Water-splitting mechanism analysis of Sr/Ca doped LaFeO3 towards commercial efficiency of solar thermochemical H2 production

Solar thermochemical (STC) technology utilizes the entire spectrum of solar energy to decompose water to produce hydrogen. This technology reduces carbonic fuels, nearly only producing hydrogen rather than hydrogen-oxygen mixture. However, low water-splitting activity of redox materials restricts improvement of water-hydrogen conversion ratio and fuel production efficiency. Recently, a kind of perovskite LaFeO₃ attracts attention, because of the good performance in photocatalysis hydrogen production. Nevertheless, how LaFeO₃ system works in STC water-splitting cycle is rarely studied. In this paper, the first principle method at density functional theory level is adopted to reveal the hydrogen production mechanism of perovskite LaFeO₃ doped with 25% Sr/Ca at A site. Hydrogen migration on material surface determines hydrogen generation rate. The activation energy of 25%-Ca-doped LaFeO₃ is relatively lower 150.09 kJ/mol. In addition, fuel production efficiency has been calculated. When water to hydrogen conversion ratio is 100%, solar-to-fuel efficiency can reach maximum 0.472. The effect of water-splitting kinetics on hydrogen production is also discussed. The results indicate that when T_red = T_oxi = T = 1200K and water to hydrogen conversion ratio is 10%, the dynamic efficiency of La_0.75Ca_0.25FeO₃ can reach 20%. This research can provide index for improving the hydrogen production performance of STC technology.     

Comparative study of activity of cerium oxide at thermal reduction temperatures of 1300–1550 °C for solar thermochemical two-step water-splitting cycle

Thermochemical two-step water-splitting using CeO₂ (cerium oxide) particles was studied to examine oxygen and hydrogen productivity and repeatability at thermal reduction (T-R) temperatures of 1300–1550 °C and water decomposition (W-D) temperatures of 400–1000 °C for the production of hydrogen from water using concentrated solar radiation as the energy source. The temperature dependency of oxygen and hydrogen productivity and the cyclic repeatability of CeO₂ are reported in this paper. The characteristic features of CeO₂ particles in the thermochemical two-step water-splitting cycle are compared with the well-known highly active reactive mediums of zirconia-supported Ni-ferrites (NiFe₂O₄/m-ZrO₂ and NiFe₂O₄/c-YSZ) and unsupported NiFe₂O₄.     

Low-temperature water-splitting by sodium redox reaction

Thermochemical water-splitting by sodium redox reactions was investigated from material science point of view as a future hydrogen production method. The reaction system consists of three separate reactions, which are hydrogen generation by NaOH-Na reaction, metal separation by thermolysis of Na₂O, and oxygen generation by hydrolysis of Na₂O₂. Although the current techniques of thermochemical water-splitting required a temperature higher than 800 °C for whole reaction cycle, the sodium system was able to be operated below only 400 °C by using nonequilibrium techniques to control the entropy of the chemical reactions. Therefore, this system should be recognized as a potential water-splitting technique that can widely utilize any heat sources in contrast to the conventional methods.     

Comparative study of the activity of nickel ferrites for solar hydrogen production by two-step thermochemical cycles

In this work, we compare the activity of unsupported and monoclinic zirconia – supported nickel ferrites, calcined at two different temperatures, for solar hydrogen production by two-step water-splitting thermochemical cycles at low thermal reduction temperature. Commercial nickel ferrite, both as-received and calcined in the laboratory, as well as laboratory made supported NiFe₂O₄, are employed for this purpose. The samples leading to higher hydrogen yields, averaged over three cycles, are those calcined at 700 °C in each group (supported and unsupported) of materials. The comparison of the two groups shows that higher chemical yields are obtained with the supported ferrites due to better utilisation of the active material. Therefore, the highest activity is obtained with ZrO₂-supported NiFe₂O₄ calcined at 700 °C.     

