Promising Photocathode Material Research Articles

Thermodynamic Stability and Native Point Defects of CuFeO2 Photocathodes in Dry and Electrochemical Environments

CuFeO2 has been recently identified as a promising photocathode material for photoelectrochemical water splitting cells. In spite of the first encouraging results, improvements in the catalytic act...

Fe2O3/NiO photocathode for photocatalytic methanol fuel cell: An insight on solar energy conversion

A promising photocathode material based on Fe2O3/NiO heterostructure was developed for efficient photoelectric conversion in a methanol fuel cell. The Fe2O3/NiO heterogeneous film was prepared via in-situ hydrothermal synthesis and then cathodic electrodeposition process. Surface photovoltage results showed that after electrodeposition of NiO on the Fe2O3 film, the separation direction of photogenerated electrons and holes was reversed. The photogenerated electrons accumulated on the surface of Fe2O3/NiO photocathode can induce the oxygen reduce reaction due to the relatively high conduction band level of the entire semiconductor composite. In addition, the Fe2O3/NiO based photocatalytic methanol fuel cell showed significant improvements under illumination of simulated sunlight, and the short-circuit current and open-circuit voltage reach 183 μA/cm2 and 0.48 V respectively. The work provided a new vision for solar - chemical energy conversion in the fuel cell.

Three-dimensional Cu2O nanorods modified by hydrogen treated Ti3C2TX MXene with enriched oxygen vacancies as a photocathode and a tandem cell for unassisted solar water splitting

Cuprous oxide (Cu2O) has been envisioned as a promising photocathode material for photoelectrochemical (PEC) water reduction due to its abundance, scalability and nontoxicity. Herein, a three-dimensional (3D) Cu2O nanorods (NRs) photocathode is prepared by in-situ growing Cu2O NRs on a copper foam substrate. The bare 3D Cu2O NRs photocathode exhibits a high photocurrent density of −4.02 mA cm−2 at 0 V versus the reversible hydrogen electrode (RHE) under AM 1.5 G simulated sunlight. The oxygen vacancies enriched Ti3C2TX flakes are obtained by reducing Ti3C2TX with H2/Ar gas (denoted as H:Ti3C2TX). The H:Ti3C2TX/Cu2O, Ti3C2TX/Cu2O as well as Au/Cu2O composite photocathodes are fabricated through a facile dip-coating method. The H:Ti3C2TX/Cu2O and Ti3C2TX/Cu2O yield a photocurrent density of −5.41 and −4.45 mA cm−2 respectively, which is higher than the bare Cu2O and Au/Cu2O (−4.31 mA cm−2). The results suggest that the Ti3C2TX is more effective than noble metal Au for improving the PEC performance of Cu2O photocathode. The superior performance of H:Ti3C2TX/Cu2O is attributed to enriched oxygen vacancies in the H:Ti3C2TX which can enhance the conductivity, light-harvesting efficiency and charge-transfer ability of fabricated photocathode. A PEC tandem cell with H:Ti3C2TX/Cu2O as the photocathode and BiVO4 as the photoanode is designed for unassisted solar water splitting. Due to the high porosity and optical transmittance of the 3D copper foam substrate, we illuminate the cell from the H:Ti3C2TX/Cu2O side rather than from the BiVO4 side in usual fashion. The present configuration exhibits a 1.6 times higher solar-to-hydrogen (STH) conversion efficiency of 0.55% than the conventional design with a STH conversion efficiency of 0.34%. The enhanced STH conversion efficiency in our configuration is due to the higher light-harvesting efficiency.

Boosting the performance of delafossite photocathode through constructing a CuFeO2/CuO heterojunction for photoelectrochemical water reduction

CuFeO2 is a promising photocathode material with favorable band gap and chemical stability in alkaline electrolytes. However, CuFeO2 is suffered from poor photoinduced electron–hole separation and collection. In this paper, we utilize a heterojunction strategy for inhibiting charge recombination and enhancing its photoelectrochemical performance. CuFeO2/CuO heterojunction photocathodes were prepared by a facile spin coating method, which were used for photoelectrochemical water reduction. A high photocurrent as much as 50 μA/cm2 after 600 s irradiation of white light at the potential of − 0.5 V versus Ag/AgCl electrode is observed, which is higher than those of the pure CuFeO2 and CuO photocathodes. The effect of the interfacial electric field on the behaviors of photoinduced charge carriers is studied. It is the interfacial electric field that extends the lifetime of photoinduced charge carriers and promotes the charge separation efficiency, which results high performance and good durability.

