Bi-Containing n-FeWO₄ Thin Films Provide the Largest Photovoltage and Highest Stability for a Sub-2 eV Band Gap Photoanode

Abstract

Photoelectrocatalysis of the oxygen evolution reaction remains a primary challenge for development of tandem-absorber solar fuel generators due to the lack of a photoanode with broad solar spectrum utilization, a large photovoltage, and stable operation. Bismuth vanadate with a 2.4−2.5 eV band gap has shown the most promise becauses its photoactivity down to 0.4 V vs RHE is sufficiently low to couple to a lower-gap photocathode for fuel synthesis. Through development of photoanodes based on the FeWO 4 structure, in particular, Fe-rich variants with addition of about 6% Bi, we demonstrate the same 0.4 V vs RHE turn-on voltage with a 2 eV band gap metal oxide, enabling a 2-fold increase in the device efficiency limit. Combinatorial exploration of materials composition and processing facilitated synthesis of n-type variants of this typical p-type semiconductor that exhibit much higher photoactivity than previous implementations of FeWO 4 in solar photochemistry. The photoanodes are particularly promising for solar fuel applications given their stable operation in acid and base.

Rational Design of a Coupled Confronting Z‐Scheme System Toward Photocatalytic Refractory Pollutant Degradation and Water Splitting Reaction

“…As shown in Figure a,c,e, E fb or quasi‐Fermi levels of B, C, and L are found to be −0.03, −0.9, and −0.75 V versus Ag/AgCl electrode, respectively, or +0.57, −0.32, and −0.15 V in normal hydrogen electrode (NHE). As per consensus, depending on the effective mass of electron ( m e ) and carrier concentration, conduction band minimum (CBM) resides 0.1 V lower than the E fb . Hence, the positions of CBM of B, C, and L are calculated to be +0.47, −0.42, and −0.25 V, respectively, in NHE.…”

Combinatorial screening yields discovery of 29 metal oxide photoanodes for solar fuel generation

“…Fig. 3 shows the 7 metal oxide phases with the highest external quantum efficiency (EQE) at 2.41 eV, which includes 2 discoveries from the present work, discoveries from our previous work, 15,16,21 and our reproduction of the previously-reported photoanodes FeBiO3. 22 While EQE is amenable to optimization, for example through tailoring carrier transport and catalyst coatings, the photon energy of photocurrent onset is more deeply engrained in the electronic structure of the metal oxide phase.…”

Successes and Opportunities for Discovery of Metal Oxide Photoanodes for Solar Fuels Generators

“…FeWO 4 was identified through combinatorial investigation of non-equilibrium synthesis conditions. 26 This Fe +2 -containing metal oxide is stabilized against oxidation by a selfpassivation surface layer that includes Fe +3 , highlighting the role of self-passivation in enabling stable operation of thermodynamically unstable photoanodes. Self-passivation was also observed in copper vanadates, where the degradation in photocurrent was found to depend on the thickness of the developed passivating film.…”

“…beyond that exhibited in this figure, include the observation of a turn-on potential (lowest potential with observed photocurrent) near 0.6 V vs RHE for FeBiO 3 25 and near 0.4 V vs RHE for Bi-alloyed FeWO 4 . 26 More detailed understanding of the semiconductor−liquid junctions and band energy alignment are needed to elucidate the limiting photovoltage and associated efficiency of each photoanode, as discussed further below.…”