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

The paper reports a study on the influence of alloying nickel by left-hand side transition metals (Fe, Mo, W) on the electrocatalytic activity in hydrogen evolution in an acidic environment. A number of experimental techniques were used in research (dc and ac electrochemical techniques, XRD, SEM, EDS, and ICP). The results clearly demonstrate that alloying nickel with Fe, Mo or W results in an increased electrocatalytic activity in hydrogen evolution when compared to pure nickel. Two effects were found to be responsible for the observed behavior; an increase in surface roughness and intrinsic activity of the material. Ni7.3Mo was found to yield the highest overall electrocatalytic activity among the investigated materials mainly due the highest surface roughness, while Ni3.4W was found to yield the highest intrinsic electrocatalytic activity, which is explained on the basis of the modification of electron density in d-orbitals upon alloying nickel with tungsten. It was also noticed that an increase in crystallinity can contribute to an increased electrocatalytic activity in hydrogen evolution.

Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium

Ni and NiMo hydrogen evolution electrocatalysts electrodeposited in a polyaniline matrix

The construction and use of “dynamic potential–pH diagrams” (DPPDs), that are intended to extend the usefulness of thermodynamic Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), an important electrochemical reaction because of its key role in energy conversion devices and biological systems (water electrolyses, photoelectrochemical water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochemical system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO2, Co3O4, Co3O4 electrodeposited in a phosphate medium (Co–Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of experimental conditions (pH, potential, current density). In addition, the experimental setup and the factors affecting the accurate collection and presentation of data (e.g., reference electrode system, correction of ohmic drops, bubble formation) are discussed.

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Dynamic potential–pH diagrams application to electrocatalysts for water oxidation

Composite materials for electrocatalysis of O2 evolution: IrO2+SnO2 in acid solution

RuO2-doped Co3O4 electrodes were prepared by thermal decomposition of the corresponding nitrates using Ni as a support. The RuO2 content was varied between 0 and 20mol%. The kinetics of hydrogen evolution from alkaline solution was studied by recording quasisteady state polarization curves at several NaOH concentrations. A mechanism is proposed on the basis of Tafel slopes and reaction orders. The electrocatalytic activity of the mixed oxides has been found to go through a maximum at intermediate RuO2 contents. Some evidence of instability has emerged.

N. Krstajic

Papers

Cathodic behaviour of RuO2-doped Ni/Co3O4 electrodes in alkaline solutions: hydrogen evolution