To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO 2 reduction (CO 2 R). There are variety of factors that impact CO 2 R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO 2 R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO 2 R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO 2 R.

Graphene oxide suspended in ethanol undergoes reduction as it accepts electrons from UV-irradiated TiO(2) suspensions. The reduction is accompanied by changes in the absorption of the graphene oxide, as the color of the suspension shifts from brown to black. The direct interaction between TiO(2) particles and graphene sheets hinders the collapse of exfoliated sheets of graphene. Solid films cast on a borosilicate glass gap separated by gold-sputtered terminations show an order of magnitude decrease in lateral resistance following reduction with the TiO(2) photocatalyst. The photocatalytic methodology not only provides an on-demand UV-assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.

Field-sensitive electrochemical reactions are controlled by electrode charging which is sensitive to the size of the electrolyte containing cations.

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The use of a 2-D carbon nanostructure, graphene, as a support material for the dispersion of Pt nanoparticles provides new ways to develop advanced electrocatalyst materials for fuel cells. Platinum nanoparticles are deposited onto graphene sheets by means of borohydride reduction of H2PtCl6 in a graphene oxide (GO) suspension. The partially reduced GO-Pt catalyst is deposited as films onto glassy carbon and carbon Toray paper by drop cast or electrophoretic deposition methods. Nearly 80% enhancement in the electrochemically active surface area (ECSA) can be achieved by exposing partially reduced GO-Pt films with hydrazine followed by heat treatment (300 °C, 8 h). The electrocatalyst performance as evaluated from the hydrogen fuel cell demonstrates the role of graphene as an effective support material in the development of an electrocatalyst.

Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor/electrolyte interface. In this work, we show that a sputtered 100 nm TiO(2) layer on top of a thin Ti metal layer may be used to protect an n(+)p Si photocathode during photocatalytic H(2) evolution. Although TiO(2) is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H(2) evolution conditions. This behavior is due to the fortunate alignment of the TiO(2) conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H(2) evolution onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated by the red part (λ > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H(2) evolution) was also significantly enhanced by the antireflective properties of the TiO(2) layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe(2+)/Fe(3+) redox couple was used to help elucidate details of the band diagram.

The past 10 years have seen great advances in the field of electrochemical hydrogen evolution. In particular, several new nonprecious metal electrocatalysts, for example, the MoS2 or the Ni2P family of materials, have emerged as contenders for electrochemical hydrogen evolution under harsh acidic conditions offering nearly platinum-like catalytic performance. The developments have been particularly fast in the last 5 years, and the present Perspective highlights key developments and discusses them, along with hydrogen evolution in general, in the context of the global energy problem.

Photoelectrochemical (PEC) solar-fuel conversion is a promising approach to provide clean and storable fuel (e.g., hydrogen and methanol) directly from sunlight, water and CO. However, major challenges still have to be overcome before commercialization can be achieved. One of the largest barriers to overcome is to achieve a stable PEC reaction in either strongly basic or acidic electrolytes without degradation of the semiconductor photoelectrodes. In this work, we discuss fundamental aspects of protection strategies for achieving stable solid/liquid interfaces. We then analyse the charge transfer mechanism through the protection layers for both photoanodes and photocathodes. In addition, we review protection layer approaches and their stabilities for a wide variety of experimental photoelectrodes for water reduction. Finally, we discuss key aspects which should be addressed in continued work on realizing stable and practical PEC solar water splitting systems.