Efficient Photoelectrochemical Hydrogen Research Articles

Building Directional Charge Transport Channel in CdTe-Based Multilayered Photocathode for Efficient Photoelectrochemical Hydrogen Evolution

Building Directional Charge Transport Channel in CdTe-Based Multilayered Photocathode for Efficient Photoelectrochemical Hydrogen Evolution

Design of hybrid Au grating/TiO2 structure for NIR enhanced photo-electrochemical water splitting

Titanium oxide is commonly considered as an effective catalyst for photo-electrochemical water splitting due to its low cost, high activity, and perfect stability. However, a wide bandgap of TiO2 prevents the utilization of visible and near-infrared photons in the photo-electrochemical process. Photosensitization of TiO2 with plasmon active nanostructures was proposed as a way to solve this problem but the optimal design of coupled TiO2-plasmonic nanostructures, as well as the exact mechanism of plasmon triggering, are still under debate. In this work, we propose a plasmon-based TiO2 photosensitization, using the gold grating, covered by homogenous TiO2 layer. The gold grating supports the excitation and propagation of surface plasmon polariton wave (SPP), which is responsible for TiO2 triggering (unlike common localized plasmon resonance commonly used for TiO2 activation). For the optimization of the present structure design, we studied the impact of TiO2 layer thickness, illumination regime, and spatial distribution of plasmonic evanescent wave energy with regards to the mechanism of TiO2 activation. Obtained results proved that the proposed structure can be used for efficient photo-electrochemical water splitting and hydrogen production under irradiation with NIR photons of 700–1000 nm wavelengths. It was also verified that for a certain TiO2 layer thickness the activation of catalytic activity is mostly due to the plasmon electric field, which is concentrated at the electrolyte/catalyst surface.

Efficient and Long-term Photoelectrochemical Hydrogen Liberation from Hydrazine Hydrate on CdS Nanorod Arrays

Efficient and Long-term Photoelectrochemical Hydrogen Liberation from Hydrazine Hydrate on CdS Nanorod Arrays

Electropolymerization process dependent poly(1,4-di(2-thienyl)benzene) based full spectrum activated photocathodes for efficient photoelectrochemical hydrogen evolution

In this work, with the help of different electrochemical polymerization methods, the surface morphology of poly-1,4-bis(2-thienyl)benzene (PDTB) photocathode and S oxidation state were modulated. Thus, PDTB photocathodes with strong light absorption (310–2500 nm) and strong charge separation ability were successfully prepared. • Controllable composition, morphology and performance of PDTB photocathodes. • A novel strategy for design photocathodes with low band gap. • The enhanced photoelectrochemical performance was confirmed by detailed experiments. Electrochemical surfaces are essential for photoelectrocatalytic processes occuring on semiconductor electrodes, which influence light harvesting capability and charge separation of photoelectrochemical (PEC) hydrogen evolution process. In this work, electrochemical processes through cyclic voltammetry (CV), chronoamperometry (CA) and amperometry ( i-t ) show different abilities to electropolymerization. It results in poly-1,4-bis(2-thienyl)benzene (PDTB) photoelectordes with various microporous structures and regulated S oxidation states. Benifiting from these effect factors, the light absorption ability and charge separation of PDTB photoelectordes are expanded and promoted. In particular, PDTB photoelectrodes prepared by CA with 1.23 eV band gap and strong light absorption (310–2500 nm) show the best photoelectrochemical activity with a charge transfer resistance of 7 Ω, a photocurrent of 0.9 mA cm −2 and increases the onset hydrogen production potential to 0.92 V. The incident photon to current efficiency (IPCE) can reach 14.16 % at 850 nm. It suggests that photoelectrodes from electropolymerization may offer a simple and efficient strategy by adjusting the degree of polymerization to get a promising low band gap semiconductor for efficient PEC water reduction.

