Comments on solid state hydrogen storage systems design for fuel cell vehicles

In recent years, significant research and development efforts were spent on hydrogen storage technologies with the goal of realizing a breakthrough for fuel cell vehicle applications. This article scrutinizes design targets and material screening criteria for solid state hydrogen storage. Adopting an automotive engineering point of view, four important, but often neglected, issues are discussed: 1) volumetric storage capacity, 2) heat transfer for desorption, 3) recharging at low temperatures and 4) cold start of the vehicle. The article shall help to understand the requirements and support the research community when screening new materials.     

The problem of solid state hydrogen storage

A short review of the materials under investigation suitable for solid state hydrogen storage is presented, with particular reference to the experimental activity carried out at the laboratory of Hydrogen Group of Padova University.     

Metal hydride hydrogen storage and compression systems for energy storage technologies

Along with a brief overview of literature data on energy storage technologies utilising hydrogen and metal hydrides, this article presents results of the related R&D activities carried out by the authors. The focus is put on proper selection of metal hydride materials on the basis of AB₅- and AB₂-type intermetallic compounds for hydrogen storage and compression applications, based on the analysis of PCT properties of the materials in systems with H₂ gas. The article also presents features of integrated energy storage systems utilising metal hydride hydrogen storage and compression, as well as their metal hydride based components developed at IPCP and HySA Systems.     

Nanoconfined mixed Li and Mg borohydrides as materials for solid state hydrogen storage

Several mixtures of LiBH₄ and Mg(BH₄)₂ borohydrides in different stoichiometric ratios (1:0, 2:1, 1:1, 1:2, 0:1), prepared by high energy ball milling, have been investigated with X-ray powder diffraction and thermal programmed desorption (TPD) volumetric analysis to test the dehydrogenation kinetics in correlation with the physical mixture composition. Afterwards mixed and unmixed borohydrides were dispersed on high specific surface area ball milled graphite by means of the solvent infiltration technique. BET and statistical thickness methods were used to characterize the support surface properties, and SEM micrographs gave a better understanding of the preparation techniques. It has been observed by TPD volumetric measurements that the confinement of the reactive borohydrides on the nanoporous supports leads to a lower dehydrogenation temperature compared to unsupported borohydrides. Moreover, a further decrease of the dehydrogenation temperature has been observed by increasing the specific surface area of the support and the pores volume and by using the prepared mixtures instead of pure materials. The dehydrogenation process seems to be favoured by the heterogeneous nucleation on the graphite surface of decomposition products or intermediate phases from melted liquid borohydrides.     

Experiments on solid state hydrogen storage device with a finned tube heat exchanger

The absorption and desorption performances of a solid state (metal hydride) hydrogen storage device with a finned tube heat exchanger are experimentally investigated. The heat exchanger design consists of two “U” shaped cooling tubes and perforated annular copper fins. Copper flakes are also inserted in between the fins to increase the overall effective thermal conductivity of the metal hydride bed. Experiments are performed on the storage device containing 1 kg of hydriding alloy LaNi₅, at various hydrogen supply pressures. Water is used as the heat transfer fluid. The performance of the storage device is investigated for different operating parameters such as hydrogen supply pressure, cooling fluid temperature and heating fluid temperature. The shortest charging time found is 490 s for the absorption capacity of 1.2 wt% at a supply pressure of 15 bar and cooling fluid temperature and velocity of 288 K and 1 m/s respectively. The effect of copper flakes on absorption performance is also investigated and compared with a similar storage device without copper flakes.     

Large-scale compressed hydrogen storage as part of renewable electricity storage systems

Storing energy in the form of hydrogen is a promising green alternative. Thus, there is a high interest to analyze the status quo of the different storage options. This paper focuses on the large-scale compressed hydrogen storage options with respect to three categories: storage vessels, geological storage, and other underground storage alternatives. In this study, we investigated a wide variety of compressed hydrogen storage technologies, discussing in fair detail their theory of operation, potential, and challenges. The analysis confirms that a techno-economic chain analysis is required to evaluate the viability of one storage option over another for a case by case. Some of the discussed technologies are immature; however, this does not rule out these technologies; rather, it portrays the research opportunities in the field and the foreseen potential of these technologies. Furthermore, we see that hydrogen would have a significant role in balancing intermittent renewable electricity production.     

Technical assessment of cryo-compressed hydrogen storage tank systems for automotive applications

On-board and off-board performance and cost of cryo-compressed hydrogen storage are assessed and compared to the targets for automotive applications. The on-board performance of the system and high-volume manufacturing cost were determined for liquid hydrogen refueling with a single-flow nozzle and a pump that delivers liquid H₂ to the insulated cryogenic tank capable of being pressurized to 272 atm. The off-board performance and cost of delivering liquid hydrogen were determined for two scenarios in which hydrogen is produced by central steam methane reforming (SMR) or by central electrolysis. The main conclusions are that the cryo-compressed storage system has the potential of meeting the ultimate target for system gravimetric capacity, mid-term target for system volumetric capacity, and the target for hydrogen loss during dormancy under certain conditions of minimum daily driving. However, the high-volume manufacturing cost and the fuel cost for the SMR hydrogen production scenario are, respectively, 2–4 and 1.6–2.4 times the current targets, and the well-to-tank efficiency is well short of the 60% target specified for off-board regenerable materials.     

