The Master Key to the Problem of Reversible Chemical Hydrogen Storage is 12 kJ (mol H2)-1

Abstract

<p>This article outlines a potent theoretical formalism illuminating the boundaries to reversible solid hydrogen storage based on the ideal gas law and classic equilibrium thermodynamics. A global picture of chemical reversible hydrogen sorption is unveiled including a thermodynamic explanation of partial reversibility. This is utilized to elucidate a multitude of issues from metal hydride chemistry: Highlights are why the substitution of a mere 4 mol % Na by K in Ti-doped NaAlH<sub>4</sub> raises the reversible storage capacity by 42 % and elaboration of the reaction pathway in (Rb/K)H-doped Mg(NH<sub>2</sub>)<sub>2</sub>/2LiH. The findings of this work allow for a change of paradigm towards the understanding of reversible chemical energy storage and provide a hitherto missing tool of ample analytic and predictive power, complementary to experiment.</p>

The Thermodynamic Way of Assessing Reversible Metal Hydride Volume Expansion: Getting a Grip on Metal Hydride Formation Overpotential

“…Advantageously simple in structure, it nonetheless offers a good approximation for hydrogen behaviour at typical metal hydride reaction conditions (T > 300 K, p < 100 bar). Since -H is equal to the excursion of the gas phase chemical potential from µ°, 7 it figures by amount to a pV m -energy term expressible via the ideal gas law which is shown in equation 2.…”

“…the density ); placing this gram-volume element in a hydrogen atmosphere at p° entails the formation of the metal hydride while the chemical potential of the gas phase changes by -H = µ H2 and pressure above the sorbent drops to a fraction x of p°. 7 In order to re-establish a pressure of p° above the sorbent, the temperature H/S = T 1bar is required. While the initial hydrogenation pressure p° drops to a fraction x, the metal volume expands by V and at the end of the absorption process, the chemical potentials of the gas and sorbent phase are on parity.…”

A Study of K/Ti-co-doped NaAlH₄ on Thermodynamic Tailoring, Surplus Hydrogen Amount and Metal Hydride Molar Volume

“…It is reasonable to relate the effects caused by KH-doping with the next stable mixed alanate K 2 NaAlH 6 , 9 as KNa 2 AlH 6 is instable. 11,12 Its density is calculated by the entropy method, 10 on basis of the data of SØRBY et al, 13 as shown in equations 3a to 3c; the required NaH and Al densities base on Römpp's chemistry lexicon, 14,15 those of KH on data provided by American Elements.…”

“…That tells something of significance although nobody except the author seemingly ever attempted to make global sense out of that: the point about the equilibrium nature of reversible chemical hydrogen storage and doping effects has already been made from a gas phase perspective. 8,9 While that general solution comes in terms of hydrogen gas chemical potential, 8,9 i.e. temperature and pressure, switching perspective from gas to sorbent phase thermodynamics entails a change in metrics towards sorbent phase molar volume: That way the flipside manifestation of a chemical potential change in the gas phase shows in the equilibrium system.…”

The Thermodynamic Way of Assessing Reversible Metal Hydride Volume Expansion: Getting a Grip on Metal Hydride Formation Overpotential

“…On basis of equation 13, the conditional pressure for [AlH 4 ]-formation can be figured out as shown in equation 14a to 14c: the change in arithmetic sign due to the perspective switch from sorbent to gas phase between equations 14a and 14b is noteworthy, 7 leading to the result that for [AlH 4 ]-formation pressure needs to be larger than 78.53 bar ≈ 79 bar.…”