Wood‐Inspired Binder Enabled Vertical 3D Printing of g‐C3N4/CNT Arrays for Highly Efficient Photoelectrochemical Hydrogen Evolution

Abstract2D nanomaterials are very attractive for photoelectrochemical applications due to their ultra‐thin structure, excellent physicochemical properties of large surface‐area‐to‐volume ratios, and the resulting abundant active sites and high charge transport capacity. However, the application of commonly used 2D nanomaterials with disordered‐stacking is always limited by high photoelectrode tortuosity, few surface‐active sites, and low mass transfer efficiency. Herein, inspired by wood structures, a vertical 3D printing strategy is developed to rapidly build vertically aligned and hierarchically porous graphitic carbon nitride/carbon nanotube (g‐C3N4/CNT) arrays by using lignin as a binder for efficient photoelectrochemical hydrogen evolution. Arising from the directional electron transport and multiple light scattering in the out‐of‐plane aligned and porous architecture, the resulting g‐C3N4/CNT arrays display an outstanding hydrogen evolution performance, with the hydrogen yield up to 4.36 µmol (cm−2 h−1) at a bias of −0.5 V versus RHE, 12.7 and 41.6 times higher than traditional thick g‐C3N4/CNT and g‐C3N4 films, respectively. Moreover, this 3D printed structure can overcome the agglomeration problem of the commonly used g‐C3N4 with powder configuration and shows desirable recyclability and stability. This facile and scalable vertical 3D printing strategy will open a new avenue to highly enhance the photoelectrochemical performance of 2D nanomaterials for sustainably production of clean energy.

Direct Z-scheme SiNWs@Co3O4 photocathode with a cocatalyst of sludge-derived carbon quantum dots for efficient photoelectrochemical hydrogen production

Solar driven photoelectrochemical (PEC) hydrogen production has attracted considerable attention, but the design of highly efficient, robust and low-cost photocathode still remains a significant challenge. Herein, we report a novel SiNWs@Co3O4Z-scheme heterojunction photocathode with carbon quantum dots eco-friendly derived from sludge (SCQDs) as the co-catalyst. The photocathode not only leads to effective separation of electron–hole pair, lower transmission resistance, and longer lifetime of charge carriers, but also elevates the stability by preventing direct contact between the SiNWs and the electrolyte as well as the self-oxidation. Simultaneously, the excellent electron transport properties of the SCQDs further improved the PEC performance. Correspondingly, a maximum current density of 14.88 mA·cm−2 was obtained at −0.67 V with the applied bias photon-to-current efficiency (ABPE) achieving 8.4% under visible light irradiations at pH = 7. This work provides a promising scheme for Si-based photocathodes for PEC hydrogen production with a high performance, reliable stability, and low-cost.

Colloidal carbon quantum dots as light absorber for efficient and stable ecofriendly photoelectrochemical hydrogen generation

Solar-driven hydrogen production is one of the most promising strategies for solar-to-hydrogen energy conversion. Compared to inorganic quantum dots (QDs), carbon quantum dots (C-dots) have attracted a lot of attention for optoelectronic devices due to their structure-dependent optical properties and green composition. However, the solar-to-hydrogen conversion efficiency of most of the photoelectrochemical (PEC) devices based on colloidal QDs is still low. Here we demonstrated a highly efficient and stable ecofriendly PEC device using C-dots sensitized TiO2 photoanode, Pt loaded on carbon nanofibers as counter electrode, and glucose aqueous solution as electrolyte. The red-color C-dots were prepared using a solvothermal reaction, with an absorption spectrum ranging from 300 to 600 nm and a quantum yield (QY) of 50%. The C-dots have excitation independent photoluminescence peak positions and highly crystalline structure. The hydroxyl group on the C-dots can strongly interact with the TiO2, forming a very stable complex. Benefiting from these features, the PEC devices based on C-dots exhibit a saturated photocurrent density as high as ~4 mA/cm2 at 0.6 V vs. RHE and the device is very stable (keeping 95% of its initial value after 10-hour illumination upon 100 mW/cm2). This work indicates the promising properties of the C-dots/TiO2 system, which holds huge potential for applications in the fields of optoelectronic and catalytic devices.

