Photoelectrochemical Water Splitting Cells Research Articles

Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO2 Photoanodes for Oxygen Evolution

AbstractAmong the various photoanode materials investigated for photoelectrochemical water splitting cells, TiO2 stands out due to its abundance, stability, and favorable valence band edge for water oxidation. In this study, the importance of introducing and combining oxygen and titanium vacancy point defects in anatase TiO2 photoanodes to improve their performance is unveiled, achieving a photocurrent density of 0.73 (±0.015) mA cm−2 at +1.23 VRHE under 100 mW cm−2 of simulated sunlight or 26.4 mA cm−2 at +1.23 VRHE under 100 mW cm−2 of 365 nm light. The characterization by X‐ray photoelectron spectroscopy, surface photovoltage, and electron paramagnetic resonance demonstrates that these oxygen and titanium vacancies can have both collective and localized positive effects on the material, leading to a narrowing of the bandgap, an increase in donor density, and an increase in hydroxyl groups on the surface of TiO2. These result in enhanced light absorption, conductivity, and photovoltage, as well as a more negative flat‐band potential and increase in hole flux to the semiconductor–electrolyte interface. These findings provide valuable insights into the role of point defects in modulating the properties of TiO2 and have important implications for the development of high‐performance TiO2‐based devices.

A Photoelectrochemical and Spectroscopic Studies of Photoelectrodes with Hematite∣Catalyst∣Electrolyte Interface Toward Oxygen Evolution Reaction

One of the recent issues in the world is climate change caused by global warming. Many countries have set an ultimate goal for carbon neutrality to prevent from further increase of the global temperatures. In this regard, research on development of renewable energy sources without any carbon emissions has been intensively conducted. Among many promising chemical fuels with sustainability, Hydrogen (H2) can be produced by water (H2O) splitting.Especially, water splitting driven by photoelectrochemical reactions has great advantages in H2 generation: photovoltage generated by light absorption in semiconductors may significantly reduce the applied overpotential which should be provided by external circuits. This photoelectrochemical system, called photoelectrochemical cell (PEC) for water splitting is basically composed of photoelectrodes, catalysts, and membranes for generation of photogenerated charge carriers, reducing overpotential by improving charge transfer kinetics, and prevention of H2 and Oxygen (O2) crossover, respectively. When photons from solar light with energies greater than band gap are incident on the photoelectrodes, an electron-hole pair (EHP) is formed in the semiconductor. These EHPs are separated and transported to the surface, driven by band bending formed at the semiconductors. In photoanodes, the most of the charge carriers arrived at the electrode surface are holes (h+s), which oxidizes H2O at the photoelectrode-electrolyte interface. The transfer of holes accompanies the proton transfers, thereby producing O2. The separated electrons (e−s) should travel to the (photo)cathode along the external circuit and generate H2. However, the overall efficiency of water splitting using PEC still confronts many challenges for wide commercialization, including the low solar-to-hydrogen (STH) efficiencies. In order to improve the performance, the semiconductor efficiently utilizes the photons in the visible light region, which takes 45% of solar irradiation. In this perspective, hematite (α-Fe2O3) is one of the most promising candidates for the semiconductor as photoanodes owing to is narrow band gap of 1.9 ~ 2.2 eV, which is in the range between 550 ~ 650 nm. Meticulous cares should be also taken in catalysts localization to prevent from the parasitic photon loss, as well as the selection of catalytic materials. Because catalysts attribute to the improved charge transfer kinetics but also simultaneously become the sites that obstruct the light penetration to the semiconductor, resulting in the lower light absorption. Furthermore, physical and chemical composition of catalysts on the semiconductor surface significantly influence the charge transfer rates for the photoelectrochemical reactions by varying the electrical field gradient formed at the semiconductor∣catalyst∣electrolyte interface. As several procedures are additionally involved in the PEC, the electrochemical reaction pathways become more complicated rather than the one in electrolyzers utilizing conductive electrodes. Therefore, underlying understanding in semiconductor∣catalyst∣electrolyte interface is crucial to achieve the high performance in PECs.Herein, we demonstrate the hematite∣catalyst∣electrolyte interface as the photoanodes to investigate the photoelectrochemical reaction pathways in water oxidation. Hematite is the perfect material as the semiconductor model, not only due to the narrow band gap in the region of visible light but also the excellent stability even when exposed to the electrolytes and electrochemical environments. The hematite is fabricated by simple hydrothermal reactions, followed by step-wise heat treatment. The structure of hematite is optimized by varying hydrothermal temperatures, affecting the crystallinity and exposed area of the conductive substrate, fluorine-doped tin oxide (FTO). The recombination by back reduction is occurred at the exposed FTO surface by pinholes in hematite, which directly contact the electrolyte. Moreover, the localization of Co-based catalysts is also easily controlled by light-guided electrodeposition. In this hematite∣catalyst∣electrolyte interface, the overall water oxidation pathways are investigated by photoelectrochemical and Raman spectroscopic studies. It is expected that our study in here contributes to the in-depth understanding on the complicated photoelectrochemical reaction mechanism, might leading to the improved performance for wide commercialization of PEC.

