Influence of Synthesis Gas Components on Hydrogen Storage Properties of Sodium Aluminum Hexahydride

Round 1

Reviewer 1 Report

This paper is about the interactions between the Na₃AlH₆ + Al + AC hydride composite and different mixtures of hydrogen + gas impurities (CO, CO₂ and N₂). The hydride composite's cycling stability was evaluated under a pure hydrogen atmosphere through several hydrogenation/dehydrogenation cycles at 170 ºC. An improvement in the cycling stability and hydrogen capacity was noticed (compared with the same hydride without the addition of Al and AC). Moreover, the capacity and cycling stability was evaluated under different H₂+gas impurities atmospheres, leading to a substantial loss of hydrogen capacity, to mainly the generation of CH₄ in the gas phase and different irreversible compounds in the solid phase, depending on the kind of impurity, i.e., CO or CO₂.   After the gas interaction, the material's characterization was mainly done by XRD for the solid phase and FT-IR for the gas phase.

 

This paper can be published after addressing the following issues: 

 

Abstract

The abstract should be re-written according to the comments and improvements required for the publication of the work. It is clear that the partial pressure of an inert compound, in this case: N₂, will influence the total pressure of the gas mixture. Thus, reducing the total pressure and preventing the hydrogenation. This statement underestimated the readers' comprehension and did not contribute to depict the main results of the work. Please, erase part of line 20, lines 21, and 22. 

Introduction

• The reference of Table 1 is not right, i.e., Ref. [8]. It is not possible to find the information reported in Table 1 in reference [8]. Please check and add the right references.

 

• Introduction: Some recent research works about the effects and conversion of CO and CO₂ on hydrides and their reaction mechanism, which are not discussed or mentioned as part of the background of the investigated subject. These works are worth mentioning since they are directly related to the topic:

 

• Tai Y, Wang L, Chen H, et al., Solid State Sciences (2020) 109.
• S. Gamba, J. Puszkiel, P. Arneodo Larochette, et al., International Journal of Hydrogen Energy (2020) 38(45) 19493.
• L. Grasso, J. Puszkiel, F. C. Gennari, et al., Phys.Chem.Chem.Phys., 2020, 22, 1944.
• Zhao J, Wei Y, Cai Y, et al., ACS Sustainable Chemistry and Engineering (2019) 7(5) 4831.
• L. Grasso, J. Puszkiel, Luisa Fernández Albanesi, et al., Phys.Chem.Chem.Phys.,2019, 21, 19825.
• Zhao J, Dong B, Teng Y, et al., International Journal of Hydrogen Energy (2018) 43(10) 5068.
• Dong B, Zhao J, Wang L, et al., Applied Energy (2017) 204 741.

 

Results

 

• Figure 2: It is a little bit confusing. It would be advisable to include some representative cycles in one Fig. and the capacity trend in another. For instance, in Fig. 2A, some cycles with point-line curves and in Fig. 2B the capacity trend. Moreover, it is reported that the 2.15 wt% hydrogen was absorbed upon hydrogenation. What is the experimental error of the machine/error band of the measurements?

 

• Fig 2 shows cycling experiments at 170 ºC under a pure hydrogen atmosphere, providing 2 hours for each hydrogenation (under 2.5 MPa) and dehydrogenation (under 0.1 MPa) process. Table 3 shows experimental results in which different hydrogenation and dehydrogenation times are tested to evaluate the obtained hydrogen capacity. It was concluded that at 170 ºC, just 1 hour for each hydrogenation and dehydrogenation process is needed since no substantial change in the capacity is noticed extending the process times. Why was done the evaluation of the cycling behavior shown in Fig. 2, considering 2 hours for each process instead of 1 hour as concluded from the data in Table 3?

Discussion

 

• The discussion must be improved since it is relatively low. There are several issues:

5.1) In the results section, it is mention that the addition of Al and activated carbon improves the capacity and stability of the composite by more than 30 % in comparison with previous work, Ref. [19] (lines 118 and 119). There is no hypothesis and explanation based on further characterization/experiments/evidence from literature about the observed cycling stability improvement. 

