Photoelectrochemical Hydrogen Evolution Research Articles

Photoelectrochemistry by Design: Tailoring the Nanoscale Structure of Pt/NiO Composites Leads to Enhanced Photoelectrochemical Hydrogen Evolution Performance.

Photoelectrochemical hydrogen evolution is a promising avenue to store the energy of sunlight in the form of chemical bonds. The recent rapid development of new synthetic approaches enables the nanoscale engineering of semiconductor photoelectrodes, thus tailoring their physicochemical properties toward efficient H2 formation. In this work, we carried out the parallel optimization of the morphological features of the semiconductor light absorber (NiO) and the cocatalyst (Pt). While nanoporous NiO films were obtained by electrochemical anodization, the monodisperse Pt nanoparticles were synthesized using wet chemical methods. The Pt/NiO nanocomposites were characterized by XRD, XPS, SEM, ED, TEM, cyclic voltammetry, photovoltammetry, EIS, etc. The relative enhancement of the photocurrent was demonstrated as a function of the nanoparticle size and loading. For mass-specific surface activity the smallest nanoparticles (2.0 and 4.8 nm) showed the best performance. After deconvoluting the trivial geometrical effects (stemming from the variation of Pt particle size and thus the electroactive surface area), however, the intermediate particle sizes (4.8 and 7.2 nm) were found to be optimal. Under optimized conditions, a 20-fold increase in the photocurrent (and thus the H2 evolution rates) was observed for the nanostructured Pt/NiO composite, compared to the benchmark nanoparticulate NiO film.

Recent Advances in Earth-Abundant Heterogeneous Electrocatalysts for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting has been considered as the most promising strategy to obtain hydrogen fuels and oxygen. Developing earth-abundant heterogeneous electrocatalysts with high catalytic activity and stability is of great importance for achieving highly efficient water splitting. Here, an overview of the recent developments in the earth-abundant heterogeneous electrocatalysts for PEC water splitting is presented. The main earth-abundant electrocatalysts for the PEC hydrogen-evolution reaction (PEC-HER) include transition-metal dichalcogenide (TMD)-based materials, transition-metal carbides/phosphatides, and N-doped graphene, whereas the earth-abundant PEC oxygen-evolution reaction (PEC-OER) electrocatalysts are focused on mixed transition-metal oxides, TMD-based materials, and transition-metal- and/or heteroatom-doped carbon catalysts. In addition, earth-abundant bifunctional (HER/OER) electrocatalysts, including NiFe layered double hydroxide and Co–Mn oxide nanoparticles/nitrogen-doped carbon frameworks, are introduced for overall water splitting. The strategies for improvement of the catalytic performance and the composition/structure–catalytic-activity relationships are highlighted. Finally, current existing challenges and the future research directions for enhancing the PEC water-splitting performance of earth-abundant heterogeneous electrocatalysts are provided.

Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%

Multiple exciton generation (MEG) in quantum dots (QDs) has the potential to greatly increase the power conversion efficiency in solar cells and in solar-fuel production. During the MEG process, two electron–hole pairs (excitons) are created from the absorption of one high-energy photon, bypassing hot-carrier cooling via phonon emission. Here we demonstrate that extra carriers produced via MEG can be used to drive a chemical reaction with quantum efficiency above 100%. We developed a lead sulfide (PbS) QD photoelectrochemical cell that is able to drive hydrogen evolution from aqueous Na2S solution with a peak external quantum efficiency exceeding 100%. QD photoelectrodes that were measured all demonstrated MEG when the incident photon energy was larger than 2.7 times the bandgap energy. Our results demonstrate a new direction in exploring high-efficiency approaches to solar fuels. Multiple exciton generation has been shown to improve the performance of quantum-dot-based solar cells. Yan et al. now apply it to photoinduced hydrogen production and present a system using PbS quantum-dot photoelectrodes that yields an external quantum efficiency of over 100%.

