Performance Of Photoanodes Research Articles

Facet-dependent activity of tailored anatase TiO2 crystals in photoanodes for photocatalytic fuel cells

A key component of the photocatalytic fuel cell (PFC) is a photoanode, typically based on highly stable semiconductor (e.g., TiO2) that is capable of effective photoelectrocatalytic oxidation of fuels. Notably, the photo(electro)catalytic activity of TiO2 materials can depend heavily on the microstructural morphology, so that tailored crystals with different shapes and exposed facets can show remarkably different, shape-dependent photoactivities. Herein, we study the PFC performance of photoanodes based on anatase crystals of various shapes – tetragonal truncated bipyramids, sheets and belts, which are characterized by different predominantly exposed facets: {101}, {001}, and {100}, respectively. We demonstrate that photoanodes based on belt-shaped crystals dominated by {100} facets, not observed in natural anatase crystals, exhibit superior performance. Analysis of open-circuit potential and impedance data suggests that this activity is related to faster transfer of photogenerated holes to methanol oxidation sites, avoiding thus the undesired surface trapping and Fermi level pinning effects that are the most likely reason of a poorer performance of materials dominated by {101} and {001} facets. This study highlights the importance of advanced crystal engineering and control over the surface catalytic properties of TiO2 materials for PFCs and other light-driven devices in which efficient alcohol photooxidation is crucial.

NiFeOx decorated Ge-hematite/perovskite for an efficient water splitting system

To boost the photoelectrochemical water oxidation performance of hematite photoanodes, high temperature annealing has been widely applied to enhance crystallinity, to improve the interface between the hematite-substrate interface, and to introduce tin-dopants from the substrate. However, when using additional dopants, the interaction between the unintentional tin and intentional dopant is poorly understood. Here, using germanium, we investigate how tin diffusion affects overall photoelectrochemical performance in germanium:tin co-doped systems. After revealing that germanium is a better dopant than tin, we develop a facile germanium-doping method which suppresses tin diffusion from the fluorine doped tin oxide substrate, significantly improving hematite performance. The NiFeOx@Ge-PH photoanode shows a photocurrent density of 4.6 mA cm−2 at 1.23 VRHE with a low turn-on voltage. After combining with a perovskite solar cell, our tandem system achieves 4.8% solar-to-hydrogen conversion efficiency (3.9 mA cm−2 in NiFeOx@Ge-PH/perovskite solar water splitting system). Our work provides important insights on a promising diagnostic tool for future co-doping system design.

Preparation of multilayer periodic nanopatterned WO3-based photoanode by reverse nanoimprinting for water splitting

Photoelectrochemical (PEC) water splitting has been studied extensively as an environmentally friendly technology for hydrogen production using solar energy. WO3 is considered a promising semiconducting material for photoanodes due to its high electron mobility, good hole diffusion length, and chemical stability. Periodic nanostructures of WO3 have been investigated for enhancing the PEC performance of WO3-based photoanodes. In this study, facile fabrication of periodic nanostructures of WO3 was achieved using reverse nanoimprint lithography, and the multilayer stacking of nanopatterned WO3 film was also confirmed. The multilayer nanopatterned WO3 films were used as photoanodes for PEC water splitting. The performance of the fabricated photoanode in PEC was 2 times higher than that of planar WO3 film due to its higher light absorbance and lower charge transfer resistance.

Conformal coating of superhydrophilic metal-organic complex toward substantially improved photoelectrochemical water oxidation

The core challenge of BiVO4 photoanodes toward water oxidation originates from their poor photoelectrochemical (PEC) kinetics and stability. Herein, we report a novel superhydrophilic metal–organic complex coating to show a significant improvement in the PEC performance of BiVO4 photoanodes. The in situ coordination of phytic acid (PA) molecules with metal ions enables uniform deposition of superhydrophilic CoFe-PA complex on the BiVO4 scaffold. The CoFe-PA could serve as an excellent cocatalyst coating to promote the surface hole transfer kinetics and armor the structural integrity of BiVO4, thus remarkably promoting the PEC activity and stability. As a result, the CoFe-PA modified BiVO4 photoelectrode exhibits a photocurrent density as high as 4.5 mA cm−2 at 1.23 V vs. RHE, which is triple of the bare counterpart. Additionally, the hybrid photoanode manifests a more negative onset potential of ca. 0.24 V vs. RHE and remarkably enhanced stability. This work will provide a fresh impetus of developing metal–organic complex films toward efficient PEC water oxidation.

