Kinetic investigation of hydrogen generation from hydrolysis of SnO and Zn solar nanopowders

The hydrolysis reaction of the two-step ZnO/Zn and SnO₂/SnO thermochemical cycles was kinetically investigated for solar hydrogen production. Nanoparticles of Zn and SnO were synthesized by solar thermal reduction of the oxides and neutral gas quenching of the vapors. They were then hydrolyzed to quantify and compare the H₂ yields and the kinetic rate laws in fixed-bed. The hydrolysis of Zn nanoparticles reached only up to 55% of H₂ yield, whereas SnO hydrolysis was almost complete. In contrast, Zn hydrolysis was much faster than SnO hydrolysis, but Zn deactivation occurred suddenly. Models of solid–gas reactions were applied to identify the controlling mechanisms and the associated kinetic parameters. The kinetic models were fitted to both isothermal and non-isothermal (temperature ramp) hydrolysis experimental data. Activation energies and reaction orders were found to be 122 ± 13 kJ/mol and 2.0 ± 0.3 for SnO, and 87 ± 7 kJ/mol and 3.5 ± 0.5 for Zn, respectively. Finally, a shrinking core approach was applied to the case of SnO to account for the reaction-controlling mechanisms.     

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%.     

Carbon-coated cobalt molybdenum oxide as a high-performance electrocatalyst for hydrogen evolution reaction

Synthesis of high-performance and cost-effective catalysts towards the hydrogen evolution reaction (HER) is critical in developing electrochemical water-splitting as a viable energy conversion technique. For non-precious metal Co- and Ni-based catalysts, hydroxides were found to form on the surface of the catalysts under alkaline environments and benefit the catalytic performance, whereas there is limited systematic study on the explicit influence of hydroxides on the electrocatalytic mechanism and performance of these catalysts. Herein, we report a close correlation observed between the amount of the surface hydroxides formed and the resulting electrocatalytic performance of a Co-Mo-O nanocatalyst through careful comprehensive structural and property characterizations. We found that an appropriate amount of hydroxide can be moderated by simply coating the catalyst surface with carbon shells to optimize the catalytic properties. As a result, a carbon-coated Co-Mo-O nanocatalyst was successfully developed and is among the best reported non-precious HER catalysts with a superior electrocatalytic activity and outstanding durability for the HER under alkaline environment. First-principles calculations were further conducted to probe the nature of the active sites and the role of hydroxides in the Co-Mo-O@C/NF catalyst towards the HER.     

Dynamic model of a solar thermochemical water-splitting reactor with integrated energy collection and storage

Water-splitting solar thermochemical cycles are important in meeting the challenges of global climate change and limited fossil fuels. However, solar radiation varies in availability, leading to unsteady state operation. We propose a solar receiver-reactor with integrated energy collection and storage. The reactor consists of a double-pipe heat exchanger placed at the focal line of a parabolic trough solar concentrator. Molten salt passes through the jacket, absorbing energy from the irradiated outer surface while driving the endothermic oxygen production step of the copper-chlorine water-splitting cycle in the reactor bore. Excess energy is stored in a thermal storage tank to buffer the reactor from changes in insolation. Dynamic simulation indicates that the reactor can sustain steady 100% conversion during 24/7 operation with a reasonable plant layout. The technology employed is extant and mature. This is important in view of the urgency to reduce dependency upon fossil fuels as primary energy sources.     

In-situ photo-reducing graphene oxide to create Zn0.5Cd0.5S porous nanosheets/RGO composites as highly stable and efficient photoelectrocatalysts for visible-light-driven water splitting

