Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

Surface raman spectroelectrochemistry: Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode

The efficiency of many energy storage technologies, such as rechargeable metal-air batteries and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen evolution reaction (OER). We found that Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3–δ (BSCF) catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than that of the state-of-the-art iridium oxide catalyst in alkaline media. The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an e g symmetry of surface transition metal cations in an oxide. The peak OER activity was predicted to be at an e g occupancy close to unity, with high covalency of transition metal–oxygen bonds.

https://science.sciencemag.org/content/sci/early/2011/10/26/science.1212858.full.pdf

A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles.

Measurements of the oxygen evolution reaction have been made on eighteen substituted perovskites containing first‐row transition metal ions. Rates are reported at equilibrium and at an overpotential of 0.3V. Electrode kinetic parameters are given, including roughness factors. The rate does not depend on semiconductor‐type properties. It increases as the pH of zero charge, at which the occupancy of OH− and H+ at the interface becomes equal, moves in an alkaline direction, with decrease of magnetic moment, with decrease of stability of the perovskite lattice, with decrease of the enthalpy of formation of the transition metal hydroxides, and with increase in the number of d‐electrons in the transition metal ion. The accuracy of the roughness factor measurements are affected by weakness in knowledge of true double layer capacities. The value assumed here, 60 μF cm−2, may be accurate to only ± 100%. Models are given which suggest that the pores are active throughout. The correlations between the rate and electronic properties are consistent with rate‐determining steps which involve desorption of OH radicals, e.g., . An MO discussion suggests that the electrocatalysis increases with increased occupancy of the antibonding orbitals of MZ‒OH. Earlier interpretations include the concept that an increase of the rate occurs because of increasing overlap between the orbitals of the transition metal ion and the spσorbital of O. However, this theory is based on only three different materials, in which rate‐determining step changes. An interpretation based upon nonstoichiometry is shown to be consistent with observed trends, but insufficient to explain their magnitude. The electrical and chemical contributions to the rate are analyzed. The value of agr; is related via bond strength considerations to the MZ‒OH bond strengths. The relative electrocatalysis discussed is limited to an overpotential which corresponds to a practical range of rates on the faster catalysts. A volcano relation for oxygen evolution on perovskites seems likely. Future electrocatalysts are predicted.

https://iopscience.iop.org/article/10.1149/1.2115565/pdf

John O’M. Bockris

Papers

The Electrocatalysis of Oxygen Evolution on Perovskites

Mechanism of oxygen evolution on perovskites

The hydrogen economy: Its history

Electrochemical methods — Fundamentals and applications, Allen J. Bard, Larry R. Faulkner. Wiley, New York (1980), xviii+718 pp., £14.70

Estimates of the price of hydrogen as a medium for wind and solar sources