Photocatalytic H2O2 generation assisted photoelectrochemical water oxidation for enhanced BiVO4 photoanode performance

A method is devised to integrate a photocatalytic and photoelectrochemical (PEC) systems to achieve enhanced water oxidation performance of a BiVO4 photoanode. The reduced gC3N4 (R-gC3N4) photocatalyst suspended in the anode compartment electrolyte of the PEC cell can serve as an agent to integrate the systems. The performance of BiVO4 photoanode in PEC water splitting is hampered by the surface states back electron-hole recombination. This is overcomed by incorporating R-gC3N4 in the electrolyte to produce hydroxyl radicals and hydrogen peroxide reactive intermediates, thereby reducing the onset potential for oxygen evolution reaction in the BiVO4 photoanode. The enhanced charge-transfer for water oxidation reaction kinetics at the photoanode/electrolyte interface is observed by suspending R-gC3N4 photocatalyst in the electrolyte. Approximately 3-fold increase in photocurrent density (105 μA/cm2 at 1.23 V vs. RHE) and a significant reduction in the onset potential (395 mV vs. RHE) of the BiVO4 photoanode is observed in R-gC3N4 + 2-propanol + 0.1 M PBS electrolyte. The application of R-gC3N4 photocatalyst to promote the reactivity of neutral pH electrolytes introduces a new paradigm for hydrogen generation.

Uniformly citrate-assisted deposition of small-sized FeOOH on BiVO4 photoanode for efficient solar water oxidation

Nanoporous BiVO4 is a promising photoanode material for driving the photoelectrochemical (PEC) water splitting to hydrogen energy, whereas its performance is limited by the sluggish surface catalysis reaction. Herein, a facile citrate-assisted method was developed for growing uniform amorphous FeOOH overlayer on nanoporous BiVO4, which significantly improved the water oxidation kinetics and resulted in a remarkable photocurrent density of 3.33 mA cm−2 at 1.23 V (vs. RHE). The activity enhancement was even superior to the well-crystallized α-FeOOH and β-FeOOH modified BiVO4. The citrate serves as the bridging agent that not only optimizes the BiVO4/FeOOH interface facilitating the hole trapping/transport, but also inhibits the particle growth of FeOOH nanolayer and provides more oxygen vacancies to promote the water oxidation. We demonstrate that the uniform FeOOH overlayer acts as the hole storage layer and catalyst, which increases the potential drop in the Helmholtz layer and greatly facilitates the interfacial hole transfer. Moreover, the citrate-assisted modification method was proved to be a general strategy to improve the PEC performance of TiO2 and α-Fe2O3 photoanodes. Our work provides a simple and effective design strategy for constructing uniform semiconductor/electrocatalysts interface that can realize efficient solar-to-hydrogen conversion.

Enhancing the performance of dye-sensitized solar cell using nano-sized erbium oxide on titanium oxide photoanode by impregnation route

Modification of conventional photoanode with oxide nanoparticles is a practical tactic to improve the performance of electrochemical devices. In this study, erbium oxide (Er2O3) nanoparticles were mixed with the TiO2 paste and photovoltaic performance of the dye-sensitized solar cell (DSSC) was investigated. The highest power conversion efficiency of 6.39% was reached with the 1 wt.% of Er2O3 in photoanode, while in the similar condition the device with the pure TiO2 photoanode achieved the power conversion efficiency of 3.54%. This improvement in the power conversion efficiency was attributed to declining in charge recombination, improved open-circuit voltage and short-circuit current due to the presence of the critical amount of Er2O3. XPS analysis confirms the structural coherence between the Er2O3 and TiO2, which enhanced the optical performance of the composite photoanode. Composite photoanodes were further characterized by FESEM-EDX, XRD, and AFM for structural and morphological studies.

