α-Fe2O3 Photoanodes Research Articles

Room-temperature photocatalytic methanol fuel cell based on one-dimension semiconductor photoanode: Intrinsic mechanism of photogenerated charge separation

We report a photocatalytic methanol fuel cell (PMFC) based on one-dimension TiO2, ZnO and α-Fe2O3 photoanode for efficient methanol oxidation reaction at room temperature. Electro-chemical analysis along with Mott-Schottky (MS) equation and cyclic voltammetry (CV) revealed thermodynamic feasibility of charge transfer and methanol oxidation in the system. Time-resolved surface photovoltage (SPV) transient gave a dynamic information of photogenerated charge carriers, which is related to types of materials, sample thickness and light intensity. The ZnO-based PMFC got the maximum short-circuit current of 1.82 mA cm−2 and TiO2-based fuel cell had an open-circuit voltage of 1.34 V. This study opens an available field to develop power cells taking advantage of both chemical energy and solar energy.

Integrated design of hematite and dye-sensitized solar cell for unbiased solar charging of an organic-inorganic redox flow battery

Integration of the photoelectrochemical cells and redox flow batteries (RFBs) is conceptualized as solar redox flow cells (SRFCs). The SRFC opens an opportunity for simultaneous electrochemical conversion and storage of solar energy in a single device. This work proposes a SRFC which integrates stable hematite (α-Fe2O3) photoanode with a near-neutral organic-inorganic RFB using organic and low-cost redox pair anthraquinone-2,7-disulfonate disodium (AQDS) on the negative side and iodide/iodine (I−/I2) redox couple on the positive side. The AQDS/iodide RFB exhibited a stable battery cycling with a cell voltage of ~0.8 V, a high coulombic efficiency of 99.2% and a peak power density of 0.22 W⋅cm−2. The hematite yielded a stable performance in contact with iodide/iodine solution with pH 5.5, despite known stability issues for pH < 7. Electrochemical impedance spectroscopy was used to elucidate the main semiconductor/electrolyte charge transfer mechanisms and rates and energy level alignment of hematite with redox pair voltages. Finally, to photocharge the AQDS/iodide SRFC unbiasedly, a tandem system was used which includes hematite photoelectrode connected in series with a dye-sensitized solar cell (DSSC). The tandem photoelectrode provided 1.6 V of photovoltage that is sufficient to fully charge the RFB.

Hydrogen sulfide splitting using solar energy and hematite photo-anodes

The mechanism was investigated of hydrogen sulfide splitting in alkaline aqueous solutions using spray-pyrolysed SnIV-doped α-Fe2O3 photo-anodes in a photo-electrochemical cell. In principle, hydrogen sulfide splitting can be used to treat hydrogen sulfide in natural and process gases and simultaneously to produce hydrogen using solar energy. A comparison with conventional water splitting demonstrated the lower energy requirements to achieve the same photocurrent densities, while producing soluble polysulfide ions and elemental sulfur from hydrogen sulfide oxidation. However, neither splitting process was spontaneous using SnIV-doped α-Fe2O3 photo-anodes without inputs of electrical energy; two judiciously chosen photo-electrodes are required to achieve that objective. The effects were also studied of stirring, hydrogen sulfide ion concentration, electrode potential and annealing of SnIV-doped α-Fe2O3 films on titanium substrates. Under potentiostatic conditions during photo-assisted electrolysis, the photo-anodes exhibited no compositional or morphological changes after 18 h. In a bench-scale reactor (0.1 dm3), stable photocurrent densities of ca. 12.5 A m−2 were recorded over 12 h at an electrode potential of 1.17 V vs. RHE and an effective irradiance of 2670 W m−2. Similarly, photocurrent densities corresponding to ca. 4.3 A m−2 were achieved in an up-scaled reactor under an effective irradiance of 457 W m−2. Charge yields for formation of polysulfide ions were close to unity when operating at optimised potentials and hydrogen sulfide ion concentrations. The shift towards lower electrode potentials of photocurrent densities for hydrogen sulfide splitting compared with those for water splitting was associated with increased charge transfer rates due to decreased interfacial electron-hole recombination rates. The potential dependences of sulfur coverage and oxygen evolution rates were also estimated.

