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

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

10.26434/chemrxiv.13857623.v1 ◽

2021 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Molar Volume ◽

Metal Hydride ◽

Chemical Potential ◽

Reaction Pathway ◽

Ideal Gas ◽

Point Of View ◽

Phase Point ◽

Desorption Enthalpy ◽

Co Doped

A remarkable finding of metal hydride hydrogen storage is that substituting 4 mol % sodium by potassium in 4 mol % Ti-doped NaAlH4 raises the reversible hydrogen storage capacity from 3.3 % w/w H to 4.7 % w/w H. This increase by 42% is concomitant with a slightly lower desorption enthalpy: intriguingly enough, it is substantially more hydrogen capacity at slightly less desorption enthalpy. The general solution to that puzzle has been already derived from a gas phase point of view, taking advantage of the equilibrium nature of the matter, which thus comes in terms of an ideal gas chemical potential. However, it is also interesting to investigate for the flipside effect in the sorbent phase, affecting molar volume. This paper elucidates by the example of K/Ti-co-doped NaAlH4 the relation of doping modifications to surplus hydrogen amount and hydride molar volume, defining the term “reaction pathway” in this context, yielding the according figures.

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

10.26434/chemrxiv.6940379.v4 ◽

2019 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Specific Energy ◽

Metal Hydride ◽

Storage Capacity ◽

Reaction Pathway ◽

Worked Examples ◽

Ideal Gas ◽

Hydrogen Electrode ◽

Battery Cell ◽

General Applicability

This article outlines a simple 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. The general applicability to experimental reality is demonstrated with pinpoint-accuracy by means of worked examples from metal hydride chemistry and electrochemistry (see ESI). Highlights of the metal hydride cases are why the substitution of 4 mol % Na by K in Ti-doped sodium alanate raises the reversible storage capacity from 3.3 to 4.7 % w/w H and the outlining of the additional reaction pathway in Mg(NH2)2/2LiH when doped with (Rb/K)H, increasing the reversible storage capacity from 3.6 to 4.4 % w/w H. The electrochemical case study derives the theoretical specific energy threshold for Li-batteries (274 Ah kg-1) from the standard hydrogen electrode potential and then figures for a practical example the reversible specific energy (a T5 NAS-battery cell by NGK Insulators Ltd.).

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On the Common Ground of Thermodynamics and Kinetics: How to Pin Down Overpotential to Reversible Metal Hydride Formation and the Complete Ideal Gas Theory of Reversible Chemical Hydrogen Storage

10.26434/chemrxiv.13308875 ◽

2020 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Metal Hydride ◽

Common Ground ◽

Chemical Potential ◽

Applied Pressure ◽

Ideal Gas ◽

Practical Significance ◽

Equilibrium Pressure ◽

Tank Design ◽

Chemical Hydrogen Storage

Ti-doped NaAlH4 requires at 125 °C for [AlH4] formation more than twice the equilibrium pressure; while it is straightforward to relate this conditional surplus in hydrogenation pressure respective chemical potential to kinetic hindrance, it appears strange that this matter has not been duly theoretically addressed in literature to this day. The interest in identifying such overpotentials is not of purely academic interest but touches a problem of very practical significance as the maximum applied pressure is an important threshold to metal hydride tank design. A theory-based tool would be a resource-efficient complement or even alternative to PCI measurements. This paper tracks the formation overpotential issue down to its root and outlines a simple yet accurate general method based on Arrhenius and van’t Hoff data. Rather unexpectedly, the result is also the final missing piece towards a comprehensive understanding of reversible chemical hydrogen storage with regard to attainable hydrogen storage capacity.