 

5.2) From line 257 to 253: "Both CO, CO2 gas impurities react with the hydride and cause a serious decline in hydrogen storage capacity by reacting with Al and NaH to form probably Al-O- and/or carboxyl and Na-O- and/or carbonates, and/or CH4, respectively. These reactions cause irreversible damage to complex hydride, which could not be regenerated with pure hydrogen under the current experimental condition. The N2 gas impurity does not react with the hydride, but it greatly reduces the partial pressure of H2 and therefore causes difficulties for the hydrogenation reaction."

 

First, the discussion based on the recent advances in the subject is not done, i.e., comparing the previous result with different hydrides used for purification and/or CO₂/CO conversion. There are similarities in different systems that might help to explain what is happening between the investigated hydride system and the employed gas mixture. Second, there is no calculation about the thermodynamic feasibility for the obtention as a final product of the proposed solid phases. For instance, it is not clear in the case of the interaction between Na₃AlH₆ + Al + AC composite and CO₂ + H₂ mixtures whether Na2CO3 or Na2C2O4 is obtained. For this reason, in this case, it would be advisable to calculate what would be a possible reaction based on the Gibbs free energy.

Moreover, there is no analysis of the possible amorphous phases formed in the solid phase; just the FTIR analyses are done for the gas phase. It would be strongly advised to apply an experimental technique that allows verifying the absence/presence of an amorphous phase in the solid products.

Finally, there is no explanation and development of a hypothesis explaining in which way the interactions between the impurities in the gas, and this hydride composite led to irreversible compounds. It is advisable to perform further characterizations, additional calculations (for instance, Gibbs free energy of a proposed reaction), and proposed a possible mechanism for the gas-hydride composite interactions.

 

 

 

Author Response

Thanks for reviewing this manuscript and give us the valuable comments.

This paper is about the interactions between the Na₃AlH₆ + Al + AC hydride composite and different mixtures of hydrogen + gas impurities (CO, CO₂ and N₂). The hydride composite's cycling stability was evaluated under a pure hydrogen atmosphere through several hydrogenation/dehydrogenation cycles at 170 ºC. An improvement in the cycling stability and hydrogen capacity was noticed (compared with the same hydride without the addition of Al and AC). Moreover, the capacity and cycling stability was evaluated under different H₂+gas impurities atmospheres, leading to a substantial loss of hydrogen capacity, to mainly the generation of CH₄ in the gas phase and different irreversible compounds in the solid phase, depending on the kind of impurity, i.e., CO or CO₂.   After the gas interaction, the material's characterization was mainly done by XRD for the solid phase and FT-IR for the gas phase.

 

This paper can be published after addressing the following issues: 

 

Abstract

The abstract should be re-written according to the comments and improvements required for the publication of the work. It is clear that the partial pressure of an inert compound, in this case: N₂, will influence the total pressure of the gas mixture. Thus, reducing the total pressure and preventing the hydrogenation. This statement underestimated the readers' comprehension and did not contribute to depict the main results of the work. Please, erase part of line 20, lines 21, and 22. 

We have erased lines 20-22 and added several sentences to the abstract.

Introduction

• The reference of Table 1 is not right, i.e., Ref. [8]. It is not possible to find the information reported in Table 1 in reference [8]. Please check and add the right references.

The reference for the Table 1 is changed to [9].

 

• Introduction: Some recent research works about the effects and conversion of CO and CO₂ on hydrides and their reaction mechanism, which are not discussed or mentioned as part of the background of the investigated subject. These works are worth mentioning since they are directly related to the topic:

We appreciate the suggested literature sources for the conversion of CO and CO2 on hydrides. We added the below mentioned literature to our references! Additional information regarding results for complex hydride and CO,CO2, H2O and O2 is added the introduction! References [10], [11]: lines 64-69

 

• Tai Y, Wang L, Chen H, et al., Solid State Sciences (2020) 109.
• S. Gamba, J. Puszkiel, P. Arneodo Larochette, et al., International Journal of Hydrogen Energy (2020) 38(45) 19493.
• L. Grasso, J. Puszkiel, F. C. Gennari, et al., Phys.Chem.Chem.Phys., 2020, 22, 1944.
• Zhao J, Wei Y, Cai Y, et al., ACS Sustainable Chemistry and Engineering (2019) 7(5) 4831.
• L. Grasso, J. Puszkiel, Luisa Fernández Albanesi, et al., Phys.Chem.Chem.Phys.,2019, 21, 19825.
• Zhao J, Dong B, Teng Y, et al., International Journal of Hydrogen Energy (2018) 43(10) 5068.
• Dong B, Zhao J, Wang L, et al., Applied Energy (2017) 204 741.