Electrodeposition of a cobalt phosphide film for the enhanced photoelectrochemical water oxidation with α-Fe2O3 photoanode

The development of photoanodes capable of photoelectrochemical water splitting is a sustainable approach for the storage of solar power to conveniently useable chemical energy. In this study, we report that amorphous CoP film can be grown on α-Fe2O3 nanorods to fabricate a core-shell structure via a simple reductive electrodeposition strategy. Under the irradiation of simulated sunlight (100 mW cm−2), the composite electrode shows that the onset potential demonstrates a 200 mV cathodic shift and the photocurrent density is enhanced from 0.45 to 0.90 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE) when compare with the bare α-Fe2O3 electrode in 1 M KOH electrolyte. Long-term photoelectrolysis reveals that this electrode has excellent stability and possesses a near-unity Faradaic efficiency toward oxygen evolution. Mechanistic studies support the role of CoP film in enhancing photoelectrochemical performance and indicate that the remarkable photoelectrochemical performance of this composite electrode is mainly due to the retardation of surface photogenerated h+/e− recombination.

Photo-deposition of cobalt-phosphate group modified hematite for efficient water splitting

In this work, Cobalt Phosphate Group (Co−Pi) Modified nanostructured hematite (α-Fe2O3) photoanodes films were deposited on fluorine-doped tin oxide (FTO) coated glass by means of the hydrothermal process. The α-Fe2O3 film morphology can be tuned from nanoparticle – based patterns to high-density cabbage-like structures as Co−Pi deposition time is increased.Systematic studies were performed to clarify the mechanisms that enhance water-splitting performance of α-Fe2O3 electrodes as Co−Pi deposition time increases. All samples followed morphological and structural studies using field-emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. Chronoamperometry measurements showed that Co-Pi/α-Fe2O3 photoanode exhibited higher photoelectrochemical activity than the α-Fe2O3 films. The maximum photocurrent density and incident photon conversion efficiencies (IPCE) were obtained for 30 s Co-Pi/α-Fe2O3 photoanode. All samples reached their best IPCE at 400 nm. IPCE values for 30 s Co-Pi hematite films were 5 times higher than that of α-Fe2O3 sample. For the α-Fe2O3 photoanode, the H2 evolution rate was ∼5.14 μmol cm−2 h−1. After decoration by Co-Pi, the rate increases to ∼7.31 μmol cm−2 h−1, showing an enhancement of about ∼ 42.2%. It was found that under illumination, the Co−Pi catalyst can efficiently collect and store photogenerated holes from the α-Fe2O3 electrode. This charge separation reduces surface state recombination, leading to the enhancement of the efficiency of water oxidation. Thicker catalytic Co−Pi films were found to increase charge separation and surface area, which in turn increase water oxidation efficiency. These results provide important new understanding of the catalytic enhancement and limitations of the Co−Pi catalyst once coupled to semiconductor electrodes for water-splitting applications.

Unraveling the role of single layer graphene as overlayer on hematite photoanodes

In this work, the role of the single layer graphene as overlayer on hematite photoanodes surface was investigated for water oxidation reaction via photoelectrochemical process. Single layer graphene (SLG) was synthesized by chemical vapour deposition (CVD) and transferred from the copper foil to the α-Fe2O3 photoanode surface. To ensure the purity and quality of SLG, elemental composition of the samples was investigated in all steps and no metal (Cu) trace was found after the transference onto the photoanode surface. The photocurrent density of α-Fe2O3/graphene photoanode was increased from 1.25 to 1.64 mA cm−2 at 1.23 VRHE in comparison to bare α-Fe2O3 photoanode. The role of SLG added on α-Fe2O3 and the charge carrier dynamics were investigated combining transient absorption spectroscopy (TAS) and surface photovoltage spectroscopy (SPS). These combined techniques revealed that the photogenerated holes had their lifetime increased as a function of applied bias and after the SLG deposition onto the α-Fe2O3 photoanode surface. As consequence, the photochemical separation and transfer of the photogenerated charge carriers became more efficient in the presence of SLG. The incorporation of SLG on α-Fe2O3 photoanode is believed to suppress the surface traps, enabling holes to diffuse into the solid-liquid interface and promoting water oxidation reaction driven by sunlight irradiation.