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On the Common Ground of Thermodynamics and Kinetics: How to Pin Down Overpotential to Reversible Metal Hydride Formation and the Complete Ideal Gas Theory of Reversible Chemical Hydrogen Storage

10.26434/chemrxiv.13308875.v1 ◽

2020 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Metal Hydride ◽

Common Ground ◽

Chemical Potential ◽

Applied Pressure ◽

Ideal Gas ◽

Practical Significance ◽

Equilibrium Pressure ◽

Tank Design ◽

Chemical Hydrogen Storage

Ti-doped NaAlH4 requires at 125 °C for [AlH4] formation more than twice the equilibrium pressure; while it is straightforward to relate this conditional surplus in hydrogenation pressure respective chemical potential to kinetic hindrance, it appears strange that this matter has not been duly theoretically addressed in literature to this day. The interest in identifying such overpotentials is not of purely academic interest but touches a problem of very practical significance as the maximum applied pressure is an important threshold to metal hydride tank design. A theory-based tool would be a resource-efficient complement or even alternative to PCI measurements. This paper tracks the formation overpotential issue down to its root and outlines a simple yet accurate general method based on Arrhenius and van’t Hoff data. Rather unexpectedly, the result is also the final missing piece towards a comprehensive understanding of reversible chemical hydrogen storage with regard to attainable hydrogen storage capacity.

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On the Common Ground of Thermodynamics and Kinetics: How to Pin Down Overpotential to Reversible Metal Hydride Formation and the Complete Ideal Gas Theory of Reversible Chemical Hydrogen Storage

10.26434/chemrxiv.13308875.v2 ◽

2020 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Metal Hydride ◽

Common Ground ◽

Chemical Potential ◽

Applied Pressure ◽

Ideal Gas ◽

Practical Significance ◽

Equilibrium Pressure ◽

Tank Design ◽

Chemical Hydrogen Storage

Ti-doped NaAlH4 requires at 125 °C for [AlH4] formation more than twice the equilibrium pressure; while it is straightforward to relate this conditional surplus in hydrogenation pressure respective chemical potential to kinetic hindrance, it appears strange that this matter has not been duly theoretically addressed in literature to this day. The interest in identifying such overpotentials is not of purely academic interest but touches a problem of very practical significance as the maximum applied pressure is an important threshold to metal hydride tank design. A theory-based tool would be a resource-efficient complement or even alternative to PCI measurements. This paper tracks the formation overpotential issue down to its root and outlines a simple yet accurate general method based on Arrhenius and van’t Hoff data. Rather unexpectedly, the result is also the final missing piece towards a comprehensive understanding of reversible chemical hydrogen storage with regard to attainable hydrogen storage capacity.

Download Full-text

On the Common Ground of Thermodynamics and Kinetics: How to Pin Down Overpotential to Reversible Metal Hydride Formation and the Complete Ideal Gas Theory of Reversible Chemical Hydrogen Storage

10.26434/chemrxiv.13308875.v3 ◽

2021 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Metal Hydride ◽

Common Ground ◽

Chemical Potential ◽

Applied Pressure ◽

Ideal Gas ◽

Practical Significance ◽

Equilibrium Pressure ◽

Tank Design ◽

Chemical Hydrogen Storage

Ti-doped NaAlH4 requires at 125 °C for [AlH4] formation more than twice the equilibrium pressure; while it is straightforward to relate this conditional surplus in hydrogenation pressure respective chemical potential to kinetic hindrance, it appears strange that this matter has not been duly theoretically addressed in literature to this day. The interest in identifying such overpotentials is not of purely academic interest but touches a problem of very practical significance as the maximum applied pressure is an important threshold to metal hydride tank design. A theory-based tool would be a resource-efficient complement or even alternative to PCI measurements. This paper tracks the formation overpotential issue down to its root and outlines a simple yet accurate general method based on Arrhenius and van’t Hoff data. Rather unexpectedly, the result is also the final missing piece towards a comprehensive understanding of reversible chemical hydrogen storage with regard to attainable hydrogen storage capacity.