Fabrication of a Cu2O/g‐C3N4/WS2 Triple‐Layer Photocathode for Photoelectrochemical Hydrogen Evolution

AbstractA Cu2O/g‐C3N4/WS2 triple‐layer photocathode was fabricated by using a layer‐by‐layer assembly method for photoelectrochemical hydrogen evolution. Cu2O was electrodeposited on the FTO as the active layer for light harvesting. The chemically inert g‐C3N4 can protect the unstable Cu2O layer and form a p‐n junction with Cu2O to increase the light‐to‐electricity conversion efficiency. Mixed‐phase WS2 nanosheets were obtained through a lithium insertion method and used as an effective hydrogen evolution catalyst for photocurrent‐to‐hydrogen conversion. This photocathode is effective for photoelectrochemical hydrogen evolution, and a photocurrent of −9.5 mA cm−2 at −0.55 V versus RHE (pH 6.0) can be obtained under visible light (λ≥420 nm, 100 mW cm−2).

Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers

Hydrogen (H2) is one of the most important fuel candidates and its low-cost production would necessitate the development of efficient electrocatalysts. In this study, we report the synthesis and evaluation of two new carbazole-containing polymers as organic photoelectrochemical (PEC) catalysts for hydrogen evolution reaction (HER). The synthesis of these new conjugated polymers, poly(N-(2-ethylhexyl)-3,6-carbazole-p-bisdodecyloxy-phenylene vinylene) (P1) and poly(N-(2-ethylhexyl)-3,6-carbazole-p-bis(2-ethylhexyloxy)-phenylene vinylene) (P2), was accomplished by the Horner–Emmons polymerization reaction and subsequently characterized by 1H NMR, FTIR, diffuse reflectance UV–vis spectroscopy (DR UV–vis), scanning electron microscope (SEM) and thermogravimetric analysis (TGA). The optical band gaps of P1 and P2, derived from the onset absorption edge, were found to be 2.10 and 2.14 eV, respectively. The chronoamperometric (CA) measurements revealed that the photo-current density generated at ∼0 V by P1 and P2, without the use of additional noble metal based cocatalysts or sacrificial electron donors, was −1.8 and −2.1 μA/cm2, respectively. The enhanced PEC performance of P2 was attributed due to its narrow band gap that enhanced light harvesting ability and the larger surface area which helped in minimizing charge recombination. The experimental observations were well supported by the drastic quenching of PL emission intensity of P2. The linear sweep voltammetry (LSV) measurements showed the onset potential at around −0.3 V for both polymers. The photocurrent density difference for P2 at −1.2 V reached to maximum value of 0.37 mA/cm2, amounting to ∼25% current enhancement under illumination. Long-term stability testing via CA measurements revealed that P2 was relatively more stable than P1, which warranted its potential as photocatalyst for solar water splitting. In addition, P1 and P2 are readily soluble in common organic solvents which make them potential candidates for photovoltaic devices application.

Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface

Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a spectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide-based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to compare directly the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving the homolytic reductive elimination of hydrogen.

Efficient and Stable Silicon Photocathodes Coated with Vertically Standing Nano-MoS2 Films for Solar Hydrogen Production.

Water splitting in a photoelectrochemical cell, which converts sunlight into hydrogen energy, has recently received intense research. Silicon is suitable as a viable light-harvesting material for constructing such cell; however, there is a need to improve its stability and explore a cheap and efficient cocatalyst. Here we fabricate highly efficient and stable photocathodes by integrating crystalline MoS2 catalyst with ∼2 nm Al2O3 protected n+p-Si. Al2O3 acts as a protective and passivative layer of the Si surface, while the sputtering method using a MoS2 target along with a postannealing leads to a vertically standing, conformal, and crystalline nano-MoS2 layer on Al2O3/n+p-Si photocathode. Efficient (0.4 V vs RHE onset potential and 35.6 mA/cm2 saturated photocurrent measured under 100 mA/cm2 Xe lamp illumination) and stable (above 120 h continuous water splitting) photocathode was obtained, which opens the door for the MoS2 catalyst to be applied in photoelectrochemical hydrogen evolution in a facile and scalable way.