Ag and N-doped graphene quantum dots co-modified CuBi2O4 submicron rod photocathodes with enhanced photoelectrochemical activity

Photoelectrochemical (PEC) water splitting that can mimic natural photosynthesis to directly harvest and convert solar energy into clean and sustainable hydrogen energy, has been known as a promising avenue for addressing environmental pollution and energy crisis. Spinel-type CuBi2O4 semiconductor as a promising photocathode material for solar water reduction has attracted much attention because of its many unique advantages. However, PEC activity of pristine CuBi2O4 is far from satisfying because of its poor charge carrier transport and transfer efficiency. Herein we report that Ag+ ions and N-doped graphene quantum dots (NGQDs) co-modified CuBi2O4 submicron rods (NGQDs/Ag-CBO SMRs) photocathode can show enhanced PEC activity. Our work demonstrated that one-dimensional structure, Ag+ ions doping, and NGQDs loading benefited the separation and transfer of photogenerated carriers and improved light harvesting ability, which were collectively contributed to enhanced PEC activity of NGQDs/Ag-CBO SMRs photocathode. As a result, the photocurrent density achieved on NGQDs/Ag-CBO SMRs photocathode under AM1.5G simulated sunlight at 0.3 V vs. RHE was 8.8, 2.3, 1.8, and 5.9-times larger than those obtained on CBO SMRs, Ag-CBO SMRs, NGQDs/CBO SMRs, and NGQDs/Ag-CBO microspheres (MSs) ones, respectively. The present study provides a new strategy for designing low-cost and highly efficient photocathodes.

Bright and Ultrafast Photoelectron Emission from Aligned Single-Wall Carbon Nanotubes through Multiphoton Exciton Resonance.

Ultrashort bunches of electrons, emitted from solid surfaces through excitation by ultrashort laser pulses, are an essential ingredient in advanced X-ray sources, and ultrafast electron diffraction and spectroscopy. Multiphoton photoemission using a noble metal as the photocathode material is typically used but more brightness is desired. Artificially structured metal photocathodes have been shown to enhance optical absorption via surface plasmon resonance but such an approach severely reduces the damage threshold in addition to requiring state-of-the-art facilities for photocathode fabrication. Here, we report ultrafast photoelectron emission from sidewalls of aligned single-wall carbon nanotubes. We utilized strong exciton resonances inherent in this prototypical one-dimensional material, and its excellent thermal conductivity and mechanical rigidity leading to a high damage threshold. We obtained unambiguous evidence for resonance-enhanced multiphoton photoemission processes with definite power-law behaviors. In addition, we observed strong polarization dependence and ultrashort photoelectron response time, both of which can be quantitatively explained by our model. These results firmly establish aligned single-wall carbon nanotube films as novel and promising ultrafast photocathode material.

Mechanistic insights of enhanced spin polaron conduction in CuO through atomic doping

The formation of a “spin polaron” stems from strong spin-charge-lattice interactions in magnetic oxides, which leads to a localization of carriers accompanied by local magnetic polarization and lattice distortion. For example, cupric oxide (CuO), which is a promising photocathode material and shares important similarities with high Tc superconductors, conducts holes through spin polaron hopping with flipped spins at Cu atoms where a spin polaron has formed. The formation of these spin polarons results in an activated hopping conduction process where the carriers must not only overcome strong electron−phonon coupling but also strong magnetic coupling. Collectively, these effects cause low carrier conduction in CuO and hinder its applications. To overcome this fundamental limitation, we demonstrate from first-principles calculations how doping can improve hopping conduction through simultaneous improvement of hole concentration and hopping mobility in magnetic oxides such as CuO. Specifically, using Li doping as an example, we show that Li has a low ionization energy that improves hole concentration, and lowers the hopping barrier through both the electron−phonon and magnetic couplings' reduction that improves hopping mobility. Finally, this improved conduction predicted by theory is validated through the synthesis of Li-doped CuO electrodes which show enhanced photocurrent compared to pristine CuO electrodes. We conclude that doping with nonmagnetic shallow impurities is an effective strategy to improve hopping conductivities in magnetic oxides.

Enhancing Durability and Photoelectrochemical Performance of the Earth Abundant Ni-Mo/TiO2 /CdS/CIGS Photocathode under Various pH Conditions.