(Digital Presentation) 2D Molybdenum Disulfide Nanosheets As Cocatalyst and Passivation Layer over CuInS2 Based Photocathode for Solar Hydrogen Evolution

An excellent strategy to improve photoelectrochemical (PEC) water splitting is to develop cost-effective co-catalysts that do not use noble metals such as Platinum (Pt), Iridium (Ir), Ruthenium (Ru). Transition metal cocatalysts have shown excellent catalytic property towards hydrogen evolution (HER) and oxygen evolution reaction (OER). The surface active sites and lower free energy of H* adsorption (GH adsorption) on the surface are the factors ameliorating and boosting their catalytic activity. Molybdenum Disulfide (MoS2), one the transition metal dichalcogenide with 2D-sheet like structure with high surface area, active sites, and excellent property to catalyze HER comparable to Pt. The property of higher stability towards the photocorrosion as compared to other cocatalysts provides MoS2 with an additional feature of passivation layer. The sheet-like structure also enhances the light absorption minimizing the effect of parasitic light absorption.Copper based oxides and chalcogenides are well known p-type semiconductors being investigated for photocathode materials to be used in unassisted PEC water splitting cell. CuInS2, a ternary copper chalcogenide material with tunable bandgap (Eg = 1.5-1.8 eV), high absorption coefficient (> 104), suitable band positions for hydrogen evolution reaction. Despite all the necessary features and properties, it suffers with the problem of high rate of recombination between photogenerated charge carriers, lower diffusional lengths and poor photostability. Several approaches have been developed to improve the PEC performance of CuInS2 that include decoration of metal NPs, heterojunction fabrication, deposition of electron (ETLs) and hole transporting layers (HTLs). CdS forms a proper heterojunction with CuInS2 owing to its conduction (CB) and valence (VB) offsets.The integration of CuInS2 with CdS could form a facile heterojunction for the effective charge separation and reduce the rate of recombination and decoration of MoS2 over the CuInS2/CdS can be beneficial in terms of improved reaction kinetics at the surface and photostability by avoiding direct contact of CuInS2 and CdS with electrolyte. A CuInS2 nanosheet array-based photocathode that is modified with CdS and the co-catalyst MoS2. This green approach enhances water splitting under solar irradiation. By introducing CdS and MoS2, we significantly improved the visible light absorption of the modified hybrid photocathode (CIS/CdS/MoS2). Photoluminescence, impedance spectroscopy, and Mott–Schottky analysis confirmed that excited electron-hole pairs were better separated, the resistance of charge transfer was minimized, and the excited-state charge carrier concentration increased, leading to increased photocurrent.Typical results demonstrated that CIS/CdS/MoS2 photocathode delivered higher photocurrent (-1.75 mA/cm2 at 0 VRHE) and HC-STH conversion efficiency (0.42% at 0.49 VRHE) than those of CIS and CIS/CdS photoelectrodes. We attribute this improved PEC performance to the synergetic impact of CdS in charge generation and transfer and MoS2 as a cocatalyst with active surface sites for proton reduction. The hydrogen evolution experiment showed CIS/CdS/MoS2 (56.9 μmol/h) showed high activity and rate of hydrogen evolution compared to CIS (22.6 μmol/h). This study not only demonstrates the promising nature of CuInS2-based light absorber photocathodes for solar energy utilization but also recommends the use of MoS2 as a cocatalyst for the proton reduction reactions for widespread applications in solar-to-hydrogen conversion. Reference (1) Kumar, M.; Meena, B.; Subramanyam, P.; Ummethala, G.; Malladi, S. R. K.; Dutta-Gupta, S.; Subrahmanyam, C. CuInS 2 Nanosheet Arrays with a MoS 2 Heterojunction as a Photocathode for PEC Water Splitting. Energy & Fuels 2023, 37 (3), 2340–2349. https://doi.org/10.1021/acs.energyfuels.2c03502. Figure 1

Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting.