Electrodeposition of Cu2O/g-C3N4 heterojunction film on an FTO substrate for enhancing visible light photoelectrochemical water splitting

An immobilized Cu2O/g-C3N4 heterojunction film was successfully made on an FTO substrate by electrophoretic deposition of g-C3N4 on a Cu2O thin film. The photoelectrochemical (PEC) performance for water splitting by the Cu2O/g-C3N4 film was better than pure g-C3N4 and pure Cu2O film. Under –0.4 V external bias and visible light irradiation, the photocurrent density and PEC hydrogen evolution efficiency of the optimized Cu2O/g-C3N4 film was –1.38 mA/cm2 and 0.48 mL h−1 cm−2, respectively. The enhanced PEC performance of Cu2O/g-C3N4 was attributed to the synergistic effect of light coupling and a matching energy band structure between g-C3N4 and Cu2O as well as the external bias.

Fe controlled charge-dynamics in ZnO for solar hydrogen generation

An efficient photoanode of Fe doped ZnO was fabricated using an economic and simple spray pyrolysis technique. It exhibited 5-fold photocurrent enhancement as compared to an un-doped photoelectrode irradiated under simulated solar radiation (AM 1.5G). The photo-conversion efficiencies of the electrodes fabricated by variation in the dopant concentration in the range 10−4–10−1% of Fe have been estimated and compared. The doping enabled to control the charge dynamics in ZnO photoanode to yield enhanced photocurrent. The photo electrochemical hydrogen evolution under solar-photons from the doped photoanode was 17-times larger in magnitude i.e. ∼307 μmol/h, than un-doped film electrodes (18 μmol/h). The film exhibited wurtzite structure (Space Group – P 63mc) which did not show any structural lattice deformation after Fe doping of ≤10−1%. Optical studies revealed a red-shift in the band-gap, while a decrease in the absorption-coefficient, with the increase in Fe concentration. These photoanodes also displayed higher Incident-Photon-Current-Conversion efficiency (IPCE) in the 400–430 nm wavelength range. Electrochemical studies revealed n-type conductivity of these photoanodes. An anodic shift in the flat-band potential was observed with an increase in the Fe dopant concentration in the ZnO lattice. The result of the experimental study is illustrated in the form of schematic diagram which demonstrates the suitability of the doped system for solar hydrogen generation. Dopant induced improved optical absorption is mainly attributed to the enhancement in photo-response of these Fe doped ZnO films. The study indicates high potential of these ZnO films for solar energy applications especially with respect to their ability to work under solar radiation.

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La5Ti2CuS5O7 Photocathodes by Surface Modification.

Particulate La5Ti2CuS5O7 (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H2 evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO2, Nb2O5, and Ta2O5 by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment.

Photoelectrochemical hydrogen evolution from water on a surface modified CdTe thin film electrode under simulated sunlight

Photoelectrochemical hydrogen evolution from water using CdTe photocathodes prepared by close spaced sublimation was investigated. A CdTe thin film deposited on a fluorine-doped tin oxide-coated (FTO) glass plate acted as the photocathode. Surface modification of this photocathode with Pt resulted in an increase in the cathodic photocurrent from only 0.01 to 0.14 mA cm−2 at 0.1 VRHE. Further surface modification with an approximately 80 nm-thick CdS layer dramatically increased both the cathodic photocurrent and the onset potential, from 0.14 to 6.0 mA cm−2 at 0.1 VRHE and from 0.2 to 0.6 VRHE, respectively, under simulated sunlight. The CdTe photocathode showed stoichiometric hydrogen evolution from water, with a half-cell solar to hydrogen conversion efficiency of 0.9% at approximately 0.2 VRHE.

Stabilizing organic photocathodes by low-temperature atomic layer deposition of TiO2

Low-temperature atomic layer deposition forms a compact TiO2 film atop a polymer light absorber for stable and efficient organic–inorganic photo-driven H2 evolution.

Epitaxial hetero-structure of CdSe/TiO2 nanotube arrays with PEDOT as a hole transfer layer for photoelectrochemical hydrogen evolution

The photocatalytic decomposition of water is believed to be able to help mitigate the crisis of fossil fuel depletion.

Platinum and indium sulfide-modified Cu3BiS3photocathode for photoelectrochemical hydrogen evolution

Platinum-deposited In2S3/Cu3BiS3film as a photoelectrode was fabricated on a molybdenum-coated glass substrate for photoelectrochemical hydrogen evolution.