The Role of Surface States on Reduced TiO2@BiVO4 Photoanodes: Enhanced Water Oxidation Performance through Improved Charge Transfer

The efficient transfer of photogenerated carriers and improved stability against corrosion are essential to maximize the performance of photoanodes. Herein, a reduced catalytic layer formed on a TiO2 protected BiVO4 (R-TiO2@BiVO4) photoanode has been prepared for progress on both fronts. Specifically, R-TiO2@BiVO4 photoanodes at pH 8 displayed a high photocurrent of 2.1 mA cm–2 at 1.23 VRHE and a more negative onset potential of 234 mVRHE compared to pristine BiVO4. We here discovered two surface states on BiVO4 photoanodes through photoelectrochemical impedance studies. In contrast, only one of them, located at higher potential, appeared on oxygen-vacancy-rich R-TiO2@BiVO4 photoanodes. For BiVO4 photoanodes, the first surface state (SS1) is located near the onset potential (∼0.45 VRHE), while the second surface state (SS2) sits near the water oxidation potential (∼1.05 VRHE). However, SS1 at lower energetics, which originated from water oxidation intermediates with slow kinetics, is passivated in R-TiO2@BiVO4 photoanodes. In contrast, the hole densities in SS2 at higher energetics were greatly enhanced in R-TiO2@BiVO4 photoanodes, due to the increased accumulation of intermediates with fast water oxidation kinetics. Therefore, SS2 is proposed as a reaction center, which is related to the amount and occupancy of oxygen vacancies. Additionally, surface recombination centers in BiVO4 photoanodes are passivated by TiO2, which prevents electron trapping into the irreversible surface conversion of VO2+ to VO2+. These observations provide fundamental understanding of the role of surface states and of the function of oxygen vacancies in BiVO4 photoanodes. Our study offers detailed insight into key strategies for optimal photoelectrochemical performance through surface property tuning.

Activation of α-Fe2O3 Photoanode by Rapid Annealing Process for Photoelectrochemical Water Splitting

Hematite (α-Fe2O3) has been attracted for PEC water splitting applications due to its chemical stability and visible light absorption property. To, further improve the PEC water splitting performance of the α-Fe2O3 photoanode, a new activation process of hybrid microwave annealing (HMA) was taken on. In the present work, an α-Fe2O3 photoanode was synthesized by the companied hydrothermal method and HMA process. The effect of HMA processing time on PEC water splitting performance of the α-Fe2O3 photoanode was investigated. The results showed that a shorter HMA processing time (30 s) of photoanode provides a higher photocurrent density (∼0.74 mAcm−2 at 1.23 V vs RHE) than the photocurrent density (∼0.11mAcm−2at 1.23 V vs RHE) of photoanode processed by longer time (3 min). This result due to the cauliflower-like morphology of α-Fe2O3(30 s) photoanode facilitates the enhanced light absorption thus provides a higher electrical conductivity with suppressed photogenerated charge carrier’s recombination rates.

Thermal Ramping Rate during Annealing of TiO2 Nanotubes Greatly Affects Performance of Photoanodes

Electrochemical Anodization Self-organised TiO2 nanotubes are synthesized on a Ti substrate using electrochemical anodization. In article number 2100040 by Patrik Schmuki and co-workers, thermal annealing of the tubes up to 450 °C in air at different ramping rates are investigated for their effect on the photoelectrochemical performance. A ramping rate of 1 °C s–1 leads to the highest performance due to a most effective charge separation in the illuminated tubes.

Effect of film morphology on water oxidation enhancement in NiFeCo modified hematite photoanodes

Hematite (α-Fe2O3) is a promising candidate for water oxidation applications due to its abundance in the earth crust and its suitable bandgap. However, hematite performance is severely limited by electron-hole recombination at its interface with the electrolyte; something that can be addressed using electrocatalysts. In this report, we evaluate the influence of a ternary NiFeCo co-catalyst to enhance the water oxidation performance of hematite photoanodes. Thus, NiFeCo co-catalyst is optimized for hematite thin films with i) dense and ii) porous (nano-rod) morphologies. Both hematite films are prepared using electron beam evaporation method followed by an annealing step and NiFeCo co-catalyst is processed using electrodeposition. The results demonstrate optimal thickness of NiFeCo co-catalyst is thinner for porous hematite layers relative to the dense ones indicating that the optimal condition of the co-catalyst is affected by hematite morphology. The optimized dense NiCoFe/hematite photoanode shows nearly than 2.8-fold enhancement relative to the pristine hematite photoanode, whereas the optimized porous NiCoFe/hematite photoanode shows 2.2-fold enhancements. Transient photocurrent measurements confirm that NiFeCo facilitates hole injection from hematite into the electrolyte. These findings have important implications for optimizing electrocatalysts for photoelectrochemical water splitting photoanodes and taking into account the effect of surface morphology.

