Conceptual Recycling Chain for Proton Exchange Membrane Water Electrolyzers—Case Study Involving Review-Derived Model Stack

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper developed a conceptual recycling chain for a proton exchange membrane water electrolyzer involving a review-derived model stack. It addresses the most valuable catalysts, such as platinum on the cathode and iridium on the anode. The paper is interesting and valuable, and the following question can be addressed.

1. The economic analysis for recycling could be addressed to highlight the advantages of precious metal resource recycling in proton exchange membrane water electrolyzers.

2 The diversity of proton exchange membrane can be discussed in the separation of anode and cathode materials.

3 Is there any environmental risk in the recycling chain？

4 The mechanical metallurgy and hydrometallurgy have been used for the separation of anode and cathode materials. Is there any pyrometallurgy method?

Author Response

Reviewer 1

Comments and Suggestions for Authors

This paper developed a conceptual recycling chain for a proton exchange membrane water electrolyzer involving a review-derived model stack. It addresses the most valuable catalysts, such as platinum on the cathode and iridium on the anode. The paper is interesting and valuable, and the following question can be addressed.

Response

Thank you very much for writing the review and for your constructive criticism and comments on the manuscript. We have tried to respond to all comments and take them into account when revising the article as best we could in the short time available. We hope that our responses have captured the essence of the points of criticism and answered them satisfactorily.

 

1. The economic analysis for recycling could be addressed to highlight the advantages of precious metal resource recycling in proton exchange membrane water electrolyzers.

Response

Many thanks for this comment. An economic analysis of the recycling of PEMWE is certainly very interesting. We have not carried out such an analysis here because the focus of the work is on presenting a basic technical approach to recycling PEMWE. Once a technology has been developed, i.e. the combination of individual sub-processes to form a complete recycling process, the technological scale-up must be carried out on the basis of which a reliable economic assessment can be made. At this point in time and at the current stage of development, such an economic assessment is neither meaningful nor credible.

 

2. The diversity of proton exchange membrane can be discussed in the separation of anode and cathode materials.

Response

Thank you for your insightful comment. We have added a paragraph discussing the diversity of membranes and its potential influence on the separation of anode and cathode materials. The corresponding addition can be found on page 19.

 

3. Is there any environmental risk in the recycling chain?

Response

Thank you for this comment. Recycling is not always a purely environmentally friendly technology, as the recovery of valuable material streams usually leaves a residual material enriched with pollutants or components that cannot be used economically. Harmful emissions can also be produced during recycling processes, which require technical solutions to prevent them. In the case of electrolyser recycling, as proposed in this article, residues containing flour, gases containing flour or even gaseous HCl can be produced. Brief reference was made to this in the sections on the individual treatment steps (see, for example, section 3.2.5). In this section, for example, alternative acids for dissolving the platinum were also examined in order to reduce the impact on the environment and the use of environmentally friendly leaching agents.

In the introductory section as well as in section 2.1 of the article, reference was also made to the formation of fluorine-containing gases, which can be found in the cited literature (see source Klose et al. 2020, Wittstock 2016). The pyrometallurgical processing of the material flows was not investigated in this study.

Klose, et al. All‐Hydrocarbon MEA for PEM Water Electrolysis Combining Low Hydrogen Crossover and High Efficiency. Advanced Energy Materials 2020, 10, doi: 10.1002/aenm.201903995.

Wittstock, et al. Challenges in Automotive Fuel Cells Recycling. Recycling 2016, 1, 343-364, doi: 10.3390/recycling1030343.

We believe that the environmental aspect has been sufficiently taken into account in the article and have therefore only made minor changes to the manuscript.

 

4. The mechanical metallurgy and hydrometallurgy have been used for the separation of anode and cathode materials. Is there any pyrometallurgy method?

Response

That is a very relevant question. Of course, there is also a pyrometallurgical way to recycle electrolysers. No investigations were carried out on this in the context of this work. The pyrometallurgical route can be based on the recycling of fuel cells, which has been investigated in the past (see Wegner, 2012). The main problem with direct pyrometallurgical recycling was the flour-containing membranes, which cause HF formation in the process. The authors write about the use of a threefold excess of the additive CaO, which was not sufficient to bind all the HF formed. From the authors' point of view, this results in considerable problems for pyrometallurgical recycling due to difficult process control, increased corrosion and high process costs. The use of pyrometallurgical processes should therefore only be carried out after prior mechanical treatment to separate the membranes containing flour.