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9mA cm-2 at 1.23V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3V.

Fully inkjet-printed large-scale photoelectrodes

Small-area photoelectrodes are used to study fundamental science and material development for photoelectrochemical (PEC) water splitting cells at the laboratory scale. For practical applications, however, one needs to develop scalable geometrical designs and architectures of large photoelectrodes as well as their fabrication using low-cost, solution-processed, scalable methods. In this perspective, we first discuss the device physics concepts for developing large photoelectrodes using dimensional engineering (size, geometry, shape, and structures) and scalable architectures (such as symmetric and asymmetric designs with gridlines as well as monolithically integrated modules with interconnections), similar to the earlier development of large thin-film photovoltaic cells. Finally, we propose a novel and strategic protocol to fabricate these designs for the development of large-photoelectrode modules via commercially deployable, fully inkjet-printing as a solution processed thin-film deposition method. Small-area photoelectrodes are used to study fundamental science and material development for photoelectrochemical (PEC) water splitting cells at the laboratory scale. For practical applications, however, one needs to develop scalable geometrical designs and architectures of large photoelectrodes as well as their fabrication using low-cost, solution-processed, scalable methods. In this perspective, we first discuss the device physics concepts for developing large photoelectrodes using dimensional engineering (size, geometry, shape, and structures) and scalable architectures (such as symmetric and asymmetric designs with gridlines as well as monolithically integrated modules with interconnections), similar to the earlier development of large thin-film photovoltaic cells. Finally, we propose a novel and strategic protocol to fabricate these designs for the development of large-photoelectrode modules via commercially deployable, fully inkjet-printing as a solution processed thin-film deposition method.

The effects of potential and solar input on Z-scheme C3N4-TiO2 nanotubes @ Ti electrode in a broad potential window

Construction of Z-scheme graphitic carbon nitride-titanium dioxide nanotubes (C3N4-TNT) has been known useful to optimize the band structure for improving photon capture and for accelerating charge carrier separation and transfer rate in photoelectrochemical water splitting (PECWS) cells. However, the reported operating potential window in a PECWS cell, often in 0 – 1.23 VRHE (volt versus reversible hydrogen electrode) plus its overpotential, is too narrow to understand the C3N4-TNT electrode. Herein, a broad potential window of −0.5 − 2.5 VRHE is applied to C3N4-TNT@Ti and recorded via the polarization test under chopped sunlight to analyze the effect of both electrons from external electrical circuit and photons from simulated sunlight. In 0 – 2.5 VRHE, the potential enhances the photocurrent density. For example, at 1.6 VRHE, the C3N4-TNT sample exhibits 1.8-time higher photocurrent density than that of pure TNT. In −0.5 − 0 VRHE, i.e., both samples do not give photo-current response. In addition, for advanced water oxidation/reduction beyond WS to oxygen/hydrogen, a large potential window will be expected. Further, the light capture ability, the charge carrier recombination rate, and the electron flow path through the C3N4-TNT junction without and with reverse/forward potentials are discussed to elucidate the effect of the applied potential.

Thermoelectric voltage triggered self-biased photoelectrochemical water splitting utilizing visible light active Ag/NaNbO3 nanocomposite photoanode

Nanostructure semiconductor photoactive materials are often used as working photoelectrodes to absorb solar light in conventional photoelectrochemical (PEC) cells for water splitting to demonstrate enhanced PEC performance. However, an external bias is required to start PEC water splitting reactions for the generation of hydrogen. Here, we demonstrate a novel way of designing a self-biased hybrid system that combines a darkened thermoelectric device (TE) and a photoelectrochemical cell with a visible light-active n-type Ag/NaNbO3 photoanode to produce hydrogen via water splitting by utilizing the entire solar spectrum. This method can combine utilization of a visible light photon energy from the Ag/NaNbO3 photoanode with thermal energy from the darkened TE device to overcome the overpotential requirements. Upon light illumination, the photoactive behaviour of the Ag/NaNbO3 photoanode and the voltage generated from the darkened TE device in our hybrid system modifies the band positions of an Ag/NaNbO3 photoanode and spontaneously provide the external overpotential requirement for the PEC process and enhances the H2 production. Compared to the stand-alone PEC device, the TE-coupled PEC hybrid system exhibited ∼17 times enhancement in the photocurrent density and H2 production at zero bias. The electrochemical impedance spectroscopy analysis also shows that the charge transfer process gets significantly enhanced in the hybrid PEC system compared to the PEC cell alone. This finding indicates the synergistic coupling between TE and PEC cells, where a single light energy source can increase H2 production in the hybrid systems without needing an external bias.