Thermal Ramping Rate during Annealing of TiO2 Nanotubes Greatly Affects Performance of Photoanodes

Herein, highly ordered TiO2 nanotube (NT) arrays on a Ti substrate is synthesized in a fluoride‐containing electrolyte, using the electrochemical anodization method, which yields amorphous oxide tubes. The effects of different thermal annealing profiles for the crystallization of the amorphous TiO2 NTs are studied. It is found that the temperature ramping rate has a significant impact on the magnitude of the resulting photocurrents (incident photon‐to‐current conversion efficiency [IPCE]) from the tubes. No appreciable changes are observed in the crystal structure and morphology of the TiO2 NTs for different annealing profiles (to a constant temperature of 450 °C). The electrochemical properties of the annealed TiO2 NTs are investigated using intensity‐modulated photocurrent spectroscopy (IMPS), open‐circuit potential decay, and Mott–Schottky analysis. The results clearly show that the annealing ramping rate of 1 °C s−1 leads to the highest IPCE performance. This beneficial effect can be ascribed to a most effective charge separation and electron transport (indicating the least amount of trapping states in the tubes). Therefore, the results suggest that controlling the annealing ramping rate is not only a key factor affecting the defect structure but also a powerful tool to tailor the physical properties, and photocurrent activity of TiO2 NTs.

Unravelling the essential difference between TiOx and AlOx interface layers on Ta3N5 photoanode for photoelectrochemical water oxidation

Tantalum nitride (Ta3N5) is a very promising photoanode material due to its narrow band gap (2.1 eV) and suitable band alignment for solar water splitting. However, it suffers from severe photocorrosion during water oxidation. In this work, it was found that surface passivation by AlOx and TiOx layers results in dramatically different PEC performance of Ta3N5 photoanode for water oxidation. The mechanism study indicates that the negative charges on AlOx can generate additional field to promote separation of photogenerated charges, while the positive charges on TiOx layer show the opposite effect. As a result, the Ta3N5 based photoanode modified with AlOx layer gives a high photocurrent of 12.5 mA cm−2 for 24 h at 1.23 V versus the reversible hydrogen electrode (RHE). Dynamic analysis implies that the hole extraction and transfer are significantly improved by the modification with the AlOx layer. This work reveals the importance of the charges on surface passivation layer in interface engineering of photoelectrodes.

Synthesis of TiO<sub>2</sub>-rGO Nanocomposite and its Application as Photoanode of Dye-Sensitized Solar Cell (DSSC)

Dye-sensitized solar cells (DSSC) are solar cells that has a great potential to be applied as renewable energy conversion. The major advantages of DSSC are the ease of fabrication process and low cost of production. Despite of these advantages, the efficiency of DSSC for converting light into electricity is still low. It is due to charge recombination in DSSC which limits the photoanode performance. Numerous efforts has been carried out to increase the efficiency of DSSC, one of which is by adding reduced graphene oxide (rGO) to titanium oxide (TiO2) to obtain TiO2-rGO nanocomposite. In this study, the synthesis of TiO2-rGO nanocomposites was carried out with concentration of rGO are 0.6, 0.8, and 1.0 wt% to amount of TiO2. We have done some characterizations to confirm the result of synthesized TiO2-rGO. UV-Vis measurement shows the addition of rGO has widened the absorption up to 400 nm. The FT-IR spectrum confirms that the rGO peaks appears at wavelength of 1400, 1600, dan 1700 cm-1 which exhibited the vibration C-O, C=C, and C=O stretching from COOH groups, respectively. The highest efficiency of DSSC with photoanode TiO2-rGO nanocomposite is 0.09% which was obtained from 0.8 wt% of rGO.

