Non-Precious Metal Catalysts for the Oxygen Reduction Reaction
To further remove Fe from NPM ORR catalysts we exposed them to chlorine gas at high temperature (600- 1000 °C). Through this treatment Fe content as low as 0.16 wt% is achieved. After the demetalation with chlorine gas the ORR onset is seen to shift to more negative potentials by as much as 250 mV when tested in acidic media. Remetalation by exposure to FeCl3 results in partial recovery of ORR activity.
We also summarize our efforts examining multinuclear copper complexes inspired by laccase, a class of blue copper oxidases that efficiently mediate the four-electron four-proton reduction of dioxygen to water.(1) We first prepare Cu complexes of 2,2'-dipicolylamine (DPA) and test its electrocatalytic activity towards the ORR. To investigate the effect of multinuclearity on the ORR, we connect two DPA units together with a flexible linker and installed a third metal-binding pocket in the ligand framework (Figure 1).
The ORR onset potentials and the diffusion-limited current densities of [Cu(1)]2+ are comparable to that of [Cu(TPA)(H2O)]2+ previously reported by our group. However, the electron transfer rate between [Cu(1)]2+ and carbon electrode surface is low, indicating that more intimate coupling to the electrode may be required for efficient ORR. Covalently linking two [Cu(DPA)]2+ cores together results in ORR activities no better than [Cu(1)]2+ alone.
This study highlights the viability of Cu complexes of DPA to mimic the T3-site of laccase and serves as a guide for designing future laccase models. The relationship between inter-site flexibility and cooperativity is long recognized; it is insufficient to simply connect reactive Cu sites. To ensure cooperativity comparable to that of metalloproteins, future catalysts will require more sophisticated designs.