Effect of Pore Size on Dehydrogenation Temperature of Carbon Cryogel-Ammoniaborane Nanocomposites
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Effect of Pore Size on Dehydrogenation Temperature of Carbon CryogelAmmoniaborane Nanocomposites Saghar Sepehri, Betzaida Batalla Garcia, and Guozhong Cao Materials Science and Engineering, University of Washington, Seattle, WA, 98195-2120 ABSTRACT This study reports the effects of pore size of porous carbon scaffold on the dehydrogenation of ammonia borane in the coherent carbon- ammonia borane nanocomposites. Porous carbon scaffold is obtained from resorcinol formaldehyde derived carbon cryogels. The nanocomposites are made by simple soaking porous carbon scaffold in ammonia borane solution. Nitrogen sorption analysis and differential scanning calorimetry are used to investigate the structure and dehydrogenation of the nanocomposites. The results reveal that dehydrogenation temperature decreases in nanocomposites as compared to neat ammonia borane, and is lower in nanocomoposites with smaller pore sizes. These findings can be used to tune the dehydrogenation temperature to meet specific hydrogen storage applications. Also, dehydrogenation kinetics of nanocomposites is enhanced as compared to neat ammonia borane. INTRODUCTION Hydrogen storage is a key issue in the hydrogen technology and considerable progress has been made in synthesizing and investigating novel materials for hydrogen storage in the past decade. Although various techniques and materials have been routinely used to store hydrogen, there is neither method nor material that satisfies all the requirements of perceived hydrogen economy [1-4]. Ideally, hydrogen should be stored in the form of liquid or solid under near the ambient conditions and hydrogenation and dehydrogenation process is reversible and rapid. Physisorption of hydrogen in porous media requires high pressure and offers relatively low storage capacity, though it is perfectly reversible and rapid in kinetics. Porous carbons have attracted considerable attention for this application [5, 6]. Hydrides are a group of materials under intensive research, as they offer high storage capacity at ambient conditions. However, hydrides suffer other drawbacks. For example, some hydrides offering high storage capacity have poor reversibility; others possess good reversibility but have low storage capacity; and most hydrides have relatively high and un-tunable dehydrogenation temperature. Hydrides also suffer from poor thermal conductivity and large latent heat of hydrogenation and dehydrogenation [7, 8]. It is not surprising that there is no single hydride meeting all the requirements for hydrogen storage. Developing of new hydrides, particularly complex hydrides, has been a very active research topic [9]. Admixing amides into hydrides has demonstrated to be an efficient approach to reduce the dehydrogenation temperature through the change of reaction pathways [10, 11]. Introducing efficient catalysts such as transition metal oxides has shown its potential [12, 13]. Carbon-hydride composites have shown improved dehydrogenation properties [14, 15]. Confining hydrides inside nanoporous scaff
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