Atoms vs. Ions: Intermediates in Reversible Electrochemical Hydrogen Evolution Reaction
Round 1
Reviewer 1 Report
In this short paper, an interesting revisited view of the fundamental reactions of hydrogen, as ion or bimolecular gas, in electrochemical systems is proposed. Arguments are discussed from fundamental chemistry, thermodynamics, and electrochemistry. However, many points are not clear by the terms used, claims and implications must be better communicated, concepts should be clarified and simplified to avoid any confusion. Specific comments are reported below.
- Lines are not numbered, so it is a little bit difficult to timely address revisions.
- The use of English is fine, but some typos can be found in the text (see last two lines of abstract). Carefully check the text for hidden mistakes.
- Introduction, first line: “…the main source of energy”, it is too short for clarity and could be misunderstood. Hydrogen, H2, is not a primary source, and unfortunately is not yet the most used fuel. Authors should specify more.
- Page 2, row 6: whether “adatoms” is a specific term, could be better to use “adsorbed atoms”.
Page 2, row 21: for the sake of accuracy, because the discussion is about this, the exact values and units that one must use to obtain 0.059 V could be indicated.
Page 2, row 45: “…for the formation of 2 moles of H atoms”. It can be debated that “H atoms”, alone, do not exist in nature, so the reasoning is again controversial. Authors should comment and clarify.
Page 3, row 20: a relatively low activation energy means that a reaction occurs easily. It is not clear how this is related to the sluggish kinetics in alkaline media.
Page 3, eqs. 5 and 6: in the text it is explained that reaction 5 is the “reversible rate determining step”, but also that “processes (5) and (6) are fast […] proceed simultaneously”. This is confusing and it is not clear how they can occur simultaneously, i.e., in parallel, at the same time, if one is the rds and one not.
Page 4, at some point, the intrinsic exchange current, i0, is abruptly mentioned without a definition or a formula (given too late in Figure 2), to explain reversible/irreversible mechanisms. Authors must revise and expose better this concept.
- Indeed, subsection 2.2 is critical, but difficult to follow because the discussion is not linear, organized, schematic. The two figures are badly positioned and mentioned because they come at the end of the subsection, and Fig. 2 is mentioned for the first time before Fig. 1. The authors are warmly recommended to reorganize this subsection to be super clear, using simple sentences structure and to illustrate and compare more the intended mechanism.
- Subsection 2.3 is clear and could be used as a reference, however, what is missing in the whole paper is a discussion about non-ideality and the role of surface material. It seems that the problem is about the intermediate scientists refer to when trying to explain the mechanism by just writing reaction steps and formulas, but the actual and real consequences claimed by the authors are not clear.
- The conclusions are clear, but in the manuscript, there is not enough evidence to support them, the discussion proposed is too brief for clarity and sometimes too qualitative. Until the conclusions section, 27 references are mentioned, but in the reference list 55 items appear, and on page 7, Appendix 3 is mentioned (Appendix 1, 2 missing, and however letters should be used to list them) and not found later. No other files have been transmitted to the reviewer.
Author Response
In this short paper, an interesting revisited view of the fundamental reactions of hydrogen, as ion or bimolecular gas, in electrochemical systems is proposed. Arguments are discussed from fundamental chemistry, thermodynamics, and electrochemistry. However, many points are not clear by the terms used, claims and implications must be better communicated, concepts should be clarified and simplified to avoid any confusion. Specific comments are reported below.
Comment: Lines are not numbered, so it is a little bit difficult to timely address revisions.
Answer: The terms are introduced on the first use. Discussion of critical points are expanded, formulas are explicitly derived with detailed steps, typos are removed.
Comment: The use of English is fine, but some typos can be found in the text (see last two lines of abstract). Carefully check the text for hidden mistakes.
Answer: Thank you. English proofreading was made.
Comment: Introduction, first line: "...the main source of energy", it is too short for clarity and could be misunderstood. Hydrogen, H2, is not a primary source, and unfortunately is not yet the most used fuel. Authors should specify more.
Answer: The sentence was rephrased to “Hydrogen, the most abundant element in the universe, is expected to become carbon neutral fuel of the future”.
Comment: Page 2, row 6: whether "adatoms" is a specific term, could be better to use "adsorbed atoms".
Answer: the term "adatoms" was replaced with "adsorbed H atoms".
Comment: Page 2, row 21: for the sake of accuracy, because the discussion is about this, the exact values and units that one must use to obtain 0.059 V could be indicated.
Answer: an explanation has been added: “..., where R is gas the constant, 8.135 J K−1 mol−1,T is the absolute temperature, taken to be equal to 298 K, F is the Faraday’s constant, 96845 C mol−1, and 2.3 is a conversion factor from the natural log to log10”.
