Water-splitting Devices Research Articles

Improving the photovoltage of Cu2O photocathodes with dual buffer layers

Cuprous oxide (Cu2O) is a promising oxide material for photoelectrochemical water splitting (PEC), and increasing its photovoltage is the key to creating efficient overall PEC water-splitting devices. Previous reports are mostly focused on optimizing the energy band alignment between Cu2O and the n-type buffer layer to improve the photovoltage of Cu2O photocathodes. However, the band alignment between the n-type buffer layer and the protective layer is often ignored. In this work, Cu2O photocathodes with a single buffer layer (Ga2O3) and dual buffer layers (Ga2O3/ZnGeOx) are fabricated, and their PEC performances are compared. Results show that after inserting the second buffer layer (ZnGeOx), the onset potential of the Cu2O photocathode increases by 0.16 V. Operando electrochemical impedance spectroscopy measurements and analysis of the energy-level diagrams of each layer show that an energy level gradient between Ga2O3 and TiO2 is created when ZnGeOx is introduced, which eliminates the potential barrier at the interface of Ga2O3/TiO2 and improves the photovoltage of the Cu2O photocathode. Our work provides an effective approach to improve the photovoltage of photoelectrodes for solar water splitting by introducing dual buffer layers.

A fine 3d transition metal regulation strategy toward high-entropy alloy mesoporous nanotubes as efficient electrocatalysts

High-entropy alloys (HEAs) are attracting considerable attention because of their interesting catalytic features, intrinsic thermodynamic stability and diversity of components. Nevertheless, the precise shaping low-dimensional architectures and fine-regulated catalytic performance are still challenging for HEAs. Herein, we propose a cost-effective general template-triggered method to prepare a class of quinary to septenary HEA mesoporous nanotubes (HEA mNTs). In particular, we explore the electron structure–activity relationship toward HEA mNTs with different 3d transition metal regulation by experiments and DFT calculations. As a proof of demonstration, Fe-regulated senary HEA mNTs (PtPdRuIrFeCu mNTs) with ∼1.5 nm wall experimentally unveil the excellent mass activity of 1.94 A/mgPt (1.22 A/mgnoble metals) @ 0.9 VRHE for ORR, which is 7.46- (5.30-) fold higher than commercial Pt/C in 0.1 M KOH. Density functional theory results further reveal that the differential oxygen adsorption energy (EO) of PtPdRuIrFeCu is closest to the theoretical optimal EO, corresponding to excellent ORR activity. Additionally, the linear relationship between the differential EO of HEA and the d band center of pure metals can provide a rapid screening method for HEA catalysts. HEA mNTs reported in this work are a class of prospective catalysts for the fuel cells and water splitting devices, and the would-be applications in sensors and energy storages look bright because of their charming characteristics.

Scaling up BiVO4 Photoanodes on Porous Ti Transport Layers for Solar Hydrogen Production.

Commercialization of photoelectrochemical (PEC) water-splitting devices requires the development of large-area, low-cost photoanodes with high efficiency and photostability. Herein, we address these challenges by using scalable fabrication techniques and porous transport layer (PTLs) electrode supports. We demonstrate the deposition of W-doped BiVO4 on Ti PTLs using successive-ionic-layer-adsorption-and-reaction methods followed by boron treatment and chemical bath deposition of NiFeOOH co-catalyst. The use of PTLs that facilitate efficient mass and charge transfer allowed the scaling of the photoanodes (100 cm2 ) while maintaining ~90 % of the performance obtained with 1 cm2 photoanodes for oxygen evolution reaction, that is, 2.10 mA cm-2 at 1.23 V vs. RHE. This is the highest reported performance to date. Integration with a polycrystalline Si PV cell leads to bias-free water splitting with a stable photocurrent of 208 mA for 6 h and 2.2 % solar-to-hydrogen efficiency. Our findings highlight the importance of photoelectrode design towards scalable PEC device development.