 

Wegner, R.; Fokkens, E.; Holdt, H.; Bukowsky, H.; Trautmann, M.; Nettesheim, S.; Jakubith, S.; Scholz, P.; Mollenhauer, T.; Theuring, S.; et al. React—Rückgewinnung und Wiedereinsatz von Edelmetallen aus Brennstoffzellen. In Recycling und Rohstoffe; Thomé-Kozmiensky, K.J., Goldmann, D., Eds.; Vivis TK-Verlag: Nietwerder, Germany, 2012; Volume 5, pp. 429–441.

 

To complete the article in this respect, we have added a short paragraph on pyrometallurgy at the beginning of the section on recycling in general (see also version in change mode).

In pyrometallurgical recycling, the generation of HF from the decomposition of flour-containing membranes is a major issue [45,46]. The use of a threefold excess of the additive CaO was insufficient to bind all the HF produced [45]. This leads to significant challenges in pyrometallurgical recycling, including difficulties in process control, increased corrosion, and higher process costs [45,46]. This paper does not go into detail about pyrometallurgical experiments, but focuses on hydrometallurgical approaches.

Reviewer 2 Report

Comments and Suggestions for Authors

The author proposes an integrated "dismantling–mechanical treatment–hydrometallurgy" process for the end-to-end recycling scheme of PEM water electrolysis (PEMWE), supplemented with several experimental verifications. The topic aligns with the urgent needs of the current green hydrogen industry for a circular economy, making it of practical significance. However, there are still notable deficiencies in the depth of mechanistic explanation, data completeness, and industrial feasibility. It is recommended that the manuscript be revised before further review.

1. The manuscript emphasizes that "impact-based" delamination can achieve 80–99% removal of the electrode layer, but does not provide actual measurements of stress/energy density, membrane–electrode interfacial adhesion strength, or quantitative characterization of failure modes (cohesive/adhesive).
2. It is recommended to determine the interfacial adhesion energy through tensile/peel tests or nanoindentation, and to use SEM/FIB-TEM to observe fracture surfaces to clarify the fracture location.
3. The manuscript claims that liquid–liquid phase separation (LLPS) of TiO₂ (IrOx) and C/Pt is achieved by exploiting differences in hydrophilicity/hydrophobicity, but only provides static contact angle data, which is strongly influenced by ionomer adsorption. It is suggested to supplement with dynamic contact angle and roll-off angle tests, and to evaluate wetting behavior in practical separation systems (including dispersants and pH adjustment).
4. The evaluation of the recycled PFSA's molecular weight, isoelectric point, IEC content, and membrane conductivity/mechanical strength after recasting is missing, making it impossible to demonstrate the feasibility of "closed-loop recycling."

Author Response

Reviewer 2

Comments and Suggestions for Authors

The author proposes an integrated "dismantling–mechanical treatment–hydrometallurgy" process for the end-to-end recycling scheme of PEM water electrolysis (PEMWE), supplemented with several experimental verifications. The topic aligns with the urgent needs of the current green hydrogen industry for a circular economy, making it of practical significance. However, there are still notable deficiencies in the depth of mechanistic explanation, data completeness, and industrial feasibility. It is recommended that the manuscript be revised before further review.

Response

Thank you very much for writing the review and for your constructive criticism and comments on the manuscript. We have tried to respond to all comments and take them into account when revising the article as best we could in the short time available. We hope that our responses have captured the essence of the points of criticism and answered them satisfactorily.

The main challenge when proposing a potential recycling technology at such an early stage of a new technology is that there are no significant waste streams or quantities of test material from the development phase. As long as a technology has not yet been widely introduced to the market or is not yet in industrial use, available small quantities must be used. These small quantities should then correspond as closely as possible to the expected return material. Due to the limited amount of test material, it is not possible to carry out extensive tests with regard to material and property characterization, process optimization or industrial scale-up. The proposed technology for recycling PEM is therefore rather a prediction based on in-depth literature research, the authors' experience from recycling similar materials and small-scale validation trials. Everything that goes in the direction of process optimization and large-scale implementation must be the subject of in-depth studies when significant material flows of PEM become available. This can only take place in the coming years.

 

1. The manuscript emphasizes that "impact-based" delamination can achieve 80–99% removal of the electrode layer, but does not provide actual measurements of stress/energy density, membrane–electrode interfacial adhesion strength, or quantitative characterization of failure modes (cohesive/adhesive).