Engineering the Interfacial Structure of Heavy Metal‐Free Colloidal Heterostructured Quantum Dots for High‐Efficiency Photoelectrochemical Water Oxidation without Co‐Catalyst

Solar‐driven photoelectrochemical (PEC) water splitting cell fabricated using colloidal quantum dots (QDs) is deemed as low‐cost and high‐efficiency solar‐to‐fuel conversion systems for future carbon neutrality. However, current QDs‐based PEC technologies are still hindered by several critical restrictions including the sluggish water oxidation kinetics and the frequent use of highly toxic QDs (e.g., Pb, Cd‐chalcogenides) as well as co‐catalysts, thus limiting their prospective commercial developments. Herein, the optoelectronic properties of heavy metal‐free InP/ZnSe core/shell QDs are tailored by introducing the interfacial GaP layers with variable thicknesses. As‐prepared InP/GaP/ZnSe core/shell QDs are used to sensitize TiO2 film as photoanodes for PEC water oxidation, showing an unprecedented photocurrent density of 4.1 mA cm−2 at 1.23 V versus reversible hydrogen electrode with excellent durability under one sun AM 1.5 G illumination. It is demonstrated that engineering the thickness of the interfacial GaP layer enables optimized optical properties and can introduce appropriate intermediate energy levels to promote the charge extraction from the InP core to the ZnSe shell for enhanced PEC water oxidation efficiency. This study paves the way for interfacial engineering of “green” core/shell QDs to realize cost‐effective, environment‐friendly, high‐performance, and co‐catalyst‐free QDs‐based PEC water splitting system.

Cobalt as a sacrificial metal to increase the photoelectrochemical stability of CuBi2O4 films for water splitting

CuBi2O4 is an excellent photocathode candidate in water-splitting photoelectrochemical cells. However, its poor photoelectrochemical stability caused by the reduction of Cu2+ to Cu metal limits its use. Here, we show a strategy to decrease the reduction of Cu2+ to Cu using cobalt as a sacrificial metal. Co-doped CuBi2O4 films were prepared by spray pyrolysis using Co2+ salt as the precursor. Co2+ ions replace Cu2+ in the CuBi2O4 structure, and subsequent heat treatment at 500 °C leads to partial oxidation of Co2+ to Co3+. As the reduction potential of Co3+/Co2+ is higher than that of Cu2+/Cu, Cu2+ reduction can be minimized. Comparatively, about 72% of the photocurrent produced by the CuBi2O4 film is lost in the first few minutes of illumination. In the Co-doped CuBi2O4 film, the photocurrent drops by less than 7%. Thus, the Co-doping can increase the CuBi2O4 photostability and be helpful for the fabrication of more stable photocathodes.

(Invited) Photochemical Energy Conversion with Integrated Plasmonics

Plasmonic nanostructures can dramatically increase the chemical reaction rates by providing more efficient solar-to-chemical energy conversion. Upon light excitation, metallic nanostructures sustain the collective oscillation of surface electrons, i.e. surface plasmons, producing local field enhancement and after one hundred femtoseconds decaying by generating a non-thermal distribution of hot carriers. Their consequent thermalization results in intense local heating of the nanostructures and their surrounding. All these effects can alter the reaction pathways and boost the reaction kinetics. Such discoveries have boosted the field of plasmonic photocatalysis, which aims at driving industrially-relevant chemical reactions using solar illumination. In this talk, I will present our recent progress in the fabrication of large-area plasmonic metasurfaces exhibiting a rich set of electromagnetic resonances and their integration in photoelectrochemical water splitting cells, direct alcohol fuel cells, and photochemical devices for hydrogen evolution. I will discuss the complex relationship between the optical resonances of the plasmonic metasurfaces with their chemical reactivity by distinguishing the various effects enhancing their photochemical performance. Finally, a new scanning photoelectrochemical method that provide the two-dimensional mapping of the chemical reactivity of plasmonic nanostructures on planar substrates with sub-micron resolution will be introduced.

