Structure and dynamics of microbial fuel cell catalyst layer

Abstract

Molecular dynamics method is used to investigate the mass transfer rules of reactants H3O+ and O2 in the cathode catalyst layer in the presence of five kinds of cations (Na+, K+, NH4+, Mg2+, Ca2+) in the microbial fuel cell solution environment. Structural characteristics of the catalyst layer Pt/C substrate/Nafion/solution three-phase interface are also analyzed. The results show that the transport mechanism and pathway of H3O+ in the catalyst layer are similar to those of monovalent cations (Na+, K+, NH4+). The minimum diffusion coefficient of H3O+ appears in the presence of K+. On the other hand, H3O+ mainly transport inside the water clusters in the presence of divalent cations (Mg2+, Ca2+). The diffusion law of metal cations (K+> Na+> Mg2+> Ca2+) is still applicable in the catalyst layer containing Nafion ionomer and unaffected by cation concentration. In general, the higher concentration of O2 molecules in the main part of the Nafion phase and the farther distribution from the carbon substrate cause a larger O2 diffusion coefficient. Moreover, the O2 transport pathways in the main part of the Nafion phase are along the Nafion hydrophilic/hydrophobic phase interface and inside the hydrophobic phase. In the presence of Ca2+, the concentration of O2 near the Pt particles is the highest. These O2 molecules tend to adsorb on the surface of Pt particles with small size. Thus, the catalyst utilization rate is high. In addition, Ca2+ has a strong cross-linking effect on the sulfonic acid groups from different Nafion molecules, which helps to enhance the stability of the catalyst layer.