Non-Precious Metal Catalysts for the Oxygen Reduction Reaction

We report on our efforts to characterize the active site of Fe-based non-precious metal (NPM) catalysts for the oxygen reduction reaction (ORR). Acid leaching using aqua regia significantly decreases the amount of Fe present in the catalyst material to levels below 1 wt% with the lowest Fe content observed at 0.34 wt%. However, no change in the ORR onset is observed following catalyst treatment with acid when tested in acidic media. This indicates that some Fe centers are buried within the carbon material making then inaccessible to the acid treatment. These remaining Fe sites are accessible to oxygen and are catalytically active to the ORR.

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.

Reference

1.             E. C. M. Tse, D. Schilter, D. L. Gray, T. B. Rauchfuss and A. A. Gewirth, Inorg. Chem., 53, 8505 (2014).