Thermochemical two-step water-splitting for hydrogen production using Fe-YSZ particles and a ceramic foam device

Fe₃O₄ supported on cubic yttria-stabilized zirconia (Fe₃O₄/c-YSZ) is proposed as a promising redox material for the production of hydrogen from water via a thermochemical two-step water-splitting cycle. In this study, the evolution of oxygen and hydrogen during the cyclic reaction was examined using Fe₃O₄/c-YSZ particles in order to demonstrate reproducible and stoichometric oxygen/hydrogen production through a repeatable two-step reaction. Subsequently, a ceramic foam device coated with Fe₃O₄ and c-YSZ particles was prepared and examined as a thermochemical water-splitting device in a directly irradiated receiver/reactor hydrogen production system. The Fe₃O₄/c-YSZ system formed a Fe-containing YSZ (Fe-YSZ) by high-temperature reaction between Fe₃O₄ and the c-YSZ support at 1400 °C in an inert atmosphere. The reaction mechanism of the two-step water-splitting cycle is associated with the redox transition of Fe²⁺–Fe³⁺ ions in the c-YSZ lattice. The Fe-YSZ particles exhibit good reproducibility for reaction with a hydrogen/oxygen ratio of approximately 2.0 throughout repeated cycles. The foam device coated with Fe-YSZ particles was also successful for continual hydrogen production through 32 repeated cycles. A 20–27% ferrite conversion was obtained using 10.5 wt% Fe₃O₄ loading over an irradiation period of 60 min.     

Solar hydrogen evolution over native visible-light-driven Sn3O4

Low-cost semiconductor photocatalysts that can efficiently harvest solar energy and generate H₂ from water or alcohols will be critical to future hydrogen economies. Co-catalyst loading and/or doping of foreign element at host material have been crucial for semiconductor photocatalyst to produce significant H₂ evolution, so far. We synthesized native-visible-light driven Sn₃O₄ photocatalyst, which significantly catalyzed hydrogen evolution from various alcohol solutions under irradiation of visible light (λ > 400 nm), without co-catalyst. The H₂ production reaction proceeded through hydroxyalkyl radical reaction in the methanol solution. The apparent quantum yield was 0.4% for the Sn₃O₄ competitive to that of visible-light-sensitive co-catalyst loaded doped photocatalyst. The enhanced hydrogen evolution is attributed to the desirable band gap and band edge positions (CBM and VBM) of the Sn₃O₄ for H₂ production in visible light, which would originate from atomically layered structure of Sn₃O₄. The Sn₃O₄ material is good promising photocatalyst for solar hydrogen production from alcohols.     

Oxygen vacancies and morphology engineered Co3O4 anchored Ru nanoparticles as efficient catalysts for ammonia borane hydrolysis

Developing efficient but facile strategies to modulate the catalytic activity of Ru deposited on metal oxides is of broad interest but remains challenging. Herein, we report the oxygen vacancies and morphological modulation of vacancy-rich Co₃O₄ stabilized Ru nanoparticles (NPs) (Ru/V_O-Co₃O₄) to boost the catalytic activity and durability for hydrogen production from the hydrolysis of ammonia borane (AB). The well-defined and small-sized Ru NPs and V_O-Co₃O₄ induced morphology transformation via in situ driving V_O-Co₃O₄ to 2D nanosheets with abundant oxygen vacancies or Co²⁺ species considerably promote the catalytic activity and durability toward hydrogen evolution from AB hydrolysis. Specifically, the Ru/V_O-Co₃O₄ pre-catalyst exhibits an excellent catalytic activity with a high turnover frequency of 2114 min^?1 at 298 K. Meanwhile, the catalyst also shows a high durability toward AB hydrolysis with six successive cycles. This work establishes a facile but efficient strategy to construct high-performance catalysts for AB hydrolysis.     

Life cycle net energy and global warming impact assessment for hydrogen production via decomposition of ammonia recovered from source-separated human urine

Decomposition of ammonia derived from source-separated human urine is a renewable approach for hydrogen production. Life cycle net energy analysis and global warming impact of scaled-up hydrogen production via this technique are studied in this paper. Ammonia decomposition processes, including fixed-bed reactors with Ru/Al₂O₃ and Ni/Al₂O₃ as catalyst options are simulated using the Aspen Plus software, and the results are compared with published data for validation. The life cycle net energy indicators are assessed for three scenarios of ammonia generation: conventional air stripping, microbial fuel cell, and electrochemical cell methods at a unit basis of 1000 kg of H₂ production. Results show that the microbial fuel cell process is more energy-efficient and emits lower greenhouse gases. The net energy ratio of the microbial fuel cell method is 1.38, and 1.12, for Ru/Al₂O₃ and Ni/Al₂O₃, respectively. A comparative assessment of ammonia generation and decomposition options for environmentally-benign hydrogen production is discussed.     

