Sluggish Water Oxidation Kinetics Research Articles

RGO decorated semiconductor heterojunction of BiVO4/NiO to enhance PEC water splitting efficiency

Monoclinic phase of BiVO4 is a promising photoanode material for photoelectrochemical (PEC) water splitting, but its sluggish water oxidation kinetics and frequent bulk charge recombination greatly reduce its efficiency of PEC water splitting. A novel BiVO4/NiO/rGO photoanode was very simply prepared by electrodeposition, solution immersion and spin coating methods, in particular, the solution immersion method to loading NiO has never been reported in PEC research. Compared with BiVO4, the photocurrent density of the ternary photoanode reaches 1.52 mA/cm2 at 1.23 V vs RHE, which is 2.41 and 1.39 times higher than that of pure BiVO4 and binary BiVO4/NiO photoanode, respectively. The onset potential of the ternary photoanode shows a significant cathodic shift of 130 mV compared with the BiVO4 photoanode. Moreover, the measured incident photon-to-current efficiency (IPCE) value reaches 50.52% at λ = 420 nm. The improvement is attributed to the type-II heterojunction formation that enhances the separation efficiency of electron/hole and the rGO decoration that accelerates the electron transfer and provides more active sites for gas adsorption.

Modification of Ti-doped hematite nanowires with a NiOx buffer layer for improved photoelectrochemical performance

Hematite (α-Fe2O3) is a promising photoanode material for photoelectrochemical (PEC) water splitting due to its appropriate bandgap, good stability, and earth-abundance. However, the poor charge transfer property and sluggish kinetics of water oxidation limit the PEC performance of α-Fe2O3 photoanodes. Herein, a thin NiOx buffer layer was introduced between the Ti doped α-Fe2O3 (Fe2O3-Ti) layer and the fluorine-doped tin oxide (FTO) substrate without affecting the nanowire morphology and light absorption property of Fe2O3-Ti. This buffer layer can apparently suppress the charge recombination by mitigating the lattice mismatching between the Fe2O3-Ti film and the FTO substrate. In addition, the good conductivity of the NiOx film from the non-stoichiometric composition is also beneficial to the charge transfer. As a consequence, the photocurrent density was greatly improved by adding the NiOx layer in the Fe2O3-Ti photoanode, reaching 1.32 mA·cm−2 at 1.23 VRHE without any co-catalyst and sacrificial agent. This work gives a detailed analysis of the back contact in the hematite-based photoanode and provides an effective strategy for underlayer interface optimization.

Improving photoelectrochemical water oxidation activity of BiFeO3 photoanode via surface passivation

Bismuth iron oxide (BiFeO3) has attracted widespread attention in the field of photoelectrochemistry due to its special ferroelectric properties. However, its high charge recombination from defects and sluggish water oxidation kinetics limit the potential application. Herein, oxygen vacancies are first introduced in BiFeO3 thin films by Ar annealing to improve the bulk charge transfer efficiency. Subsequently, a p-type conductive poly(3, 4-ethylenedioxythiophene) (PEDOT) is employed to suppress the surface-defect-related recombination of photogenerated carriers and to form a p–n junction to increase the charge extraction efficiency. As a result, the photocurrent density of the optimized NiCoFe-Bi/PEDOT/BiFeO3 photoanode reaches 0.17 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE) under simulated 1 sun irradiation for water oxidation, which is enhanced by six times compared with that of the photoanode without PEDOT modification. The photocurrent density is further improved to 0.23 mA cm−2 at 1.23 V vs RHE through polarization of the ferroelectric material. This study strongly suggests that surface passivation is an effective way to improve the activity of the BiFeO3 photoanode for solar water splitting.

