Electrocatalytic Oxygen Evolution Reaction in Acidic Environments Reaction Mechanisms and Catalysts

DOI: 10.1002/aenm.201601275 order to bridge the time gap between supply and demand. Molecular fuels like hydrogen or hydrocarbons produced from renewable electricity and water or CO2, respectively, can provide such a longterm chemical energy storage solution.[1–3] Considering hydrogen, its reconversion to electrical energy can be efficiently performed in fuel cells.[4] In a transition period, hydrogen can additionally be used as fuel for combustion engines, which underlines its versatility. Besides that, hydrogen and hydrocarbons can be appropriately used for mobile applications due to their comparably high gravimetric energy density.[2] The electrocatalytic production of molecular fuels like hydrogen from water or hydrocarbons from CO2 is based on reduction reactions which, in turn, require an electron donating counter reaction. In this context, the electrocatalytic oxidation of water to mole cular oxygen, the oxygen evolution reaction (OER), is the most promising candidate with regard to availability and sustainability.[5] Additionally, the OER constitutes a common counter reaction in metal electrowinning.[6] Hence, the OER is not only a key step for electricity storage but is furthermore of outmost importance in other processes. Unfortunately, the OER is a complex multistep reaction, which adds a considerably large overpotential to the actual process and, thus, distinctly reduces the process efficiency even if current benchmark catalysts are applied.[5] Additionally, the inherent high electrode potentials during the OER are demanding with respect to the catalysts stability. In the context of water electrolysis for renewable electricity storage, proton exchange membrane (PEM) electrolyzers offer great advantages compared to alkaline electrolyzers such as lower ohmic losses, higher voltage efficiency, higher gas purity, a more compact system design, higher current density, a faster system response and a larger partial load range.[7,8] The aforementioned advantages are directly or indirectly related to the PEM, which is an acidic solid polymer electrolyte membrane. In particular, the PEM ensures a small gas crossover and provides a high proton conductivity.[7] Since the gas crossover rate is rather independent of the applied load, gas crossover becomes problematic at low loads where gas production rates are low.[7] In this case, the transport rate of H2 through the membrane can be high enough to form H2-O2 mixtures that exceed the explosion limit, which has to be strictly avoided for The low efficiency of the electrocatalytic oxidation of water to O2 (oxygen evolution reaction-OER) is considered as one of the major roadblocks for the storage of electricity from renewable sources in form of molecular fuels like H2 or hydrocarbons. Especially in acidic environments, compatible with the powerful proton exchange membrane (PEM), an earth-abundant OER catalyst that combines high activity and high stability is still unknown. Current PEM-compatible OER catalysts still rely mostly on Ir and/or Ru as active components, which are both very scarce elements of the platinum group. Hence, the Ir and/or Ru amount in OER catalysts has to be strictly minimized. Unfortunately, the OER mechanism, which is the most powerful tool for OER catalyst optimization, still remains unclear. In this review, we first summarize the current state of our understanding of the OER mechanism on PEM-compatible heterogeneous electrocatalysts, before we compare and contrast that to the OER mechanism on homogenous catalysts. Thereafter, an overview over monometallic OER catalysts is provided to obtain insights into structure-function relations followed by a review of current material optimization concepts and support materials. Moreover, missing links required to complete the mechanistic picture as well as the most promising material optimization concepts are pointed out.