(Invited) In Situ Time-Resolved Vibrational Spectroscopic Probe of Catalyst Structures, Dynamics and Reaction Mechanisms on Semiconductor Photoelectrodes

Attachment of molecular catalysts onto semiconductor or metal electrodes is a promising approach for rational design of efficient photoelectrochemical cells for solar water splitting and CO2 reduction. The understanding and improvement of such devices requires fundamental understanding of the structure and dynamics of catalysts as well as the interfacial electrolyte environment at electrode-liquid interfaces. Vibrational sum frequency generation (VSFG) spectroscopy is an in situ interface specific technique for probing interfacial properties at the semiconductor electrode/electrolyte interface at the molecular level. In this talk, we discuss our recent efforts in developing and applying in situ time-resolved VSFG for studying structural and dynamics of rhenium bipyridyl and other molecular CO2 reduction catalysts on semiconductor and metal electrodes. We have shown that by combining VSFG experiment with advanced computational modeling, catalyst adsorption geometry can be determined; through electrochemical Stark effect, local electric field and the potential profile in the electric double layer under light and dark conditions can be directly measured; using VSFG as a time-dependent probe, the interfacial charge transfer dynamics can be directly measured and key reactive intermediates can be identified.

Emerging Copper-Based Semiconducting Materials for Photocathodic Applications in Solar Driven Water Splitting

Hydrogen production through solar-driven water splitting is a promising approach and an alternative to the conventional steam reforming of natural gas and coal gasification. The growing energy demand and environmental degradation through carbon-emitting fossil fuels urge a transition in the usage of non-renewable to renewable sources of energy. The photocathodes in a photoelectrochemical (PEC) water-splitting cell are essential for the direct evolution of hydrogen. Among the known photocathodes, Cu-based p-type semiconducting materials are the most promising photo-absorber materials owing to their low-cost, low toxicity, natural abundance, suitable bandgaps, and favorable band edges for reduction. Moreover, the chemical stability and the rate of recombination significantly limit the longevity, the PEC performance, and practical applicability of Cu-based photocathodes. To overcome these problems, it is critical to have a thorough understanding of the constraints, improvement strategies, and an assessment of current developments in order to construct and design highly stable and efficient photocathodes. Here, in this review we have summarized the development of Cu-based metal oxide and sulfide photocathodes with the significant operational challenges and strategies that have successfully been employed to enhance the PEC performance. Furthermore, the emphasis is placed on recent reports and future perspectives regarding emerging challenges.

An Organic Semiconductor Photoelectrochemical Tandem Cell for Solar Water Splitting

AbstractPhotoelectrochemical cells employing organic semiconductors (OS) are promising for solar‐to‐fuel conversion via water splitting. However, despite encouraging advances with the half reactions, complete overall water splitting remains a challenge. Herein, a robust organic photocathode operating in near‐neutral pH electrolyte by careful selections of a semiconducting polymer bulk heterojunction (BHJ) blend and organic charge‐selective layer is realized. The optimized photocathode produces a photocurrent density of >4 mA cm−2 at 0 V vs the reversible hydrogen electrode (VRHE) for solar water reduction with noticeable operational stability (retaining ≈90% of the initial performance over 6 h) at pH 9. Combining the optimized BHJ photocathode with a benchmark BHJ photoanode leads to the demonstration of a large‐area (2.4 cm2) organic photoelectrochemical tandem cell for complete solar water splitting, with a predicted solar‐to‐hydrogen (STH) conversion efficiency of 0.8%. Under unassisted two‐electrode operation (1 Sun illumination) a stabilized photocurrent of 0.6 mA and an STH of 0.3% are observed together with near unity Faradaic efficiency of H2 and O2 production.

Effects of Fluorination and Molybdenum Codoping on Monoclinic BiVO4 Photocatalyst by HSE Calculations.

