Dehydration through pervaporation from HIx solution (HI-H2O-I2 mixture) using a cation exchange membrane for thermochemical water-splitting iodine-sulfur process

Pervaporation (PV) of water from HIx solution (HI-H₂O-I₂ mixture) using Nafion-117 was evaluated aiming at the application to dehydrate the azeotropic composition in HI decomposition reaction of thermochemical IS process. PV experiment was carried out by using HI solutions of 40–65 wt% and an I₂/HI molar ratio of 0–3 in the feed at the room temperature. The permeation flux decreased with increasing HI weight fraction in the feed. The permeation flux is dependent on the I₂ concentration in the feed having an I₂/HI molar ratio. A long time PV experiment was carried out using I₂/HI molar ratio of 1 (in HI solution of 55.9%) in the feed at room temperature. It is expected that the permeation component in the permeate zone using the PV process was mainly H₂O, and H₂O permeation was constant with increasing operation time.     

Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2

PtSe₂ monolayer is previously predicted to be a two-dimensional water-splitting photocatalyst. However, the weak van der Waals (vdW) interaction between H₂O and the basal surface of PtSe₂ significantly undermines its photocatalytic water-splitting activities. In this work, we explore the possibility of various intrinsic defects of PtSe₂ in remedying this deficiency on the basis of first-principles calculations. It is interesting to find that the introduction of Pt@Se, Se@Pt, and Se interstitial defect not only fully retain the water redox abilities of pure PtSe₂ and realize spatial separation of photogenerated electrons and holes, but also can extend optical absorption range and absorption coefficients. Moreover, introduction of the three kinds of defects increase the initial weak vdW interactions between H₂O and the PtSe₂ surface to different extent. In particular, Pt@Se anti-site defect transform the initial weak vdW to strong chemical interaction between H₂O and PtSe₂ surface, and function as active reaction site. These insights demonstrate that introduction of intrinsic defects, especially the Pt@Se anti-site defect, are effective means for improving the photocatalytic water-splitting activities of PtSe₂ monolayer.     

Techno-economic analysis of thermochemical water-splitting system for Co-production of hydrogen and electricity

The two-step thermochemical metal oxide water-splitting cycle with the state-of-the-art material ceria inevitably produces unutilized high-quality heat, in addition to hydrogen (H₂). This study explores whether the ceria cycle can be of greater value by using the excess heat for co-production of electricity. Specially, this technoeconomic study estimates the H₂ production cost in a hybrid ceria cycle, in which excess heat produces electricity in an organic Rankine cycle, to increase revenue and decrease H₂ cost. The estimated H₂ cost from such a co-generation multi-tower plant is still relatively high at $4.55/kg, with an average H₂ production of 1431 kg/day per 27.74 MW_th tower. Sensitivity analyses show opportunities and challenges to achieving $2/kg H₂ through improvements such as increased solar field efficiency, increased revenue from electricity sales, and a decreased capital recovery factor from baseline assumptions. While co-production improves overall system efficiency and economics, achieving $2/kg H₂ remains challenging with ceria as the active material and likely will require a new material.     

A review on the recent developments in zirconium and carbon-based catalysts for photoelectrochemical water-splitting

In the recent years, considerable interest in the development of clean and renewable alternative energy resources has been observed to overcome the problems of dwindling fossil reserves, environmental pollution and increasing energy demand for a sustainable future. In this respect, hydrogen is considered a sustainable, clean, and energy-rich fuel. Photoelectrochemical (PEC) water-splitting is deemed to be a very promising technology hydrogen production. A number of research endeavors have been dedicated to develop efficient catalysts for this process. An optimum photoelectrocatalyst drives down the energy needed for the disassociation of water by lowering the overpotential of the process and make it competent for commercial applications. Recently, a lot of Zirconium (Zr) and Carbon (C) based compounds have been analyzed for PEC water-splitting. This review article intends to offer insight and timely reference for the progress on Zr and C based catalyst for practical PEC water-splitting in a comprehensive and concise manner. With emphasis on the photoelectrochemical performance, relative design strategies and different approaches to improve or optimize the photoelectrocatalyst materials with Zr and C are discussed. Research approach and recommendations for future PEC water-splitting are also proposed.     

Pd-YSZ cermet membranes with self-repairing capability in extreme H2S conditions

A Pd-YSZ cermet membrane that performs coupled operations of hydrogen separation from a mixed-gas stream and simultaneous hydrogen production by non-galvanic water-splitting, and have high sulfur tolerance is fabricated. It is proved that in H₂S containing atmosphere the Pd-YSZ membrane has self-repairing capability, originating mainly from the conversion of Pd₄S back to metallic Pd and SO₂ by ambipolar-diffused oxygen obtained from water-splitting. The performance of membrane was analyzed at different temperatures in high H₂S containing (0–4000 ppm H₂S) mixed gas feed during the operation as a hydrogen separation membrane as well as during the coupled operation of hydrogen separation and hydrogen production. At 900 °C with the feed-stream having ≥2000 ppm H₂S, the hydrogen flux was severely affected due to the formation of some liquid phase of Pd₄S, resulting in the segregation of hydrogen permeating Pd-phase at the membrane surface. But at 800 °C, though the membrane was affected by the Pd₄S formation in high H₂S environment (up to 1200 ppm H₂S), its self-repairing capability and additional hydrogen production by water-splitting is capable of maintaining the hydrogen flux around ~1.24 cm³ (STP)/min.cm², a value expected by the same membrane while performing only the hydrogen separation function in H₂S-free environment.     

