Mobility — the transport of people and goods — is a socioeconomic reality that will surely increase in the coming years. It should be safe, economic and reasonably clean. Little energy needs to be expended to overcome potential energy changes, but a great deal is lost through friction (for cars about 10 kWh per 100 km) and low-efficiency energy conversion. Vehicles can be run either by connecting them to a continuous supply of energy or by storing energy on board. Hydrogen would be ideal as a synthetic fuel because it is lightweight, highly abundant and its oxidation product (water) is environmentally benign, but storage remains a problem. Here we present recent developments in the search for innovative materials with high hydrogen-storage capacity.

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Hydrogen-storage materials for mobile applications

Metal hydride materials for solid hydrogen storage: a review

A review: Hydrogen generation from borohydride hydrolysis reaction

Kinetics of hydrolysis of sodium borohydride for hydrogen production in fuel cell applications: A review

The emergence of high-entropy materials (HEMs) with their excellent mechanical properties, stability at high temperatures, and high chemical stability is poised to yield new advancement in the performance of energy storage and conversion technologies. This review covers the recent developments in catalysis, water splitting, fuel cells, batteries, supercapacitors, and hydrogen storage enabled by HEMs covering metallic, oxide, and non-oxide alloys. Here, first, the primary rules for the proper selection of the elements and the formation of a favorable single solid solution phase in HEMs are defined. Furthermore, recent developments in different fields of energy conversion and storage achieved by HEMs are discussed. Higher electrocatalytic and catalytic activities with longer cycling stability and durability compared to conventional noble metal-based catalysts are reported for high-entropy materials. In electrochemical energy storage systems, high-entropy oxides and alloys have shown superior performance as anode and cathode materials with long cycling stability and high capacity retention. Also, when used as metal hydrides for hydrogen storage, remarkably high hydrogen storage capacity and structural stability are observed for HEMs. In the end, future directions and new energy-related technologies that can be enabled by the application of HEMs are outlined.

Recent progress of high-entropy materials for energy storage and conversion

New V-based alloys with high protium absorption and desorption capacity

Protium absorption properties of Ti–V–Cr–Mn alloys with a b.c.c. structure

Vanadium-based BCC alloys are known to absorb a high amount of protium. But, they do not desorb absorbed-protium completely. This paper aims to clarify the effects of alloy composition and heat-treatment on the protium absorption-desorption properties of Ti-(0-35) at%V-Cr alloys. A Ti-35V-40Cr alloy was chosen as the starting composition. The plateau pressure of the Ti-35V-(40+x)Cr (x=-3, 0, 3) alloys increased with increasing Cr content. For the alloys containing more than 15 at%V, a BCC phase was formed in the as-cast state, which was found to have an effective protium capacity of about 2.4 mass%H. However, a Laves phase was formed for alloys containing less than 10 at%V, which only had an effective protium capacity of about 1.8 mass%H. The effect of heat-treatment on the microstructures and protium absorption-desorption properties of the Ti-35V-40Cr alloy were studied. It was found that the optimum condition for heat-treatment was annealing at 1573 K for I min. The Ti-35V-40Cr alloy heat-treated under this condition exhibited the highest effective protium capacity of 2.6 mass%H. This effective protium capacity is higher than any other reported values at 313 K. However, further annealing at 1573 K led to a decrease in protium capacity, which may result from an increase in the amount of a Ti-rich phase. A Ti-10V-55.4Cr alloy, which contained a Laves phase in the as-cast state, became single-phase BCC after heat-treatment. Even though the alloy contains only 10 at%V, this heat-treated alloy showed an effective protium capacity of 2.5 mass%H, which is higher than any other reported values for BCC alloys.

Protium Absorption-Desorption Properties of Ti–V–Cr Alloys with a BCC Structure

The protium absorption-desorption properties of Ti-V-Cr-(Mn, Ni) alloys were investigated in conjunction with Mn, Ni and V contents and heat-treatment conditions. The compositions studied were (1) Ti-35V-(37-x)Cr-xM (M=Mn and Ni; 0 ≤ x ≤ 15), and (2) Ti-xV-Cr-M (M=Mn and Ni; 15 ≤ x ≤ 45) in which the content of Ti, Cr, and M was fixed at a ratio of 28: 32: 5. The addition of Ni reduced the protium storage capacity and increased the plateau pressure due to the formation of a Ti 2 Ni secondary phase, and to the incorporation of Mn into the BCC matrix phase. Increasing the V content was found to be an effective way of stabilizing the BCC phase, and thereby increasing the protium storage capacity. For Ti-V-Cr-Ni alloys, the slope of the plateau region was also improved by increasing the V content. In addition, the Mn-added alloy after annealing at 1573 K exhibited an almost uniform BCC solid solution and a storage capacity as high as 2.6 mass%: however, the Ni-added alloy still contained the Ti 2 Ni phase. The addition of Mn was found to be an effective way of decreasing the slope of the plateau region.

Protium Absorption-Desorption Properties of Ti–V–Cr–(Mn, Ni) Alloys

Vanadium-based alloys with BCC structure are known to absorb high content of protium. The reduction of costs of the alloys requires to lower V content and to have higher protium capacity. The decrease of the V content in the alloys appears to form Laves phases, which results in reduced protium capacity. It is expected that BCC former addition to the alloys will form BCC phase with less V content inhibiting formation of Laves phase. The effects of the addition of BCC former elements such as Nb, Ta, Mo, and W on protium absorbing properties and the phases of Cr-Ti-V-M alloys have been studied. It has been found that Nb, Ta, and Mo additions are effective in forming BCC structures with improved homogeneity of the alloys, resulting in flattening the slope of plateau region of their PCT curves.

Takeshi Fuda

Papers

New V-based alloys with high protium absorption and desorption capacity

Protium absorption properties of Ti–V–Cr–Mn alloys with a b.c.c. structure

Protium Absorption-Desorption Properties of Ti–V–Cr Alloys with a BCC Structure

Protium Absorption-Desorption Properties of Ti–V–Cr–(Mn, Ni) Alloys

Effects of Additions of BCC Former Elements on Protium Absorbing Properties of Cr–Ti–V Alloys