Hydrogen

Among the processes for producing hydrogen and oxygen from water via the use of solar energy, water splitting has the advantage of being carried out in onestep. According to thermodynamics, this process exhibits conversions of practical interest at very high temperatures and needs efficient separation systems in order to separate the reaction products, hydrogen and oxygen. In this conceptual work, the behavior of a membrane reactor that uses two membranes perm-selective to hydrogen and oxygen is investigated in the temperature range 2000–2500 °C of interest for coupling this device with solar receivers. The effect of the reaction pressure has been evaluated at 0.5 and 1 bar while the permeate pressure has been fixed at 100 Pa. As a first result, the use of the membrane perm-selective to oxygen in addition to the hydrogen one has improved significantly the reaction conversion that, for instance, at 0.5 bar and 2000 °C, moves from 9.8% up to 18.8%. Based on these critical data, a preliminary design of a membrane reactor consisting of a Ta tubular membrane separating the hydrogen and a hafnia camera separating the oxygen is presented: optimaloperating temperature of the reactor results in being around 2500 °C, a value making impracticable its coupling with solar receivers even in view of an optimistic development of this technology. The study has verified that at 2000 °C with a water feed flow rate of 1000 kg h⁻¹ about 200 and 100 m³ h⁻¹ of hydrogen and oxygen are produced. In this case, a surface of the hafnia membrane of the order of hundreds m² is required: the design of such a membrane device may be feasible when considering special reactor configurations. Full article

Low-cost, high-performance coatings for hydrogen production via electrolytic water-splitting are of great importance for de-carbonising energy. In this study the Raney2.0 coating was analysed using various electrochemical techniques to assess its absolute performance, and it was confirmed to have an extremely low overpotential for hydrogen evolution of just 28 mV at 10 mA/cm². It was also confirmed to be an acceptable catalyst for oxygen evolution, making it the highest performing simple bifunctional electrocatalyst known. The coating exhibits an extremely high capacitance of up to 1.7 F/cm${}^2$, as well as being able to store 0.61 J/cm${}^2$ in the form of temporary hydride deposits. A new technique is presented that performs a best-fit of a transient simulation of an equivalent circuit containing a constant phase element to cyclic voltammetry measurements. From this the roughness factor of the coating was calculated to be approximately 40,000, which is the highest figure ever reported for this type of material. The coating is therefore an extremely useful improved bifunctional coating for the continued roll-out of alkaline electrolysis for large-scale renewable energy capture via hydrogen production. Full article

The aim of this work is to study the influence of the sulphur source (elemental sulphur, thiourea and L-cysteine) in the solvothermal synthesis of Ag-CdS over its growth, structuration and state of Ag and how these changes influence on its photoactivity. The differences in the generation rate of the S²⁻ from the sulphur sources during the solvothermal synthesis determine the nucleation and growth pathways of CdS affecting to the silver state and its incorporation into the CdS lattice. The hydrogen production on Ag-CdS photocatalysts decreases according the sequence: thiourea > elemental sulphur >> L-cysteine. The changes in the photoactivity of Ag-CdS samples are analysed in terms of the differences in the insertion of Ag⁺ into the CdS lattice, the formation of composites between CdS and Ag₂S and the formation of CdS crystalline domains with strong confinement effect derived from the different sulphur source used in the solvothermal synthesis. Full article

Solid-state hydrogen storage covers a broad range of materials praised for their gravimetric, volumetric and kinetic properties, as well as for the safety they confer compared to gaseous or liquid hydrogen storage methods. Among them, A_xB_y intermetallics show outstanding performances, notably for stationary storage applications. Elemental substitution, whether on the A or B site of these alloys, allows the effective tailoring of key properties such as gravimetric density, equilibrium pressure, hysteresis and cyclic stability for instance. In this review, we present a brief overview of partial substitution in several A_xB_y alloys, from the long-established AB₅ and AB₂-types, to the recently attractive and extensively studied AB and AB₃ alloys, including the largely documented solid-solution alloy systems. We not only present classical and pioneering investigations, but also report recent developments for each A_xB_y category. Special care is brought to the influence of composition engineering on desorption equilibrium pressure and hydrogen storage capacity. A simple overview of the A_xB_y operating conditions is provided, hence giving a sense of the range of possible applications, whether for low- or high-pressure systems. Full article

A novel Pd/templated carbon catalyst (Pd/TC) was developed, characterized, and tested in the dehydrogenation of formic acid (FA) under mild conditions, with the possibility to control the H₂ generation rate, in the absence or presence of HCOONa (SF), by adjusting the Pd:FA and/or FA:SF ratios. The characterization results of the templated carbon obtained by the chemical vapor deposition of acetylene on NaY zeolite revealed different structural and morphological properties compared to other C-based supports. Therefore, it was expected to induce a different catalytic behavior for the Pd/TC catalyst. Indeed, the TC-supported Pd catalyst exhibited superior activity in the decomposition of FA, even at room temperature, with turnover frequencies (TOFs) of up to 143.7 and 218.8 h⁻¹ at 60 °C. The H₂ generation rate increased with an increasing temperature, while the H₂ yield increased with a decreasing FA concentration. Constant generation of gaseous flow (H₂ + CO₂) was achieved for 11 days, by the complete dehydrogenation of FA at room temperature using a 2 M FA solution and Pd:FA = 1:2100. The presence of SF in the reaction medium significantly enhanced the H₂ generation rate (535 h⁻¹ for FA:SF = 3:1 and 60 °C). Full article

Metal alloys have become a ubiquitous choice as catalysts for electrochemical hydrogen evolution in alkaline media. However, scarce and expensive Pt remains the key electrocatalyst in acidic electrolytes, making the search for earth-abundant and cheaper alternatives important. Herein, we present a facile and efficient synthetic route towards polycrystalline Co₃Mo and Co₇Mo₆ alloys. The single-phased nature of the alloys is confirmed by X-ray diffraction and electron microscopy. When electrochemically tested, they achieve competitively low overpotentials of 115 mV (Co₃Mo) and 160 mV (Co₇Mo₆) at 10 mA cm⁻² in 0.5 M H₂SO₄, and 120 mV (Co₃Mo) and 160 mV (Co₇Mo₆) at 10 mA cm⁻² in 1 M KOH. Both alloys outperform Co and Mo metals, which showed significantly higher overpotentials and lower current densities when tested under identical conditions, confirming the synergistic effect of the alloying. However, the low overpotential in Co₃Mo comes at the price of stability. It rapidly becomes inactive when tested under applied potential bias. On the other hand, Co₇Mo₆ retains the current density over time without evidence of current decay. The findings demonstrate that even in free-standing form and without nanostructuring, polycrystalline bimetallic electrocatalysts could challenge the dominance of Pt in acidic media if ways for improving their stability were found. Full article

Liquid organic hydrogen carriers (LOHC) store hydrogen by reversible hydrogenation of a carrier material. Water can enter the system via wet hydrogen coming from electrolysis as well as via moisture on the catalyst. Removing this water is important for reliable operation of the LOHC system. Different approaches for doing this have been evaluated on three stages of the process. Drying of the hydrogen, before entering the LOHC system itself, is preferable. A membrane drying process turns out to be the most efficient way. If the water content in the LOHC system is still so high that liquid–liquid demixing occurs, it is crucial for water removal to enhance the slow settling. Introduction of an appropriate packing can help to separate the two phases as long as the volume flow is not too high. Further drying below the rather low solubility limit is challenging. Introduction of zeolites into the system is a possible option. Water adsorbs on the surface of the zeolite and moisture content is therefore decreased. Full article