Understanding water-splitting thermochemical cycles based on nickel and cobalt ferrites for hydrogen production

Two step water-splitting cycles by using metal ferrites are considered as a clean and sustainable hydrogen production method, when concentrated solar energy is used to drive the thermochemical reactions. This process involves the reduction at very high temperature of the ferrite, followed by the water reoxidation to the original phase at moderate temperature, with the release of hydrogen. In order to decrease the temperature required to decompose the oxide, mixed ferrites of the type MFe₂O₄ with spinel crystal structure have been examined. In this sense, ferrites with the partial substitution of Co and Ni for Fe appear as successful materials in terms of hydrogen production and cyclability. In this work, commercial Ni and synthetic Co ferrites have been subjected to two water splitting cycles. The solid products obtained after thermal reduction and water decomposition reactions have been chemically and structurally characterized by WDXRF, XRD, XPS and SEM techniques, in order to get a deeper understanding of the mechanisms controlling the water splitting process. This knowledge contributes to improve the process involved in thermochemical cycles and to understand the lower efficiencies (H₂/O₂) for Co ferrite thermochemical cycles in comparison with those corresponding to Ni ferrite.     

Nanostructured materials for the visible-light driven hydrogen evolution by water splitting: A review

The conversion of abundantly available photonic energy into useful chemical energy is considered to be a greener protocol for addressing the energy shortage. Recently, since most of the emphasis has been centralized on the semiconductor-based photocatalysis; the designing and fabrication of the novel semiconductor photocatalytic material is happening at a blistering rate. Recently, the nanostructured materials have attracted ever-growing research attention as photocatalytic material for hydrogen generation reaction by dissociation of water. Such photocatalytic nanomaterials are known to exhibit superior activity than their corresponding bulk counter-parts because of the improved interfacial charge separation and the broad surface area providing sufficient active sites. However, the improvement in the efficiency and selectivity towards hydrogen production reaction under solar or visible light radiation always remains a challenging assignment. In the present review, the segregation of the so far reported nanostructured photocatalysts into different categories, based on their dimensionality such as 0-D, 1-D and 2-D materials, is implemented. Furthermore, their synthetic route and the photocatalytic hydrogen evolving efficiencies are explored and briefly summarized. Moreover, the methodology of development of nanocomposite materials leading to the construction of heterojunctions including Type-I, Type-II, Type-III, Z-Scheme and S-Scheme system is also discussed. In addition, an in-depth investigation on the charge carrier's generation, separation and their transportation is also reviewed. Finally, the future perspectives regarding the designing of an efficient, stable and economic photoactive nano-architecture material for the efficient hydrogen production via photocatalytic dissociation of water are also pointed.     

Cadmium sulfide embedded Prussian Blue as highly active bifunctional electrocatalyst for water-splitting process

Hydrogen production by water-splitting has limited commercial application as substantial amount of energy is required for the favorable kinetics of the process. We present an interface engineering strategy for constructing a bifunctional electrode material for an efficient water splitting process. Designed cadmium sulphide and Prussian blue nanorods (CdS-NRs@PBNPs) heterostructures acts as bifunctional electrocatalyst improved water splitting performance, for both HER and OER. For HER, the optimized hybrid CdS-NRs@PBNPs (1:1) showed significantly a low overpotentials of 126 mV and 181 mV at current densities of 10 mA cm^?2 and 20 mA cm^?2 respectively. For OER it displays an overpotential of 250 mV and 316 mV at current densities of 10 mA cm^?2 and 20 mA cm^?2. Additionally, the CdS-NRs@PBNPs (1:1) has demonstrated long-term stability. The hybrid's enhanced OER and HER activity is attributable to a synergetic impact between CdS-NRs and PBNPs, as well as the active site modification due to the presence of Cadmium and iron in the hybrid.     

Activity engineering to transition metal phosphides as bifunctional electrocatalysts for efficient water-splitting

At present, water splitting has been regarded as one of the most promising ways for hydrogen production. Therefore, exploitation of cost-effective electrocatalysts is essential to realize the industrialization of electrocatalytic techniques. In recent years, transition metal phosphides (TMPs) as non-noble metal electrocatalysts have gained a great deal of attention owing to their multifunctional active sites, tunable structure and composition, as well as unique physicochemical properties. However, the poor electrical conductivity of TMPs and high adsorption energy of hydrogen intermediates during the hydrogen evolution severely restrict its large-scale application. Therefore, it is of great importance to develop effective activity engineering to TMPs. Herein, the reaction mechanisms of water splitting including hydrogen evolution and oxygen evolution reactions and the key performance parameters are briefly clarified. Then, the strategies to improve the performance of TMPs are summarized in four aspects, including modulation of electronic structure, tailoring microstructures, selection of working electrode, and replacing OER with an energy-saving reaction. Finally, a summary and perspective for further opportunities and challenges are highlighted for the TMPs from the point of characterization methodologies, theoretical calculation and practical application.     

Thermochemical two-step water-splitting reactor with internally circulating fluidized bed for thermal reduction of ferrite particles

A thermochemical two-step water-splitting cycle using a redox system of iron-based oxides or ferrites was examined on hydrogen productivity and reactivity of ferrite in order to convert solar energy into hydrogen in sunbelt regions. In the present paper, a new concept is proposed for a windowed thermochemical water-splitting reactor, using an internally circulating fluidized bed of NiFe₂O₄/m-ZrO₂ particles, and thermal reduction of the bed is demonstrated on a laboratory scale by using a solar-simulating Xe-beam irradiation. The concept is that concentrated solar radiation passes through the transparent window and directly heats the internally circulating fluidized bed. The fluidized bed reactor enabled the NiFe₂O₄/m-ZrO₂ sample to remain in powder form without sintering and agglomerating during direct Xe-beam irradiation over 30 min. Approximately 45% of the NiFe₂O₄ was converted to the reduced phase by the solar-simulated high-flux beam, and was then completely reoxidized with steam at 1000 °C to generate hydrogen.