Hydrogen storage in novel carbon materials

I. Motivation

Electrical vehicles using hydrogen fuel cells constitute, in the mid to long term, the only means to completely eliminate the harmful emissions of the transportation sector while satisfying market concerns on vehicle range and speed of refueling.

Improvement in energy storage of hydrogen is necessary in order to achieve acceptable performances and to compete with established liquid fuels. An electric vehicle powered by a hydrogen fuel cell will require 3.1 kg of hydrogen to achieve a range of 500 km. This amount, when stored in a typical gasoline tank, would correspond to a hydrogen density of approx. 65 kg/m3 and 6.5 wt%. Only liquid hydrogen systems are close to this target. However, the cost of using liquid hydrogen as a transportation fuel is nearly twice that of gaseous hydrogen when costs due to liquefaction, fuel transportation and manipulation are factored in. Present compression and metal hydride storage technologies are far from the target. Any significant improvement can only be achieved at the cost of more severe operating conditions of pressure and temperature, or of compromises on range and fuel consumption.

The physical adsorption of hydrogen on activated carbon can be used to reduce the pressure required to store it compressed, making that an adsorption-based storage system of hydrogen for vehicular use becomes viable, although the application of this technology requires operation at cryogenic temperature in order to achieve high efficiency. For comparison, Fig. 1 shows the gain of adsorption over same pressure and temperature compression storage at 77, 150 and 293 K. The gain is defined as follows:

II. Theory

The phenomenon of adsorption is essentially the accumulation of atoms or molecules of a gaseous or liquid fluid on a surface of a solid or liquid, as shown in Fig. 2. The inverse process is called desorption.

Interaction between absorbate (adsorbed molecules) and adsorbent (a porous solid) consists of molecular forces known as van der Waal's forces. The preferential concentration of molecules in the proximity of the surface arises because the surface forces of an adsorbent solid are unsaturated and therefore short range (repulsive) and longer range (attractive) forces between adsorbate and adsorbent only become balanced when there is a concentration of molecules on the surface.

III. Goals

In order to demonstrate novel carbon materials as storage medium for hydrogen, the joint project partners - IRH (Institut de Recherche sur l'Hydrogene, Québec, Canada), FutureCamp GmbH (Munich, Germany) and Lehrstuhl für Thermodynamik (TD) (Munich, Germany) - will design and build a hydrogen tank based on activated carbon for a small hydrogen-fueled vehicle.

Before such a device can be built, several questions on the design of cryo-adsorptive storage devices suitable for use outside the laboratory have to be addressed.

Both storage (adsorption) and extraction (desorption) of hydrogen are subject to inhibitory effects, which determine the time required for charging a cryo-adsorptive system and ultimately limit the maximum hydrogen (or power) output. So far only the themodynamics of adsorption has been investigated in detail, required models for the kinetics of ad-/desorption have to be developed in order to get a handle on the dynamics of a cryo-adsorptive storage device.

Furthermore, convective and diffusive transport within the storage device can limit charge and discharge rates and sorption processes require the addition or removal of heat, therefore thermal management of a storage device is more than just providing optimal insulation for maximum long-term storage.

In a first step, a model for the cryo-adsorption storage system will be developed and numerical simulation will be performed to optimize the design of the reservoir by the TD and IRH.

Based on the simulation results, FutureCamp will then develop a cryogenic storage system, which works at low temperatures and moderate pressures. Different tests with the storage system will be performed to validate the modeling assumptions and measure the performance of the system. In a next step, will integrate such a storage system in a demonstrator like a bike or a scooter.