Solid Polymer Electrolyte-Coated Macroporous Titania Nanotube Photoelectrode for Gas-Phase Water Splitting

Photoelectrochemical Conversion of Methane to Ethane and Hydrogen under Visible Light Using Functionalized Tungsten Trioxide Photoanodes with Proton Exchange Membrane

Developing methane utilization technologies is desired to convert abundant and renewable carbon resources, such as natural gas and biogas, into value-added chemical products. This study provides insights into emerging photoelectrochemical (PEC) technology for the photocatalytic transformation of methane to C2H6 and H2 using visible light at room temperature. The PEC conversion of methane to oxygenates has been investigated in aqueous electrolytes. Herein, we demonstrate the gas-phase PEC methane conversion using a proton exchange membrane (PEM) as a solid polymer electrolyte and a gas-diffusion photoanode for methane oxidation. Tungsten trioxide (WO3), a semiconductor photocatalyst responsive to visible light, is utilized as the photoanode material. Ultraviolet light (∼365 nm) excitation predominantly results in CO2 production with lower C2H6 selectivity in humidified methane. In contrast, visible light (∼453 nm) effectively promotes C2H6 production over the WO3 photoanode, attributed to preferential hydroxyl radical (•OH) formation compared to UV irradiation. Photogenerated holes formed near the valence band maximum of WO3 contribute to •OH formation through a single-electron water oxidation. The photogenerated •OH activates gaseous methane molecules to methyl radicals, subsequently coupled into C2H6 at the gas-electrolyte-semiconductor boundary. H2 is concurrently formed on the cathode electrocatalyst. Improving the selectivity for the dehydrogenative coupling of methane is pivotal for enhancing the energy efficiency in the PEM-PEC system.

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.

Sorption-Based Atmospheric Water Harvesting: Materials, Components, Systems, and Applications

Freshwater scarcity is a global challenge posing threats to the lives and daily activities of humankind such that two-thirds of the global population currently experience water shortages. Atmospheric water, irrespective of geographical location, is considered as an alternative water source. Sorption-based atmospheric water harvesting (SAWH) has recently emerged as an efficient strategy for decentralized water production. SAWH thus opens up a self-sustaining source of freshwater that can potentially support the global population for various applications. In this review, the state-of-the-art of SAWH, considering its operation principle, thermodynamic analysis, energy assessment, materials, components, different designs, productivity improvement, scale-up, and application for drinking water, is first extensively explored. Thereafter, the practical integration and potential application of SAWH, beyond drinking water, for wide range of utilities in agriculture, fuel/electricity production, thermal management in building services, electronic devices, and textile are comprehensively discussed. The various strategies to reduce human reliance on natural water resources by integrating SAWH into existing technologies, particularly in underdeveloped countries, in order to satisfy the interconnected needs for food, energy, and water are also examined. This study further highlights the urgent need and future research directions to intensify the design and development of hybrid-SAWH systems for sustainability and diverse applications.

Transparent Porous Conductive Substrates for Gas-Phase Photoelectrochemical Hydrogen Production

Gas diffusion electrodes are essential components of common fuel and electrolysis cells but are typically made from graphitic carbon or metallic materials, which do not allow light transmittance and thus limit the development of gas-phase based photoelectrochemical devices. Herein, the simple and scalable preparation of F-doped SnO₂ (FTO) coated SiO₂ interconnected fiber felt substrates is reported. Using 2-5 µm diameter fibers at a loading of 4 mg cm^-2 , the resulting substrates have porosity of 90%, roughness factor of 15.8, and Young's Modulus of 0.2 GPa. A 100 nm conformal coating of FTO via atmospheric chemical vapor deposition gives sheet resistivity of 20 ± 3 Ω sq^-1 and loss of incident light of 41% at illumination wavelength of 550 nm. The coating of various semiconductors on the substrates is established including Fe₂ O₃ (chemical bath deposition), CuSCN and Cu₂ O (electrodeposition), and conjugated polymers (dip coating), and liquid-phase photoelectrochemical performance commensurate with flat FTO substrates is confirmed. Finally, gas phase H₂ production is demonstrated with a polymer semiconductor photocathode membrane assembly at 1-Sun photocurrent density on the order of 1 mA cm^-2 and Faradaic efficiency of 40%.

Water Vapor Photoelectrolysis in a Solid-State Photoelectrochemical Cell with TiO2 Nanotubes Loaded with CdS and CdSe Nanoparticles

