Nanoscale Limitations in Metal Oxide Electrocatalysts for Oxygen Evolution

Abstract

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/cm(2). 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.

Creating Highly Active Atomic Layer Deposited NiO Electrocatalysts for the Oxygen Evolution Reaction

“…[ 25 ] The change in the slope has been attributed to a change in the fi lm that affects the rate-limiting step of the reaction, or even to problems with charge transport. [ 52 ] The similar behavior of the Tafel slopes suggests that there are similar active sites within the two fi lms, and that the main difference can be attributed to the density of the active sites. Interestingly, a comparison of the redox features of the NiO/Fe trace and NiO/Fe satd (Figure 2 b) consistently shows that the number of electrochemically accessible sites is greater in the NiO/Fe trace , the opposite of what would be expected from the Tafel slopes alone.…”

“…Its fi ne control over fi lm thickness and uniformity will allow for the minimization of parasitic light absorption and charge transport resistances, and improved stability of unstable light absorbers. [ 1,52,62 ] As illustrated in this work, care must be taken to condition the ALD catalyst to the desired surface area and active state. Many variables can be used to optimize this process, helping make ALD of electrocatalysts a versatile method suitable for many material systems.…”

Unraveling the Intrinsic Structures that Influence the Transport of Charges in TiO2 Electrodes

“…Electron tunneling offers a potential solution to reduce the internal energy dissipation for this electronic process. [88,95] Efficient electron tunneling requires reduced width and height of the potential barriers to increase the probability of electron tunneling as well as to reduce the internal energy dissipation. Generally, reduction in the width of the potential barriers can be realized by increasing the density of electronic doping in the semiconductor (Figure 28).…”

Hierarchically porous Co3O4 architectures with honeycomb-like structures for efficient oxygen generation from electrochemical water splitting

“…To further enhance its electrocatalytic OER activity, considerable effort has been devoted to shaping and morphology-engineering Co 3 O 4 catalyst in nanoscale, since nanostructuring not only provides more abundant active sites and multiple accessible channels for interaction, diffusion and transport of reactive species (e.g. H 2 O, OH À and O 2 ) [22,23], but also allows the effective use of such kind of oxide materials with low electronic conductivity to overcome the charge transport limitations [24]. As a result, various Co 3 O 4 structures such as mesoporous nanowires [25], nanocrystalline film [26], porous atomically-thick sheets [27], hollow nanosheet microspheres [28], windmill-shaped microcrystals [29], ordered mesoporous nanocast [30] and mesoporous nanoflakes [31], have been exploited as active electrocatalysts for catalyzing OER in alkaline media.…”