 

 

Results

 

• Figure 2: It is a little bit confusing. It would be advisable to include some representative cycles in one Fig. and the capacity trend in another. For instance, in Fig. 2A, some cycles with point-line curves and in Fig. 2B the capacity trend.

Thank you for your suggestion! Figure 2a represents combined data files from several consequent tests which resulted from interruptions due to power shutdown or other external reasons, therefore the reader can see the short irregularities among the blue lines of the 750 hour life-cycling plot. We extended the graph 2 showing below 7 cycles for clarity (figure 2b). Since the total capacity isn’t changing over time, but stays rather constant ca.2.15 wt%, from our point of view the capacity trend is redundant.

• Moreover, it is reported that the 2.15 wt% hydrogen was absorbed upon hydrogenation. What is the experimental error of the machine/error band of the measurements?
•  
• We calculated and added the experimental error to the displayed value:
  2.15±0.02 wt%.

 

• Fig 2 shows cycling experiments at 170 ºC under a pure hydrogen atmosphere, providing 2 hours for each hydrogenation (under 2.5 MPa) and dehydrogenation (under 0.1 MPa) process. Table 3 shows experimental results in which different hydrogenation and dehydrogenation times are tested to evaluate the obtained hydrogen capacity. It was concluded that at 170 ºC, just 1 hour for each hydrogenation and dehydrogenation process is needed since no substantial change in the capacity is noticed extending the process times. Why was done the evaluation of the cycling behavior shown in Fig. 2, considering 2 hours for each process instead of 1 hour as concluded from the data in Table 3?

The long cycling test over 750 hours was performed on the automated Sievert apparatus, but decision to investigate the cycle time dependency was made at the later date and was performed manually using hydrogenation/dehydrogenation set-up. In our future work, we can test the time dependency on the Sievert apparatus as well.

Discussion

 

• The discussion must be improved since it is relatively low. There are several issues:

5.1) In the results section, it is mention that the addition of Al and activated carbon improves the capacity and stability of the composite by more than 30 % in comparison with previous work, Ref. [19] (lines 118 and 119). There is no hypothesis and explanation based on further characterization/experiments/evidence from literature about the observed cycling stability improvement. 

We thank the reviewer for is statement, but we have clearly explained why additional aluminium is important for an increased storage capacity. This explanation can be found in lines 111-113.

 To replace this consumed Al metal and as the result maintain the reversibility of the system, the amount of 8 mol% Al is added”

In addition we have added an explanation and one important literature why we have added carbon material to our composite. This can be found in lines 122-126.

“In general, the addition of carbon materials to metal hydride compounds result in improved kinetics of dehydrogenation and hydrogenation and the ability to transport and to release the produced heat during the reaction with hydrogen [Lit.].”

[Lit.] Adelhelm P, de Jongh PE (2011) The impact of carbon materials on the hydrogen storage properties of light metal hydrides. J Mater Chem 21:2417–2427

 

5.2) From line 257 to 253: "Both CO, CO2 gas impurities react with the hydride and cause a serious decline in hydrogen storage capacity by reacting with Al and NaH to form probably Al-O- and/or carboxyl and Na-O- and/or carbonates, and/or CH4, respectively. These reactions cause irreversible damage to complex hydride, which could not be regenerated with pure hydrogen under the current experimental condition. The N2 gas impurity does not react with the hydride, but it greatly reduces the partial pressure of H2 and therefore causes difficulties for the hydrogenation reaction."

 

First, the discussion based on the recent advances in the subject is not done, i.e., comparing the previous result with different hydrides used for purification and/or CO₂/CO conversion. There are similarities in different systems that might help to explain what is happening between the investigated hydride system and the employed gas mixture. Second, there is no calculation about the thermodynamic feasibility for the obtention as a final product of the proposed solid phases. For instance, it is not clear in the case of the interaction between Na₃AlH₆ + Al + AC composite and CO₂ + H₂ mixtures whether Na2CO3 or Na2C2O4 is obtained. For this reason, in this case, it would be advisable to calculate what would be a possible reaction based on the Gibbs free energy.