Electrochemical synthesis of trimetallic nickel-iron-copper nanoparticles via potential-cycling for high current density anion exchange membrane water-splitting applications

Hydrogen is known for its elevated energy density and environmental compatibility and is a promising alternative to fossil fuels. Alkaline water electrolysis utilizing renewable energy sources has emerged as a means to obtain high-purity hydrogen. Nevertheless, electrocatalysts used in the process are fabricated using conventional wet chemical synthesis methods, such as sol–gel, hydrothermal, or surfactant-assisted approaches, which often necessitate intricate pretreatment procedures and are vulnerable to post-treatment contamination. Therefore, this study introduces a streamlined and environmentally conscious one-step potential-cycling approach to generate a highly efficient trimetallic nickel-iron-copper electrocatalyst in situ on nickel foam. The synthesized material exhibited remarkable performance, requiring a mere 476 mV to drive electrochemical water splitting at 100 mA cm−2 current density in alkaline solution. Furthermore, this material was integrated into an anion exchange membrane water-splitting device and achieved an exceptionally high current density of 1 A cm−2 at a low cell voltage of 2.13 V, outperforming the noble-metal benchmark (2.51 V). Additionally, ex situ characterizations were employed to detect transformations in the active sites during the catalytic process, revealing the structural transformations and providing inspiration for further design of electrocatalysts.

Self-supporting Co-MoOx@N-doped carbon/expanded graphite paper for efficient water splitting catalyst

Molybdenum oxides are widely used as electrocatalysts due to their polymetallic oxidation state, abundant reactive sites and low production cost. However, the low electrical conductivity and easy agglomeration has limited their practical application. Here, MoO3 nanowires encapsulated with ZIF-67 polyhedra were grown in situ on expanded graphite paper (EGP) by a simple hydrothermal synthesis method, and then the precursors were pyrolyzed to obtain self-supported Co-MoOx@N-doped carbon/EGP composites. Thanks to the three-dimensional porous structure and the synergistic effect of Co and multi-valance MoOx, the as-prepared Co-MoOx@N-doped carbon/EGP exhibits the excellent performances of OER (η10 = 256 mV) and HER (η10 = 138 mV), and a small Tafel slope and good durability. Furthermore, the assembled water splitting device achieved a current density of 10 mA cm−2 at a low voltage of 1.62 V and the excellent stability after 24 h continuous water splitting.

In situ construction of a dendritic CuBi2O4/CuO heterojunction photocathode for overall water splitting

A desirable heterogeneous structure is crucial in decreasing the photocurrent density loss of CuBi2O4-based photocathodes. However, the introduction of a cover layer that is prone to falling off and a complicated preparation process increases the cost of preparing the photocathode. Therefore, we propose a two-step electrodeposition method for constructing dendritic CuBi2O4/CuO heterojunction photocathodes in situ which achieve a photocurrent density up to − 1.13 mA cm−2 at 0.3 V vs. RHE. The enhancement performance of PEC is attributed to the presence of an interfacial electric field at the heterojunction, which promotes the separation of photogenerated electron-hole pairs and prolongs the charge lifetime. Furthermore, we loaded MoS2 on CuBi2O4/CuO to obtain a CuBi2O4/CuO/MoS2 photocathode. An unbiased tandem overall water splitting device was constructed with CuBi2O4/CuO/MoS2 as the photocathode as well as BiVO4/FeOOH/NiOOH as the photoanode, attaining a solar-hydrogen (STH) conversion efficiency of about 1.47%. With our results, the in-situ construction of heterogeneous structures has the potential to bring about the development of efficient and economical water splitting techniques.

Structural and optical properties of composition-graded InGaN nanowires

At the moment, InGaN ternary compounds are of a great interest for the development of devices for sunlight driven water splitting. However, the synthesis of such materials is hindered by the fact that InxGa1–xN layers are susceptible to phase decomposition at x from 20 to 80%. Nanowires can be a promising solution to this problem. The purpose of our study was to analyze the structural and optical properties of InxGa1–xN nanowires with a gradient x content being inside the miscibility gap. InxGa1–xN nanowires were grown on silicon substrates using plasma-assisted molecular beam epitaxy. The structural properties of nanowires were studied using scanning and transmission electron microscopy. The chemical composition and optical properties of nanowires were analyzed using energy-dispersive microanalysis and photoluminescence spectroscopy. It was shown for the first time that the composition-graded InxGa1–xN nanowires with x from 40 to 60% can be grown using plasma-assisted molecular beam epitaxy. The grown samples exhibit photoluminescence at room temperature with a maximum at about 890 nm, which corresponds to an In content of about 62% according to the modified Vegard’s rule and the transmission electron microscopy data. The obtained results can be of practical interest for the development of devices for water splitting induced by sunlight or sources of near IR radiation

