FUELS – HYDROGEN STORAGE | Chemical Carriers

Abstract

Combustion processes have been the world's primary source of energy for over 700 000 years. Since the discovery of fire (i.e., combustion) the applications of combustion expanded from heating, cooking, and lighting to metallurgy, steam engines, electricity, and eventually to the internal combustion engine. The internal combustion engine became technologically viable because of the discovery of a readily available liquid fuel called petroleum. For over 100 years the internal combustion engine has been powered by combusting petroleum-derived fuels. With ever-growing concerns of environmental pollution, energy security, and future oil supplies, the global community is now seeking nonpetroleum-based alternative fuels, along with more advanced energy technologies to increase the efficiency of energy use. A fuel cell (FC) operating on hydrogen is such a device that offers increased efficiency, while also producing eco-friendly by-products (i.e., water). The source of hydrogen may be generated from clean-coal gasification, methane steam reforming, biomass reforming, advanced solar water splitting, etc. Storing enough hydrogen onboard a vehicle for a 300 mile range is a difficult task. A two-pronged approach has been employed to increase the gravimetric storage density of hydrogen; these being chemical hydrides and hydrogen carriers. The hydrogen carrier approach utilizes nonpetroleum-derived fuels (e.g., methanol, dimethyl ether (DME), ethanol, etc.) that are readily reformed to hydrogen through onboard automotive fuel processors. The chemical hydride approach employs engineered materials to store hydrogen onboard that can be released on demand. Both approaches have their own specific US Department of Energy (DOE) technical targets that must be met for commercial viability. Common targets include gravimetric and volumetric densities, cost, hydrogen delivery rate, start-up energy, and transient response to name a few. The primary difference between the hydrogen carrier approach and the chemical hydride approach is the anticipated facile regeneration of chemical hydrides. Chemical hydrides are expected to be regenerated with hydrogen. In contrast, hydrogen carriers currently require a hydrogen source and a carbon source. This article broadly describes the thermodynamic and kinetics aspects of hydrogen production/release of candidate chemical hydrides and candidate hydrogen carriers. This article also presents some engineering aspects related to chemical hydrides and hydrogen carriers that are often neglected in the literature base. The purpose of this article is to present the ‘forest’ and not the ‘trees’ that will allow, perhaps, for a deeper understanding of the complex and nontrivial nature of our current energy crisis.