System simulation models for on-board hydrogen storage systems

System simulation models for automotive on-board hydrogen storage systems provide a measure of the ability of an engineered system and storage media to meet system performance targets. Thoughtful engineering design for a particular storage media can help the system achieve desired performance goals. This paper presents system simulation models for two different advanced hydrogen storage technologies – a cryo-adsorption system and a metal hydride system. AX-21 superactivated carbon and sodium alanate are employed as representative storage media for the cryo-adsorbent system and the metal hydride system respectively. Lumped parameter models incorporating guidance from detailed transport models are employed in building the system simulation models.     

Superalkali NLi4 decorated graphene: A promising hydrogen storage material with high reversible capacity at ambient temperature

This paper investigates the decoration of superalkali NLi₄ on graphene and the hydrogen storage properties by using first principles calculations. The results show that the NLi₄ units can be stably anchored on graphene while the Li atoms are strongly bound together in the superalkali clusters. Decoration using the superalkali clusters not only solve the aggregation of metal atoms, it also provide more adsorption sites for hydrogen. Each NLi₄ unit can adsorb up to 10 H₂ molecules, and the NLi₄ decorated graphene can reach a hydrogen storage capacity 10.75 wt% with an average adsorption energy ?0.21 eV/H₂. We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies. After considering the entropy effect, the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures. So NLi₄ decorated graphene can be promising hydrogen storage material with high reversible storage capacities.     

Hierarchical methodology for modeling hydrogen storage systems. Part I: Scoping models

Detailed models for hydrogen storage systems provide essential design information about flow and temperature distributions, as well as, the utilization of a hydrogen storage media. However, before constructing a detailed model it is necessary to know the geometry and length scales of the system, along with its heat transfer requirements, which depend on the limiting reaction kinetics. More fundamentally, before committing significant time and resources to the development of a detailed model, it is necessary to know whether a conceptual storage system design is viable. For this reason, a hierarchical system of models progressing from scoping models to detailed analyses was developed. This paper, which discusses the scoping models, is the first in a two part series that presents a collection of hierarchical models for the design and evaluation of hydrogen storage systems.     

Hierarchical methodology for modeling hydrogen storage systems. Part II: Detailed models

There is significant interest in hydrogen storage systems that employ a media which either adsorbs, absorbs or reacts with hydrogen in a nearly reversible manner. In any media based storage system the rate of hydrogen uptake and the system capacity is governed by a number of complex, coupled physical processes. To design and evaluate such storage systems, a comprehensive methodology was developed, consisting of a hierarchical sequence of models that range from scoping calculations to numerical models that couple reaction kinetics with heat and mass transfer for both the hydrogen charging and discharging phases. The scoping models were presented in Part I [Hardy BJ, Anton DL. Hierarchical methodology for modeling hydrogen storage systems, Part I: scoping models. Int J Hydrogen Energy 2009;34(5):2269–77.] of this two part series of papers. This paper describes a detailed numerical model that integrates the phenomena occurring when hydrogen is charged and discharged. A specific application of the methodology is made to a system using NaAlH₄ as the storage media.     

Development of a hydrogen catalytic heater for heating metal hydride hydrogen storage systems

This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30 kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid.     

Al-decorated carbon nanotube as the molecular hydrogen storage medium

Al-decorated, single-walled carbon nanotube has been investigated for hydrogen storage applications by using Density Functional Theory (DFT) based calculations. Single Al atom-decorated on (8,0) CNT adsorbs upto six H₂ molecules with a binding energy of 0.201 eV/H₂. Uniform decoration of Al atom is considered for hydrogen adsorption. The first Al atom has a binding energy of 1.98 eV on (8,0) CNT and it decreases to 1.33 eV/Al and 0.922 eV/Al respectively, when the number of Al atoms is increased to four and eight. Each Al atom in (8,0) CNT-8Al adsorbs four H₂ molecules, without clustering of Al atoms, and the storage capacity reaches to 6.15 wt%. This gravimetric storage capacity is higher than the revised 2015 target of U.S Department of Energy (DOE). The average adsorption binding energy of H₂ in (8,0) CNT-8(Al+4H₂), i.e. 0.214 eV/H₂, lies between 0.20 and 0.60 eV/H₂ which is required for adsorbing and desorbing H₂ molecules at near ambient conditions. Thus, Al-decorated (8,0) CNT is proposed as a good hydrogen storage medium which could be useful for onboard automobile applications, at near ambient conditions.     

Designing novel nanoporous architectures of carbon nanotubes for hydrogen storage

A multi-technique theoretical approach was used to investigate hydrogen storage in a three-dimensional diamond-like architecture composed by interconnected carbon nanotubes (CNT). This is achieved with nodes formed by four nanotubes joined together by the inclusion of heptagonal rings placed appropriately. This novel nanoporous material, named Super Diamond has, by design, tunable pore size and exhibit large free volume and surface area, which can reach the values of 95% and 2535 g/m² respectively. The interaction and the adsorption properties of this material with hydrogen were studied thoroughly via ab-initio and Grand Canonical Monte Carlo simulations. Our results show that a large pore Super Diamond can surpass the gravimetric capacity of 20% at 77 K and can reach the high value of 8% at room temperature.     