Download Full-text

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

10.26434/chemrxiv.6940379 ◽

2019 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Energy Storage ◽

Hydrogen Storage ◽

Predictive Power ◽

Storage Capacity ◽

Reaction Pathway ◽

Ideal Gas ◽

Equilibrium Thermodynamics ◽

Ideal Gas Law ◽

Chemical Hydrogen Storage ◽

The Ideal

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 NaAlH4 raises the reversible storage capacity by 42 % and elaboration of the reaction pathway in (Rb/K)H-doped Mg(NH2)2/2LiH. 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.

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How to Assess the Relative Volume Change of Reversible Metal Hydrides Easily, Speedily and Concisely (Enough) with a Surprising Relevance for Clarifying Thermodynamic Tailoring Effects

10.26434/chemrxiv.10316819 ◽

2019 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Volume Change ◽

Reaction Pathway ◽

Metal Hydrides ◽

Relative Volume ◽

Reaction Parameters ◽

Relative Volume Change ◽

Open Issue ◽

Chemical Hydrogen Storage ◽

Co Doped

A practical way for assessing the relative volume change of reversible metal hydrides upon hydrogenation, based on the van’t Hoff reaction parameters, is outlined. Hitherto computational methods can provide that information only at a much higher level of complexity. By that method, the open issue of assessing the minimum pressure for complete [AlH4]-formation in Ti-doped NaAlH4 is resolved and the nature of the additional reaction pathway in KH/Ti-co-doped NaAlH4 elucidated. This work summarizes the essentials for the thermodynamic tailoring of metal hydrides in nine points and adds thus a central missing piece to the puzzle of understanding reversible chemical hydrogen storage in metal hydrides.

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

10.26434/chemrxiv.6940379.v10 ◽

2021 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Storage Capacity ◽

Ideal Gas ◽

Equilibrium Thermodynamics ◽

Ideal Gas Law ◽

Chemical Hydrogen Storage ◽

Co Doped ◽

Normative Role ◽

The Ideal

This article shows up the intrinsic thermodynamic boundaries to reversible mass transfer on basis of the ideal gas law and classic equilibrium thermodynamics in relation to chemical hydrogen storage. In the event, a global picture of reversible chemical hydrogen storage is unveiled, including an explanation of partial reversibility. The findings of this work help to clarify problems of metal hydride chemistry which otherwise are difficult if not impossible to solve in convergent manner, e.g. why the substitution of 4 mol % Na by K in Ti-doped NaAlH4 raises the reversible storage capacity by 42 % or the way the dopants take effect in (Rb/K)-co-doped Mg(NH2)2/2LiH. This work's result is of a wider significance since based on two cornerstones of physical chemistry and particularly for the normative role of hydrogen electrodes to electrochemistry.

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How to Assess the Relative Volume Change of Reversible Metal Hydrides Easily, Speedily and Concisely (Enough) with a Surprising Relevance for Clarifying Thermodynamic Tailoring Effects

10.26434/chemrxiv.10316819.v1 ◽

2019 ◽

Author(s):

Roland Hermann Pawelke

Keyword(s):

Hydrogen Storage ◽

Volume Change ◽

Reaction Pathway ◽

Metal Hydrides ◽

Relative Volume ◽

Reaction Parameters ◽

Relative Volume Change ◽

Open Issue ◽

Chemical Hydrogen Storage ◽

Co Doped

A practical way for assessing the relative volume change of reversible metal hydrides upon hydrogenation, based on the van’t Hoff reaction parameters, is outlined. Hitherto computational methods can provide that information only at a much higher level of complexity. By that method, the open issue of assessing the minimum pressure for complete [AlH4]-formation in Ti-doped NaAlH4 is resolved and the nature of the additional reaction pathway in KH/Ti-co-doped NaAlH4 elucidated. This work summarizes the essentials for the thermodynamic tailoring of metal hydrides in nine points and adds thus a central missing piece to the puzzle of understanding reversible chemical hydrogen storage in metal hydrides.

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