Noble Metal Modification of CdS-Covered CuInS2 Electrodes for Improved Photoelectrochemical Activity and Stability

In this paper, efficient and stable photoelectrochemical (PEC) hydrogen (H2) evolution using copper indium sulfide (CuInS2) thin film electrodes was studied. Modification with a cadmium sulfide (CdS) layer led to improved charge separation at the interface between CuInS2 and CdS; however, the photocorrosive nature of CdS induced poor stability of the photocathode. Further surface coating with an electrodeposited Pt layer over the CdS-covered CuInS2 photocathode prevented the CdS layer from making contact with the electrolyte solution, and enabled efficient PEC H2 evolution without appreciable degradation. This indicates that the Pt layer functioned not only as a reaction site for H2 evolution, but also as a protection layer. In addition, it was found that surface protection using a noble metal layer was also effective for stable PEC carbon dioxide (CO2) reduction when appropriate noble metal cocatalysts were selected. When Au or Ag was used, carbon monoxide was obtained as a product of PEC CO2 reduction.

Hybrid surface passivation of PbS/CdS quantum dots for efficient photoelectrochemical hydrogen generation

Surface ligands facilitate the high dispersibility of colloidal quantum dots (QDs) during synthesis and storage, and also play a key role in passivating surface traps. The most commonly used ligands [oleic acid (OA) or oleylamine (OLA)] are insulating long organic linkers, which prevent efficient charge transfer between QDs and exhibit limited surface passivation, thus significantly reducing the overall performance of QDs based optoelectronic devices. Here we develop a solid-state ligands exchange process for PbS/CdS QDs sensitized TiO2 photoanode with hybrid inorganic ligands (I− and SCN−) to replace the original insulating OA ligands and passivate the surface traps. The hybrid surface passivation of PbS/CdS QDs removes ~75% of insulating OA ligands. The saturated photocurrent density of the photoelectrochemical (PEC) cells improves significantly through the hybrid ligand exchange process, reaching a value of 4.0 mA cm−2 which is more than two times higher compared to the control PEC cells. In addition, the long-term stability of PEC cells is better for the hybrid ligand exchange process (only 15% loss of its initial photocurrent density) than the control PEC cells (38% loss). Our results indicate that hybrid ligands engineering is a promising pathway to achieve high performance and stable PEC cells for hydrogen generation.

Enhanced Charge Carrier Lifetime of TiS3 Photoanode by Introduction of S22− Vacancies for Efficient Photoelectrochemical Hydrogen Evolution

AbstractRecent advances in layered TiS3 has shown appealing potential for photoelectrochemical hydrogen evolution due to its excellent optical and electronic properties. Here, an excellent photoanode, composed of TiS3 nanoribbon arrays with moderate S22− vacancies, is proposed to achieve efficient photoelectrochemical hydrogen evolution. These unique S22− vacancies are introduced in the TiS3 photoanode by a simple vacuum re‐annealing method, which is inspired by crystal structure analyses of TiS3 and TiS2. The existing S22− vacancies are revealed to significantly promote the electron conductivity, charge separation, and transport of the photoanode simultaneously. The resulting TiS3−x photoanode with ≈22% S22− vacancies exhibits a twofold increase in the electron diffusion length for efficient electron collection and a remarkably high photocurrent density of 15.35 mA cm−2 at 1.4 V versus reversible hydrogen electrode.

Cu2O nanoparticles sensitize TiO2/CdS nanowire arrays to prolong charge carrier lifetime and highly enhance unassisted photoelectrochemical hydrogen generation with 4.3% efficiency.

Effective separation of charge carriers and substantial prolongation of charge carrier lifetime are two vital issues for a photoanode to achieve highly efficient photoelectrochemical (PEC) hydrogen generation. Herein, a Cu2O-nanoparticle-sensitized TiO2/CdS (TiO2/CdS/Cu2O) nanowire array photoanode is fabricated via subtly combining successive ionic layer adsorption and reaction with a chemical bath deposition method. Both the type-II band alignment with a stair-like structure and a p-n junction are integrated into this ternary photoanode. The fabricated photoanode shows a significant enhancement in the PEC H2 production with 4.3% efficiency. The measurements of light absorption, photoluminescence and electrochemical spectra undoubtedly demonstrate that the enhanced PEC H2 generation is ascribed to the remarkable enhancement of visible-light absorbing ability, efficient space charge separation and substantial prolongation of charge carrier lifetime, which are achieved by the synergetic effects of the type-II band alignment with a stair-like structure and the p-n junction. The enhanced PEC H2 generation demonstrates the potential of the TiO2/CdS/Cu2O nanowire arrays as a photoanode to efficiently convert solar energy into chemical fuels.