Comment: Page 2, row 45: "...for the formation of 2 moles of H atoms". It can be debated that "H atoms", alone, do not exist in nature, so the reasoning is again controversial. Authors should comment and clarify.
Answer: The discussion indicated by reviewer is about the expenditure of energy formally required to produce H2 from H+ ions. This is purely theoretical/thermodynamical reasoning not necessarily related with actual mechanism of the process. Following discussion has been added at the end of p.3 of the revised version of the manuscript:
“Under standard conditions the actual onset of hydrogen evolution on the surface of platinum group metals is observed at E=0 V (SHE). From the thermodynamic point of view, the transfer of the first electron at E=0 V (SHE) to form H atom should require adequate energetic compensation. This is logical, since H atoms alone do not exist at normal conditions (room pressure and temperature) due to they highly reactive nature, which is based on an unpaired electron (can be considered a radical) with a short lifetime due to a strong reduction potential. In the context of current study, a hypothetical existence of H atoms could be contemplated if the huge 2.1 eV energy deficit is accounted for (see Fig. 1). However, even then, the surface-trapped H-atoms should be immobile due to such chemisorption preventing H2 formation.”
Comment: Page 3, row 20: a relatively low activation energy means that a reaction occurs easily. It is not clear how this is related to the sluggish kinetics in alkaline media.
Answer: An explanation has been added: “According to literature [4], the activation energy of reaction (4) is about 75 kJ mol−1,which is not high, however the necessity of step (4) to produce hydronium ion in solutions with pH > 7 can explain why kinetics of hydrogen evolution reaction in alkaline medium is more sluggish [4].”
Comment: Page 3, eqs. 5 and 6: in the text it is explained that reaction 5 is the "reversible rate determining step", but also that "processes (5) and (6) are fast [...] proceed simultaneously". This is confusing and it is not clear how they can occur simultaneously, i.e., in parallel, at the same time, if one is the rds and one not.
Answer: The confusion was removed. In fact, under conditions of reversibility, both processes (5) and (6) should proceed simultaneously, whereas reaction (5) becomes the rate limiting step upon transition to irreversible HER, i.e. at i > 10 i0.
An explanation has been added at the end of section 2.3:
“The E limits, where hydrogen evolution and oxidation reactions proceed reversibly approximately are ±2.3RTnF, whereas i should not exceed 10×i0 (Fig. 2(a-b)). When cathodic current significantly exceeds the magnitude of exchange current of reaction (7), i.e. when i >> i0, transition to irreversible H2 evolution occurs. In such case the transfer of first electron becomes hindered (eq. (5)) and turns to be the rate determining step, whereas the transfer of second electron proceeds instantaneously. The slope dE/dlgi increases up to 2.3RT/αF=120mV, given that charge transfer coefficient α = 0.5, and is referred to as theTafel slope.”
Comment: Page 4, at some point, the intrinsic exchange current, i0, is abruptly mentioned without a definition or a formula (given too late in Figure 2), to explain reversible/irreversible mechanisms. Authors must revise and expose this concept better.
Answer: Agreed. Definition of the exchange current is explicitly given on the first use. The i0 at the equilibrium potential is equal to the cathodic and anodic current density, which are equal and makes the cumulative electrode current equal to zero. Link of discussion with FIg 2 is added. The explicit expression is also given in Sec 2.3.
Comment: Indeed, subsection 2.2 is critical, but difficult to follow because the discussion is not linear, organized, schematic. The two figures are badly positioned and mentioned because they come at the end of the subsection, and Fig. 2 is mentioned for the first time before Fig. 1. The authors are warmly recommended to reorganize this subsection to be super clear, using simple sentence structure and to illustrate and compare more the intended mechanism.
Answer: Thank you for the remark. Fig. 2 is updated to have better presentation of the panels. Text is re edited.
Comment: Subsection 2.3 is clear and could be used as a reference, however, what is missing in the whole paper is a discussion about non-ideality and the role of surface material. It seems that the problem is about the intermediate scientists refer to when trying to explain the mechanism by just writing reaction steps and formulas, but the actual and real consequences claimed by the authors are not clear.
Answer: Thank you for the comment. Indeed non-ideality is surely important. However, this is a theoretic/concept paper, where we discuss Pt on which H2ER is the most close to the ideal conditions (also discussion applies to Pd). The conclusions change the understanding of hydrogen evolution mechanism and effects of non-ideality can be next target for practical applications. The results of experimental studies corroborating our theoretical predictions are referenced in the manuscript.
Comment: The conclusions are clear, but in the manuscript, there is not enough evidence to support them, the discussion proposed is too brief for clarity and sometimes too qualitative. Until the conclusions section, 27 references are mentioned, but in the reference list 55 items appear, and on page 7, Appendix 3 is mentioned (Appendix 1, 2 missing, and however letters should be used to list them) and not found later. No other files have been transmitted to the reviewer.)