Fe2O3 Blocking Layer Produced by Cyclic Voltammetry Leads to Improved Photoelectrochemical Performance of Hematite Nanorods

Hematite is a low band gap, earth abundant semiconductor and it is considered to be a promising choice for photoelectrochemical water splitting. However, as a bulk material its efficiency is low because of excessive bulk, surface, and interface recombination. In the present work, we propose a strategy to prepare a hematite (α-Fe2O3) photoanode consisting of hematite nanorods grown onto an iron oxide blocking layer. This blocking layer is formed from a sputter deposited thin metallic iron film on fluorine doped tin oxide (FTO) by using cyclic voltammetry to fully convert the film into an anodic oxide. In a second step, hematite nanorods (NR) are grown onto the layer using a hydrothermal approach. In this geometry, the hematite sub-layer works as a barrier for electron back diffusion (a blocking layer). This suppresses recombination, and the maximum of the incident photon to current efficiency is increased from 12% to 17%. Under AM 1.5 conditions, the photocurrent density reaches approximately 1.2 mA/cm2 at 1.5 V vs. RHE and the onset potential changes to 0.8 V vs. RHE (using a Zn-Co co-catalyst).

Anion engineering of exfoliated CoAl layered double hydroxides on hematite photoanode toward highly efficient photoelectrochemical water splitting

The exploration of highly efficient oxygen evolution cocatalysts is believed to be an efficient strategy to enhance the photoelectrochemical (PEC) water splitting of hematite (α-Fe2O3) photoanode. Herein, exfoliated CoAl layered double hydroxides (CoAl-LDHs) were successfully decorated on α-Fe2O3 films by using layer-by-layer assembly method with the assistance of common anions (An−), such as NO3−, CO32−, SO42− and PO43−. Results indicated An−-CoAl-LDHs/α-Fe2O3 photoanodes exhibited strong anion-dependent performance for PEC water oxidation. The NO3−, CO32−, SO42− and PO43− engineered CoAl-LDHs/α-Fe2O3 displayed the photocurrent density at 1.23 V in 1 mol L−1 KOH of 1.90, 2.36, 3.19 and 4.30 mA cm−2, which were ca. 2.5, 3.1, 4.2 and 5.6 times that of bare α-Fe2O3 (0.76 mA cm−2) respectively. Experimental studies indicated tetrahedral PO43− and SO42− were much more feasible for the assembly of CoAl-LDHs than that of triangular NO3− and CO32−, thus caused higher electrochemical surface area (ECSA). The anion with higher charge number and the tetrahedral configuration were more beneficial for the charge separation in bulk α-Fe2O3 and the charge transfer between CoAl-LDHs slabs. Mott-Schottky tests suggested p-n heterojunctions were formed at the interface of α-Fe2O3 and CoAl-LDHs, which could inhibit the surface charge recombination and further increase charge separation. As a result, PO43−-CoAl-LDHs/α-Fe2O3 displays the best performance for PEC water oxidation. The strategy of engineering exfoliated CoAl-LDHs with anions on α-Fe2O3 in this work will stimulate us to fabricate highly efficient photoanodes with various anions and LDHs materials.