Cu(In,Ga)(S,Se)2 (CIGS) is a promising photocathode material owing to its high absorption coefficient, adjustable band gap, and suitable band edge for the hydrogen evolution reaction (HER). However, most CIGS photocathodes have suffered from instability in applications that require a wide range of pH conditions and have utilized noble metal HER catalysts to achieve a high performance. Thus, improving the durability of the CIGS photocathode under various pH conditions and developing a cost-effective non-noble metal catalyst are critical issues in the photoelectrochemical (PEC) application of this promising photocathode material. Here, we catalyze the CIGS photocathode with Ni-Mo as a non-noble metal to enhance the PEC efficiency, and we employ atomically grown TiO2 to passivate the CdS/CIGS surface and improve the stability under a wide range of pH conditions. Our Ni-Mo alloy exhibits the best HER catalytic activity among reported earth-abundant HER catalysts in both acidic and alkaline solutions. The Ni-Mo/CdS/CIGS photocathode yields an onset potential of 0.5 V (vs. RHE) and a short-circuit photocurrent density as high as 15-25 mA cm-2 under various pH conditions ranging from 0.4 to 14, which is highly comparable to that of Pt/CdS/CIGS. Furthermore, the passivation of CdS/CIGS with a thin TiO2 layer, obtained by atomic layer deposition, effectively prevents the photocorrosion of CdS and the dissolution of the Mo back contact, which are the main causes of the degradation of the photocathode. The optimized Ni-Mo/TiO2 /CdS/CIGS photocathode produces a stable photocurrent density at 0 VRHE for 100 minutes except under strong alkaline conditions. The current work presents a very useful method to improve the efficiency and durability of the CIGS photocathodes with an earth-abundant metal catalyst, which completely replaces Pt.

Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron.

Cupric oxide (CuO) is considered as a promising photocathode material for photo(electro)chemical water splitting because of its suitable band gap, low cost related to copper earth abundancy, and straightforward fabrication. The main challenge for the development of practical CuO-based photocathodes for solar hydrogen evolution is to enhance its stability against photocorrosion. In this work, stable and efficient CuO photocathodes have been developed by using a simple and cost-effective methodology. CuO films, composed of nanowires and prepared by chemical oxidation of electrodeposited Cu, develop relatively high photocurrents in 1 M NaOH. However, this photocurrent appears to be partly associated with photocorrosion of CuO. It is significant though that, even unprotected, a faradaic efficiency for hydrogen evolution of ∼45% is attained. The incorporation of iron through an impregnation method, followed by a high-temperature thermal treatment for promoting the external phase transition of the nanowires from CuO to ternary copper iron oxide, was found to provide an improved stability at the expense of photocurrent, which decreases to about one-third of its initial value. In contrast, a faradaic efficiency for hydrogen evolution of ∼100% is achieved even in the absence of co-catalysts, which is ascribable to the favorable band positions of CuO and the iron copper ternary oxide in the core-shell structure of the nanowires.

Fabrication, characterization and photoelectrochemical properties of cuprous oxide-reduced graphene oxide photocatalysts for hydrogen generation

Reduced graphene oxide protected Cu2O/Cu foil (rGO/Cu2O/Cu foil) photocathodes have been synthesized via a two-step strategy. (NH4)2S2O8 was first used to oxidize Cu foil and prepare Cu(OH)2 nanostructures/Cu foil. In the second step, graphene oxide (GO) solution and the prepared Cu(OH)2/Cu foil sample were hydrothermally reacted in a Teflon lined stainless steel autoclave for different periods of time at 120 °C, followed by calcination at 450 °C under N2 atmosphere for 3 h. The effect of modification of rGO on the photoelectrochemical activity of different rGO/Cu2O/Cu foil samples has been studied by FESEM, XRD, FTIR, Raman spectroscopy and UV–Vis diffuse reflectance spectroscopy. Continuous and transparent layer of GO sheets are formed on the surface of Cu(OH)2 nanostructures/Cu foil, according to FESEM results. The photoelectrochemical properties of the prepared samples in the dark and under illumination conditions were investigated using linear sweep voltammetry and chronoamperometry techniques. The rGO/Cu2O/Cu foil photocathodes exhibit enhanced photocurrent density under illumination (85 mW/cm2) in comparison with the bare Cu2O/Cu foil photocathode. The improved separation efficiency of photogenerated charge carriers may be mainly accounted for this enhanced photoelectrochemical performance. A low photostability of 12.5% of the initial photocurrent density was observed for the bare Cu2O/Cu foil photocathode after 1000s of illumination whereas this value reached to almost 77% after modification of rGO. The new rGO/Cu2O/Cu foil nanocomposite prepared in this study is believed to be a promising photocathode material in photoelectrochemical cells for efficient water splitting.

