Density Functional Theory-Based Predictions and Experimental Evaluations of Ferrocene Derivatives Considered as Mediator for Anodic Catalysts of Glucose and Oxygen Enzymatic Biofuel Cells

The redox potential (ERedox) of a ferrocene (Fc) derivative differs, depending on its functional group. In this study, the various Fc derivatives are considered as mediators of anodic catalysts to promote glucose oxidation reaction (GOR) in glucose/oxygen enzymatic biofuel cells (EBFCs). Initially, their lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energies are calculated using density functional theory to predict their ERedox pattern. According to the calculations, the LUMO and HOMO energies of Fc derivatives combined with electron-donating groups (EDGs) are higher than those of Fc derivatives combined with electron-withdrawing groups (EWGs), including the results that Fc(NH2) has the highest molecular orbital (MO), while Fc(CHO) has the lowest. To verify the prediction for ERedox pattern, electrochemical evaluations are conducted. When glucose is provided, the onset potential (EOnset) of GOR is measured, while the ERedox of Fc derivatives and EOnset of GOR are linearly proportional to each other ( $R^2=0.98$ ) and DFT calculations. As the energies increase, the above two potentials are shifted more negatively. More specifically, the ERedox of Fc(NH2) and EOnset of GOR show the most negative values at (−0.112 and −0.17) V, respectively, while the ERedox of Fc(CHO) and EOnset of GOR show the most positive values at (0.496 and 0.40) V, respectively. Then, when this correlation is adopted for EBFCs, EBFCs using Fc(NH2) combined with EDG show 3.7 times higher maximum power density than those using Fc(COOH), as representatives combined with EWG. Based on that, it is well established how DFT and electrochemical evaluations should be used to design anodic catalysts including Fc derivatives as mediators for GOR.