Moreover, there is no analysis of the possible amorphous phases formed in the solid phase; just the FTIR analyses are done for the gas phase. It would be strongly advised to apply an experimental technique that allows verifying the absence/presence of an amorphous phase in the solid products.

We agree with the reviewer that a more precise description and discovery of the produced oxides/carbonates/etc. would be interesting. The aim of the investigations was to make a statement about the stability and degradation of activated-Na₃AlH₆ in the presence of reactive syn gas components. Additional work will be done in the near future to characterize the degradation products, also with solid state-NMR spectroscopy (²⁷Al and ¹³C). We have added the following to in the conclusion, lines 262-269.

“Hugelshofer et al. discovered the reaction of LiAlH₄ with CO₂ and postulated the formation of various amorphous metal oxides, which can’t be identified by XRD. The similar reactivity of the complex aluminum hydrides LilH₄ and Na₃AlH₆ could explain the formation of some amorphous metal oxides compounds with the general formula Na_xAl_YO_z. The structure and the amount of produced oxides-compounds depends strongly on the amount of CO or CO2 in the gas mixture, because reduction products are CH4 and H2O. Water seems to be the main reason for the production of metal-oxide-compounds and the reduced reversibility of the storage material.  “

Finally, there is no explanation and development of a hypothesis explaining in which way the interactions between the impurities in the gas, and this hydride composite led to irreversible compounds. It is advisable to perform further characterizations, additional calculations (for instance, Gibbs free energy of a proposed reaction), and proposed a possible mechanism for the gas-hydride composite interactions.

We have clearly stated the following, which could explain the process of irreversibility of the complex hydride with CO/CO2 in the conclusion:

“Both CO, CO₂ gas impurities react with the hydride and cause a serious decline in hydrogen storage capacity by reacting with Al and NaH to form probably Al-O- and/or carboxyl and Na-O- and/or carbonates” lines 260-262.

Reviewer 2 Report

The manuscript presents an experimental work evaluating the effect of syngas composition (CO/CO2/N2 levels) upon the performance of hydrogen storage Sodium Aluminium Hexahydride. The manuscript is overall well written and sound, the experimental setup is clearly described as well as the experimental procedures and experiments taken out to assess the storage performance. The results for CO, CO2 and N2 are suitably described and discussed. Although the experimental campaign is quite limited (effect of CH4 and H2O is missing, effect of contaminants is missing) the paper provides suitable insight in the fact that hydrides do not seem to be a viable solution for hydrogen storage from syngas – requiring a compulsory syngas cleaning into pure H2.

The authors should address some general comments listed below, providing – where possible – explanation or justification of why the experimental campaign is somewhat limited respect to the existing reformate/partial oxidation/biomass-derived syngas compositions and characteristics. The authors are encouraged to complete the analysis of impact of CH4 and H2O in present or further work.

Comments

1. I would not say that CO, CO2, N2, CH4 and H2O are impurities of synthesis gas, while they are basic constituents of the syngas itself. In fact, if a syngas would be only H2 it would not be a syngas anymore. I strongly suggest modifying “synthetic gas impurities” with “synthetic gas composition”. This is a major issue
2. Instead, syngas impurities are notably tar compounds (benzene, toluene, naphthalene, etc.), sulphides (H2S especially) chlorides (HCl, KCl), PM or solid carbon (C). Has any analysis been done on these components. If not I further suggest changining the scope of the paper as suggested above. The presence and effects of such contaminants (tar, sulphides, chlorides, etc)
3. Text talks about biomass derived syngas (line 43) but Table 2 does not report typical syngas compositions for biomass gasification. Please include it in Table 2. Find typical biomass gasification compositions in https://www.mdpi.com/2079-3197/8/4/86 or any other comparable literature.
4. Table 3 compositions are nowhere near Table 2 typical syngas compositions (except mixture 7). Justify why this is so: hydride is already irreversibly deactivated by small amounts of CO and CO2, experimental setup limitations (gas from cylinder), etc.
5. What about effects of CH4 and H2O ? Have they been analysed? If not please briefly mention the expected results and elaborate in future work.
6. Paper is missing the conclusion section! Consider renaming the Discussion section to Conclusions
7. Since the results of the paper is that syngas is not suitable for hydrides storage a brief overview to syngas conditioning and clean-up should be at least mentioned in the discussion/conclusion section. For example see https://doi.org/10.3390/en13102594 or https://doi.org/10.1016/j.ijhydene.2019.12.142

Author Response

Thanks for the reviewer giving the valuable comments for our manuscript.

