Nanoscale Limitations in Metal Oxide Electrocatalysts for Oxygen Evolution

Metal oxides are attractive candidates for low cost, earth-abundant electrocatalysts. However, owing to their insulating nature, their widespread application has been limited. Nanostructuring allows the use of insulating materials by enabling tunneling as a possible charge transport mechanism. We demonstrate this using TiO₂ as a model system identifying a critical thickness, based on theoretical analysis, of about ∼4 nm for tunneling at a current density of ∼1 mA/cm². This is corroborated by electrochemical measurements on conformal thin films synthesized using atomic layer deposition (ALD) identifying a similar critical thickness. We generalize the theoretical analysis deriving a relation between the critical thickness and the location of valence band maximum relative to the limiting potential of the electrochemical surface process. The critical thickness sets the optimum size of the nanoparticle oxide electrocatalyst and this provides an important nanostructuring requirement for metal oxide electrocatalyst design

Electrocatalytic oxygen evolution over supported small amorphous ni-fe nanoparticles in alkaline electrolyte

The electrocatalytic oxygen evolution reaction (OER) is a critical anode reaction often coupled with electron or photoelectron CO2 reduction and H2 evolution reactions at the cathode for renewable energy conversion and storage. However, the sluggish OER kinetics and the utilization of precious metal catalysts are key obstacles in the broad deployment of these energy technologies. Herein, inexpensive supported 4 nm Ni-Fe nanoparticles (NiyFe1-yOx/C) featuring amorphous structures have been prepared via a solution-phase nanocapsule method for active and durable OER electrocatalysts in alkaline electrolyte. The Ni-Fe nanoparticle catalyst containing 31% Fe (Ni0.69Fe0.31Ox/C) shows the highest activity, exhibiting a 280 mV overpotential at 10 mA cm -2 (equivalent to 10% efficiency of solar-to-fuel conversion) and a Tafel slope of 30 mV dec-1 in 1.0 M KOH solution. The achieved OER activity outperforms NiOx/C and commercial Ir/C catalysts and is close to the highest performance of crystalline Ni-Fe thin films reported in the literature. In addition, a Faradaic efficiency of 97% measured on Ni 0.69Fe0.31Ox/C suggests that carbon support corrosion and further oxidation of nanoparticle catalysts are negligible during the electrocatalytic OER tests. Ni0.69Fe0.31O x/C further demonstrates high stability as there is no apparent OER activity loss (based on a chronoamperometry test) or particle aggregation (based on TEM image observation) after a 6 h anodization test. The high efficiency and durability make these supported amorphous Ni-Fe nanoparticles potentially applicable in the (photo)electrochemical cells for water splitting to make H2 fuel or CO2 reduction to produce usable fuels and chemicals. © 2014 American Chemical Society

Semiconductor Nanowires for Artificial Photosynthesis

In this Perspective, we discuss current challenges in artificial photosynthesis research, with a focus on the benefits of a nanowire morphology. Matching the flux between electrocatalysts and light-absorbers, and between individual semiconducting light-absorbers, are two major issues to design economically viable devices for artificial photosynthesis. With the knowledge that natural photosynthesis is an integrated nanosystem, individual building blocks of biomimetic artificial photosynthesis are discussed. Possible research directions are presented under an integrated device design scheme, with examples of our current progress in these areas. Coupling all of the components together, including electrocatalysts, light-absorbers, and charge transport units, is crucial due to both fundamental and practical considerations. Given the advantages of one-dimensional nanostructures, it is evident that semiconductor nanowires can function as essential building blocks and help to solve many of the issues in artificial photosynthesis. © 2013 American Chemical Society