BiVO4 Photoanodes Enhanced with Metal Phosphide Co-Catalysts: Relevant Properties to Boost Photoanode Performance.

Achieving highly performant photoanodes for oxygen evolution is key to developing photoelectrochemical devices for solar water splitting. In this work, BiVO4 photoanodes are enhanced with a series of core-shell structured bimetallic nickel-cobalt phosphides (MPs), and key insights into the role of co-catalysts are provided. The best BiVO4 /Ni1.5 Co0.5 P and BiVO4 /Ni0.5 Co1.5 P photoanodes achieve a 3.5-fold increase in photocurrent compared with bare BiVO4 . It is discovered that this enhanced performance arises from a synergy between work function, catalytic activity, and capacitive ability of the MPs. Distribution of relaxation times analysis reveals that the contact between the MPs, BiVO4 , and the electrolyte gives rise to three routes for hole injection into the electrolyte, all of which are significantly improved by the presence of a second metal cation in the co-catalyst. Kinetic studies demonstrate that the significantly improved interfacial charge injection is due to a lower charge-transfer resistance, enhanced oxygen-evolution reaction kinetics, and larger surface hole concentrations, providing deeper insights into the carrier dynamics in these photoanode/co-catalyst systems for their rational design.

Investigation of the Solar Hydrogen Sensitivity of GeSe Thin Film Photoelectrode with Photoelectrochemical Environment.

GeSe photovoltaic thin films are very promising for photoelectrochemical (PEC) hydrogen evolution. The GeSe-based PEC water splitting device is a system containing a photoelectrode, electrolyte, and other packages, and the performance of the GeSe photoelectrode inside the system is very sensitive to the PEC system environment, such as the electrolyte temperature, pH, and concentration. Here, we reveal how the electrolyte environment at the electrolyte/photoelectrode interface influences the optoelectronic/PEC properties of GeSe photoelectrodes. It was found that the photocurrent density of the GeSe photoelectrode increased with temperature between 10 and 50 °C but decreased when the temperature was over 50 °C. In addition, the pH values of the electrolyte were inversely proportional to the photocurrent density of the GeSe photoelectrode. Moreover, the PEC performance improved as the sodium ion concentration of the electrolyte increased. The results in this work should provide a new direction for further optimizing the performance of photoelectrodes.

Renewable Energy from Livestock Waste Valorization: Amyloid-Based Feather Keratin Fuel Cells.

Increasing carbon emissions have accelerated climate change, resulting in devastating effects that are now tangible on an everyday basis. This is mirrored by a projected increase in global energy demand of approximately 50% within a single generation, urging a shift from fossil-fuel-derived materials toward greener materials and more sustainable manufacturing processes. Biobased industrial byproducts, such as side streams from the food industry, are attractive alternatives with strong potential for valorization due to their large volume, low cost, renewability, biodegradability, and intrinsic material properties. Here, we demonstrate the reutilization of industrial chicken feather waste into proton-conductive membranes for fuel cells, protonic transistors, and water-splitting devices. Keratin was isolated from chicken feathers via a fast and economical process, converted into amyloid fibrils through heat treatment, and further processed into membranes with an imparted proton conductivity of 6.3 mS cm-1 using a simple oxidative method. The functionality of the membranes is demonstrated by assembling them into a hydrogen fuel cell capable of generating 25 mW cm-2 of power density to operate various types of devices using hydrogen and air as fuel. Additionally, these membranes were used to generate hydrogen through water splitting and in protonic field-effect transistors as thin-film modulators of protonic conductivity via the electrostatic gating effect. We believe that by converting industrial waste into renewable energy materials at low cost and high scalability, our green manufacturing process can contribute to a fully circular economy with a neutral carbon footprint.