Alkali Treatment for Enhanced Photoelectrochemical Water Oxidation on Hematite Photoanode

As one of the most popular photoanode materials for photoelectrochemical (PEC) water splitting, hematite (α-Fe2O3) suffers from low conductivity, severe electron–hole recombination, and sluggish water oxidation kinetics unfortunately. Herein, we report an alkali-treatment method to effectively accelerate the water oxidation kinetics of α-Fe2O3 and titanium doped α-Fe2O3 (Ti:α-Fe2O3) nanorod array photoanodes. The purpose of Ti-doping is to increase the conductivity of α-Fe2O3. The photocurrent densities increased 3- and 2-times for α-Fe2O3 and Ti:α-Fe2O3 photoanodes after KOH treatment, respectively. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses demonstrated that a conformal thin layer grafted with hydroxyl (−OH) groups was formed on the hematite surface. Linear sweep voltammetry (LSV) curves under light irradiation and in the dark indicated that the thin OH-grafted overlayer behaved like an electrocatalyst to accelerate the water oxidation kinetics on hematite...

Self-improvement of solar water oxidation for the continuously-irradiated hematite photoanode.

Improving bulk- or surface-properties has been found as an effective route to regulate and enhance the photoelectrochemical (PEC) performances of some metal-oxide photoelectrodes. However, both bulk and surface self-improvement resulting from the photocharging (PC) effect is rarely reported and as a result the underlying mechanism of the PC effect is not fully understood. Here, we demonstrate that the hematite photoanode integrated with Sn doping and a TiO2 underlayer shows a substantial increase in the photocurrent density (i.e., from 0.69 to 1.12 mA cm-2 at 1.23 V relative to the standard hydrogen electrode) and a cathodic shift of the onset potential after being irradiated by a one-sun simulator for 12 h. The primary reasons for these can be categorized into two fundamental factors: (1) the enhanced bulk conductivity and the resulting decrease in carrier bulk recombination from the gradually increasing ratio of Fe2+ and Fe3+; (2) the reduced carrier surface recombination from the photogenerated passivation layer. Ultimately, both the bulk and surface electrical properties of the hematite photoanode are substantially self-improved under continuous irradiation. This work deepens the understanding of the PC effect and proves that it is a promising technique for the PEC-performance enhancement of the hematite photoanode.

Introduction of oxygen vacancies into hematite in local reducing atmosphere for solar water oxidation

Sn Doping and creation of oxygen vacancies have been adopted universally to overcome the poor electric conductivity and unfavorable hole diffusion length of α-Fe2O3 photoanodes. Generally, Sn doping is realized via longitudinal migration of tin element from FTO (fluorine-doped tin oxide) substrates into α-Fe2O3 at high temperature. To introduce oxygen vacancies along with Sn into hematite for further promoting its electric conductivity, we have created a local reducing atmosphere via partial oxidation of graphite while doping hematite with Sn. The donor density of the resultant Fe2O3 photoanode annealed on graphite (G-Fe2O3) at 770 °C for 20 min is increased to ∼1.7 times that of the counterpart annealed on SiO2 powders (S-Fe2O3), indicating that the electric conductivity of hematite is improved after introduction of oxygen vacancies. Moreover, oxygen vacancies have been demonstrated to significantly reduce the charge transfer resistance of Sn doped hematite. Consequently, the photocurrent density of G-Fe2O3 is enhanced remarkably (∼70%) compared with S-Fe2O3. However, the improvement in photocurrent density due to oxygen vacancies becomes less significant when more Sn is doped into hematite. The strategy for creation of oxygen vacancies reported here can be extended to other photoanodes for better understanding the effect of oxygen vacancies on PEC performance.

Enhanced Charge Transport and Increased Active Sites on α-Fe2O3 (110) Nanorod Surface Containing Oxygen Vacancies for Improved Solar Water Oxidation Performance.

The effect of oxygen vacancies (VO) on α-Fe2O3 (110) facet on the performance of photoelectrochemical (PEC) water splitting is researched by both experiments and density functional theory (DFT) calculations. The experimental results manifest that the enhancement in photocurrent density by the presence of VO is related with increased charge separation and charge-transfer efficiencies. The electrochemical analysis reveals that the sample with VO demonstrates an enhanced carrier density and reduced charge-transfer resistance. The results of DFT calculation indicate that the better charge separation is also contributed by the decrease of potential on the VO surface, which improves the hole transport from the bulk to the surface. The reduced charge-transfer resistance is owing to the greatly increased number of active sites. The current study provides important insight into the roles of VO on α-Fe2O3 photoanode, especially on its surface catalysis. The generated lesson is also helpful for the improvement of other PEC photoanode materials.