The manuscript presents an experimental work evaluating the effect of syngas composition (CO/CO2/N2 levels) upon the performance of hydrogen storage Sodium Aluminium Hexahydride. The manuscript is overall well written and sound, the experimental setup is clearly described as well as the experimental procedures and experiments taken out to assess the storage performance. The results for CO, CO2 and N2 are suitably described and discussed. Although the experimental campaign is quite limited (effect of CH4 and H2O is missing, effect of contaminants is missing) the paper provides suitable insight in the fact that hydrides do not seem to be a viable solution for hydrogen storage from syngas – requiring a compulsory syngas cleaning into pure H2.

The authors should address some general comments listed below, providing – where possible – explanation or justification of why the experimental campaign is somewhat limited respect to the existing reformate/partial oxidation/biomass-derived syngas compositions and characteristics. The authors are encouraged to complete the analysis of impact of CH4 and H2O in present or further work.

Comments

1.I would not say that CO, CO2, N2, CH4 and H2O are impurities of synthesis gas, while they are basic constituents of the syngas itself. In fact, if a syngas would be only H2 it would not be a syngas anymore. I strongly suggest modifying “synthetic gas impurities” with “synthetic gas composition”. This is a major issue

 

2.Instead, syngas impurities are notably tar compounds (benzene, toluene,      naphthalene, etc.), sulphides (H2S especially) chlorides (HCl, KCl), PM or solid      carbon (C). Has any analysis been done on these components. If not I further  suggest changining the scope of the paper as suggested above. The presence and  effects of such contaminants (tar, sulphides, chlorides, etc)

Thanks to the reviewer for a very valuable correction regarding the meaning of impurities of syngas. Indeed the gases we analyzed were not the impurities, as it is mentioned by the reviewer, rather synthetic gas components! The word “impurities” in the title is changed to “components” according to the definition of synthesis gas.

We also agree with the reviewer in this case, that impurities have a much less concentration compared to the main components in syn gas mixtures. It would be interesting to describe the behavior of complex aluminium hydrides against these impurities, but it can’t be done in a short run. We plan to do these investigations in future project.

 

3. Text talks about biomass derived syngas (line 43) but Table 2 does not report typical syngas compositions for biomass gasification. Please include it in Table 2. Find typical biomass gasification compositions in https://www.mdpi.com/2079-3197/8/4/86 or any other comparable literature.

From our view the gasification of biomass is important process of synthesis gas production, but not one of the main ones, therefore it’s not listed in the Table 2. The literature reference suggested by the reviewer is added to the references[5]. We have added one more article about biomass gasification [4].

4.Table 3 compositions are nowhere near Table 2 typical syngas compositions (except mixture 7). Justify why this is so: hydride is already irreversibly deactivated by small amounts of CO and CO2, experimental setup limitations (gas from cylinder), etc.

The purpose of this investigation was to find out how individual gas components react with the complex hydride and how many hydrogenation/dehydrogenation cycles are possible to achieve, therefore we tested different compositions of gasesous constituences separately prior to testing a combined mixture #7.

5.What about effects of CH4 and H2O ? Have they been analysed? If not please briefly mention the expected results and elaborate in future work.

H2O violently reacts with the complex hydride and irreversibly oxidizes it to produce Na-oxides and H2, which was investigated in 2007 by Sandia lab. Methane is likely does not react with complex hydride, but act as well as N2 reducing the partial pressure if present in a mixture. Some preliminary test were done after writing this article. The results will be presented at the later date.

6.Paper is missing the conclusion section! Consider renaming the Discussion section to Conclusions

Discussion section is renamed to Conclusions

7.Since the results of the paper is that syngas is not suitable for hydrides storage a brief overview to syngas conditioning and clean-up should be at least mentioned in the discussion/conclusion section. For example see https://doi.org/10.3390/en13102594 or https://doi.org/10.1016/j.ijhydene.2019.12.142

 

The cleaning technologies of syngas are now mentioned in the conclusion part and we give an outlook for alternative uses of doped Na3AlH6.