Single-atomic-Co cocatalyst on (040) facet of BiVO4 toward efficient photoelectrochemical water splitting

Monoclinic bismuth vanadate (BiVO4) shows promising application prospect in photoelectrochemical (PEC) water splitting on account of its relatively ideal band gap to harvest sunlight. However, the poor charge migrate property and sluggish water oxidation kinetics severely limit the PEC performance of BiVO4 photoelectrodes. In this work, BiVO4 photoanode with highly exposed (040) facets for superior charge transfer was prepared by a seed assisted hydrothermal method. Moreover, cobalt single atoms stabilized in N-doped carbon nanosheet (Co SAs-NC) was well modified on BiVO4 (040) (denoted as BiVO4 (040)/Co SAs-NC) with much improved water oxidation efficiency. The as-prepared BiVO4 (040)/Co SAs-NC photoanode generated 2.2 times higher photocurrent density than that of pristine BiVO4 (040) at 1.23 V vs. RHE, and presented nearly 100% charge injection efficiency. The detail kinetic measurements reveal that the modified Co SAs-NC cocatalyst effectively suppresses the carrier recombination and promotes the surface reaction kinetics. This work provides a promising strategy to fabricate composite photoanodes for solar energy conversion based on facet engineering matched with single-atomic-catalyst.

In-situ coating of multifunctional FeCo-bimetal organic framework nanolayers on hematite photoanode for superior oxygen evolution

Owing to the high theoretical photocurrent density, the hematite (α-Fe2O3) based photoanode has been intensively concerned in photoelectrochemical (PEC) water splitting, but its serious charge recombination and sluggish water oxidation kinetics are still the stumbling blocks. This work reports a high-performance Fe2O3-based photoanode achieved by coating multifunctional FeCo-bimetal organic framework (MOF) nanolayers (NLs) on Fe2O3 nanoarrays via an in-situ solvothermal process. The FeCo MOF NLs introduced not only effectively passivate the surface states of Fe2O3 photoanode and boost the water oxidation kinetics serving as the cocatalyst, but also construct p-n heterojunctions with Fe2O3 to accelerate the directional migration and separation of photogenerated charge carriers. Expectedly, the as-obtained FeCo-MOF/Fe2O3 photoanode exhibits an apparent negative shift of onset potential, an excellent long-term PEC stability, and highly improved photocurrent density. This finding provides a novel and effective strategy to introduce advanced multifunctional surface coating for enhancing the PEC performance of photoelectrodes.

Plasmon Au modified 3D-nanostructrue CoOx decorated hematite for highly efficient photoelectrochemical water splitting

Surface modification and interface engineering are efficient strategies to address the serious charge recombination and the sluggish water oxidation kinetics in photoelectrochemical water splitting. In this work, CoOx decorated hematite nanosheets (Fe2O3/CoOx) are deposited on Nickel foam by the in-situ hydrothermal process. Au nanoparticles are incorporated on Fe2O3/CoOx semiconductors (Fe2O3/CoOx/Au) by electrochemical deposition. In photoelectrochemical test, Fe2O3/CoOx attains a photocurrent density of 1.87 mA cm−2 at 1.23 VRHE, which is 4.45 times that for α-Fe2O3. The onset potential of Fe2O3/CoOx decreases by 266 mV compared with α-Fe2O3. The 3D-nanostructrue Fe2O3/CoOx/Au attains a photocurrent density of 3.88 mA·cm−2 at 1.23 VRHE, which is 9.24 times that of ɑ-Fe2O3. The applied bias photon-to-current efficiency, charge separation and charge injection efficiency of Fe2O3/CoOx/Au are improved. EIS studies show the co-modification of CoOx and Au reduces charge transfer resistance. This strategy would provide a potential approach to promote light absorption and charge separation for photoelectrochemical catalyst.

An Inorganic–Organic Hybrid Polymer Cocatalyst for Photoelectrochemical Water Oxidation with Dual Functions of Accelerating Kinetics and Improving Charge Transfer

Oxygen evolution cocatalysts (OECs) play important roles in improving the efficiency of photocatalysts in solar water splitting. Inorganic–organic hybrid polymers (IOHPs), which have good electroly...