Design and characterization of novel 2D TlPt2X3 (X = S, Se, Te) semiconductor monolayers for electronic and optoelectronic applications

The energy crisis and environmental issues have stimulated an increasing interest in developing novel materials, such as two-dimensional (2D) materials, with customized photocatalytic and photovoltaic features for sustainable energy production. In this study, we thoroughly investigated the stability, physical characteristics, electronic properties, optical behaviors, and photocatalytic potential of TlPt2X3 (X = S, Se, Te) monolayers (MLs) using state-of-the-art density functional theory calculations at HSE06 and PBE-GGA levels of theory. Our calculations show that these MLs are dynamically, thermally, mechanically, and thermodynamically stable. The elastic constant calculations indicate that these MLs possess high mechanical flexibility owing to their small Young's modulus. The HSE06 approach calculations reveal that the TlPt2S3, TlPt2Se3, and TlPt2Te3 MLs possess band gaps of 1.364 eV (indirect), 1.896 eV (indirect), and 1.174 eV (direct), respectively. The optical characteristics of these MLs demonstrate a wide absorption spectrum that can be activated by visible light, with considerable absorption intensity in the near ultraviolet range. Our findings demonstrate that the TlPt2Se3 (TlPt2Te3) ML exhibits remarkable hydrogen-oxygen (oxygen) evolution performance with an increase (decrease) in pH value, suggesting they can be used for photocatalytic water splitting under visible light. These results provide new directions for the use of these materials in photocatalytic water splitting and optical devices.

Facile Synthesis of Co Nanoparticles Embedded in N-Doped Carbon Nanotubes/Graphitic Nanosheets as Bifunctional Electrocatalysts for Electrocatalytic Water Splitting.

Developing robust and cost-effective electrocatalysts to boost hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs) is crucially important to electrocatalytic water splitting. Herein, bifunctional electrocatalysts, by coupling Co nanoparticles and N-doped carbon nanotubes/graphitic nanosheets (Co@NCNTs/NG), were successfully synthesized via facile high-temperature pyrolysis and evaluated for water splitting. The morphology and particle size of products were influenced by the precursor type of the cobalt source (cobalt oxide or cobalt nitrate). The pyrolysis product prepared using cobalt oxide as a cobalt source (Co@NCNTs/NG-1) exhibited the smaller particle size and higher specific surface area than that of the pyrolysis products prepared using cobalt nitrate as a cobalt source (Co@NCNTs/NG-2). Notably, Co@NCNTs/NG-1 displayed much lower potential -0.222 V vs. RHE for HER and 1.547 V vs. RHE for OER at the benchmark current density of 10 mA cm-2 than that of Co@NCNTs/NG-2, which indicates the higher bifunctional catalytic activities of Co@NCNTs/NG-1. The water-splitting device using Co@NCNTs/NG-1 as both an anode and cathode demonstrated a potential of 1.92 V to attain 10 mA cm-2 with outstanding stability for 100 h. This work provides a facile pyrolysis strategy to explore highly efficient and stable bifunctional electrocatalysts for water splitting.

TiO2 Nanotubes Decorated with Mo2C for Enhanced Photoelectrochemical Water-Splitting Properties.