Facet engineering of WO3 arrays toward highly efficient and stable photoelectrochemical hydrogen generation from natural seawater

Hydrogen generation via photoelectrochemical (PEC) technology is one of the most ideal strategies for providing sustainable fuel, in particular, from abundant natural resource (such as seawater). Herein, we demonstrate facet-controlled WO3 array decorated with reasonable distribution of dual-cocatalysts, which shows highly efficient and stable PEC H2 generation from natural seawater. The experimental and theoretical results illustrate that photo-generated electrons and holes accumulate directionally on different crystal facets of WO3 to achieve effective spatial separation, owing to the divergent energy levels of each crystal facet. Moreover, the charge utilization is further enhanced by selective modification of Ag nanoparticles and ZnFe-layered double hydroxide on different crystal facets reasonably, getting an average PEC H2 production of 38.18 μmol h−1 from natural seawater with excellent stability. This work opens up a new facet engineering pathway for the fabrication of novel photoelectrodes, which have potential applications in the fields of solar energy conversion and storage.

Electrochemically controlled CdS@CdSe nanoparticles on ITO@TiO2 dual core–shell nanowires for enhanced photoelectrochemical hydrogen production

Here, we report a novel dual heterostructured photoanode consisting of CdS@CdSe core–shell nanoparticles (NPs) and 1D-structure tin-doped indium oxide (ITO)@TiO2 core–shell nanowires (ITO@TiO2@CdS@CdSe) for highly efficient photoelectrochemical (PEC) hydrogen production. The finely controlled hierarchical core–shell CdS@CdSe sensitization from consecutive electrochemical deposition on the ITO@TiO2 core–shell nanowire has synergistic effects of visible-light utilization and efficient charge transport on the PEC response. The rationally designed dual core–shell heterostructure leads to cascade charge migration throughout the aligned energy band edges with rapid charge extraction through the hierarchical heterostructure of ITO@TiO2@CdS@CdSe, alleviating the crucial charge accumulation. As a result, the dual heterostructured photoanode exhibits a maximum photocurrent density of 20.11 mA/cm2 at 1.23 V vs. the reversible hydrogen electrode (RHE) and a dramatic enhancement in the incident photon-to-current efficiency (IPCE) over the extended absorption spectrum. The time-resolved photoluminescence (TRPL) characterization indicates the realized multiple-band cascade charge migration throughout ITO@TiO2@CdS@CdSe could promote an 8-fold increase in the charge separation efficiency. This rational design of dual-heterojunction-structured photoelectrodes via electrochemical deposition provides a demonstration of modifying conventional light-harvesting photoelectrodes with stagnate solar energy conversion and PEC hydrogen production.

Electrochemically prepared copper/indium oxides photocathode for efficient photoelectrochemical hydrogen production

A novel photocathodic composite has been studied towards photoelectrochemical (PEC) hydrogen production from water. Copper oxide/indium oxide (CuO/In2O3) thin film photocathodes have been prepared by electrochemical method under various potentials, followed by the thermal treatment. Photocathodes were characterized using X-ray powder diffraction (XRD) and scanning electron microscopy with elemental mapping. Under irradiation in alkaline electrolyte Cu/In material showed an impressive cathodic photocurrent, up to 4 mA cm−2, which can be associated with efficient electron-hole pairs separation and low rate of unfavorable recombination. Photogenerated electrons induced a water reduction to hydrogen, as confirmed by gas chromatography. Photoactivity of Cu/In was observed in a broad wavelength range, partially corresponding to the UV–Vis spectrum obtained from diffuse reflectance spectroscopy. Advanced techniques such as synchrotron-radiation X-ray absorption spectroscopy enabled the investigation of materials stability. Under irradiation of Cu/In composite, CuII is partially reduced to CuI, leading to a formation of self-stabilized composite.

