Toward Settling the Debate on the Role of Fe₂O₃ Surface States for Water Splitting

Abstract

Understanding the chemical nature and role of electrode surface states is crucial for improved electrochemical cell operation. For iron(III) oxide (α-Fe 2 O 3 ), which is one of the most widely studied anode electrodes used for water splitting, surface states were related to the appearance of a dominant absorption peak during water splitting. The chemical origin of this signature is still unclear, and this open question has provoked tremendous debate. In order to pin down the origin and role of surface states, we perform first-principles calculations with density functional theory + U on several possible adsorbates at the α-Fe 2 O 3 (0001) surface. We rule out the existence of a stable peroxo Fe−O−O−Fe adsorbate and show that the origin of the surface absorption peak could be a Fe−O• type bond that functions as an essential intermediate of water oxidation.

Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy

“…On the basis of these assignments, a H 2 O-oxidation mechanism can be established that involves the formation of an iron-oxo group as the product of the first oxidation reaction on the surface by valence-band holes. This initial step is further supported by recent theoretical work that indicates the Fe IV =O groups as probable intermediate species in water oxidation on haematite after the first hole transfer-bridging peroxide species were ruled out as intermediates 26 . Subsequent attack by a water molecule and oxidative dissociation of a proton are the probable next steps, which would produce a surface peroxide intermediate that releases O 2 on further oxidation.…”

“…Given the potential energy of haematite's valence-band holes, reactions (2)-(5) are thermodynamically favourable based on the calculated free energy for water splitting on fully hydroxylated surfaces 25,26 . The proposed mechanism is analogous to the mechanism recently established for water oxidation on cobalt oxide catalysts 4,34 .…”

Highly Selective Electrochemical Reduction of Dinitrogen to Ammonia at Ambient Temperature and Pressure over Iron Oxide Catalysts

“…Furthermore, chronoamperometry tests of the four catalysts at more negative potentials were carried out. [34][35][36] In our case, the slight changes in crystal orientation may be acontributing factor in the higherN H 3 formation rates that were observedf or the hematite samples than fort he untreated hematite analogues after thermala nnealing (see the Supporting Information,T ableS2), yet they were unable to explain the large difference in NRR activity between the two hematite samples after thermala nnealing. All catalysts displayed the highest averageN H 3 formation rate at À0.9 Vv s. Ag/AgCl, with the highest rate (0.459 mgh À1 cm À2 )o btainedb yt he o-Fe 2 O 3 -Ar/ CNT catalyst.…”

Visible‐Light‐Driven Water Oxidation with a Polyoxometalate‐Complexed Hematite Core of 275 Iron Atoms

“…[36] At the same time,the first-and third-order dependence of hole formation reported by Durrant, [7] and the first-and second-order kinetics and operando observations of electrochemical water oxidation by Chen and Song, [6] support O À Oformation both at as ingle Fe atom, and at larger hole densities and/or highly basic pH values via coupling between adjacent surfacetrapped holes.F or solutions of 1 at pH 8, OÀOf ormation is more likely to occur at asingle Fe atom, giving the iron(III)- Computational results argue that the distances between adjacent Fe atoms at the hematite surface are too large to give stable peroxidebridged di-iron intermediates,Fe III ÀOOÀFe III .…”