Preparation,scale-up and testing of nanoscale,doped amide systems for hydrogen storage

A comparative study of the LiNH₂–MgH₂ hydrogen storage system has been made, and several additives (LiBH₄, KH and ZrCoH₃) have been tested as single catalysts and in various combinations in order to study potential synergistic effects.     

Technical assessment of compressed hydrogen storage tank systems for automotive applications

The performance and cost of compressed hydrogen storage tank systems has been assessed and compared to the U.S. Department of Energy (DOE) 2010, 2015, and ultimate targets for automotive applications. The on-board performance and high-volume manufacturing cost were determined for compressed hydrogen tanks with design pressures of 350 bar (∼5000 psi) and 700 bar (∼10,000 psi) capable of storing 5.6 kg of usable hydrogen. The off-board performance and cost of delivering compressed hydrogen was determined for hydrogen produced by central steam methane reforming (SMR). The main conclusions of the assessment are that the 350-bar compressed storage system has the potential to meet the 2010 and 2015 targets for system gravimetric capacity but will not likely meet any of the system targets for volumetric capacity or cost, given our base case assumptions. The 700-bar compressed storage system has the potential to meet only the 2010 target for system gravimetric capacity and is not likely to meet any of the system targets for volumetric capacity or cost, despite the fact that its volumetric capacity is much higher than that of the 350-bar system. Both the 350-bar and 700-bar systems come close to meeting the Well-to-Tank (WTT) efficiency target, but fall short by about 5%.     

Multi-objective optimization of batteries and hydrogen storage technologies for remote photovoltaic systems

Stand-alone photovoltaic (PV) systems comprise one of the promising electrification solutions to cover the demand of remote consumers, especially when it is coupled with a storage solution that would both increase the productivity of power plants and reduce the areas dedicated to energy production.     

Separation and storage of hydrogen by steam-iron process: Effect of added metals upon hydrogen release and solid stability

During the last decade, the steam-iron process has re-emerged as a possible way to separate and/or storage pure hydrogen through the use of metallic oxides subjected to redox cycles. The most renamed candidate to achieve this goal has traditionally been iron oxide. Nevertheless, the study of its behaviour along repetitive reduction/oxidation stages has shown that the hydrogen storage density diminishes abruptly from the first cycle on.To cope with this problem, the inclusion of a second metal oxide in the solid structure has been tried. Isothermal experiments of reduction with hydrogen rich flows and oxidation with steam have been carried out with Al, Cr and Ce as second metals, in nominal amounts from 1% to 10 mol% added to the hematite structure, which has been synthesized in laboratory by coprecipitation. Series of up to seven cycles (reductions followed by oxidations in a thermogravimetric system acting as differential reactor for the gas) have shown that to that point, an almost repetitive behaviour can be obtained, recovering the magnetite (Fe₃O₄) structure after each oxidation step.Since the second metal oxide does not intervene in the reduction/oxidation process, the optimum content of second metal for each species has been determined with the aim to keep the highest hydrogen storage density along cycles.     

High performance material for hydrogen storage: Graphenelike Si2BN solid

Recently, two dimensional graphenelike i.e. Si₂BN solid monolayer have attracted much attention for the use of hydrogen developments. The work is based on first principles calculations using density functional theory with long range van der Waal (vdW) interactions. The optimized structure is energetically more stable due to high formation energy 45.39 eV with PBE and 50.82 eV with HSE06 functionals, respectively. Our ab-initio studies show that Pd (palladium) adatoms secured graphenelike Si₂BN solid via two types of interactions; physisorption and chemisorptions reactions, which engrossing up to 3H₂ molecules signifying gravimetric limits of ≈6.95–10.21 wt %. The absorption energies vary from ?0.31 eV to ?1.93 eV with Pd-adatom and without Pd-adatom respectively, and it varies up to ?1.24 eV. The work function of pure Si₂BN is 5.36 eV while metal-adatom on monolayer Si₂BN with (1 to 6)H₂ molecules is 3.53 eV–4.99 eV and reaches up to 5.85 eV. The theoretical study suggests that the functionalized graphenelike Si₂BN is efficient for hydrogen storage and propose a possible improvement for advantageous storage of hydrogen at ambient conditions.     

Metal hydride material requirements for automotive hydrogen storage systems

The United States Department of Energy (DOE) has published a progression of technical targets to be satisfied by on-board rechargeable hydrogen storage systems in light-duty vehicles. By combining simplified storage system and vehicle models with interpolated data from metal hydride databases, we obtain material-level requirements for metal hydrides that can be assembled into systems that satisfy the DOE targets for 2017. We assume minimal balance-of-plant components for systems with and without a hydrogen combustion loop for supplemental heating. Tank weight and volume are driven by the stringent requirements for refueling time. The resulting requirements suggest that, at least for this specific application, no current on-board rechargeable metal hydride satisfies these requirements.