Role of Cocatalysts on Hematite Photoanodes in Photoelectrocatalytic Water Splitting: Challenges and Future Perspectives

Abstract

The efficiency of a nanostructure applied to photo‐electrocatalysis is fundamentally governed by the capability of the nanostructure surface to sustain a reaction without the occurrence of electron trapping or a recombination of the photogenerated holes. This Minireview summarizes the latest progress in the use of cocatalysts on hematite electrodes and their application in water splitting induced by sunlight. The major drawback of hole diffusion through a solid‐liquid interface is addressed through the selection of the best cocatalyst for increasing the efficiency of an iron‐based material. Finally, the most promising modification routes and outstanding materials for enhancing the low kinetics of the oxygen evolution reaction during sunlight‐driven water oxidation reactions in hematite are provided and those materials that may have a significant impact on the overall photoelectrochemical performance are discussed.

Interface engineering of nanoceramic hematite photoelectrode for solar energy conversion

“…In this perspective, Sn‐contribution in the overall nanoceramic hematite enhancement can be addressed by two different main effects: (a) reducing the energetic barrier decreasing the built‐in resistances and/or (b) enhancing the electrical field on the solid‐liquid interface which lead to an increment on the band bending (improving the charge separation) 42,25 …”

“…43 Undoubtedly, FeNi-based cocatalysts have promoted so far a higher photocurrent density at lower potential with reported onset cathodic shift raging from ~100 to 300 mV, without compromise the semiconductor light absorption due to their transparency at the hematite absorption range. 42,[44][45][46][47] A developed methodology to deposit transparent FeNi-based material over nanoceramic hematite surface photoelectrode significantly increases the electrode activity at low overpotential, which is a typical signature of a cocatalyst. 46 Nevertheless, the task of finding suitable modifiers is only part of the challenge.…”

“…Indeed, the surface state passivation may reduce the charge recombination at the semiconductor‐electrolyte interface facilitating the hole collection at lower potential improving the η cat 40,41 . Among several materials, Co‐based and noble‐metal system have been widely employed to promote a faster kinetics for hole collection achieving the best reported values in the literature, but with no long‐term photoelectrochemical stability 42 . In this aspect the transition metal oxides have stood out as interesting alternatives with high‐stable performances and reasonable cost‐benefits 42 …”

“…In fact, the authors have shown that water oxidation kinetics was favored by increased species adsorption at the surface and bond strength of the OH* intermediate during OER, mediated by the presence of oxygen and cation vacancies acting synergistically 43 . Undoubtedly, FeNi‐based cocatalysts have promoted so far a higher photocurrent density at lower potential with reported onset cathodic shift raging from ~100 to 300 mV, without compromise the semiconductor light absorption due to their transparency at the hematite absorption range 42,44‐47 . A developed methodology to deposit transparent FeNi‐based material over nanoceramic hematite surface photoelectrode significantly increases the electrode activity at low overpotential, which is a typical signature of a cocatalyst 46 …”

Revealing the synergy of Sn insertion in hematite for next‐generation solar water splitting nanoceramics

“…Currently, cocatalyst materials showed an alternative to collect the photogenerated charge at low potentials by partially passivating the low kinetic surface states acquired by Sn intentional modification 29,31 . As already discussed in the literature and also by our group, 1,106 there is a great prospect in selecting a suitable cocatalyst that can act synergistically with Sn to decrease the E onset maintaining the satisfactory charge transport efficiency, as it has been achieved.…”