Unusual effects of vacuum annealing on large-area Ag3PO4 microcrystalline film photoanode boosting cocatalyst- and scavenger-free water splitting

Ag3PO4 as a novel photoanode material, despite its arguably highest photoactivity, suffers for its poor light absorption and stability for photoelectrochemical (PEC) water oxidation. In this work, 4.5 × 4.5 cm2 Ag3PO4 microcrystalline films are grown via a room-temperature solution process, and vacuum annealing is proposed to solve the stability and light absorption issues. It is found that the major process below 400 °C of vacuum annealing is the |Recovery| process for Ag3PO4 microcrystals, when lattice defects and Ag0 surface species get reduced. Next, |Recrystallization| stage occurs at >400 °C. The recovery of native defects of silver vacancies, with both density functional theory calculation and experimental results, could simultaneously improve the light absorption and catalytic activity of Ag3PO4. The 400 °C-annealed Ag3PO4 photoanode, with enhanced light harvesting and crystal quality, exhibits 88% increase in (Jlight − Jdark) value (1.94 mA cm−2) than non-annealed photoanode (1.03 mA cm−2). Moreover, it retains >99% current density after a 4000-s stability test. These results suggest that vacuum annealing can substantially improve the PEC performance of Ag3PO4 microcrystalline film photoanodes due to mitigated effects of native defects, improved light harvesting, and inhibited Ag3PO4 decomposition during water oxidation reaction.

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...

Surface Passivation Effect of Ferrihydrite with Hole-Storage Ability in Water Oxidation on BiVO4 Photoanode

Ferrihydrite (Fh) is an effective surface modifier for enhancing photoelectrochemical (PEC) performance of photoanode, while the exact role of Fh, serving as cocatalyst, hole-storage layer, or pass...

Unravelling the bulk and interfacial charge transfer effects of molybdenum doping in BiVO4 photoanodes

The role of Mo6+ doping on the photoelectrochemical (PEC) performance of BiVO4 photoanodes was investigated both in the presence and in the absence of sulfite as hole scavenger. Optically transparent, flat BiVO4 photoanodes containing different amounts of Mo6+ dopant were synthesized by spin coating. An increase of Mo6+ dopant amount was found to both improve the electron transport in the BiVO4 bulk by increasing its conductivity, as unequivocally ascertained when employing a Ni/Fe oxyhydroxide co-catalyst, and facilitate the charge transfer at the electrode/electrolyte interface in water oxidation, in the absence of hole scavenger. On the other hand, increasing amounts of the Mo6+ dopant in BiVO4 induced an unexpected decrease in PEC performance per unit surface area in sulfite oxidation, resulting from enhanced interfacial charge transfer resistance, as demonstrated by electrochemical impedance spectroscopy. First evidence is thus provided of a different behaviour observed upon Mo6+ doping of BiVO4 depending on the nature of the involved electron donor species, together with an intriguing multifaceted role played by Mo6+ doping in enhancing the PEC performance of modified BiVO4 electrodes.

Boosting the Photoelectrochemical Performance of BiVO4 Photoanodes by Modulating Bulk and Interfacial Charge Transfer

BiVO4 is an attractive candidate that is used as a photoanode for solar water splitting. Unfortunately, its photoelectrochemical (PEC) performance is restricted by the slow charge transfer ability in the bulk and the interface. In this study, we accelerated bulk charge transfer via introducing oxygen vacancies by Ar plasma treatment and facilitated interfacial carrier transfer by loading a WCoFe oxyhydroxide electrocatalyst. A larger enhancement was achieved compared to the improvement results from the individual modification, and a photocurrent of 5.49 mA/cm2 was obtained under light irradiation in a potassium phosphate (KPi) buffer solution. The results demonstrate that the PEC performance of the BiVO4-based film photoanode is enhanced by integrating synergy effects of favorable factors by introducing oxygen vacancies and loading electrocatalysts.

Controlling Charge Carrier Trapping and Recombination in BiVO4 with the Oxygen Vacancy Oxidation State.

The lack of an in-depth understanding of the intrinsic oxygen vacancy (OV) defect properties in the photoanode BiVO4 limits the further improvement of its photoelectrochemical water splitting performance. To address this issue, nonadiabatic molecular dynamics simulations are performed to study the impact of OV on charge carrier lifetimes in BiVO4. The simulations show that a neutral OV gives rise to local structural distortions due to the formation of V-O-V bonds, forcing the electrons trapped on the nearer of the two V atoms to form two deep polaron-like V4+ hole traps. These localized midgap states greatly accelerate nonradiative electron-hole recombination compared to that of pristine BiVO4, reaching a time scale of several nanoseconds in good agreement with experiments. The ionized OV state restores the bandgap to its value in pristine BiVO4 and restores the charge carrier lifetimes due to the fast loss of coherence time. Our study reveals the mechanism of the detrimental role of OV in BiVO4 and provides valuable insights for improving the performance of the BiVO4 photoanode by ionizing the oxygen vacancy.