Oxygen reduction reaction

In chemistry, the Oxygen reduction reaction (ORR) refers to the reduction half reaction whereby O2 is reduced to water or hydrogen peroxide. In fuel cells, the reduction to water is preferred because the current is higher. ORR is well demonstrated and highly efficient in nature.
Stoichiometry.
The stoichiometries of ORR, which depends on the medium, are shown:
4e- pathway in acid medium: <chem>O2 + 4 e- + 4H+ -> 2 H2O</chem>
2e- pathway in acid medium: <chem>O2 + 2e- + 2H+ -> H2O2</chem>
4e- pathway in alkaline medium: <chem>O2 + 4e- + 2H2O -> 4 OH-</chem>
2e- pathway in alkaline medium: <chem>O2 + 2e- + H2O -> HO2- + OH-</chem>
4e- pathway in solid oxide: <chem>O2 + 4e- -> 2 O^2-</chem>
The 4e- pathway reaction is the cathode reaction in fuel cell especially in proton-exchange membrane fuel cells, alkaline fuel cell and solid oxide fuel cell. While the 2e- pathway reaction is often the side reaction of 4e- pathway or can be used in synthesis of H2O2. 
Catalysts.
Biocatalysts.
ORR is an essential reaction for aerbobic organisms. Such organisms are powered by the heat of combustion of fuel (food) by O2. Rather than combustion, organisms rely on elaborate sequences of electron-transfer reactions, often coupled to proton transfer. The direct reaction of O2 with fuel is precluded by ORR, which produces water and ATP. Cytrochrome oxidase effects ORR by binding O2 in a heme-Cu complex. In laccase, O2 is engaged and reduced by a four-copper aggregate. Three Cu centers bind O2, and one Cu center functions as an electron donor.
Heterogeneous catalysts.
In fuel cells, platinum is the most common catalyst. Because platinum is expensive, it is dispersed on a carbon support. Certain facets of Pt are more active than others.ref name=Dodelet/>
Coordination complexes.
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 ORR catalysts for ORR were based on cobalt phthalocyanines. Many related coordination complexes have been tested. as ORR 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 ORR electrocatalyst. Besides phthalocyanine, porphyrin is also a suitable ligand for metal center to provide N4 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. 
Recent development and modification.
Since ORR in fuel cells need to be catalyzed heterogeneously, conductive substrates such as carbon materials is always needed in constructing ORR 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. Therefore, increasing the active site density and creating atomic level dispersed catalyst is a key step to improve the catalyst activity. To solve this problem, we can use some porous substrates to confine the active sites or use some defect or ligands to prevent the migration of the active site. In the mean time, the porous structure or the defect will also be beneficial to the oxygen absorption process. 
Besides active site density, the electron configuration of M center in M-N4 active site also plays an important role in the activity and stability of ORR catalyst. Because the electron configuration of M center can affects the redox potential, which determines the activation energy of the ORR. 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. Besides, some heteroatoms such as S, P other than N can also be used to modulate the electron configuration too, since these atoms have different electronegativity and electron configuration. 