Preparation of TiO2-Decorated Boron Particles by Wet Ball Milling and their Photoelectrochemical Hydrogen and Oxygen Evolution Reactions

TiO2-coated boron particles were prepared by a wet ball milling method, with the particle size distribution and average particle size being easily controlled by varying the milling operation time. Based on the results from X-ray photoelectron spectroscopy, transmission electron microscopy, energy dispersive X-ray analysis, and Fourier transform infrared spectroscopy, it was confirmed that the initial oxide layer on the boron particles surface was removed by the wet milling process, and that a new B–O–Ti bond was formed on the boron surface. The uniform TiO2 layer on the 150 nm boron particles was estimated to be 10 nm thick. Based on linear sweep voltammetry, cyclic voltammetry, current-time amperometry, and electrochemical impedance analyses, the potential for the application of TiO2-coated boron particles as a photoelectrochemical catalyst was demonstrated. A current of 250 μA was obtained at a potential of 0.5 V for hydrogen evolution, with an onset potential near to 0.0 V. Finally, a current of 220 μA was obtained at a potential of 1.0 V for oxygen evolution.

Highly efficient photocatalytic and photoelectrocatalytic activity of solar light driven WO3/g-C3N4 nanocomposite

In this work, using compositing of two semiconductors WO3 and g-C3N4 (WO3:g-C3N4=1:1, 1:3, and 3:1), we achieved the best solar light driven photocatalyst. The photocatalytic activity of WO3/g-C3N4 nanocomposite was investigated using the photodegradation of an azo dye, Reactive black 5 (RB5) and photoelectrochemical hydrogen evolution. The WO3/g-C3N4 nanocomposite with a mass ratio of 1:3 (W1G3) showed the highest photocatalytic activity in degrading the most stable component (benzene component) of RB5 under solar light irradiation. By using different scavengers, it was determined that superoxide radical anion is the main reactive species for the photocatalytic degradation of RB5. The cyclic voltammetry (CV) and linear sweep voltammetry techniques were used to investigate electrochemical and photoelectrochemical properties of WO3/g-C3N4 nanocomposite. W1G3 showed the best electrochemical property compared with WO3, g-C3N4, and other nanocomposites because of the strongest anodic current density in the cyclic voltammetry. The anodic currents of W1G3 in CV exhibited an enhancement of 3.4 and 1.2 fold compared to those of the WO3 and g-C3N4, respectively. The electrode parameters for photoelectrochemical properties, obtained by linear sweep voltammetry technique, confirmed that W1G3 nanocomposite is an effective photocatalyst for photoelectrochemical water reduction reaction. The excellent photocatalytic properties of this nanocomposite is due to both the increase of the light absorption and the presence of WO3 NPs on the g-C3N4 surface nanosheets, decrease of the recombination rate of electron and hole because of Z-scheme configuration and suitable band positions.

Sunlight‐Driven Overall Water Splitting by the Combination of Surface‐Modified La5Ti2Cu0.9Ag0.1S5O7 and BaTaO2N Photoelectrodes

AbstractA photoelectrochemical (PEC) cell composed of an Al‐doped La5Ti2Cu0.9Ag0.1S5O7 (Al‐LTCA) photocathode and a BaTaO2N (BTON) photoanode was employed for overall water splitting. The surface of the Al‐LTCA photocathode was modified first with CdS by a chemical bath treatment (CBT) and subsequently with TiO2 and Pt (to form Pt/TiO2/CBT‐CdS/Al‐LTCA). The modified photocathode Pt/TiO2/CBT‐CdS/Al‐LTCA showed a significantly enhanced photocurrent for the PEC hydrogen evolution reaction in strongly alkaline aqueous phosphate solutions. This enhancement was attributed to surface stabilization by TiO2 and to the enhanced charge separation caused by the formation of a p–n junction at the CBT‐CdS/Al‐LTCA interface. The BTON photoanode surface modified with Co and Ir species (Ir/Co/BTON) showed a lower onset potential than that of a Co‐loaded BTON photoanode for the PEC oxygen evolution reaction. A PEC cell incorporating the Pt/TiO2/CBT‐CdS/Al‐LTCA photocathode and the Ir/Co/BTON photoanode exhibited stoichiometric H2 and O2 generation. The solar‐to‐hydrogen energy conversion efficiency of this PEC cell was found to be 0.14 % following a minute of simulated sunlight irradiation.