In this study, polyol-made CdS and CdSe crystalline nanoparticles (NPs) are loaded by impregnation on TiO₂ nanotube arrays (TNTAs) for solar-simulated light-driven photoelectrochemical (PEC) water vapor splitting. For the first time, we introduce a safe way to utilize toxic, yet efficient photocatalysts by integration in solid-state PEC (SSPEC) cells. The enabling features of SSPEC cells are the surface protonic conduction mechanism on TiO₂ and the use of polymeric electrolytes, such as Nafion instead of liquid ones, for operation with gaseous reactants, like water vapor from ambient humidity. Herein, we studied the effects of both the operating conditions in gaseous ambient atmospheres and the surface modifications of TNTAs-based photoanodes with well-crystallized CdS and CdSe NPs. We showed 3.6 and 2.5 times increase in the photocurrent density of defective TNTAs modified with CdS and CdSe, respectively, compared to the pristine TNTAs. Electrochemical impedance spectroscopy and structural characterizations attributed the improved performance to the higher conductivity induced by intrinsic defects as well as to the enhanced electron/hole separation at the TiO₂/CdS heterojunction under gaseous operating conditions. The SSPEC cells were evaluated by cycling between high relative humidity (RH) (80%) and low RH levels (40%), providing direct evidence of the effect of RH and, in turn, adsorbed water, on the cell performance. Online mass spectrometry indicated the corresponding difference in the H₂ production rate. In addition, a complete restoration of the SSPEC cell performance from low to high RH levels was also achieved. The presented system can be employed in off-grid, water depleted, and air-polluted areas for the production of hydrogen from renewable energy and provides a solution for the safe use of toxic, yet efficient photocatalysts.

Enhanced Photoelectrochemical Water-Splitting Property on TiO2 Nanotubes by Surface Chemical Modification and Wettability Control

We report a simple method to control the photoelectrochemical (PEC) water-splitting performance of TiO₂ nanotube arrays (NTs) by surface chemical modification. Four types of modifier molecules with different surface energy and functional groups, including amine (-NH₂), n-alkane (-C_nH_2n+1), perfluoroalkyl (-F), and polymer molecule (-polymer), were self-assembled to the surface of TiO₂ NTs, which could change the surface chemical composition and wettability from superhydrophilicity to hydrophobicity. Interestingly, different from expected results, photoelectrochemical measurement results show that the n-octadecyltrichlorosilane-modified TiO₂ nanotube arrays with a contact angle of about 134° present the highest PEC property with doubled photocurrent density and more negative onset potential. And the total PEC performance order of the monolayers-modified TiO₂ NTs is (-C_nH_2n+1) > (-F) > (-NH₂) > (-OH, pristine TiO₂) > (-polymer), which is due to the molecular monolayers modification being able to suppress the recombination of photogenerated electrons and holes and facilitate water oxidation by regulating the interface electric double layer, whereas a thick polymer layer on the photoelectrode surface would affect the light absorbance and decrease the PEC performance. Further investigation indicates that the surface energy and wettability of the TiO₂ photoelectrode adjusted by surface modification also have an important influence on the interface reaction of water oxidation and the adsorption/desorption of newly formed oxygen, which also provides a new method for controlling the surface photocatalytic reactions.

Solar Hydrogen Generation from Ambient Humidity Using Functionalized Porous Photoanodes

Solar hydrogen is a promising sustainable energy vector, and steady progress has been made in the development of photoelectrochemical (PEC) cells. Most research in this field has focused on using acidic or alkaline liquid electrolytes for ionic transfer. However, the performance is limited by (i) scattering of light and blocking of catalytic sites by gas bubbles and (ii) mass transport limitations. An attractive alternative to a liquid water feedstock is to use the water vapor present as humidity in ambient air, which has been demonstrated to mitigate the above problems and can expand the geographical range where these devices can be utilized. Here, we show how the functionalization of porous TiO₂ and WO₃ photoanodes with solid electrolytes-proton conducting Aquivion and Nafion ionomers-enables the capture of water from ambient air and allows subsequent PEC hydrogen production. The optimization strategy of photoanode functionalization was examined through testing the effect of ionomer loading and the ionomer composition. Optimized functionalized photoanodes operating at 60% relative humidity (RH) and T_cell = 30-70 °C were able to recover up to 90% of the performance obtained at 1.23 V versus reverse hydrogen electrode (RHE) when water is introduced in the liquid phase (i.e., conventional PEC operation). Full performance recovery is achieved at a higher applied potential. In addition, long-term experiments have shown remarkable stability at 60% RH for 64 h of cycling (8 h continuous illumination-8 h dark), demonstrating that the concept can be applicable outdoors.

Photoelectrochemical Gas-Electrolyte-Solid Phase Boundary for Hydrogen Production From Water Vapor

Hydrogen production from humidity in the ambient air reduces the maintenance costs for sustainable solar-driven water splitting. We report a gas-diffusion porous photoelectrode consisting of tungsten trioxide (WO₃) nanoparticles coated with a proton-conducting polymer electrolyte thin film for visible-light-driven photoelectrochemical water vapor splitting. The gas-electrolyte-solid triple phase boundary enhanced not only the incident photon-to-current conversion efficiency (IPCE) of the WO₃ photoanode but also the Faraday efficiency (FE) of oxygen evolution in the gas-phase water oxidation process. The IPCE was 7.5% at an applied voltage of 1.2 V under 453 nm blue light irradiation. The FE of hydrogen evolution in the proton exchange membrane photoelectrochemical cell was close to 100%, and the produced hydrogen was separated from the photoanode reaction by the membrane. A comparison of the gas-phase photoelectrochemical reaction with that in liquid-phase aqueous media confirmed the importance of the triple phase boundary for realizing water vapor splitting.