In fuel cells, the reduction to water is preferred because the current is higher.
The oxygen reduction reaction is well demonstrated and highly efficient in nature.
Rather than combustion, organisms rely on elaborate sequences of electron-transfer reactions, often coupled to proton transfer.
Cytochrome c oxidase affects the oxygen reduction reaction by binding O2 in a heme–Cu complex.
[2] Detailed mechanistic work results from studies on transition metal dioxygen complexes, which represent models for the initial encounter between O2 and the metal catalyst.
Early catalysts for the oxygen reduction reaction were based on cobalt phthalocyanines.
[5] as the oxygen reduction reaction catalyst and different electrocatalysis performance was achieved by these small molecules.
These exciting results trigger further research of the non-noble metal contained small molecules used for the oxygen reduction reaction electrocatalyst.
[6] Besides phthalocyanine, porphyrin is also a suitable ligand for metal center to provide N4[definition needed] part in the M-N4 site.
In biosystems, many oxygen related physical chemical reactions are carried by proteins containing the metal-prophyrin unit such as O2 delivery, O2 storage, O2 reduction and H2O2 oxidation.
Since the oxygen reduction reaction in fuel cells need to be catalyzed heterogeneously, conductive substrates such as carbon materials is always needed in constructing electrocatalysts.
To increase the conductivity and enhance the substrate-loading interaction, thermal treatment is usually performed before application.
During the treatment, M-N4 active sites turn to aggregate spontaneously due to the high intrinsic energy, which will dramatically decrease the active site density.
In the mean time, the porous structure or the defect will also be beneficial to the oxygen absorption process.
Because the electron configuration of M center can affects the redox potential, which determines the activation energy of the oxygen reduction reaction.
To modulate the electron configuration, a simple way is to change the ligands of the metal center.
For example, researchers found that whether the N atoms in M-N4 active sites are pyrrolic or pyridinic can affect the performance of the catalyst.