Hydrogen production by water splitting on manganese ferrite-sodium carbonate mixture: Feasibility tests in a packed bed solar reactor-receiver

The sodium manganese mixed ferrite thermochemical cycle Na(Mn_1/3Fe_2/3)O₂/(MnFe₂O₄ + Na₂CO₃) for sustainable hydrogen production has been implemented in a solar reactor-receiver, packed with indirectly heated MnFe₂O₄/Na₂CO₃ mixture pellets, with the aim of verifying its feasibility and of determining the critical aspects of the process. The reactor operates at nearly constant temperature in the range 750–800 °C; the shift between the hydrogen-producing and regeneration steps is obtained by switching the reactive gas from water to carbon dioxide. Hydrogen produced during 1-h operation of the reactor is in the range of 130–460 μmol/g of mixture, depending on experimental conditions. Compared to other existing prototypes, the implemented process obtains comparable production efficiencies while operating at lower temperature both in the hydrogen production and regeneration phases.     

A critical review on integrated system design of solar thermochemical water-splitting cycle for hydrogen production

The development of clean hydrogen production methods is important for large-scale hydrogen production applications. The solar thermochemical water-splitting cycle is a promising method that uses the heat provided by solar collectors for clean, efficient, and large-scale hydrogen production. This review summarizes state-of-the-art concentrated solar thermal, thermal storage, and thermochemical water-splitting cycle technologies that can be used for system integration from the perspective of integrated design. Possible schemes for combining these three technologies are also presented. The key issues of the solar copper-chlorine (Cu–Cl) and sulfur-iodine (S–I) cycles, which are the most-studied cycles, have been summarized from system composition, operation strategy, thermal and economic performance, and multi-scenario applications. Moreover, existing design ideas, schemes, and performances of solar thermochemical water-splitting cycles are summarized. The energy efficiency of the solar thermochemical water-splitting cycle is 15–30%. The costs of the solar Cu–Cl and S–I hydrogen production systems are 1.63–9.47 $/kg H₂ and 5.41–10.40 $/kg H₂, respectively. This work also discusses the future challenges for system integration and offers an essential reference and guidance for building a clean, efficient, and large-scale hydrogen production system.     

Direct high-purity hydrogen production from ammonia by using a membrane reactor combining V-10mol%Fe hydrogen permeable alloy membrane with Ru/Cs2O/Pr6O11 ammonia decomposition catalyst

The hydrogen production capabilities of the membrane reactor combining V-10 mol%Fe hydrogen permeable alloy membrane with Ru/Cs₂O/Pr₆O₁₁ ammonia decomposition catalyst are studied. The ammonia conversion is improved by 1.7 times compared to the Ru/Cs₂O/Pr₆O₁₁ catalyst alone by removing the produced hydrogen through the V-10mol%Fe alloy membrane during the ammonia decomposition. 79% of the hydrogen atoms contained in the ammonia gas are extracted directly as high-purity hydrogen gas. Both the Ru/Cs₂O/Pr₆O₁₁ catalyst and the V-10 mol% Fe alloy membrane are highly durable, and the initial performance of the hydrogen separation rate lasts for more than 3000 h. The produced hydrogen gas conforms to ISO 14687–2:2019 Grade D for fuel cell vehicles because the ammonia and nitrogen concentrations are less than 0.1 ppm and 100 ppm, respectively.     

Ferrite/zirconia-coated foam device prepared by spin coating for solar demonstration of thermochemical water-splitting

A Ferrite/zirconia foam device in which reticulated ceramic foam was coated with zirconia-supported Fe₃O₄ or NiFe₂O₄ as a reactive material was prepared by a spin-coating method. The spin-coating method can shorten the preparation period and reduce the coating process as compared to the previous wash-coating method. The foam devices were examined for hydrogen productivity and cyclic reactivity in thermochemical two-step water-splitting. The reactivity of these foam devices were studied for the thermal reduction of ferrite on a laboratory scale using a sun simulator to simulate concentrated solar radiation, while the thermally reduced foam devices were reacted with steam in another quartz reactor under homogeneous heating in an infrared furnace. The most reactive foam device, NiFe₂O₄/m-ZrO₂/MPSZ, was tested for successive two-step water-splitting in a windowed single reactor using solar-simulated Xe-beam irradiation with a power input of 0.4-0.7 kW_th. The production of hydrogen continued successfully in the 20 cycles that were demonstrated using the NiFe₂O₄/m-ZrO₂/MPSZ foam device. The NiFe₂O₄/m-ZrO₂/MPSZ foam device produced hydrogen at a rate of 1.1-4.6 cm³ per gram of device through 20 cycles and reached a maximum ferrite conversion of 60%.     