Boosting the photocatalytic hydrogen evolution performance via an atomically thin 2D heterojunction visualized by scanning photoelectrochemical microscopy

Heterojunction catalyst can facilitate efficient photoelectrochemical (PEC) hydrogen evolution by reducing a potential barrier for charge transfer at the semiconductor/electrolyte interface. Such a heterojunction effect at the atomic thickness limit has not yet been explored although it can be strengthened because of strong built-in field and ultrafast charge transfer across the junction. Here, we first investigate a novel strategy to boost the hydrogen evolution performance of the p-type WSe2 photocathode via reducing the overpotential with an atomically thin heterojunction catalyst comprising MoS2 and WS2 monolayers. To unveil an effective role of the heterojunction by isolating its kinetic contribution from other collective catalytic effects, we develop and utilize an in situ scanning PEC microscopy, which enables the spatially-resolved visualization of the enhanced photocatalytic hydrogen evolution performance of the heterojunction. Notably, significant reduction in overpotential, from +0.28 ± 0.03 to −0.04 ± 0.05 V versus (vs.) the reversible hydrogen electrode (RHE), is achieved when the MoS2/WS2 heterojunction is introduced as a catalyst even without intentional generation of catalytic sites. As a result, the photocurrent of ~4.0 mA cm−2 occurs even at 0 V vs. RHE. Furthermore, the beneficial effect of the atomically scaled vertical heterojunction is explained by the built-in potential resulted from efficient charge transfer in type-II heterojunctions with the support of first-principles calculations. Our demonstration not only offers an unprecedented approach to investigating the fundamental PEC characteristics in relation to the tailored properties of a catalyst but also proposes a new catalytic architecture, thereby enabling the design of highly efficient PEC systems.

CdS branched TiO2: Rods-on-rods nanoarrays for efficient photoelectrochemical (PEC) and self-bias photocatalytic (PC) hydrogen production

Rationally designing and constructing high-efficiency composite photocatalysts with advantaged structure play significant roles in efficient photoelectrochemical or photocatalytic solar energy conversion systems. In this work, contrasted to the typical CdS nanoparticles modified TiO2 nanomaterials, a novel rods-on-rods CdS branched TiO2 nanoarrays have been firstly synthesized by a simple in-situ growth strategy. The controllable surface-to-volume ratios of CdS nanorods, combining with their original intimate connection with TiO2 nanorods, not only enhance both the solar light scattering and absorption performance, but also effectively promote the separation and transportation of photogenerated charges. In addition, this structure ensures adequate exposure of CdS to the electrolyte, thus increasing the photooxidation reactive sites. In a typical photoelectrochemical water splitting system, the maximum photon-to-current conversion efficiency reaches 34% under 450 nm wavelength light irradiation, and the best applied bias photo to current conversion efficiency of the samples runs up to 1.5% under solar light irradiation. Interestingly, by using a ‘cocatalysts-separated-loading’ strategy, the novel CdS branched TiO2 nanoarrays films can be further employed as self-bias photocatalytic films, which also obtain an enhanced photocatalytic hydrogen production activity and stability.

Visible light active CZTS sensitized CdS/TiO2 tandem photoanode for highly efficient photoelectrochemical hydrogen generation

A tandem photoanode consisting of Cu2ZnSnS4 (CZTS) nanoparticles layer over CdS/TiO2 heterostructure thin film on FTO substrate has been fabricated for photoelectrochemical hydrogen generation. CdS thin film has been deposited by chemical bath deposition over the spin-coated TiO2 thin film, which is followed by the deposition of CZTS nanoparticles layer by spray coating technique resulting in a CZTS/CdS/TiO2/FTO photoanode. The coating of CZTS nanoparticles layer over CdS/TiO2 films resulted in the tandem structure of different band level positions, which enhances the optical absorption in the visible region and also leads to higher separation of the photogenerated charge carriers. A substantial enhancement (39 times) in the PEC activity has been demonstrated for the CZTS/CdS/TiO2/FTO photoanode as compared to TiO2/FTO photoanode, which is due to cascade band gap from 3.2 eV to 1.7 eV and formation of heterojunction at interfaces. The enhancement in the charge transport properties of CZTS/CdS/TiO2/FTO photoanode has been confirmed from the electrochemical impedance spectroscopy (EIS) measurement. The EIS results confirm that the CZTS/CdS/TiO2/FTO photoanode exhibited lowest charge transfer resistance (Rct = 302 Ω/cm2) as compared to CdS/TiO2 (Rct = 615 Ω/cm2) and TiO2 (Rct = 1700 Ω/cm2) photoanode. A mechanism depicting the enhanced the performance of photoelectrochemical (PEC) CZTS/CdS/TiO2/FTO photoanode has been proposed.