Response

Many thanks for this comment. It is correct that this is mainly an analysis of the decoating results achieved, without an in-depth investigation of the failure mechanisms or a characterization of the bonding conditions. As already described above, this is a study that demonstrates the general feasibility and proves this using a model stack and individual tests.

Data on stress energies in connection with the achieved degree of decoating of the membranes must be obtained in the context of extensive individual studies if sufficiently large quantities of test material are available. This is not the aim of the submitted publication and would go beyond the scope.

Unfortunately, we cannot make any changes to the manuscript here that would do justice to the review commentary. Rather, we must refer to future studies that can specifically address this topic.

 

2. It is recommended to determine the interfacial adhesion energy through tensile/peel tests or nanoindentation, and to use SEM/FIB-TEM to observe fracture surfaces to clarify the fracture location.

Response

This is a very valid objection and we would like to be able to consider this point in greater depth. Unfortunately, such investigations are very time-consuming and need to be carried out on different membrane composites in order to be statistically robust. In the past, we have already carried out such and similar investigations as part of various other research projects on anodes and cathodes of LIBs^1,2, on slags^3,4 and on natural ores⁴. The scientific expertise for this is therefore available.

From our point of view, such investigations would be very useful and should definitely be carried out if a sufficient amount of different test material is available. This should be carried out as part of a separate study. It is beyond the scope of this article. Unfortunately, we can only refer to the coming years.

¹ Wuschke, L.; Jäckel, H.-G.; Leißner, T.; Peuker, U.A. Crushing of large Li-ion battery cells. Waste Management 2019, 85, 317-326, doi: https://doi.org/10.1016/j.wasman.2018.12.042.

² Wuschke, L. Mechanische Aufbereitung von Lithium-Ionen-Batteriezellen. Dissertation, 2018, TU Bergakademie Freiberg

³ Võ, T.T.; Leißner, T.; Peuker, U.A. Utilizing X-ray Computed Tomography for Lithium Slag: A Guide to Analyzing Microstructure and Its Potential Influence on Liberation. Minerals 2024, 14, doi: 10.3390/min14010042.

⁴ Leißner, T.; Hoang, D.H.; Rudolph, M.; Heinig, T.; Bachmann, K.; Gutzmer, J.; Schubert, H.; Peuker, U.A. A mineral liberation study of grain boundary fracture based on measurements of the surface exposure after milling. Int J of Mineral Processing 2016, 156, 3-13, doi: 10.1016/j.minpro.2016.08.014.

 

We have added the following sentence to the manuscript:

“Future investigations must also consider different stress mechanisms and combine these with characterizations from material testing (tensile, compression and peel tests) and analyze the failure mechanism using suitable high-resolution microscopic methods.”

 

3. The manuscript claims that liquid–liquid phase separation (LLPS) of TiO₂ (IrOx) and C/Pt is achieved by exploiting differences in hydrophilicity/hydrophobicity, but only provides static contact angle data, which is strongly influenced by ionomer adsorption. It is suggested to supplement with dynamic contact angle and roll-off angle tests, and to evaluate wetting behavior in practical separation systems (including dispersants and pH adjustment).

Response

Thank you very much for this valuable comment. In Figure 5, our aim was to demonstrate that, although the degree may vary depending on the type of MEA, cathode materials generally exhibit hydrophobic behavior while anode materials tend to be more hydrophilic. We agree that wettability can be characterized using various approaches, and we have cited the study from Ahn and Rudolph, in which different techniques were applied to assess the surface wettability of the used materials. However, the focus of the present manuscript is not on a detailed characterization of wettability, but rather on proposing a conceptual recycling chain for the components of PEM electrolyzers. Therefore, we kindly ask for your understanding that a more extensive analysis was beyond the scope of this work.

 

4. The evaluation of the recycled PFSA's molecular weight, isoelectric point, IEC content, and membrane conductivity/mechanical strength after recasting is missing, making it impossible to demonstrate the feasibility of "closed-loop recycling."

Response

Thank you for this comment. It is true that no characterization of recycled membranes or membranes made from recyclates has been carried out here. Of course, such a characterization of membranes from closed-loop recycling is necessary to prove the closed loop for this component. At this point, we must again refer to the objective of the article, according to which we present a concept for the general recycling of PEMWE. A characterization of the membranes is not part of the objective. Furthermore, there would not be sufficient sample material available for such investigations at the current time and at the current stage of development of recycling.