The presence of Ti3+ in the structure of TiO2 nanotube arrays (NTs) has been shown to enhance the photoelectrochemical (PEC) water-splitting performance of these NTs, leading to improved results compared to pristine anatase TiO2 NTs. To further improve the properties related to PEC performance, we successfully produced TiO2 NTs using a two-step electrochemical anodization technique, followed by annealing at a temperature of 450 °C. Subsequently, Mo2C was decorated onto the NTs by dip coating them with precursors at varying concentrations and times. The presence of anatase TiO2 and Ti3O5 phases within the TiO2 NTs was confirmed through X-ray diffraction (XRD) analysis. The TiO2 NTs that were decorated with Mo2C demonstrated a photocurrent density of approximately 1.4 mA cm-2, a value that is approximately five times greater than the photocurrent density exhibited by the bare TiO2 NTs, which was approximately 0.21 mA cm-2. The observed increase in photocurrent density can be ascribed to the incorporation of Mo2C as a cocatalyst, which significantly enhances the photocatalytic characteristics of the TiO2 NTs. The successful deposition of Mo2C onto the TiO2 NTs was further corroborated by the characterization techniques utilized. The utilization of field emission scanning electron microscopy (FESEM) allowed for the observation of Mo2C particles on the surface of TiO2 NTs. To validate the composition and optical characteristics of the decorated NTs, X-ray photoelectron spectroscopy (XPS) and UV absorbance analysis were performed. This study introduces a potentially effective method for developing efficient photoelectrodes based on TiO2 for environmentally sustainable hydrogen production through the use of photoelectrochemical water-splitting devices. The utilization of Mo2C as a cocatalyst on TiO2 NTs presents opportunities for the advancement of effective and environmentally friendly photoelectrochemical (PEC) systems.

Enhancing the photoelectrochemical activity and stability of plate-like WO3 photoanode in neutral electrolyte solution using optimum loading of BiVO4 layer and NiFe–LDH electrodeposition

Tungsten trioxide (WO3) is an attractive candidate that can be used as a photoanode in the photoelectrochemical (PEC) water splitting device. However, WO3 photoanode is known for its high thermodynamic and chemical stability only in strong acidic (low pH) electrolyte solution and its relatively wide bandgap energy (2.8 eV) leads to insufficient absorption of visible light. The current study explored a WO3/BiVO4 heterojunction photoanode, consisting of plate-like WO3 covered with a uniform and highly porous BiVO4 thin layer, with electrodeposition of NiFe–LDH as a co-catalyst for oxygen evolution reaction. This top layer of BiVO4 nanostructure not only exhibited an enhancement in absorption of visible light but also protected the underneath plate-like WO3 from being attacked by the neutral environment of electrolyte solution (pH=∼7), resulting in significant improvement in its stability. The electrodeposition of NiFe–LDH using an acidic aqueous electrolyte solution (pH=3.7) was employed to overcome the sluggish kinetics of water oxidation on the surface of BiVO4. To overcome the weakness of the chemical dissolution of BiVO4 nanostructure in strong acidic solution, the electrodeposition time was carefully optimized for 50 s, leading to a uniform distribution of co-catalyst on the surface of WO3/BiVO4 heterostructure without the dissolving of BiVO4 in the acidic media. The WO3/BVO5/NiFe50 photoanode with the highest PEC performance (photocurrent density of around 1.78 mA/cm2 at 1.23 V vs. RHE) could maintain around 75% of its initial photocurrent after 24 h.

Hierarchical NiCo-LDH@MoS2/CuS composite as efficient trifunctional electrocatalyst for overall water splitting and asymmetric supercapacitor

The development of highly active and long-lived multifunctional electrocatalysts is a key priority to boost clean renewable energy technologies. It is difficult to construct such hierarchically structured materials with an efficient synergistic heterostructure technique to improve electrochemical performance. Here, a novel hierarchical NiCo-LDH@MoS2/CuS heterostructure (HS) was prepared by facile two-step hydrothermal synthesis for water splitting and supercapacitor applications. The hierarchical network of NiCo-LDH is grown on MoS2/CuS and is put forward based on various physio-chemical analysis. The hierarchical structure and heterostructure synergistic effect result in excellent HER (114 mV@10 mA cm−2) and OER (310 mV@10 mA cm−2) activity. Two electrode water-splitting cells (NiCo-LDH@MoS2/CuS||NiCo-LDH@MoS2/CuS) achieved a cell potential of 1.57 V@10 mA cm−2. NiCo-LDH@MoS2/CuS also exhibits the electrochemical charge storage property of 147.16 mAh g−1@0.2 A g−1. NiCo@MoS2/CuS||AC flexible solid-state asymmetric supercapacitor (FSSC) demonstrated charge storage of 226.08 mAh g−1 and energy density of 152.60 W h kg−1@0.2 A g−1 with 90.05 % retention rate, demonstrated the excellent cyclic and structural stability. The acquired knowledge from this study could be a novel strategy for designing hierarchical LDH-based heterostructures as electrode materials for water splitting and energy storage devices.