Monoclinic phase bismuth vanadate (BiVO4) is one of the most promising photoelectrochemical materials used in water-splitting photoelectrochemical cells. It could be even better if its band gap and charge transport characteristics were optimized. Although codoping of BiVO4 has proven to be an effective strategy, its effects are remarkably poorly understood. Using the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional, we estimate the formation energy, electronic properties, and photocatalytic activities of F and Mo codoped BiVO4. We find that Mo atoms prefer to replace V atoms, whereas F atoms prefer to replace O atoms (FOMoV-doped BiVO4) under oxygen-poor conditions according to calculated formation energies. BiVO4 doped with FOMoV is found to be shallow-level doped, occurring with some continuum states above the conduction band edge, which is advantageous for photochemical catalysis. Moreover, FOMoV-doped BiVO4 shows absorption stronger than that of pure BiVO4 in the visible spectrum. Based on the band-edge calculation, BiVO4 doped with FOMoV still retains a high oxidizing capacity. It has been shown that FOMoV-doped BiVO4 exhibits a very high photocatalytic activity under visible light.

Hydrothermal Synthesis in Gap: Conformal Deposition of Textured Hematite Thin Films for Efficient Photoelectrochemical Water Splitting.

Obtaining high performance of hematite (α-Fe2O3) in a photoelectrochemical (PEC) water splitting cell is a challenging task because of its poor electrical conductivity and extremely short carrier lifetime. Here, we introduce a new hydrothermal method, called gap hydrothermal synthesis (GAP-HS), to obtain textured hematite thin films with an outstanding PEC water oxidation performance. GAP-HS proceeds in a precursor-solution-filled narrow gap to induce an anisotropic ion supply. This gives rise to an interesting phenomenon associated with the growth of nanomaterials that reflect the texture of the used substrates. Also, GAP-HS causes the preferential growth of hematite crystal along the [110] direction, leading to improved electrical conductivity within the (001) basal plane. The hematite thin films obtained via GAP-HS exhibit a very high photocurrent of more than 1.3 mA cm-2 at 1.23 V with respect to the reversible hydrogen electrode with 550 °C annealing only. It is the highest photocurrent, to the best of our knowledge, obtained for the hydrothermally synthesized pristine hematite photoanode. Because the low-temperature annealing allows avoiding of substrate deformation, the hematite thin films obtained via GAP-HS are expected to be advantageous for tandem-cell configuration.

Highly active titanium oxide photocathode for photoelectrochemical water reduction in alkaline solution

Developing low-cost, efficient and stable metal oxide cathode catalyst for hydrogen evolution reaction (HER) is of great importance to overcome the major challenge in photoelectrochemical water splitting (PECWS) cell. Anodic titanium dioxide nanotube (TNT) as the electrode has shown promising for its flexibility, economy and efficiency. Herein, a highly active and stable photocathode for HER was achieved with a technique of stepwise cathodic reduction of anodic TNT without loading any cocatalyst. It exhibits a self-improving PECWS performance after the five-step cathodic reduction process. The onset voltage displays an anodic shift from −0.79 to 0.19 V versus the reversible hydrogen electrode (RHE) and the overall current shows an enhancement from −110.06 to −210.66 mA at −1.0 VRHE in 1 M NaOH aqueous solution (pH = 13.6). The photocathode also withstands a voltage @ −1.0 VRHE and a current @ −100 mA in the WS cell for more than 12,000 s sequentially. This work demonstrates a feasible strategy for forming a multijunction composite for efficient HER with the anodic TNT as the precursor.

Research priorities for seasonal energy storage using electrolyzers and fuel cells

Paul A. Kempler is a research assistant professor at the University of Oregon and the Associate Director for the Oregon Center for Electrochemistry (OCE). At the OCE, he has developed three graduate laboratory courses in electrochemistry and electrochemical engineering and serves as the liaison to industry partners. His current research interests include interfacial ion transfer in energy conversion and the technoeconomic analysis of decarbonized energy systems. Paul received his PhD in chemical engineering from the California Institute of Technology for research on catalyst integration and gas evolution in photoelectrochemical cells for water splitting. John James Slack is a senior scientist at Nikola Corporation. John specializes in developing novel materials, studying their interactions, and designing nanostructured architectures within fuel cell cores. In the first two years of his career, he authored two successful proposals worth $2.7 million and has co-authored several patents related to advancing the field of fuel cell membrane electrode assemblies. He is active in the fuel cell community, regularly presenting to the Department of Energy and to the Million Mile Fuel Cell Truck Consortium. John's background is in chemical engineering, and he obtained his PhD at Vanderbilt University in 2019. Andrew M. Baker is a senior scientist at Nikola Corporation. His current research is focused on improving the performance and durability of membrane-electrode-assembly materials and accelerated stress test development for heavy-duty PEM fuel cell applications. Prior to working at Nikola, he was a scientist at Los Alamos National Laboratory. He received his PhD in mechanical engineering from the University of Delaware in 2017.