Unfortunately, we are therefore unable to continue working on this point and ask for your understanding.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presented highlight a very relevant topic about recycling proton exchange membrane according to the increasing research of this technology and the scarcity of PGM. However, low quantitative analysis were carried out that make it difficult to properly follow the relevant data. Even, some sections seems to long and make it difficult for readers to follow them. More charts, schemes would be included specially when there are steps. Thus, more specific comments are included and should be clarified before publishing this manuscript in recycling journal:

• Some information about dimensions, sites, production rate should be included to shed light on the dimensions of this study.
• Please clarify sentences from 47-51 lines.
• Table 1 has many information but some references about these studies has to be included.
• Some references in the recycling process of membrane about the environmental problems of fluorinated compounds in water should be included
• Line 244 define GORE
• Line 245. We need more precise values, what is the operating range? How much is reduced the thickness?
• Line 264 define which are the toxic gases and the average concentration.
• Some graphs or schemes to clarify and make more easy to be read would be necessary in section 3 about the steps.
• Which are the key recycling process and which process has been more studied, which recoveries were obtained in each step?
• Line 560 a reference chart about lifetime and main component prices would clarify the whole discussion.
• Conclusion should be summarized and specific details about the useful data supplied by this manuscript has to be highlighted.

Author Response

Reviewer 3

Comments and Suggestions for Authors

The manuscript presented highlight a very relevant topic about recycling proton exchange membrane according to the increasing research of this technology and the scarcity of PGM. However, low quantitative analysis were carried out that make it difficult to properly follow the relevant data. Even, some sections seems to long and make it difficult for readers to follow them. More charts, schemes would be included specially when there are steps. Thus, more specific comments are included and should be clarified before publishing this manuscript in recycling journal:

Response:

Thank you very much for writing the review and for your constructive criticism and comments on the manuscript. We have tried to respond to all comments and take them into account when revising the article as best we could in the short time available. We hope that our responses have captured the essence of the points of criticism and answered them satisfactorily.

When examining the recycling of a technology at such an early stage of development and before a broad market launch, large quantities of material are not available. It is therefore difficult to carry out extensive tests that provide a solid statistical basis. Instead, the observations are based on the expertise of the authors, a thorough analysis of the published scientific articles and individual validation tests. We can well understand the criticism here. Unfortunately, however, it is not currently possible for us to generate and provide comprehensive data on recycling technology and the individual process steps.

We have shortened the manuscript slightly in various places in order to reduce the length and improve readability. More detailed answers can be found in the individual review comments.

 

1. Some information about dimensions, sites, production rate should be included to shed light on the dimensions of this study.

Response

The information has been added to the text. the facts can be used to make a better assessment of the importance of recycling.

“According to the International Energy Agency (IEA), global electrolysis capacity for hydrogen production has grown in recent years, reaching an installed capacity of 1.4 GW by the end of 2023. The production capacity of electrolysers at that time reached 25 GW per year. The electrolysis capacity is increasing from a low starting point and must be significantly accelerated to align with the Net Zero Emissions by 2050 (NZE) scenario. This scenario requires the installation of 560 GW of electrolysis capacity by 2030. The increasing demand for electrolyzers with a simultaneously constant production volume of only approx. 8 tons of iridium per year worldwide illustrates the importance of an efficient recycling chain.”

 

2. Please clarify sentences from 47-51 lines.

Line 46 to 51: “As the installation capacity of electrolyzers increases, the recovery and recycling of critical raw materials used in PEM water electrolyzers (PEMWE) like iridium or platinum will also become more important in the future. The criticality of the raw materials themselves [3], resulting from the security of supply and resource situation, and a sustainability claim associated with hydrogen technology are motivation for the development of an adapted circular economy for PEMWE stacks.”

Response

We rephrased the above sentences as follows and replaced it in the text.

“As the capacity of electrolyzer installations expands, the recovery and recycling of critical raw materials used in PEM water electrolyzers (PEMWE), such as iridium and platinum, will become increasingly important in the future. The criticality of these raw materials [3] —stemming from supply security concerns, resource availability, and sustainability considerations associated with hydrogen technology—drives the need to develop an adapted circular economy for PEMWE stacks.”

 

3. Table 1 has many information but some references about these studies has to be included.

Response

The contents of Table 1 are explained again in detail in Chapter 2. The various components of an electrolysis stack and the criteria of the morphological box from Table 1 are discussed in each of the sub-chapters in Chapter 2. The references can be found in the text. Including the references in Table 1 would make it more confusing. We have therefore decided to keep the table lean and readable and to list the references in the text.

 

4. Some references in the recycling process of membrane about the environmental problems of fluorinated compounds in water should be included

Response

Thank you for the important note on the environmental aspects of perflourinated hydrocarbons. We added the following sentences to the document.