Answer: Discussion and clarifications have been added. References corrected.
Reviewer 2 Report
Juodkazyte et al. have written a paper on the reversible hydrogen evolution reaction and its mechanism. The major claim of the authors is that the reaction proceeds via H2+ as intermediate species rather than H_ads as reaction intermediate.
While opposing views may stimulate the scientific discipline of electrocatalysis, and I am open to think out of box, I cannot recommend this article for publication, neither in this journal nor in any other journals. There are serious flaws in the presentation and discussion of the authors.
i) The authors start the discussion by referring to the equilibrium potentials of the intermediate species H_ads or H2+. Their main claim is that the adsorption or chemisorption of these species occur in equilibrium, and thus Delta G = 0 should be fulfilled. This is more likely the case for H2+ rather than for H_ads, and thus the authors put forth the hydrogen electrocatalysis proceeds via H2+ rather than H_ads,
What the authors have not understood is that an electrochemical system is an open system, and thus the equilibrium condition cannot be met and is never met. as The hydrogen electrocatalysis is a non-equilibrium process that is driven by Delta G_overall < 0 so that an overpotential \eta < 0 or \eta > 0 for the HER or HOR, respectively manifests. Therefore, the authors' discussion that each step should meet Delta G_step = 0 is incorrect and wrong.
In the hydrogen electrocatalysis or any other electrocatalytic reaction, rather the presence of quasi equilibrium is observed. Quasi equilibrium denotes that reaction intermediates before the rate-determining step are equilibrated, but the quasi-equilibrium condition does not necessitate Delta G_step = 0. Therefore, the arguments of the authors in the introduction are not valid. I recommend studying the book of Bockris and Reddy.
ii) The authors put forth equations (5) and (6) as reaction mechanism and state equation (5) is the RDS. There is no reasoning why (5) is the RDS. Actually, it is a well-known fact that the RDS for electrocatalytic reactions is a function of the applied overpotential, and the RDS can change with increasing driving force.
iii) "Reaction (5) is reversible rate determining step." --> RDS is not reversible!
iv) HER on Pt has two Tafel slope regimes, one small Tafel slope about 30-40 mV/dec and one large Tafel slope of about 120 mV/dec. This is not considered by the authors. Some recent study in this field on Ooka, ACS Catal 2021, could be of interest to the authors.
v) The Tafel slopes of 60 mV/dec and 30 mV/dec indicate that a chemical step is the RDS rather than an electrochemical step. This is not encountered with reaction equations (5) and (6). The papers of Fletcher, J Solid State Electrochem 2010 or 2011 could be of interest to the authors.
vi) The transition from equation (14) to (15) is clearly wrong!
vii) Equation (19): the authors cannot predict a Tafel slope based on thermodynamic considerations. This is not possible. The authors should study the origin of the Tafel slope and its meaning. The work of Parsons, Farad Trans 1951 provides an introduction, and the book of Bockris and Reddy provides further clarification on this aspect: the Tafel slope is determined by the transition from the intermediate with smallest to the transition state with highest free energy.
If the authors are convinced that the hydrogen electrocatalysis consists of a H2+ intermediate, then they should provide some theoretical proof, such as encountered with density functional theory calculations for a Pt electrode. Here, the authors missed the study by Laasonen, ACS Catal 2021, which demonstrates that H2+ is a precursor for the HER, but the main intermediate is H_ads. This was demonstrated by DFT-based ab initio MD, thereby also including dynamic effects in the analysis.
The authors also overlooked the works from Feliu and coworkers, which have discussed the energetics of H_ads adsorption, JPC B 2004. It becomes clear that the energetics for the adsorption of H_ads is a function of H_ads coverage, which also is a counter argument to the authors' argumentation of using Delta G_step = 0 as benchmark.
Making a long story short: I do not object the presence of the H2+ adsorbate as an intermediate in the hydrogen electrocatalysis, either for the HER or HOR, but at the current moment there is no evidence that H2+ is the main intermediate because a) the main argument of the authors based on Delta G = 0 is erroneous; b) the mathematical derivation is wrong; c) ab initio MD-based DFT studies show that H_ads is the main intermediate; d) experimental works from the literature also point toward H_ads as main intermediate.
Author Response
Thank you for the remarks, which are addressed below.
While opposing views may stimulate the scientific discipline of electrocatalysis, and I am open to think out of box, I cannot recommend this article for publication, neither in this journal nor in any other journals. There are serious flaws in the presentation and discussion of the authors.