Reviewer 3 Report

The paper by Sun et al. reports an interesting and sound investigation of the effects of the occurrence of impurities (CO, CO2 and N2) in hydrogen gas on the hydrogenation/deydrogenation properties of Na3AlH6. The paper is well organized and well written.   I have only some minor suggestions for the authors: 1) Lines 67-70: a systematic investigation of the occurrence of impurites in H2Gas has been performed in the case of Pd-Ag and some other Pd alloys which are commonly used in hydrogen purification systems; please add some references about this issue; 2) Figure 2: due to the presence of empty symbols, it seems that long hydrogenation/dehydrogenation cycles taking 200 hours may take place. I understand that it is not true, but the graph may be misleading; 3) If the IR measurements were performed in transmission mode, possibly the authors mixed the powders with KBr or some other salt. Are they sure that no chemical reaction occurs between them? Maybe ATR measurements would have been more appropriate.

Author Response

Thanks for giving valuable comments for our manuscript.

The paper by Sun et al. reports an interesting and sound investigation of the effects of the occurrence of impurities (CO, CO2 and N2) in hydrogen gas on the hydrogenation/deydrogenation properties of Na3AlH6. The paper is well organized and well written.   I have only some minor suggestions for the authors:

1) Lines 67-70: a systematic investigation of the occurrence of impurites in H2Gas has been performed in the case of Pd-Ag and some other Pd alloys which are commonly used in hydrogen purification systems; please add some references about this issue;

 

The literature sources [5,17] are added to the references! Thank you for this valuable recommendation!

 

2) Figure 2: due to the presence of empty symbols, it seems that long hydrogenation/dehydrogenation cycles taking 200 hours may take place. I understand that it is not true, but the graph may be misleading;

 

The plot is changed to eliminate the empty symbols! The individual cycles are added to Figure 2b to clarify the Figure 2a.

 

3) If the IR measurements were performed in transmission mode, possibly the authors mixed the powders with KBr or some other salt. Are they sure that no chemical reaction occurs between them? Maybe ATR measurements would have been more appropriate.

 

The gas phases produced were investigated in a transmission mode in a SMART OMNI transmission cell with KBr windows. Some changes to the experimental procedure are done: lines 86-90.

Reviewer 4 Report

Sun et al. report the influenece of synthesis gas impurities on the hydrogen storage performance of Na3AlH6, which is an important concern for the adoption of synthesis gas as the hydrogen source. Overall, this work is well structured and hence I recommend the publication of this paper after addressing the query as follow. 

Besides the cycling capacity, the impact of impurities on the hydrogenation and dehydrogenation kinetics should be considered. 

Author Response

Thank you for giving the valuable comments on our manuscript.

et al. report the influenece of synthesis gas impurities on the hydrogen storage performance of Na3AlH6, which is an important concern for the adoption of synthesis gas as the hydrogen source. Overall, this work is well structured and hence I recommend the publication of this paper after addressing the query as follow. 

Besides the cycling capacity, the impact of impurities on the hydrogenation and dehydrogenation kinetics should be considered. 

Some parts of the text were changed to address the English grammar corrections.

The kinetics study is beyond the scope of this investigation, but it can be considered in future work! From our point of view it was important to show that deactivation/degradation of material occurs if reacted with CO, CO2.

Round 2

Reviewer 1 Report

Report

• Question asked in the first round: In the results section, it is mention that the addition of Al andactivated carbon improves the capacity and stability of thecomposite by more than 30 % in comparison with previous work,Ref. [19] (lines 118 and 119). There is no hypothesis andexplanation based on furthercharacterization/experiments/evidence from literature about theobserved cycling stability improvement.

Regarding this issue, it is stated by the authors:

 “but we have clearly explained why additional aluminium is important for an increased storage capacity. This explanation can be found in lines 111-113.”

 Explanation in the manuscript:

 Lines 108-113:

 Activated carbon (8 mol%) and aluminium (8 mol%) were added to Na3AlH6 and TiCl3 (4 mol%) to enhance heat transfer of the composite material, that could improve the hydrogenation and dehydrogenation kinetic as well as dispersion, which can prevent the material from sintering during long time cycling. The other reason for addition of aluminium is that the forming of Ti-Al alloy during doping processes would consume part of Al from Na3AlH6. To replace this consumed Al metal and as the result to maintain the reversibility of the system, the amount of 8 mol% Al is added [25].