Two-site H2O2 photo-oxidation on haematite photoanodes

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.

Effects of cathodic electrodeposition conditions on morphology and photoelectrochemical response of α-Fe2O3 photoanode

α-Fe2O3 photoanodes were prepared on Fluorine-doped tin oxide glass by annealing of electrodeposited Fe films. By controlling the synthesis parameters, the α-Fe2O3 film prepared from the Fe film electrodeposited in 0.05 M ferrous sulfate solution (pH = 2.1) for 50 s and then annealed at 600 °C for 3 h has the optimal photocurrent density around 257.08 μA cm−2 at 1.23 V vs. RHE (Reversible Hydrogen Electrode). In this paper, the effects of different synthesis parameters on α-Fe2O3 photoanodes were studied systematically. The morphology and properties of the sample were characterized with scanning electron microscopy, UV–vis spectra, X-ray diffractometry and photoelectrical measurements.

Toward Tandem Solar Cells for Water Splitting Using Polymer Electrolytes

Tandem photoelectrochemical cells, formed by two photoelectrodes with complementary light absorption, have been proposed to be a viable approach for obtaining clean hydrogen. This requires the development of new designs that allow for upscaling, which would be favored by the use of transparent polymer electrolyte membranes (PEMs) instead of conventional liquid electrolytes. This article focuses on the photoelectrochemical performance of a water-splitting tandem cell based on a phosphorus-modified α-Fe2O3 photoanode and on an iron-modified CuO photocathode, with the employment of an alkaline PEM. Such a photoelectrochemical cell works even in the absence of bias, although significant effort should be directed to the optimization of the photoelectrode/PEM interface. In addition, the results reveal that the employment of polymer electrolytes increases the stability of the device, especially in the case of the photocathode.

Fabrication of Fe-doped Co2P nanoparticles as efficient electrocatalyst for electrochemical and photoelectrochemical water oxidation

Fe-doped Co2P nanoparticles (Fe-Co2P) are successfully synthesized using a facile and scalable solid reaction method and demonstrate highly efficient oxygen evolution reaction (OER) activity towards water splitting. Benefiting from the synergistic effect of Fe, Co and P as well as enlarged electrochemical active surface area (ECSA), the resultant Fe-Co2P nanoparticles yield extraordinary OER performance with an overpotential as low as 289 mV at 10 mA cm−2, a small Tafel slope of 40 mV dec−1, and excellent OER stability, which are superior to the individual Co2P, Fe2P as well as the commercial RuO2 electrocatalyst. Moreover, Fe-Co2P nanoparticles are also shown to be a resultful co-catalyst to promote the photoelectrochemical (PEC) water oxidation performance of α-Fe2O3 photoanode, leading to a cathodic potential shift of ∼200 mV at 1.0 mA cm−2 and a significantly promoted photocurrent density of 1.40 mA cm−2 at 1.23 VRHE with respect to that of bare α-Fe2O3. The effective synthesis method and electronic modulate-induced enhancement of OER by hetero-atom doping highlight the promising design and fabricate of various efficient bimetallic or polymetallic phosphides for photo-/electrochemical water splitting.

Synergistic effect of Ti(OBu)4 and annealing regime on the structure, morphology and photoelectrochemical response of α-Fe2O3 photoanode

Ti-modified α-Fe2O3 photoanodes are prepared by one-step hydrothermal method with Ti(OBu)4 solution as the Ti precursor followed by annealing treatment. Through the synergistic effect of small amount of Ti(OBu)4 and high annealing temperature, the 0.3TiFe-H photoanode has a significant enhanced photocurrent response as 1.31 mA cm−2, which is almost 33 times than that of the Bare-Fe photoanode as 0.04 mA cm−2 and 3 times than that of the Bare-Fe-H photoanode as 0.46 mA cm−2. The enhanced photoelectrochemical response is mainly attributed to the increased bulk charge separation caused by excellent conductivity, and surface charge injection caused by synergistic effect of decreased resistance and increased capacitance at the surface. This work provides an indepth understanding of the effect of Ti(OBu)4 and annealing regime on morphology, crystalline structure and photoelectrochemical performance of Ti-modified hematite photoanode via systematical characterization with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, UV–vis spectra, X-ray diffractometry (XRD), X-ray photoelectron spectra (XPS), and photoelectrical measurements.

