Abstract
Nanoporous carbons remain the most promising candidates for effective hydrogen storage by physisorption in currently foreseen hydrogen-based scenarios of the world’s energy future. An optimal sorbent meeting the current technological requirement has not been developed yet. Here we first review the storage limitations of currently available nanoporous carbons, then we discuss possible ways to improve their storage performance. We focus on two fundamental parameters determining the storage (the surface accessible for adsorption and hydrogen adsorption energy). We define numerically the values nanoporous carbons have to show to satisfy mobile application requirements at pressures lower than 120 bar. Possible necessary modifications of the topology and chemical compositions of carbon nanostructures are proposed and discussed. We indicate that pore wall fragmentation (nano-size graphene scaffolds) is a partial solution only, and chemical modifications of the carbon pore walls are required. The positive effects (and their limits) of the carbon substitutions by B and Be atoms are described. The experimental ‘proof of concept’ of the proposed strategies is also presented. We show that boron substituted nanoporous carbons prepared by a simple arc-discharge technique show a hydrogen adsorption energy twice as high as their pure carbon analogs. These preliminary results justify the continuation of the joint experimental and numerical research effort in this field.
Highlights
In the face of progressive depletion of fossil fuels, increasing pollution, and global warming, hydrogen has been widely considered as a major element of our energy future.Today the benefits of being able to produce hydrogen in a ‘clean’ way, and of its clean combustion have started to counterbalance the substantial industrial difficulties resulting from hydrogen’s low volumetric energy density
In this paper we summarize our 15-year effort to design model nanoporous carbon with structural and energetic characteristics optimized to attain hydrogen storage objectives formulated for automotive applications
As the experimental data show that locally their structure contains randomly oriented graphene fragments that form slit shaped pores of nanometric size (especially in nanoporous carbons with specific surface area (SSA) larger than 2000 m2 /g) [7,16], we focused our analysis of hydrogen adsorption on this type of structure
Summary
In the face of progressive depletion of fossil fuels, increasing pollution, and global warming, hydrogen has been widely considered as a major element of our energy future. Nanomaterials 2021, 11, 2173 three decades, the early euphoric reports of over 60 wt% hydrogen storage in carbon nanofibers at room temperature and relatively low pressure of ~110 bars [2] were (realistically) scaled down to ~1.5–2 wt%. This low storage capacity is a consequence of weak attraction between hydrogen and carbon (low heat of hydrogen physisorption on carbonbased materials, ~5 kJ/mol); on the other hand, this weak physical interaction between sorbent and hydrogen guarantees process reversibility, essential for effective application in real devices. We finish our analysis with experimental validation of numerical predictions, and we present the first experimental evidence that boron substituted carbons, synthetized using the arc-discharge between boron containing graphite electrodes, exhibit hydrogen adsorption energy twice as high as their unsubstituted analogs
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