Mono-Doped and Co-Doped Nanostructured Hematite for Improved Photoelectrochemical Water Splitting.

In this study, zinc-doped (-FeO:Zn), silver-doped (-FeO:Ag) and zinc/silver co-doped hematite (-FeO:Zn/Ag) nanostructures were synthesized by spray pyrolysis. The synthesized nanostructures were used as photoanodes in the photoelectrochemical (PEC) cell for water-splitting. A significant improvement in photocurrent density of 0.470 mAcm at 1.23 V vs. reversible hydrogen electrode (RHE) was recorded for -FeO:Zn/Ag. The -FeO:Ag, -FeO:Zn and pristine hematite samples produced photocurrent densities of 0.270, 0.160, and 0.033 mAcm, respectively. Mott–Schottky analysis showed that -FeO:Zn/Ag had the highest free carrier density of 8.75 × 10 cm, while pristine -FeO, -FeO:Zn, -FeO:Ag had carrier densities of 1.57 × 10, 5.63 × 10, and 6.91 × 10 cm, respectively. Electrochemical impedance spectra revealed a low impedance for -FeO:Zn/Ag. X-ray diffraction confirmed the rhombohedral corundum structure of hematite. Scanning electron microscopy micrographs, on the other hand, showed uniformly distributed grains with an average size of <30 nm. The films were absorbing in the visible region with an absorption onset ranging from 652 to 590 nm, corresponding to a bandgap range of 1.9 to 2.1 eV. Global analysis of ultrafast transient absorption spectroscopy data revealed four decay lifetimes, with a reduction in the electron-hole recombination rate of the doped samples on a timescale of tens of picoseconds.

Considerations for the Accurate Measurement of Incident Photon to Current Efficiency in Photoelectrochemical Cells

In this paper we review some of the considerations and potential sources of error when conducting Incident Photon to Current Efficiency (IPCE) measurements, with focus on photoelectrochemical (PEC) cells for water splitting. The PEC aspect introduces challenges for accurate measurements often not encountered in dry PV cells. These can include slow charge transfer dynamics and, depending on conditions (such as a white light bias, which is important for samples with non-linear response to light intensity), possible composition changes, mostly at the surface, that a sample may gradually undergo as a result of chemical interactions with the aqueous electrolyte. These can introduce often-overlooked dependencies related to the timing of the measurement, such as a slower measurement requirement in the case of slow charge transfer dynamics, to accurately capture the steady-state response of the system. Fluctuations of the probe beam can be particularly acute when a Xe lamp with monochromator is used, and longer scanning times also allow for appreciable changes in the sample environment, especially when the sample is under realistically strong white light bias. The IPCE measurement system and procedure need to be capable of providing accurate measurements under specific conditions, according to sample and operating requirements. To illustrate these issues, complications, and solution options, we present example measurements of hematite photoanodes, leading to the use of a motorized rotating mirror stage to solve the inherent fluctuation and drift-related problems. For an example of potential pitfalls in IPCE measurements of metastable samples, we present measurements of BiVO4 photoanodes, which had changing IPCE spectral shapes under white-light bias.

Chemical reactions of graphitic carbon nitride films with glass surfaces and their impact on photocatalytic activity.

Thin films of graphitic carbon nitride (g-CN), a visible-light-driven photocatalyst, have recently attracted interest for application in photoelectrochemical cells for water splitting and high-throughput photocatalysis. In typical syntheses, g-CN films are formed by heating the nitrogen-rich precursor and substrate to 500-600 °C. The heated substrate should affect the polycondensation of the precursor and thereby alter the properties of the g-CN film. In this paper, we demonstrate that soda-lime glass, such as commercial glass slides, modifies the chemical structure of g-CN. The terminal amino groups of g-CN are partially substituted with cyanamide and hydroxyl groups. The electron-withdrawing groups provide the energy offsets of the frontier orbitals between the modified and unmodified molecules, facilitating exciton dissociation. After alkali metals are removed, the modified g-CN film exhibits a faster photodegradation of methyl orange compared with a melon film. The simple protocol to activate a g-CN film without co-catalysts paves a new way to enhance photocatalytic activity via selections of substrates, including waste glass.