Nanoporous Zn_0.5Cd_0.5S nanosheets/reduced graphene oxide (Zn_0.5Cd_0.5S/RGO) composites were prepared by a facile in-situ photoreduction method of graphene oxide (GO) in the presence of nanoporous Zn_0.5Cd_0.5S single-crystal-like nanosheets under visible light irradiation. The Zn_0.5Cd_0.5S/RGO photoelectrodes was characterized by TEM, IR and Raman spectra. Electrochemical measurements demonstrated that Zn_0.5Cd_0.5S/RGO photoelectrodes own a higher anodic photocurrent density, a lower zero current potential, and a higher photoelectrochemical response than that of pure Zn_0.5Cd_0.5S photoelectrodes under visible light irradiation under the same conditions. This high photochemical activity is predominately ascribed to the presence of RGO, which serves as the electron collector to efficiently prolong the lifetime of photoinduced electrons from the excited Zn_0.5Cd_0.5S nanosheets. In addition, the content of RGO in the composites had a remarkable influence on the photoelectrochemical behaviors of the photoelectrodes and the optimal RGO content was found to be 5 wt%. Zn_0.5Cd_0.5S/RGO composites at RGO content of 5 wt% reached a stable hydrogen production rate of 12.05 μmol h⁻¹ cm⁻² at an externally applied bias of 0.6 V. Furthermore, the Zn_0.5Cd_0.5S/RGO composites as photoelectrodes were found to be highly stable for hydrogen evolution reaction. The electrons stored in RGO are readily discharged or scavenged on demand by the applied positive bias to the counter electrode, and thus rectify the flow of electrons. Importantly, this work may open up a facile in-situ method for using RGO scaffold to create a stable photoelectrode with enhanced photoelectrochemical activities.     

Efficient STEP (solar thermal electrochemical photo) production of hydrogen – an economic assessment

A consideration of the economic viability of hydrogen fuel production is important in the STEP (Solar Thermal Electrochemical Photo) production of hydrogen fuel. STEP is an innovative way to decrease costs and increase the efficiency of hydrogen fuel production, which is a synergistic process that can use concentrating photovoltaics (CPV) and solar thermal energy to drive a high temperature, low voltage, electrolysis (water-splitting), resulting in H₂ at decreased energy and higher solar efficiency. This study provides evidence that the STEP system is an economically viable solution for the production of hydrogen. STEP occurs at both higher electrolysis and solar conversion efficiencies than conventional room temperature photovoltaic (PV) generation of hydrogen. This paper probes the economic viability of this process, by comparing four different systems: (1) 10% or (2) 14% flat plate PV driven aqueous alkaline electrolysis H₂ production, (3) 25% CPV driven molten electrolysis H₂ production, and (4) 35% CPV driven solid oxide electrolysis H₂ production. The molten and solid oxide electrolysers are high temperature systems that can make use of light, normally discarded, for heating. This significantly increases system efficiency. Using levelized cost analysis, this study shows significant cost reduction using the STEP system. The total price per kg of hydrogen is shown to decrease from $5.74 to $4.96 to $3.01 to $2.61 with the four alternative systems. The advanced STEP plant requires less than one seventh of the land area of the 10% flat cell plant. To generate the 216 million kg H₂/year required by 1 million fuel cell vehicles, the 35% CPV driven solid oxide electrolysis requires a plant only 9.6 mi² in area. While PV and electrolysis components dominate the cost of conventional PV generated hydrogen, they do not dominate the cost of the STEP-generated hydrogen. The lower cost of STEP hydrogen is driven by residual distribution and gate costs.     

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.     

Mn-containing ZSM-5 type zeolite as a water-oxidizing catalyst: New findings and current controversies

Water-splitting for H₂ and O₂ production is a key reaction toward a clean energy future. However, the water-oxidation is one of the major limiting factors affecting the efficiency of this important reaction. Thus, the development of low-cost and efficient water-oxidizing catalysts is a key challenge in the artificial photosynthesis field. In this work, Mn-containing ZSM-5 type zeolite was synthesized using silicic acid, manganese(II) chloride tetrahydrate, potassium carbonate and tetrapropylammonium bromide (TPAB). Samples were characterized by different methods. The water-oxidizing activity of the catalysts were studied in the presence of cerium(IV) ammonium nitrate (Ce(IV)) and in the electrochemical water-oxidation condition. The effect of K⁺ ions on the structural properties and the catalytic performance of the compounds was also investigated. It was observed that after treating of manganosilicate with a Ce(IV) solution or applying a 2 V constant potential to the sample, in a convective–suspension–collision system, framework located Mn ions completely leaked. These results are important to reconsider the true catalysts in the different catalytic reactions in the presence of manganosilicates.     

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.