Direct Fabrication of a CuFeO2/Fe Photocathode for Solar Hydrogen Production by Hydrothermal Method

CuFeO2 is a promising photocathode material for photoelectrochemical water splitting. In this study, a hydrothermal method was used for the first time to directly fabricate CuFeO2 films on an Fe substrate, and the prepared films worked as photocathodes. To enhance the performance of CuFeO2 photocathodes, it is important to optimize their film thickness and surface morphology.

Silicon microwire arrays decorated with amorphous heterometal-doped molybdenum sulfide for water photoelectrolysis

Silicon is a promising photocathode material for solar hydrogen evolution because of its small band gap, negative conduction band position, and ideal theoretical current density. In this study, p-type Si microwire (p-Si MW) arrays were prepared as photocathodes because of the large surface area and high light-harvesting capability. However, Si MWs suffered from low photocatalytic activity because of slow photo-induced carriers during driving of water-splitting reaction. Therefore, molybdenum sulfide (MoS2) with appropriate band alignment with p-Si material was employed for surface modification to function as a co-catalyst for collecting photo-generated minority carriers and reducing recombination possibility. The onset potential and current density at 0V versus reversible hydrogen electrode (RHE) of Si@MoS2 MWs were +0.122V and −8.41mAcm−2. Heterometal atoms were employed to dope MoS2 co-catalyst and expose more sulfur-terminated active sites to further boost photoelectrochemical performance. Optimal activity of Si@MMoSx (M = Fe, Co, Ni) was achieved by doping Co heteroatoms, and its turn-on voltage and photocurrent density at 0V (vs. RHE) were respectively increased to +0.192V and −17.2mAcm−2. X-ray absorption spectroscopy was applied to demonstrate that Fe ions of FeMoSx were dichalcogenide materials, forming a composite with MoS2 and contributing better photoelectrolytic efficiency. By contrast, two-valent heteroatoms of CoMoSx and NiMoSx substituted the Mo4+ ions in MoS2. For charge compensation, more defects and edges were revealed as active sites of solar hydrogen production by adding Co or Ni dopants in MoS2 co-catalyst, which led to lower overpotential.

Material informatics driven design and experimental validation of lead titanate as an aqueous solar photocathode

Materials informatics is a rapidly emerging data- and knowledge-driven approach for the identification of novel materials for a range of applications, including solar energy conversion. Despite significant experimental effort, the development of highly efficient, stable, and cost-effective photovoltaic materials remains a challenging scientific problem. The quest for precisely defined semiconductor properties revolves around an immensely broad landscape of structural parameters. Here, we have resolved this challenge by applying material informatics to design a novel photocathode material for dye-sensitized solar cells (DSSCs). By conducting a virtual screening of 50,000 known inorganic compounds, we have identified lead titanate (PbTiO3), a perovskite, as the most promising photocathode material. Notably, lead titanate is significantly different from the traditional base elements or crystal structures used for photocathodes. The fabricated PbTiO3 DSSC devices exhibited the best performance in aqueous solution, showing remarkably high fill factors compared to typical photocathode systems. The results highlight the pivotal role materials informatics can play in streamlining the experimental development of materials with the desired properties.

Oxygen-Intercalated CuFeO2 Photocathode Fabricated by Hybrid Microwave Annealing for Efficient Solar Hydrogen Production

Delafossite CuFeO2 is a promising photocathode material for solar hydrogen production, but its performance is low because of poor charge transport properties. When the prepared CuFeO2 electrode is annealed by hybrid microwave annealing (HMA), its photoelectrochemical water reduction activity increases by more than 4 times (−1.3 mA cm–2 @ 0.4 VRHE), while the conventional thermal annealing (CTA) improves the performance by only 2 times (−0.62 mA cm–2 @0.4 VRHE). The postannealing of the electrode intercalates extra oxygen into the CuFeO2 lattice to form CuFeO2+1.5δ, which increases the charge carrier density and thus improves charge transport properties. The oxygen intercalation with HMA takes place more uniformly over the whole solid and is more effective than CTA. In addition, HMA post-treated CuFeO2 is modified with a NiFe-layered double hydroxide/reduced graphene oxide electrocatalyst, which exhibits a high photoactivity of −2.4 mA cm–2 @ 0.4 VRHE, unprecedented for CuFeO2-based photocathodes.

