Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping

Abstract

Metal oxides are promising for photoelectrochemical (PEC) water splitting due to their robustness and low cost. However, poor charge carrier transport impedes their activity, particularly at low-bias voltage. Here we demonstrate the unusual effectiveness of phosphorus doping into bismuth vanadate (BiVO4) photoanode for efficient low-bias PEC water splitting. The resulting BiVO4 photoanode shows a separation efficiency of 80% and 99% at potentials as low as 0.6 and 1.0 VRHE, respectively. Theoretical simulation and experimental analysis collectively verify that the record performance originates from the unique phosphorus-doped BiVO4 configuration with concurrently mediated carrier density, trap states, and small polaron hopping. With NiFeOx cocatalyst, the BiVO4 photoanode achieves an applied bias photon-to-current efficiency of 2.21% at 0.6 VRHE. The mechanistic understanding of the enhancement of BiVO4 properties provides key insights in trap state passivation and polaron hopping for most photoactive metal oxides.

Unraveling the Site-Selective Doping Mechanism in Single-Crystalline BiVO4 Thin Films for Photoelectrochemical Water Splitting

“…32 Solid-state electronic characterizations provide more insights into the difference in the effects of Gd and Mo dopants on the electron transport in BVO. 33 Current−voltage (I−V) characteristics are measured for pristine and doped BVO films over YSZ substrates, and the transport behavior is mainly governed by the majority carriers (electrons) within BVO grains. As shown in Figure 5a−c, the Gd-BVO film presents a roomtemperature resistance similar to that of the pristine BVO film, which are both nearly 2 orders of magnitude higher than those of the Mo-BVO films.…”

“…Solid-state electronic characterizations provide more insights into the difference in the effects of Gd and Mo dopants on the electron transport in BVO . Current–voltage ( I – V ) characteristics are measured for pristine and doped BVO films over YSZ substrates, and the transport behavior is mainly governed by the majority carriers (electrons) within BVO grains.…”

Unconventional Substitution for BiVO4 to Enhance Photoelectrocatalytic Performance by Accelerating Polaron Hopping

“…W 6+ - and Mo 6+ -doped BiVO 4 could effectively increase the major carrier density and enhance PEC water splitting. , P-doped BiVO 4 significantly promotes charge transfer kinetics with a remarkable increase of photocurrent density . Substituting V 5+ could suppress bulk recombination with improved electron mobility, which is attributed to the decreased polaron hopping energy barrier by the local polarization of the lattice. , The conventional substitution of V 5+ involves doping with higher valent metal ions (W 6+ or Mo 6+ ), , but there are few studies on the doping with lower valent metal ions, for example, Ti 4+ , which has been doped into BiVO 4 for photocatalysis rather than photo-electrocatalysis. , …”

“…22 Substituting V 5+ could suppress bulk recombination with improved electron mobility, which is attributed to the decreased polaron hopping energy barrier by the local polarization of the lattice. 23,24 The conventional substitution of V 5+ involves doping with higher valent metal ions (W 6+ or Mo 6+ ), 20,21 but there are few studies on the doping with lower valent metal ions, for example, Ti 4+ , which has been doped into BiVO 4 for photocatalysis rather than photo-electrocatalysis. 25,26 Here, we propose an unconventional substitution of V 5+ sites by lower valent Ti 4+ ions with the similar ionic radii to synthesize the Ti:BiVO 4 photoanode, which increases the photocurrent density 1.90 times up to 2.51 mA cm −2 at 1.23 V vs RHE and increases the charge carrier density 1.81 times to 5.86 × 10 18 cm −3 , compared with bare BiVO 4 .…”

Polaron-Mediated Transport in BiVO4 Photoanodes for Solar Water Oxidation

“…These values of E a are comparable to other experimental and theoretical calculations of the small polaron hopping barrier of BiVO 4 photoanode in the literature. 41,46 The smaller E a in the hydrogenated BiVO 4 means that electrons require less energy to hop from one BiVO 4 lattice site to the next, which arises from the changes in the coordination environment induced by the hydrogen dopant. We note that the activation energy of hole polaron hopping in BiVO 4 has been experimentally and theoretically proved to be much smaller than that of the electron small polaron hopping.…”