Efficient Photoelectrochemical Hydrogen Evolution on Silicon Photocathodes Interfaced with Nanostructured NiP2 Cocatalyst Films.

Increasing attention has now been focused on the photoelectrochemical (PEC) hydrogen evolution as a promising route to transforming solar energy into chemical fuels. Silicon is one of the most studied PEC electrode materials, but its performance is still limited by its inherent PEC instability and electrochemical inertness toward water splitting. To achieve significant PEC activities, silicon-based photoelectrodes usually have to be coupled with proper cocatalysts, and thus, the formed semiconductor-cocatalyst interface presents a critical structural parameter in the rational design of efficient PEC devices. In this study, we directly grow nanostructured pyrite-phase nickel phosphide (NiP2) cocatalyst films on textured pn+-Si photocathodes via on-surface reaction at high temperatures. The areal loading of the cocatalyst film can be tailored to achieve an optimal balance between its optical transparency and electrocatalytic activity. As a result, our pn+-Si/Ti/NiP2 photocathodes demonstrate a great PEC onset potential of 0.41 V versus reversible hydrogen electrode (RHE), a decent photocurrent density of ∼12 mA/cm2 at the thermodynamic potential of hydrogen evolution, and an impressive operation durability for at least 6 h in 0.5 M H2SO4. Comparable PEC performance is also observed in 1 M potassium borate buffer (pH = 9.5) using this device.

Ultrasmall and phase-pure W2C nanoparticles for efficient electrocatalytic and photoelectrochemical hydrogen evolution.

Earlier research has been primarily focused on WC as one of the most promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER), whereas the other compound in this carbide family—W2C—has received far less attention. Our theoretical calculations suggest that such a focus is misplaced and W2C is potentially more HER-active than WC. Nevertheless, the preparation of phase pure and sintering-free W2C nanostructures represents a formidable challenge. Here we develop an improved carburization method and successfully prepare ultrasmall and phase-pure W2C nanoparticles. When evaluated for HER electrocatalysis, W2C nanoparticles exhibit a small onset overpotential of 50 mV, a Tafel slope of 45 mV dec−1 and outstanding long-term cycling stability, which are dramatically improved over all existing WC-based materials. In addition, the integration of W2C nanoparticles with p-type Si nanowires enables highly active and sustainable solar-driven hydrogen production. Our results highlight the great potential of this traditionally non-popular material in HER electrocatalysis.

Cu2O/CuO Bilayered Composite as a High-Efficiency Photocathode for Photoelectrochemical Hydrogen Evolution Reaction

Solar powered hydrogen evolution reaction (HER) is one of the key reactions in solar-to-chemical energy conversion. It is desirable to develop photocathodic materials that exhibit high activity toward photoelectrochemical (PEC) HER at more positive potentials because a higher potential means a lower overpotential for HER. In this work, the Cu2O/CuO bilayered composites were prepared by a facile method that involved an electrodeposition and a subsequent thermal oxidation. The resulting Cu2O/CuO bilayered composites exhibited a surprisingly high activity and good stability toward PEC HER, expecially at high potentials in alkaline solution. The photocurrent density for HER was 3.15 mA·cm−2 at the potential of 0.40 V vs. RHE, which was one of the two highest reported at the same potential on copper-oxide-based photocathode. The high photoactivity of the bilayered composite was ascribed to the following three advantages of the Cu2O/CuO heterojunction: (1) the broadened light absorption band that made more efficient use of solar energy, (2) the large space-charge-region potential that enabled a high efficiency for electron-hole separation, and (3) the high majority carrier density that ensured a faster charge transportation rate. This work reveals the potential of the Cu2O/CuO bilayered composite as a promising photocathodic material for solar water splitting.