Limiting and achievable efficiencies for solar thermal hydrogen production

For reversible conversion, we derive theoretical Sun-to-H₂ (STH₂) efficiencies for water-splitting processes harnessing solar energy as heat predominantly. At solar concentration ratios (C) of 2,000–10,000, the derived STH₂ efficiency limits are 72.4–80.1%. For real processes with irreversibilities, we conceptualize direct and two-stage thermal water-splitting processes to estimate the achievable STH₂ efficiency, the favorable operating conditions and design challenges that must be overcome. For direct thermal water-splitting, achievable STH₂ efficiencies between 35 and 50% are possible at reaction temperatures of 1300–2000 K, and C = 2,000–10,000. This STH₂ efficiency range is greater than the estimates of achievable values available for low and high temperature water electrolysis or single bandgap methods for generating H₂. The direct process requires efficient heat integration, and high temperature membranes for H₂(g) and O₂(g) separation to surpass reaction equilibrium limitations. Alternatively, for two-stage water-splitting using Fe₃O₄/FeO with solar heat recovered at 1600–2300 K, the calculated estimates for the achievable STH₂ efficiency are 38–54%.     

Catalyst support effect on ammonia decomposition over Ni/MgAl2O4 towards hydrogen production

Ammonia is a prospective fuel for hydrogen storage and production, but its application is limited by the high cost of the catalysts (Ru, etc.) to decompose NH₃. Decomposing ammonia using non-precious Ni as catalysts can therefore improve its prospects to produce hydrogen. This work proposes several Ni/MgAl₂O₄ with the support properties tuned and investigates the support effect on the catalytic performance. Ni/MgAl₂O₄-LDH shows high NH₃ conversion (～88.7%) and H₂ production rate (～1782.6 mmol g^?1 h^?1) at 30,000 L. kg^?1 h^?1 and 600 °C, which is 1.68 times as large as that of Ni/MgAl₂O₄-MM. The performance remains stable over 30 h. The characterizations manifest that the high specific surface area of Ni/MgAl₂O₄-LDH can introduce highly dispersed Ni on the surface. Kinetics analysis implies promoted NH₃ decomposition reaction and alleviated H₂ poisoning for Ni/MgAl₂O₄-LDH. A roughly linear relationship is obtained by fitting the curves of dispersed Ni on the surface vs the reaction orders regarding H₂ and NH₃. This indicates that enhanced NH₃ decomposition performance can be ascribed to the strengthened NH₃ decomposition reaction and weakened H₂ poisoning by the highly dispersed Ni on the MgAl₂O₄-LDH surface. This work provides an opportunity to develop highly active and cost-effective catalysts to produce hydrogen via NH₃ decomposition.     

KW-GO,a graphene-like carbon extracted from kitchen waste (KW) advances the hydrogen production of V2O5

In this paper, graphene-like carbon (KW-GO) extracted from kitchen waste (KW) is used to reduce the agglomeration of V₂O₅ and improve the separation rate of photogenerated electron-hole pairs from V₂O₅. We found that the V₂O₅-KW-GO composite material (VKW-GO) could significantly enhance the photocatalytic activity and H₂ production rate under visible light irradiation compared to pure V₂O₅. To analyze the composition and morphology of the materials, XRD, SEM, BET, UV–Vis, XPS, and Raman were measured. The results showed that the addition of KW-GO reduced the aggregation of V₂O₅ powder. At the same time, the specific surface area of the composite sample increased providing more active sites for photocatalytic hydrogen production. In addition, the visible absorption range of the composite sample also increased. As a result, the hydrogen production rate of V₂O₅ increased from 247.52 mol h^?1 g^?1 to 354.15 mol h^?1 g^?1. The method using V₂O₅ and VKW-GO as a catalyst for H₂ production is innovative, and the conclusion may provide important theoretical guidance for photocatalytic hydrogen production.     

Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production

In this paper, solar reactor efficiency analysis of the solar thermochemical two-step zinc oxide–zinc sulfate (ZnO–ZnSO₄) water splitting cycle. In step-1, the ZnSO₄ is thermally decomposed into ZnO, SO₂, and O₂ using solar energy input. In step-2, the ZnO is re-oxidized into ZnSO₄ via water splitting reaction producing H₂. The ZnSO₄ is recycled back to the solar reactor and hence can be re-used in multiple cycles. The equilibrium compositions associated with the thermal reduction and water-splitting steps are identified by performing HSC simulations. The effect of Ar towards decreasing the required thermal reduction temperature is also explored. The total solar energy input and the re-radiation losses from the ZnO–ZnSO₄ water splitting cycle are estimated. Likewise, the amount of heat energy released by different coolers and water splitting reactor is also determined. Thermodynamic calculations indicate that the cycle (η_cycle) and solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) of the ZnO–ZnSO₄ water splitting cycle are equal to 40.6% and 48.9% (without heat recuperation). These efficiency values are higher than previously investigated thermochemical water splitting cycles and can be increased further by employing heat recuperation.     