Improving photo-oxidation activity of water by introducing Ti3+ in self-ordered TiO2 nanotube arrays treated with Ar/NH3

The synthesis of TiO2 nanostructures with visible light absorption is critical for realizing low-cost and efficient photoelectrochemical hydrogen generation from water splitting. In this work, we report an ArNH3Ar treatment method capable to generate Ti3+ species in anodic self-ordered TiO2 nanotube arrays for the synthesis of substoichiometric TiO2 nanotube arrays. The presence of Ti3+ is verified by XPS, EPR and EDS measurements while no detectable N is observed in the material. The modified TiO2 exhibits a 5-fold enhancement in photocurrent compared to pristine TiO2 at 1.23V vs. RHE due to the increased absorption of the solar spectrum. For the 4-h treated sample, a photocurrent of 1 mAcm−2 is obtained at 1.18V vs. RHE, owing to the reduced flat band potential and the increased density of charge carriers evaluated using EIS measurements. The photocatalytic activity can be further optimized by increasing the layer thickness, leading also to a cathodic shift of the onset potential. Furthermore, the modified TiO2 remains intact in air for no less than 1 week. This enhancement in photo-catalytic activity is proven to be closely relevant to the Ti3+ induced by the ArNH3Ar treatment.

Organic-inorganic hybrid perovskite – TiO2 nanorod arrays for efficient and stable photoelectrochemical hydrogen evolution from HI splitting

Solar-driven photoelectrochemical (PEC) hydrogen production offers a promising solution to simultaneously tackle the global energy crisis and the environmental pollution. Herein, we report a PEC cell of organic-inorganic hybrid perovskite (methylammonium lead iodide, MAPbI3)-TiO2 nanorod array (TNAs) for efficient and stable hydrogen evolution in aqueous hydrogen iodide (HI) solution. The built-in electric field created across the MAPbI3-TiO2 junction is able to efficiently separate the electron-hole pairs photogenerated in MAPbI3 with electrons quickly injected from MAPbI3 to TiO2, which are then transported along the one-dimensional TiO2 nanorod channels to the counter electrode to reduce proton to evolve hydrogen. The optimized MAPbI3-TNA PEC cell exhibits a high photocurrent density of 1.75 mA cm−2 at 0.14 V (vs. Ag/AgCl) under AM 1.5G illumination, which is able to stably produce molecular hydrogen at a rate of 33.3 μmol cm−2 h−1 for more than 8 h.

Packaging BiVO4 nanoparticles in ZnO microbelts for efficient photoelectrochemical hydrogen production

Constructing semiconductor heterojunction with optimal structure and composition is highly desired to maximize the solar light utilization for photoelectrochemical (PEC) water splitting. Here, we reported the fabrication of BiVO4@ZnO heterojunction with a novel nanostructure for PEC water splitting via foaming-assisted electrospinning and subsequent atomic layer deposition (ALD) techniques. In such BiVO4@ZnO heterojunction, the isolated BiVO4 nanoparticles were packaged within the ZnO microbelt matrix. During PEC water splitting, the BiVO4 acts as the primary light absorber for wider solar spectral harvesting, and the ZnO prompts the transfer of the photo-excited high-energy electrons, which would render them with prolonged lifetime and enhanced separation of the photogenerated charge carriers. In addition, the microbelts architecture with a hollow channel can also effectively improve the interfacial charge separation and transportation. Accordingly, the PEC performances of BiVO4@ZnO hybrid microbelts were significantly enhanced with a photocurrent density up to ∼0.46 mA cm−2 (at 1.23 V vs. reversible hydrogen electrode (RHE) under simulated sunlight illumination), which is 15.3 times to that of pure BiVO4 counterpart (∼0.03 mA cm−2). The photocurrent density of the BiVO4@ZnO electrode can be further increased to 1.07 mA cm−2 at 1.2 V vs. RHE by adding hole scavenger (NaSO3) in the electrolyte solution under AM 1.5 G irradiation.