Co-Ru alloy nanoparticles decorated onto two-dimensional nitrogen doped carbon nanosheets towards hydrogen/oxygen evolution reaction and oxygen reduction reaction

Constructing highly-efficient electrocatalysts toward hydrogen evolution reaction (HER)/oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) with excellent stability is quite important for the development of renewable energy-related applications. Herein, Co-Ru based compounds supported on nitrogen doped two-dimensional (2D) carbon nanosheets (NCN) are developed via one step pyrolysis procedure (Co-Ru/NCN) for HER/ORR and following low-temperature oxidation process (Co-Ru@RuOx/NCN) for OER. The specific 2D morphology guarantees abundant active sites exposure. Furthermore, the synergistic effects arising from the interaction between Co and Ru are crucial in enhancing the catalytic performance. Thus, the resulting Co-Ru/NCN shows remarkable electrocatalytic performance for HER (70 mV at 10 mA cm−2) in 1 M KOH and ORR (half-wave potential E1/2 = 0.81 V) in 0.1 M KOH. Especially, the Co-Ru@RuOx/NCN obtained by oxidation exhibits splendid OER performance in both acid (230 mV at 10 mA cm−2) and alkaline media (270 mV at 10 mA cm−2) coupled with excellent stability. Consequently, the fabricated two-electrode water-splitting device exhibits excellent performance in both acidic and alkaline environments. This research provides a promising avenue for the advancement of multifunctional nanomaterials.

Coupled Lead Halide Perovskite Photoelectrode for Unbiased Photoelectrochemical Water Splitting

In search for new technology to preserve global warming caused by fossil fuel, solar energy driven devices have been high in demand. For instance, photovoltaics (PV) are a promising technology that can produce high power conversion efficiency (PCE). In addition, photoelectrochemical (PEC) water splitting, a promising artificial photosynthesis method, is a potential alternative technique to generate renewable fuel. PEC devices can generate renewable energy sources such as hydrogen, which can alleviate CO2 emission from fossil fuels. However, most readily employed PEC devices require external bias to function as a hydrogen producer. Thus, utilizing PV active layer as a photo absorber, PEC system is a suitable approach to the breakthrough of efficient hydrogen production.In the past decade of PV technology advances, organic-inorganic lead halide perovskite (LHP) can be arguably the most progressive research because of its tunable bandgap, fast charge separation, broad light absorption spectra, low cost, facile fabrication, and high efficiency. These advantageous properties led LHPs to be used as an active layer of water splitting devices. However, LHP-based PECs have flaws to be addressed to be viable material for PEC. LHP are known for their fragile nature from exposure to water. Hence, LHP-based PECs are commonly protected with metal foil encapsulation while covering the edges with epoxy resins. However, the metal foil provides poor contact with the LHP leading to poor charge transfer for water splitting. To solve this problem, we applied carbon conductive powder (CCP) as the interlayer to successfully facilitate charge transfer.With the proposed device modeling, monolithic PEC device that is capable of generating hydrogen in presence of low external applied bias is fabricated. Nonetheless, single monolithic PEC devices still need external applied bias to function as water splitting device which seek for further research. Therefore, adding another photoelectrode will eliminate the necessity of external bias resulting in an only solar powered water splitting device. Herein, we report unassisted solar to hydrogen conversion by using inverted and planar type LHP solar cells as photocathode and photoanode, respectively. With the integration of catalyst, 3D Ni.NiFe and NiPCoP, LHP-based PEC with exceptional performance was attained. We demonstrated unbiased PEC water splitting using a coupled LHP-based photoelectrode under alkaline conditions. Each LHP-based photoelectrode exhibited a outstanding performance of 22.42 mA cm-2 at 1.23 VRHE and 21.96 mA cm-2 at 0 VRHE for the photoanode and photocathode, respectively. Thus, we achieved solar to hydrogen (STH) of 10.64% and unassisted photo-current density of 8.65 mA/cm2. In addition, unbiased catalytic reactivity over 20 h retaining about 60% of photo-current density was attained.