One-Dimensional Co(OH)F as a Noble Metal-Free Redox Mediator and Hole Extractor for Boosted Photoelectrochemical Water Oxidation in Worm-like Bismuth Vanadate

In this work, a noble metal-free one-dimensional Co(OH)F and hierarchical BiVO₄ combination as a model system is proposed for an efficient photoelectrochemical catalyst. BiVO₄, known for its superior theoretical current density of 7.5 mA/cm², suffers from poor photoelectrochemical performance due to the sluggish water oxidation kinetics, recombination of photogenerated carriers, and results in reduced photoelectrochemical water oxidation. BiVO₄/Co(OH)F photoanode exhibits an enhanced photocurrent and a cathodic shift of 160 mV in onset potential as compared to the pristine BiVO₄ photoanode. Cyclic voltametric studies reveal that cobalt exists in the mixed valence state. The presence of fluorine by virtue of its high electronegativity induces facile positive charge on the metal center, i.e., on cobalt, thereby Co²⁺ ions as an active site accept holes more efficiently from the semiconductor and oxidized to Co³⁺ or/and Co⁴⁺. Subsequently, these active species, Co³⁺ or/and Co⁴⁺, deliver the positive charge to produce O₂ and recover to the initial state to regenerate redox couple. A mechanistic study reveals that Co(OH)F nanorods as an efficient hole extractor by virtue of its redox ability, suppresses the recombination of photogenerated electron–hole at the electrolyte/semiconductor interface and accelerates the water-oxidation kinetics. Co(OH)F modification of the BiVO₄ surface is able to utilize a higher number of holes, which have reached the semiconductor/electrolyte interface for water oxidation that resulted in a photocurrent of 3.4 mA/cm². Investigations on the hole transfer efficiency reveals a faster oxidation kinetics, resulting in improved charge injection in the presence of Co(OH)F. Electrochemical impedance measurements shows a low interfacial charge transfer resistance leading to better photoelectrochemical performance. A Faradaic yield of ∼95% suggests that the generated charge carriers (anodic photocurrent) in the BiVO₄/Co(OH)F photoanode is predominantly due to water oxidation.

Ti-doped hematite films coupled with ultrathin nickel-borate layer as photoanode for enhanced photoelectrochemical water oxidation

Hematite (α-Fe2O3) is a very attractive photoanode material for photoelectrochemical (PEC) water oxidation application due to its remarkable chemical stability and appropriate bandgap. However, the low charge separation efficiency, poor conductivity, and sluggish water oxidation kinetics still severely limit its expected performance. In this study, we constructed a photo-electrodeposited amorphous nickel-borate (Ni-Bi) thin layer on Ti-doped α-Fe2O3 photoanode. The fabricated heterostructure photoanode developed a built-in electric field, which will accelerate charge transport and improve the water oxidation kinetics significantly, resulting in the greatly enhanced photocurrent density of about 348% at 1.23 V vs. reversible hydrogen electrode (RHE) and the improvement of onset potential advance of ~ 80 mV. The surface morphology of samples was characterized with field-emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscope (HRTEM). The Mott–Schottky (M-S) and electrochemical impedance spectroscopy (EIS) results confirmed the donor density and the conductivity of photoanodes with Ti-doping and Ni-Bi coated. This work provides a novel strategy to develop various heterojunction photoanodes for practical PEC application of α-Fe2O3.