Fabricating of Fe2O3/BiVO4 heterojunction based photoanode modified with NiFe-LDH nanosheets for efficient solar water splitting

It is essential to develop a high activity and stability photoanode for enhancing PEC water splitting performance. According to energy band matching principle the Fe2O3/BiVO4 heterojunction based photoanode is constructed by electrodeposition and metal organic decomposition, following by electrodeposited NiFe-LDH on heterojunction to structure a trinary integrating photoanode. The structure of material and PEC properties of the photoanode are systemically studied, the results show that the photoanode achieves the highest photocurrent density that is about 4.25 times and 2 times higher than that of pristine α-Fe2O3 and BiVO4 photoanodes. The IPCE value is 3.7 times higher than α-Fe2O3 and the onset potential has a significant cathodic shift of 400 mV compared to the pristine α-Fe2O3 for the ternary integrating photoanode. These results are higher than or are comparable to other α-Fe2O3 composites reported in literatures, which is attributed to the efficient separation of the photoexcited electron-hole pairs and alleviates holes accumulation at the surface of electrode due to the double action of semiconductor junction and co-catalyst of NiFe-LDH.

Nanocomposites Based on CoSe2-Decorated FeSe2 Nanoparticles Supported on Reduced Graphene Oxide as High-Performance Electrocatalysts toward Oxygen Evolution Reaction.

FeCo-based materials are promising candidates as efficient, affordable, and sustainable electrocatalysts for oxygen evolution reaction (OER). Herein, a composite based on FeSe2@CoSe2 particles supported on reduced graphene oxide (rGO) was successfully prepared as an OER catalyst. In the catalyst, the CoSe2 phase was located on the FeSe2 surface, forming a large number of exposed heterointerfaces with acidic iron sites because of strong charge interaction between CoSe2 and FeSe2. It is believed that the exposed heterointerfaces act as catalytic active sites for OER via a two-site mechanism, manifesting an overpotential as low as 260 mV to reach the current density of 10 mA cm-2 in 1 M KOH and excellent stability for at least 6 h, which is superior to those of CoSe2/rGO, FeSe2/rGO, as well as most of the FeNi- and FeCo-based electrocatalysts reported in recent literatures. It was demonstrated that the most optimal composite electrocatalysts release more Fe species into the electrolyte during the OER process, whereas the releasing of Co species is negligible. When the FeSe2@CoSe2/rGO catalysts were loaded on a α-Fe2O3 photoanode, the photocurrent density was increased by three times. These results may open up a promising avenue into the design and engineering of highly active and durable catalysts for water oxidation.

Enhanced efficiency of hematite photoanode for water splitting with the doping of Ge

Ge doped α-Fe2O3 nanowires are synthesized through a hydrothermal procedure with GeO2 as a precursor and investigated as photoanodes for water splitting. The content of Ge in the photoanode rises with the increase of the amount of GeO2 in the precursor solution. A proper amount of Ge facilities the preferred oriented growth of the (110) plane of α-Fe2O3, while excessive Ge hinders the growth of α-Fe2O3 crystals. The doping of Ge increases the absorption efficiency and decreases the recombining rate of the photogenerated electrons and holes. Ge also improves the density and transfer rate of the charge carriers in the photoanode. Ge doped α-Fe2O3 photoanode exhibits a highest photocurrent density of 0.92 mA cm−2 at 1.23 V vs. reversible hydrogen electrode under AM 1.5 G simulated sunlight, which is nearly twice of that obtained by pure α-Fe2O3 under the same condition.