“Perflourinated hydrocarbons are stable over the long term and accumulate in the environment [43]. They have already been detected in drinking water and are suspected of causing damage to health [43].”

 

5. Line 244 define GORE

Response

GORE refers to W. L. Gore & Associates, Inc. We have edited the text as follows:

“W. L. Gore & Associates, Inc. also offers an ePTFE-reinforced PEM with a thickness of 80 µm [15],…”

 

6. Line 245. We need more precise values, what is the operating range? How much is reduced the thickness?

Response

Thank you very much for this comment. It is of course always desirable to be able to take all interesting information directly from the article and not have to look in the referenced literature first. In this case, we have referred directly to the GORE data sheet available online (https://www.gore.com/system/files/2024-02/GORE-PEM-Water-Electrolysis-Datasheet-EN.pdf), from which you can obtain more detailed specifications. As we have also only specified the thickness of the other membrane materials mentioned in the text (see previous sentences), we have also refrained from mentioning any additional parameters here, but have instead referred to the data sheet.

 

7. Line 264 define which are the toxic gases and the average concentration.

Response

Line 264 ff: “Since PFSA in CCM releases toxic fluorine gases during metallurgical processing [40,43], it is important for environmental reasons to separate as much membrane material as possible from the electrodes before these are submitted to a metallurgical process step.”

The referenced literature is not very specific on this topic. It can be found that the pyrometallurgical processing of PFSA membranes causes the generation of HF or fluorine gases in general. However, there is no published information on the concentrations. The concentrations will depend heavily on how the process is designed (amount of fluorine-rich material, volume of the process chamber, volume flow, etc.).

The original literature contains information on the pyrometallurgical processing of the flour-containing membranes, which describes the formation of HF, whereby CaO used in triple excess was not sufficient to bind the released HF. Pyrometallurgical extraction of the valuable materials would cause considerable problems in process control, corrosion and costs in the presence of flour-containing membranes (see Wegner, et al. 2012). This reference now also is included in the article.

Wegner, R.; Fokkens, E.; Holdt, H.; Bukowsky, H.; Trautmann, M.; Nettesheim, S.; Jakubith, S.; Scholz, P.; Mollenhauer, T.; Theuring, S.; et al. React—Rückgewinnung und Wiedereinsatz von Edelmetallen aus Brennstoffzellen. In Recycling und Rohstoffe; Thomé-Kozmiensky, K.J., Goldmann, D., Eds.; Vivis TK-Verlag: Nietwerder, Germany, 2012; Volume 5, pp. 429–441.

 

8. Some graphs or schemes to clarify and make more easy to be read would be necessary in section 3 about the steps.

Response

All subsections related to the recycling steps in Section 3 are supported by figures and tables. The relevant information is presented there. We therefore believe that no additional figures and tables should be included there.

 

9. Which are the key recycling process and which process has been more studied, which recoveries were obtained in each step?

Response

From a practical point of view, these are of course very relevant questions and it is right to ask them here. There is no significant sub-process that needs to be emphasized here, but all sub-processes only work together as a recycling process (process chain). If you want to enrich using physical or physicochemical processes, a very good mechanical liberation must be available in advance. It is also important for recycling to dismantle into relevant sub-units before the actual mechanical or metallurgical processing.

It should be emphasized once again that the investigations were carried out on very small quantities or model materials and must therefore be understood as orientation values or proof of feasibility. The recoveries of the individual tests are given in the corresponding sections. It is not possible to transfer these values to a technical scale without further ado. Rather, optimization tests must be carried out if relevant quantities become available in recycling.

 

10. Line 560 a reference chart about lifetime and main component prices would clarify the whole discussion.

Response

This comment is also entirely justified as tables typically simplify the presentation of data and correlations. However, the data in the cited source on cost trends in the production of PEMWE and the price trends are relatively complex and are themselves based on model assumptions. From our point of view, the interested reader should refer to the literature cited, which is freely available on the Internet, rather than inserting a highly simplified statement out of context here.

 

11. Conclusion should be summarized and specific details about the useful data supplied by this manuscript has to be highlighted.

Response

The Conclusion section was restructured in order to better highlight the findings.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

It can be accepted now.

Reviewer 3 Report

Comments and Suggestions for Authors

Authors have carefully considered most of my comments and they have improved the quality of the manuscript. 

Thus, i can recommend the publication of this preliminary study about recycling PEMWE raw materials in this journal