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%.

Hydrogen production from the air

Green hydrogen produced by water splitting using renewable energy is the most promising energy carrier of the low-carbon economy. However, the geographic mismatch between renewables distribution and freshwater availability poses a significant challenge to its production. Here, we demonstrate a method of direct hydrogen production from the air, namely, in situ capture of freshwater from the atmosphere using hygroscopic electrolyte and electrolysis powered by solar or wind with a current density up to 574 mA cm⁻². A prototype of such has been established and operated for 12 consecutive days with a stable performance at a Faradaic efficiency around 95%. This so-called direct air electrolysis (DAE) module can work under a bone-dry environment with a relative humidity of 4%, overcoming water supply issues and producing green hydrogen sustainably with minimal impact to the environment. The DAE modules can be easily scaled to provide hydrogen to remote, (semi-) arid, and scattered areas.

While obtaining H₂ from water splitting offers a promising strategy for renewable fuel production, current technologies rely on liquid freshwater. Here, authors use a hygroscopic electrolyte to achieve electrocatalytic water vapor splitting driven by renewable resources without liquid water.

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.

Prospects for Hermetic Sealing of Scaled-Up Photoelectrochemical Hydrogen Generators for Reliable and Risk Free Operation

Photo-electrochemical (PEC) systems have the potential to contribute to de-carbonation of the global energy supply because solar energy can be directly converted to hydrogen, which can be burnt without the release of greenhouse gases. However, meaningful deployment of PEC technology in the global energy system, even when highly efficient scaled up devices become available, shall only be a reality when their safe and reliable operation can be guaranteed over several years of service life. The first part of this review discusses the importance of hermetic sealing of up scaled PEC device provided by the casing and sealing joints from a reliability and risk perspective. The second part of the review presents a survey of fully functional devices and early stage demonstrators and uses this to establish the extent to which the state of the art in PEC device design address the issue of hermetic sealing. The survey revealed that current material choices and sealing techniques are still unsuitable for scale–up and commercialization. Accordingly, we examined possible synergies with related photovoltaic and electrochemical devices that have been commericalised, and derived therefrom, recommendations for future research routes that could accelerate the development of hermetic seals of PEC devices.

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

Photoelectrochemical (PEC) water vapor splitting by using n-type semiconductor electrodes with a proton exchange membrane (PEM) enabled pure hydrogen production from humidity in ambient air. We proved a design concept that the gas-electrolyte-semiconductor triple-phase boundary on a nanostructured photoanode is important for the photoinduced gas-phase reaction. A surface coating of solid-polymer electrolyte on a macroporous titania-nanotube array (TNTA) electrode markedly enhanced the incident photon-to-current conversion efficiency (IPCE) at the gas-solid interface. This indicates that proton-coupled electron transfer is the rate-determining step on the bare TNTA electrode for the gas-phase PEC reaction. The perfluorosulfonate ionomer-coated TNTA photoanode exhibited an IPCE of 26 % at an applied voltage of 1.2 V under 365 nm ultraviolet irradiation. The hydrogen production rate in a large PEM-PEC cell (16 cm² ) was 10 μmol min^-1 .

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.

Unlocking the observation of different proton populations in fluorinated polymers by solid-state 1H and 19F double resonance NMR spectroscopy

Nafion proton exchange membranes (PEMs) for fuel cell applications are extensively studied and commercially applied, but their unique proton conduction capabilities are still somewhat unexplained. For studying proton dynamics in situ, molecular level spectroscopic techniques have been of limited utility so far. By solid-state ¹H and ¹⁹F double resonance nuclear magnetic resonance (NMR) spectroscopy using the recently revived multiple contact cross-polarization (MC-CP) pulse sequence along with double-quantum ¹H-¹H filtering, high resolution proton populations distinct from the dominant water resonance were observed in Nafion for the first time. This methodology quenches signal decay due to spin-lattice relaxation in the rotating frame and enables magnetization transfer between the relatively mobile ¹H and ¹⁹F spin baths in Nafion. Further studies of these previously unrevealed proton populations will lead to a better understanding of the Nafion proton conduction mechanism and proton exchange processes in general.

Surface Water Dependent Properties of Sulfur-Rich Molybdenum Sulfides: Electrolyteless Gas Phase Water Splitting

Sulfur-rich molybdenum sulfides are an emerging class of inorganic coordination polymers that are predominantly utilized for their superior catalytic properties. Here we investigate surface water dependent properties of sulfur-rich MoS_x (x = 3²/₃) and its interaction with water vapor. We report that MoS_x is a highly hygroscopic semiconductor, which can reversibly bind up to 0.9 H₂O molecule per Mo. The presence of surface water is found to have a profound influence on the semiconductor's properties, modulating the material's photoluminescence by over 1 order of magnitude, in transition from dry to moist ambient. Furthermore, the conductivity of a MoS_x-based moisture sensor is modulated in excess of 2 orders of magnitude for 30% increase in humidity. As the core application, we utilize the discovered properties of MoS_x to develop an electrolyteless water splitting photocatalyst that relies entirely on the hygroscopic nature of MoS_x as the water source. The catalyst is formulated as an ink that can be coated onto insulating substrates, such as glass, leading to efficient hydrogen and oxygen evolution from water vapor. The concept has the potential to be widely adopted for future solar fuel production.

Harvesting Hydrogen Gas from Air Pollutants with an Unbiased Gas Phase Photoelectrochemical Cell

The concept of an all-gas-phase photoelectrochemical (PEC) cell producing hydrogen gas from volatile organic contaminated gas and light is presented. Without applying any external bias, organic contaminants are degraded and hydrogen gas is produced in separate electrode compartments. The system works most efficiently with organic pollutants in inert carrier gas. In the presence of oxygen, the cell performs less efficiently but still significant photocurrents are generated, showing the cell can be run on organic contaminated air. The purpose of this study is to demonstrate new application opportunities of PEC technology and to encourage further advancement toward PEC remediation of air pollution with the attractive feature of simultaneous energy recovery and pollution abatement.

Vapor-fed microfluidic hydrogen generator

Water-splitting devices that operate with humid air feeds are an attractive alternative for hydrogen production as the required water input can be obtained directly from ambient air. This article presents a novel proof-of-concept microfluidic platform that makes use of polymeric ion conductor (Nafion®) thin films to absorb water from air and performs the electrochemical water-splitting process. Modelling and experimental tools are used to demonstrate that these microstructured devices can achieve the delicate balance between water, gas, and ionic transport processes required for vapor-fed devices to operate continuously and at steady state, at current densities above 3 mA cm(-2). The results presented here show that factors such as the thickness of the Nafion films covering the electrodes, convection of air streams, and water content of the ionomer can significantly affect the device performance. The insights presented in this work provide important guidelines for the material requirements and device designs that can be used to create practical electrochemical hydrogen generators that work directly under ambient air.