Operando unraveling photothermal-promoted dynamic active-sites generation in NiFe2O4 for markedly enhanced oxygen evolution

The ability to develop highly active and low-cost electrocatalysts represents an important endeavor toward accelerating sluggish water-oxidation kinetics. Herein, we report the implementation and unraveling of the photothermal effect of spinel nanoparticles (NPs) on promoting dynamic active-sites generation to markedly enhance their oxygen evolution reaction (OER) activity via an integrated operando Raman and density functional theory (DFT) study. Specifically, NiFe2O4 (NFO) NPs are first synthesized by capitalizing on amphiphilic star-like diblock copolymers as nanoreactors. Upon the near-infrared light irradiation, the photothermal heating of the NFO-based electrode progressively raises the temperature, accompanied by a marked decrease of overpotential. Accordingly, only an overpotential of 309 mV is required to yield a high current density of 100 mA cm-2, greatly lower than recently reported earth-abundant electrocatalysts. More importantly, the photothermal effect of NFO NPs facilitates surface reconstruction into high-active oxyhydroxides at lower potential (1.36 V) under OER conditions, as revealed by operando Raman spectroelectrochemistry. The DFT calculation corroborates that these reconstructed (Ni,Fe)oxyhydroxides are electrocatalytically active sites as the kinetics barrier is largely reduced over pure NFO without surface reconstruction. Given the diversity of materials (metal oxides, sulfides, phosphides, etc.) possessing the photo-to-thermal conversion, this effect may thus provide a unique and robust platform to boost highly active surface species in nanomaterials for a fundamental understanding of enhanced performance that may underpin future advances in electrocatalysis, photocatalysis, solar-energy conversion, and renewable-energy production.

A co-activation strategy for enhancing the performance of hematite in photoelectrochemical water oxidation

Hematite (α-Fe2O3) is a promising photoanode for photoelectrochemical (PEC) water splitting. However, the severe charge recombination and sluggish water oxidation kinetics extremely limit its use in photo-hydrogen conversion. Herein, a co-activation strategy is proposed, namely through phosphorus (P) doping and the loading of CoAl-layered double hydroxides (CoAl-LDHs) cocatalysts. Unexpectedly, the integrated system, CoAl-LDHs/P-Fe2O3 photoanode, exhibits an outstanding photocurrent density of 1.56 mA/cm2 at 1.23 V (vs. reversible hydrogen electrode, RHE), under AM 1.5 G, which is 2.6 times of pure α-Fe2O3. Systematic studies reveal that the remarkable PEC performance is attributed to accelerated surface OER kinetics and enhanced carrier separation efficiency. This work provides a feasible strategy to enhance the PEC performance of hematite photoanodes.

Atomically targeting NiFe LDH to create multivacancies for OER catalysis with a small organic anchor

Abstract The fabrication of porous structure in the ultrathin materials still faces high difficulties. In particular, the precise modulations in the porosity and size are highly challenging. In this work, we have introduced small molecules to overcome such a challenge. And this substantially contributes to the energy related applications, especially to the water-energy (WE) treatment. Electrocatalytic water-splitting is hindered by the sluggish kinetics of water oxidation, requiring efficient earth-abundant electrocatalysts for the oxygen evolution reaction (OER). Herein we demonstrate the robust OER activity by introducing metal and oxygen multivacancies in noble-metal-free layered double hydroxides (LDHs) through the specific electron-withdrawing organic molecule methyl-isorhodanate (CH3NCS). Our work reveals that the metal and oxygen vacancies endow NiFe LDH with enhanced electron transfer and modulate the H2O adsorption, thereby boosting the OER electrocatalytic properties. Remarkably, the best-performing laminar NiFe LDH nanosheets with metal and oxygen multivacancies (v- L -LDHs) show an ultra-low overpotential of 230 mV at 100 mA cm−2 and Tafel slope of 37.1 mV dec−1. Density functional theory (DFT) has revealed the improved OER performance is realized by the co-existence of metal and O vacancies in NiFe LDH, where the defective region activates the electroactivity of Ni sites and O sites to promote the electron transfer and intermediate transformation. The Fe sites play a key role to preserve the high electroactivity of the Ni sites in long-term applications. The superior OER performance underpins the high potential of the reported facile organic anchor strategy for designing and synthesizing advanced electrocatalysts in both LDH and other potential 2D layered materials.