Comparative Studies of Ir-Based PEM Electrolyzers Via Operando ‘Tender’ X-Ray AP-XPS

Ir-based catalysts are considered the state-of-the-art cathode material for the oxygen evolution reaction (OER) in water splitting devices. However, large scale hydrogen production from such devices will require reduced Ir loadings and/or new materials due to factors such as Ir scarcity, cost and loss during electrolysis operation. In the hopes of enabling progress of rational design for OER catalysts, effective characterization of the electrode-liquid interface is crucial. Synchrotron-based X-ray Photoelectron Spectroscopy (XPS) offers a versatile way to obtain surface chemical and elemental characterization of these materials. The use of X-rays in the ‘tender’ regime (~2-6 keV) allows for deeper probing than the conventional ‘soft’ x-ray (< 2000 eV) set up that is common at synchrotrons and lab-based sources. The photoelectrons generated from tender x-rays have enough energy to penetrate tens of nanometers through low density components such as a layer of liquid or polymeric electrolyte. This opens the door to access the chemistry at the interface of liquid and ionomer electrolyte-electrocatalyst materials. With the use of a fully operational two-electrode system, we are able to study these membrane electrode assemblies under active water splitting conditions.My talk will highlight recent AP-XPS results on iridium-based polymer electrolyte membranes for water splitting. With application of elevated potential, we can correlate the iridium valency and surface species with the appearance of product traces in a mass spectrum. I will show trends within and between Ir catalyst types that lend to a better understanding of these OER cathodes. Figure 1

(Invited) (Photo-)Electrochemical Reactions on Semiconductor Surfaces:Si and III-V Surfaces – Atomic and Electronic Structure

Semiconductor/electrolyte interfaces are used in many application areas where strong non linear variations in the electric performance depending on applied potential or due to illumination play a decisive role. Often a simplified explanation on charge redistribution across the semiconductor/electrolyte junctions is applied considering only the charge redistribution involving the bulk and the electrolyte following an idealized semiconductor /metal Schottky barrier model. But as will be addressed in our contribution strong deviations are usually observed in real devices which are due to Fermi level pinning effects and adsorbate related double layer potential shifts of the band edges. This is especially true for photoelectrochemical cells used in water splitting devices. For this reason we will present detailed surface science studies on the properties of Si and III-V surfaces espcially by applying photoelectron spectroscopy.In so-called model experiments the effect of surface states found on differently doped Si(111) and (100) surfaces is investigated in relation to the resulting surface electronic structure after appying different surface preparation and processing procedures. Additionally the reactivity and the changes in electronic structure following a „frozen electrolyte“ junction formation will be presented after water exposure at low sample temperature. The results are compared to photoelectrochemical cells based on Si single crystals as well as thin film multiabsorber cells used as photocathodes.For III-V semiconductors at first the surface properties of etched InP, GaP and GaInP (100) surfaces will be compared and further modified by subsequent vacuum based processing steps as heating and sputtering. The best surfaces are in their surface composition very close to samples prepared by MOCVD. Also for the III-V materials the interaction with H2O was investigated. In all cases Fermi level pinning close to the valence band maximum was observed and compared to the resulting band diagrams of photoelectrochemical devices used for HER.In combining photoelectrochemical studies with surface science investigations on the electrode properties using directly contacted but also buried junction photoelectrodes it can be shown that defect levels related to remaining dangling bond states play a dominant role for the performance of the devices. The concentration of the remaining defect states influence the obtained performance in dependence of the interfacial contact composition preferring the formation of buried junctions. In conclusion we suggest that surface science studies of photoelectrodes before and after electrochemical studies are essential and need to be further developed for in operando experiments with special emphasis also in the valence band regime to obtain improved insights into semiconductor/electrolyte junction properties.