In reference [25], in the conclusions, it is written in the conclusions:

 

“Ball-milled sample containing NaAlH4 + 2NaH + 4mol% TiCl3 + 8 mol% Al + 8 mol% AC gave better results compared to the sample containing NaAlH4 + 2NaH + 4mol% TiCl3, with hydrogen capacities of 2.2 and 1.7 wt%, respectively. The addition of Al allowed for the remaining NaH to react further into Na3AlH6 during the hydrogenation process, thereby increasing the hydrogen capacity.”

It is possible to deduce that the addition of Al might induce an increase of capacity. Though, there is not systematic study about this issue, for example, additional measurements in which the progressive addition of Al shows such hypothesis. It would be great to have experimental evidence that it is the reason of the improvement of the capacity. Moreover, it highly possible that you have an increase of the capacity owing to the better thermal conductivity attributed to the addition of AC. Dissipating more energy upon the exothermal hydrogenation reaction would lead to a milder temperature increase leading to a better driving force condition. In m opinion, the augment is not as solid.

• There is still no analysis of the possible amorphous phases formed in the solid phase; just the FTIR analyses are done for thegas phase. I understand that it is plan to do NMR and include it in other work. However, the dicussion-conclusions of this work lack of scientific soudness and need further experimenal work as well as better structure and discussion. As stated before, there are some „hypothesis“ that are based on expected results, not included in this work.

 

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for their helpful comments, which help us to improve the quality of our paper. 

Question asked in the first round: In the results section, it is mention that the addition of Al andactivated carbon improves the capacity and stability of thecomposite by more than 30 % in comparison with previous work,Ref. [19] (lines 118 and 119). There is no hypothesis andexplanation based on furthercharacterization/experiments/evidence from literature about theobserved cycling stability improvement.

 

Regarding this issue, it is stated by the authors:

 

 “but we have clearly explained why additional aluminium is important for an increased storage capacity. This explanation can be found in lines 111-113.”

 

 Explanation in the manuscript:

 

 Lines 108-113:

 

 Activated carbon (8 mol%) and aluminium (8 mol%) were added to Na3AlH6 and TiCl3 (4 mol%) to enhance heat transfer of the composite material, that could improve the hydrogenation and dehydrogenation kinetic as well as dispersion, which can prevent the material from sintering during long time cycling. The other reason for addition of aluminium is that the forming of Ti-Al alloy during doping processes would consume part of Al from Na3AlH6. To replace this consumed Al metal and as the result to maintain the reversibility of the system, the amount of 8 mol% Al is added [25].

 

In reference [25], in the conclusions, it is written in the conclusions:

 

 

 

“Ball-milled sample containing NaAlH4 + 2NaH + 4mol% TiCl3 + 8 mol% Al + 8 mol% AC gave better results compared to the sample containing NaAlH4 + 2NaH + 4mol% TiCl3, with hydrogen capacities of 2.2 and 1.7 wt%, respectively. The addition of Al allowed for the remaining NaH to react further into Na3AlH6 during the hydrogenation process, thereby increasing the hydrogen capacity.”

 

It is possible to deduce that the addition of Al might induce an increase of capacity. Though, there is not systematic study about this issue, for example, additional measurements in which the progressive addition of Al shows such hypothesis. It would be great to have experimental evidence that it is the reason of the improvement of the capacity. Moreover, it highly possible that you have an increase of the capacity owing to the better thermal conductivity attributed to the addition of AC. Dissipating more energy upon the exothermal hydrogenation reaction would lead to a milder temperature increase leading to a better driving force condition. In m opinion, the augment is not as solid.

 

 

Answer:

In this case, we do not agree with the reviewer, that no systematic study regarding addition of Al and AC was done to support our hypothesis. In our cited paper [25] we have shown and explained the positive effect of additional Al-powder on the storage capacity of doped-Na3AlH6. The doping process of Na3AlH6 with 0,04 TiCl3 is a stoichiometric reaction ([25], reaction 3)where Ti3+ Al3+ are reduced to [TiAl3] and Cl- forming NaCl. This Al amount from [TiAl3] is no longer available for the absorption of hydrogen. Therefore, the addition of 0.08 Al-metal is necessary to react with 0,24NaH to form 0,08Na3AlH6 (reaction 5, [25]) and as a result increasing the amount of hexahydride (0.96Na3AlH6 vs. 0.88Na3AlH6) for complete rehydrogenation of the decomposed Na3AlH6 ! The experimental evidence of advantage of addition of Al and AC is shown in Figures 8 and 9 of Ref,[25] where cycling performance is described. It’s also experimentally shown in our previous work Ref. [3], Figure 7, where 2,0 wt% H2 was achieved in 1,9 kg Na3AlH6 + 8 mol% Al +8mol% AC Al-alloy tank in comparison to 1,7 wt% H2 in 0,213 kg Na3AlH6+4mol%TiCl3 tank (Figure 6 and Figure 8, [26]).

 

In addition we discussed now the positive effect of grain refiners (Discussions).and have cited relevant literature [28] describing this well-known phenomenon on the cycle stability of NaAlH4 + carbon materials. Based on these results we have added activated carbon as grain refiner for our storage material. Altogether, the use of grain refiners does not only improve the dehydrogenation and rehydrogenation kinetics, but also enhanced the cycle stability ([10] in our [25]).

We added Ref.[27] regarding benefits of addition of carbon to light alanates.

From all these information it should be clear that the addition of Al and activated carbon is based on comprehensive research results from our group and other colleagues working on complex hydrides.

From our point of view, we have cited the relevant and important papers supporting our hypothesis experimental results.

 

There is still no analysis of the possible amorphous phases formed in the solid phase; just the FTIR analyses are done for thegas phase. I understand that it is plan to do NMR and include it in other work. However, the dicussion-conclusions of this work lack of scientific soudness and need further experimenal work as well as better structure and discussion. As stated before, there are some „hypothesis“ that are based on expected results, not included in this work.

Answer:

In this case, we agree with the reviewer, that we do not have analyzed the amorphous phases in our paper, but we have cited several papers, which describe a similar behavior. The focus of our paper the storage behavior of complex aluminium hydride against syn gas components. The precise analysis of amorphous Al-oxide and Na-O-compound goes beyond the claim of the publication.

We have rewritten the Discussions in our paper and hope that the new structure will satisfy the requirements of the reviewer. This part is highlighted in our paper.

 

Kind regards!

On behalf of all authors

Michael Felderhoff

Round 3

Reviewer 1 Report

This manuscript mainly presents experimental work evaluating the effect of gas composed of different ratios of CO/CO2/N2 upon the performance of hydrogen storage sodium aluminum Hexahydride. 

The systematic study of the improvement observed upon cycling due to the Al and AC addition is arguable and somehow acceptable based on the cited references. Despite this fact, and going through the cited literature, some points are not addressed. However, it is possible to accept the last proposed explanation.

Nonetheless, as mentioned in the last two review reports, analyses through FTIR or other techniques allow identifying the amorphous phases in the solid-state after the interaction with syngas and CO2 is demanding. I do not conceive the paper's contribution to the scientific community without the complete characterization of the solid phase (which would allow me to have a better idea about what is happening in such interaction between sodium aluminum hexahydride and the mixture of gas. Therefore, I consider that this paper must still be evaluated with major corrections.  

 

Comments for author File: Comments.pdf

Author Response

Thank you for reviewing our manuscript.

Sorry for the late reply, but in the meantime we did additional solid state NMR-measurements to confirm our description of the reaction of the complex hydride with reactive gas components, like CO. The solid state 13CNMR spectra shows, that we produce Al-O-CH3 compounds as we have described in the paper. Nevertheless, this is now very clear and precise proofed with the NMR spectra. We hope that the reviewer is now satisfied with these additional results which are included in the paper. In addition we have changed the following parts and we added a conclusion to the paper. The most important changes are highlighted in yellow. 1. NMR description procedure is added in Lines 99-103 2. In Figure 4, ↓Na2CO3 removed from the legend and order of colors is changed (blue-red-black), therefore new Figure 4 is inserted Lines 185-188 3. Figure 5 is added: NMR spectrum Lines 191-194 4. Description of NMR results is added Lines 196-204 5. NMR spectrum references [28], [29] are added Lines 200, 203 respectively 6. Discussions chapter is re-written Lines 284-317 7. Conclusion chapter is added Lines 278-311. 8. Literature references [28], [29] are added into References chapter Lines 421-426. With kind regards!