Dual-bonding interactions between MnO2 cocatalyst and TiO2 photoanodes for efficient solar water splitting

Severe charge recombination and sluggish water oxidation kinetics greatly limited the practical applications of photoelectrochemical (PEC) water splitting. Herein, we demonstrated the nitrogen plasma treatment for simultaneously achieving oxygen vacancies and N-doping on TiO2 nanotube arrays. More importantly, the oxygen vacancies and N-doping could effectively promote the selective construction of MnO2 cocatalyst anchored with Mn-O/N bonds on TiO2 nanotube arrays, which could efficiently narrow charge depletion layer and promote the holes transfer from TiO2 to MnO2. As expected, this photoanode achieves a remarkably increased PEC performance (1.95 mA cm−2 at 1.23 VRHE), up to 3.4 times higher than that of pristine TiO2 (0.57 mA cm−2 at 1.23 VRHE), and excellent stability for water splitting. These demonstrations may provide a new insight for designing highly efficient TiO2-based photoanodes for solar energy conversion.

Selectively Etching Vanadium Oxide to Modulate Surface Vacancies of Unary Metal–Based Electrocatalysts for High‐Performance Water Oxidation

AbstractElectrocatalytic water splitting for hydrogen generation is hindered by the sluggish kinetics of water oxidation, and highly efficient electrocatalysts for the oxygen evolution reaction (OER) are urgently required. Numerous bi‐ and multimetal‐based, low‐cost, high‐performance OER electrocatalysts have been developed. However, unary metal–based high‐performance electrocatalysts are seldom reported. In the present study, Co2(OH)3Cl/vanadium oxide (VOy) composites are synthesized, from which VOy is completely etched out by a simple cyclic voltammetry treatment, which simultaneously transforms Co2(OH)3Cl in situ to ultrafine CoOOH. The selective removal of VOy modulates the nature of the surface in the obtained CoOOH by creating surface oxygen vacancies (Vo), along with disordered grain boundaries. The best‐performing CoOOH with optimum Vo is found to be associated with a low overpotential of 282 mV at 10 mA cm−2 catalytic current density on a simple glassy carbon electrode for OER. This facile protocol of selectively etching VOy to modulate the nature of the surface is successfully applied to synthesize another Fe‐based electrocatalyst with high OER performance, thus establishing its utility for unary metal–based electrocatalyst synthesis.

Modulating water oxidation kinetics utilizing h-BN quantum dots as an efficient hole extractor on fluorine doped hematite photoanode

The sluggish water oxidation kinetics in hematite photoanode is dominated by unfavorable recombination of photogenerated holes which restrict the efficiency of photoelectrochemical (PEC) water splitting. Herein, we demonstrate a superior photoelectrochemical performance using boron nitride quantum dots (BNQDs), which is incorporated over fluorine doped hematite (F–Fe2O3) photoanode. An impressive carrier density of 2.08 × 1020 cm−3 is achieved, which is two orders of magnitude higher than that of the bare α-Fe2O3 photo anode, resulting in a photocurrent density of 2.24 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE) for F–Fe2O3-BNQDs, which is six-fold higher than bare hematite. It is found that BNQD acts as an efficient hole extractor, which enhances the carrier separation on hematite surface and decreases the hole trapping probability. Present work emphasizes on two important parameters, i.e. (i) Enhancement in carrier density via fluorine doping directly onto the substrate using soluble precursor, organic Selectflour (ii) Modification with BNQDs to provide a simple, novel and effective strategy for the design and development of more efficient PEC water splitting systems.

