Structure and dynamics of microbial fuel cell catalyst layer

Molecular dynamics method is used to investigate the mass transfer rules of reactants H ₃ O ⁺ and O ₂ in the cathode catalyst layer in the presence of five kinds of cations (Na ⁺ , K ⁺ , NH ₄ ⁺ , Mg ²⁺ , Ca ²⁺ ) 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 H ₃ O ⁺ in the catalyst layer are similar to those of monovalent cations (Na ⁺ , K ⁺ , NH ₄ ⁺ ). The minimum diffusion coefficient of H ₃ O ⁺ appears in the presence of K ⁺ . On the other hand, H ₃ O ⁺ mainly transport inside the water clusters in the presence of divalent cations (Mg ²⁺ , Ca ²⁺ ). The diffusion law of metal cations (K ⁺ > Na ⁺ > Mg ²⁺ > Ca ²⁺ ) is still applicable in the catalyst layer containing Nafion ionomer and unaffected by cation concentration. In general, the higher concentration of O ₂ molecules in the main part of the Nafion phase and the farther distribution from the carbon substrate cause a larger O ₂ diffusion coefficient. Moreover, the O ₂ 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 Ca ²⁺ , the concentration of O ₂ near the Pt particles is the highest. These O ₂ molecules tend to adsorb on the surface of Pt particles with small size. Thus, the catalyst utilization rate is high. In addition, Ca ²⁺ 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.