Engineering MoSx/Ti/InP Hybrid Photocathode for Improved Solar Hydrogen Production

Due to its direct band gap of ~1.35 eV, appropriate energy band-edge positions, and low surface-recombination velocity, p-type InP has attracted considerable attention as a promising photocathode material for solar hydrogen generation. However, challenges remain with p-type InP for achieving high and stable photoelectrochemical (PEC) performances. Here, we demonstrate that surface modifications of InP photocathodes with Ti thin layers and amorphous MoSx nanoparticles can remarkably improve their PEC performances. A high photocurrent density with an improved PEC onset potential is obtained. Electrochemical impedance analyses reveal that the largely improved PEC performance of MoSx/Ti/InP is attributed to the reduced charge-transfer resistance and the increased band bending at the MoSx/Ti/InP/electrolyte interface. In addition, the MoSx/Ti/InP photocathodes function stably for PEC water reduction under continuous light illumination over 2 h. Our study demonstrates an effective approach to develop high-PEC-performance InP photocathodes towards stable solar hydrogen production.

Comprehensive Evaluation of CuBi2O4 as a Photocathode Material for Photoelectrochemical Water Splitting

CuBi2O4 is a multinary p-type semiconductor that has recently been identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. It has an optimal bandgap energy (∼1.8 eV) and an exceptionally positive photocurrent onset potential (>1 V vs RHE), making it an ideal candidate for the top absorber in a dual absorber PEC device. However, photocathodes made from CuBi2O4 have not yet demonstrated high photoconversion efficiencies, and the factors that limit the efficiency have not yet been fully identified. In this work we characterize CuBi2O4 photocathodes synthesized by a straightforward drop-casting procedure and for the first time report many of the quintessential material properties that are relevant to PEC water splitting. Our results provide important insights into the limitations of CuBi2O4 in regards to optical absorption, charge carrier transport, reaction kinetics, and stability. This information will be valuable in future work to optimize CuBi2O4 as a PEC material. ...

A Novel Photocathode Material for Sunlight‐Driven Overall Water Splitting: Solid Solution of ZnSe and Cu(In,Ga)Se2

Thin films of a solid solution of ZnSe and CuIn0.7Ga0.3Se2 ((ZnSe) x (CIGS) 1–x ) are prepared by co‐evaporation. Structural characterization reveals that the ZnSe and CIGS form a solid solution with no phase separation. (ZnSe)0.85(CIGS)0.15‐based photocathodes modified with Pt, Mo, Ti, and CdS exhibit a photocurrent of 7.1 mA cm−2 at 0 VRHE, and a relatively high onset potential of 0.89 VRHE under simulated sunlight. A two‐electrode cell containing a (ZnSe)0.85(CIGS)0.15 photocathode and a BiVO4‐based photoanode has an initial solar‐to‐hydrogen conversion efficiency of 0.91%, which is one of the highest values reported for a photoanode–photocathode combination. Thus, (ZnSe)0.85(CIGS)0.15 is a promising photocathode material for efficient photoelectrochemical water splitting.

Polyaniline films photoelectrochemically reduce CO2 to alcohols.

In this communication, we demonstrate that polyaniline, the very first example of an organic semiconductor, is a promising photocathode material for the conversion of carbon dioxide (CO2) to alcohol fuels. CO2 is a greenhouse gas; thus using solar energy to convert CO2 to transportation fuels (such as methanol or ethanol) is a value-added approach to simultaneous generation of alternative fuels and environmental remediation of carbon emissions. Insights into its unique behavior obtained from photoelectrochemical measurements and adsorption studies, together with spectroscopic data, are presented. Through a comparative study involving various conducting polymers, a set of criteria is developed for an organic semiconductor to function as a photocathode for generation of solar fuels from CO2.

Photoelectric quantum efficiency of niobium for λ = 193 nm and λ = 248 nm

We report a measurement of the quantum efficiency of a niobium photocathode for the wavelengths λ = 193 nm and λ = 248 nm. Charge yields for a 20 ns excimer laser pulse were measured as a function of DC accelerating anode voltage. Our experimental results for the quantum efficiency are ε (248) = 3.2±0.6 × 10 −6 and ε (193) = 4.5±0.5 × 10 −4 . We conclude that niobium is a promising photocathode material for a superconducting rf electron gun if the laser wavelength is RE200 nm. Laser pulse energy requirements are estimated for such a gun.