Cooperative Catalytic Behavior of SnO2 and NiWO4 over BiVO4 Photoanodes for Enhanced Photoelectrochemical Water Splitting Performance

n-BiVO4 is a favorable photoelectrode candidate for a photoelectrochemical (PEC) water splitting reaction owing to its suitable energy level edge locations for an oxygen evolution reaction. On the other hand, the sluggish water oxidation kinetics of BiVO4 photoanodes when used individually make it necessary to use a hole blocking layer as well as water oxidation catalysts to overcome the high kinetic barrier for the PEC water oxidation reaction. Here, we describe a very simple synthetic strategy to fabricate nanocomposite photoanodes that synergistically address both of these critical limitations. In particular, we examine the effect of a SnO2 buffer layer over BiVO4 films and further modify the photoanode surface with a crystalline nickel tungstate (NiWO4) nanoparticle film to boost PEC water oxidation. When NiWO4 is incorporated over BiVO4/SnO2 films, the PEC performance of the resultant triple-layer NiWO4/BiVO4/SnO2 films for the oxygen evolution reaction (OER) is further improved. The enhanced performance for the PEC OER is credited to the synergetic effect of the individual layers and the introduction of a SnO2 buffer layer over the BiVO4 film. The optimized NiWO4/BiVO4/SnO2 electrode demonstrated both enriched visible light absorption and achieves charge separation and transfer efficiencies of 23% and 30%, respectively. The photoanodic current density for the OER on optimized NiWO4/BiVO4/SnO2 photoanode shows a maximum photocurrent of 0.93 mA/cm2 at 1.23 V vs. RHE in a phosphate buffer solution (pH~7.5) under an AM1.5G solar simulator, which is an incredible five-fold and two-fold enhancement compared to its parent BiVO4 photoanode and BiVO4/SnO2 photoanodes, respectively. Further, the incorporation of the NiWO4 co-catalyst over the BiVO4/SnO2 film increases the interfacial electron transfer rate across the composite/solution interface.

(Photo) electrochemical water oxidation at anodic TiO2 nanotubes modified by electrodeposited NiFe oxy-hydroxides catalysts

Anodic TiO2 nanotubes are potential candidates for industrial scale and stable photoelectrochemical water oxidation. Major roadblocks for the realization of this technology however are the limited light absorption, as this material usually absorbs only in the UV, and the sluggish water oxidation kinetics at the electrode/electrolyte interface. Various efforts have nonetheless demonstrated absorption and photocurrent generation with visible light while a typical solution for the slow water oxidation is electrode modification via oxygen evolution catalysts; NiFe (oxy) hydroxides in particular are among the highest turn-over efficiency and low-cost catalysts. Studies about loading NiFe (oxy) hydroxides onto anodic TiO2 nanotubes by electrodeposition methods are rarely reported and the influence of the loaded catalysts on the photoanode performance is not fully understood. In this work, NiFe (oxy) hydroxides are first electrodeposited on Au substrates; the as-deposited catalysts indicate enhanced oxygen evolution catalytic activity. The same deposition method is then applied at various TiO2 nanotube systems. Specifically, mouths, walls and whole bodies of anodic TiO2 nanotubes are electrodeposited with NiFe (oxy) hydroxides and the corresponding photo response to water oxidation are investigated. We show that at relatively low applied potential (≤1.5 V vs. RHE) the NiFe (oxy) hydroxides layer significantly reduces photon absorption and facilitates interfacial recombination, leading to a decreased photocurrent density. In contrast, at relatively high bias (>1.5 V vs. RHE), the oxygen evolution is enhanced due to the intrinsic electro-catalytic properties of the loaded catalysts. Synergistic effect are not found in the three NiFe (oxy) hydroxide films deposited at TiO2 nanotube systems.

2D Co-incorporated hydroxyapatite nanoarchitecture as a potential efficient oxygen evolution cocatalyst for boosting photoelectrochemical water splitting on Fe2O3 photoanode