A low-temperature electro-thermochemical water-splitting cycle for hydrogen production based on LiFeO2/Fe redox pair

The thermochemical water-splitting cycles have been paid more attention in recent years because they directly convert thermal energy into stored chemical energy as H₂. However, most thermochemical cycles require extremely high temperatures as well as a temperature switch between reduction and oxidation steps, which are the main obstacles for their development. Herein, we introduced an electrochemical reaction into the thermochemical cycle and established a novel two-step water-splitting cycle based on LiFeO₂/Fe redox pair. The two-step water-splitting process involves a cyclic operation of electrochemical reduction and water-splitting steps. The feasibility of the water-splitting cycle for the hydrogen production was thermodynamically and experimentally investigated. A mechanism of hydrogen production based on LiFeO₂/Fe redox pair was developed. Compared with the traditional high-temperature thermochemical cycles, the electrochemical reduction and water-splitting steps of the process can be isothermally operated in the same cell at a relatively low temperature of 500 °C. The main advantages of the cycle are not only easily available heat sources without involvement of the associated engineering and materials issues, but also without any temperature swings. This is a promising method to achieve water splitting for hydrogen production in the future.     

Structural properties of CuO/TiO2 nanorod in relation to their catalytic activity for simultaneous hydrogen production under solar light

CuO was introduced into porous TiO₂ nanorod through impregnation method. Before the impregnation step, TiO₂ nanorod was hydrothermally synthesized from TiO₂ powder in aqueous NaOH solution and followed by thermal treatment at 450 °C. The structures and properties of impregnated samples were characterized using various techniques, including XRD, BET, XAS, TEM, and UV-DRS. Their photocatalytic performance on simultaneous hydrogen production from pure water and aqueous methanol solution was also investigated under solar light. It was found that CuO/TiO₂ nanorod possessed a high surface area, good photocatalytic property and excellent hydrogen generation activity. Incorporation of Cu ions into the lattice framework of anatase TiO₂ nanorod enhanced the efficiency in visible region at 438–730 nm. Moreover, the XAS results showed that some Cu ions formed solid solution in the TiO₂ nanorod (Cu_xT_1−xO₂). However, the excessive incorporation of Cu ions did not improve any ability of anatase TiO₂ nanorod for production of hydrogen from pure water splitting. This could be due to the excessive CuO agglomeration at outside-pores which blocked the sensitization of TiO₂ nanorod. Only 1% Cu/TiO₂ nanorod was found to be a remarkable and an efficient photocatalyst for hydrogen production under solar light from both pure water and sacrificial methanol splitting. The highest rate of hydrogen production of 139.03 μmol h⁻¹ g_catalyst⁻¹ was found in sacrificial methanol which was 3.24% higher than in pure water.     

Hydrogen production by biomass gasification in supercritical water with bimetallic Ni-M/γAl2O3 catalysts (M = Cu, Co and Sn)

Supercritical water gasification (SCWG) of wet biomass is a very promising technology for hydrogen energy and the utilization of biomass resources. Ni-based catalysts are effective in catalyzing SCWG of original biomass and organic compounds for hydrogen production. In this paper, hydrogen production by SCWG of glucose over alumina-supported nickel catalysts modified with Cu, Co and Sn was studied. The bimetallic Ni-M (M = Cu, Co and Sn) catalysts were prepared by a co-impregnation method and tested in an autoclave reactor at 673 K with a feedstock concentration of 9.09 wt.%. XRD, XRF, N₂ adsorption/desorption, SEM and TGA were adopted to investigate the changes of chemical properties between Ni and Ni-M catalysts and the deactivation mechanism of catalysts. According to the experimental results, the hydrogen yield followed this order: Ni-Cu/γAl₂O₃ > Ni/γAl₂O₃ > Ni-Co/γAl₂O₃ > Ni-Sn/γAl₂O₃. The results show that Cu could improve the catalytic activity of Ni catalyst in reforming reaction of methane to produce hydrogen in SCWG. In addition, Cu can mitigate the sintering of alumina detected by SEM. Co was found to be an excellent promoter of Ni-based catalyst in relation to hydrogen selectivity.