generation, oxygen is the preferred electron acceptor due to its abundant availability and high redox potential (Zhao et al, 2006). MFCs that use oxygen as electron acceptor suffer from some severe limitations: (1) the high cathodic overpotential (i.e.…”

“…MFCs that use oxygen as electron acceptor suffer from some severe limitations: (1) the high cathodic overpotential (i.e. drop of cathodic potential due to the activation energy of the reaction) of oxygen reduction at the cathode limits the energetic efficiency (Zhao et al, 2006); (2) the inability of any currently available CEM to selectively transfer protons causes a pH gradient, acidic at the anode and alkaline at the cathode (due to the fact that one proton is produced/ consumed per electron transferred, respectively), reducing the thermodynamic potential difference between anode and cathode and causing an inhibition of anodophiles (Rabaey et al, 2003); (3) due to the incomplete chemical oxygen demand (COD) removal often experienced at the anodes , the addition of an extra polishing step downstream of the MFC anode is required. The high cathodic overpotential of O 2 reduction on graphite can be reduced by the use of chemical catalysts, such as platinum (Cheng et al, 2006), transition metal porphyrins and phtalocyanines (Zhao et al, 2005) and ferric ions at low pH (Ter Heijne et al, 2006).…”