The serious charge recombination together with the sluggish water oxidation kinetics have largely limited the practical application of hematite (Fe2O3) in photoelectrochemical (PEC) water splitting. Surface modification with oxygen evolution cocatalyst is an efficient strategy to address the both issues. Herein, a novel 2D oxygen evolution cocatalyst, namely Co-incorporated hydroxyapatite (Co-HAP) nanoarchitecture, was rationally designed and decorated on Fe2O3 photoanode. The resulting Co-HAP decorated Fe2O3 (Co-HAP/Fe2O3) exhibited excellent PEC water splitting with a high photocurrent density of 2.25 mA cm−2 at 1.23 V vs. RHE in neutral electrolyte, which is ca. 9.78 times that for bare Fe2O3. Moreover, the onset potential displayed a 200 mV cathodic shift, indicating an accelerated water oxidation kinetics over Fe2O3. PEC characterizations revealed Co-HAP could not only significantly improve the charge-separation efficiency but also could enhance the surface charge-separation efficiency in the bulk and on the surface of Fe2O3. Comprehensive investigations unveiled the interfacial negative electrostatic field and the increased electrical conductivity arising from Co-HAP decoration were of great benefit to improve the charge separation and inhibit surface charge recombination, while the 2D architecture of Co-HAP offered high surface area and abundant exposed Co active sites, ultimately boosted PEC water splitting over Fe2O3. Owing to the superior ion-exchange ability of HAP, the strategy presented here would open a new vane to explore highly efficient oxygen evolution cocatalyst.

Boosting solar water oxidation activity and stability of BiVO4 photoanode through the Co-catalytic effect of CuCoO2

The poor surface charge separation and transfer properties as well as sluggish water oxidation kinetics jointly limit the performance of BiVO4 photoanode for water oxidation. In this work, p-type CuCoO2 with high-spin Co3+(Oh) was investigated as co-catalyst to synergistically improve the surface charge separation and transfer efficiencies as well as reaction kinetics of BiVO4 film for photoelectrochemical water oxidation. In comparison with the photocurrent on BiVO4 photoanode for water oxidation (1.21 mA/cm2 at 1.23 V vs. RHE), the CuCoO2-coupled BiVO4 (CuCoO2/BiVO4) photoanode exhibits a higher photocurrent density of 3.32 mA/cm2 at 1.23 V vs. RHE under AM 1.5G illumination. In addition, a significant improvement on the reaction stability is achieved on the CuCoO2/BiVO4 photoanode, about ∼79% water oxidation activity is retained on the CuCoO2/BiVO4 photoanode after operating at 0.8 V vs. RHE for 5 h, while only ∼9% activity is retained on the BiVO4 photoanode. The boosted water oxidation activity and stability on CuCoO2/BiVO4 photoanode could be attributed the synergistic effect that originated from CuCoO2-electrocatalysis and BiVO4-photocatalysis in thermodynamics and kinetics. Specifically, p-n heterojunctions are formed in the coupling interface between CuCoO2 (p-type) and BiVO4 (n-type), which thermodynamically improve the surface charge separation and transfer efficiencies of BiVO4 photoanode during water oxidation. Simultaneously, the high-spin Co3+(Oh) of CuCoO2 could act as active sites to accelerate the water oxidation of CuCoO2/BiVO4 photoanode in kinetics. In addition, Cu2+ active sites are formed for water oxidation through the oxidation reaction of photogenerated holes with the Cu+ of CuCoO2.

Tailoring surface states by sequential doping of Ti and Mg for kinetically enhanced hematite photoanode

Hematite (α-Fe2O3) is the most promising photoanode with a high theoretical photocurrent of 12.6 mA cm−2. However, the photocurrent of Fe2O3 achieved now is far below its theoretical value, which is mainly due to its poor electronic conductivity and sluggish water oxidation kinetics. Herein, a co-doping method by sequential in-situ Ti-doping and ex-situ Mg-doping is used to tailor the surface states of Fe2O3 photoanode for great improvement of the charge transfer at the interface and the followed transport ability by the suppressed charge recombination, resulting in about 11-folds and 6.5 times higher than that of the undoped Fe2O3 and Ti:Fe2O3 at 1.23 V vs. RHE, respectively. This is mainly due to Mg and Ti-doping into Fe2O3 modifying the electrode surface states for more holes participation in water oxidation and better kinetics that enhanced charge transfer and suppressed charge recombination for the efficient water oxidation.