Solar-Driven Upgrading of Biomass by Coupled Hydrogenation Using in Situ Photoelectrochemically Generated H2

Green H2 has been recognized as an important element in efforts to decarbonize our fossil fuel-dependent society. One approach to produce green H2 is solar water splitting in a photoelectrochemical (PEC) device. Solar-to-hydrogen (STH) efficiencies of up to 30% have been demonstrated1 but studies have shown that this approach still results in a relatively high levelized cost of hydrogen (LCOH) of ~10 US$ per kg of H2.2-3 This is ca. one order of magnitude higher than that of hydrogen produced via steam methane reforming (SMR), which forms the bulk of the currently produced H2. A possible solution is to incorporate an upgrading process of biomass feedstock that generates valuable chemicals into the solar water splitting device. This is expected to not only decrease the overall LCOH but also introduce an alternative renewable pathway in chemical manufacturing. In this study, we propose the concept of solar-driven hydrogenation of biomass-derived feedstock. Photoelectrochemically generated H2 in our solar water splitting device is coupled in situ with the homogenously catalyzed hydrogenation of itaconic acid (IA) to methyl succinic acid (MSA). IA has been identified by the US Department of Energy as one of the twelve building blocks that possess the potential to be transformed subsequently into several high-value bio-based chemicals or materials.4 MSA is a valuable chemical compound with an estimated global market size of up to ~15,000 tonnes, whose derivatives are ubiquitously used as solvents in cosmetics,5 polymer synthesis,6 binders in powder coatings,7 and organic synthesis, especially for pharmaceutical synthesis.8-9 Our coupled hydrogenation approach—performed in the PV-electrolyzer and PEC configurations using III-V PV cells and BiVO4-based photoelectrode, respectively—successfully demonstrate solar-driven H2-to-MSA conversion efficiencies as high as 60%. In comparison to the non-coupled approach (i.e., direct hydrogenation), our coupled system offers synergistic benefits in terms of prolonged durability and a higher degree of flexibility toward other important chemical transformation reactions. In addition, life-cycle net energy assessment and technoeconomic analysis results show that adding the coupled hydrogenation process significantly lowers the energy payback time and the LCOH, respectively, to a point that is competitive even with SMR-produced H2. Further implications and optimization potentials of the coupled PEC hydrogenation approach will be discussed. References Kim, J. H.; Hansora, D.; Sharma, P.; Jang, J.-W.; Lee, J. S., Toward practical solar hydrogen production – an artificial photosynthetic leaf-to-farm challenge. Chem. Soc. Rev. 2019, 48 (7), 1908-1971.Shaner, M. R.; Atwater, H. A.; Lewis, N. S.; McFarland, E. W., A comparative technoeconomic analysis of renewable hydrogen production using solar energy. Energy Environ. Sci. 2016, 9 (7), 2354-2371.Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F., Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry. Energy Environ. Sci. 2013, 6 (7), 1983-2002.Werpy, T.; Petersen, G. Top value added chemicals from biomass: volume I--results of screening for potential candidates from sugars and synthesis gas; National Renewable Energy Lab., Golden, CO (US): 2004.Richard, H.; Muller, B. Use of a 2-methylsuccinic acid diester derivative as solvent in cosmetic compositions; cosmetic compositions containing the same. WO2012119861A3, 2012.Verduyckt, J.; De Vos, D. Method for the production of methylsuccinic acid and the anhydride thereof from citric acid. WO2018065475A1, 2017.Mijolovic, D.; Szarka, Z. J.; Heimann, J.; Garnier, S. Powder coating useful as a coating agent, and for coating metallic- and non-metallic surfaces, comprises a binder comprising methyl succinic acid. DE102011080722A1, 2012.Okabe, M.; Lies, D.; Kanamasa, S.; Park, E. Y., Biotechnological production of itaconic acid and its biosynthesis in Aspergillus terreus. Appl. Microbiol. Biotechnol. 2009, 84 (4), 597-606.Willke, T.; Vorlop, K.-D., Biotechnological production of itaconic acid. Appl. Microbiol. Biotechnol. 2001, 56 (3), 289-295.