Comment: i) The authors start the discussion by referring to the equilibrium potentials of the intermediate species H_ads or H2+. Their main claim is that the adsorption or chemisorption of these species occur in equilibrium, and thus Delta G = 0 should be fulfilled. This is more likely the case for H2+ rather than for H_ads, and thus the authors put forth the hydrogen electrocatalysis proceeds via H2+ rather than H_ads, What the authors have not understood is that an electrochemical system is an open system, and thus the equilibrium condition cannot be met and is never met. as The hydrogen electrocatalysis is a non-equilibrium process that is driven by Delta G_overall < 0 so that an overpotential \eta < 0 or \eta > 0 for the HER or HOR, respectively manifests. Therefore, the authors' discussion that each step should meet Delta G_step = 0 is incorrect and wrong.
In the hydrogen electrocatalysis or any other electrocatalytic reaction, rather the presence of quasi equilibrium is observed. Quasi equilibrium denotes that reaction intermediates before the rate-determining step are equilibrated, but the quasi-equilibrium condition does not necessitate Delta G_step = 0. Therefore, the arguments of the authors in the introduction are not valid. I recommend studying the book of Bockris and Reddy.
Answer: We agree that the majority of the electrochemical processes dealt with in reality are irreversible or quasi reversible, however the reversibility of HER and HOR occurring on platinum electrode surface under standard conditions and the existence of standard hydrogen electrode cannot be negated. Our analysis is defined only for this reversible range of potentials, denoted as “Nernst region” in Fig.1 of the revised manuscript.
Comment: ii) The authors put forth equations (5) and (6) as reaction mechanism and state equation (5) is the RDS. There is no reasoning why (5) is the RDS. Actually, it is a well-known fact that the RDS for electrocatalytic reactions is a function of the applied overpotential, and the RDS can change with increasing driving force
Comment: iii) "Reaction (5) is reversible rate determining step." --> RDS is not reversible!
Answer (to both remarks): The confusion was removed. In fact, under conditions of reversibility, both processes (5) and (6) should proceed simultaneously, whereas reaction (5) becomes the rate limiting step upon transition to irreversible HER, i.e. at i > 10i0.
An explanation has been added at the end of section 2.3:
“The E limits, where hydrogen evolution and oxidation reactions proceed reversibly approximately are ±2.3RTnF, whereas i should not exceed 10×i0 (Fig. 2(a-b)). When cathodic current significantly exceeds the magnitude of exchange current of reaction (7), i.e. when i >> i0, transition to irreversible H2 evolution occurs. In such case the transfer of first electron becomes hindered (eq. (5)) and turns to be the rate determining step, whereas the transfer of second electron proceeds instantaneously. The slope dE/dlgi increases up to 2.3RT/αF=120mV, given that charge transfer coefficient α = 0.5, and is referred to as the Tafel slope.”
Comment: iv) HER on Pt has two Tafel slope regimes, one small Tafel slope about 30-40 mV/dec and one large Tafel slope of about 120 mV/dec. This is not considered by the authors. Some recent study in this field on Ooka, ACS Catal 2021, could be of interest to the authors.
Answer: Tafel slope refers to conditions where reaction proceeds irreversibly and in the case of irreversible HER on Pt electrode Tafel slope is about 120 mV, as indicated in the text. Our study is devoted to analysis of situations where hydrogen processes on the platinum surface proceed reversibly in the vicinity of E0.
An explanation has been added at the end of p.7 of the revised manuscript:
“Thus in the case of reversible hydrogen evolution process, the slope dE/dlg(c)=2.3RT/F is equal to the slope dE/dlg(i), therefore it is referred to as the Nernst slope in this study.”
Thank you for referring us to the study by Ooka et al ACS Catal 2021, which is of great relevance, in fact, as it presents the experimental proof that hydrogen binding energy of platinum is +0.094 eV (about 9 kJ/mol), which is by far less than 200 - 260 kJ/mol required to justify the existence of Had on Pt at E close to 0 V (SHE). The energies of interaction between Pt and (H2+)ads ranging between 10 - 30 kJ/mol have been reported in our previous study (Appl. Surf. Sci. 2014, 290, 13–17.)
Comment: v) The Tafel slopes of 60 mV/dec and 30 mV/dec indicate that a chemical step is the RDS rather than an electrochemical step. This is not encountered with reaction equations (5) and (6). The papers of Fletcher, J Solid State Electrochem 2010 or 2011 could be of interest to the authors.
Answer: Please refer to previous answers regarding the Tafel and Nernst slopes.
Comment: vi) The transition from equation (14) to (15) is clearly wrong!
Answer: The transition from eq. (14) to (15) is corrected. The revised version has all detailed steps of derivation explicitly shown. Now, it is easier to follow.
Comment: vii) Equation (19): the authors cannot predict a Tafel slope based on thermodynamic considerations. This is not possible. The authors should study the origin of the Tafel slope and its meaning. The work of Parsons, Farad Trans 1951 provides an introduction, and the book of Bockris and Reddy provides further clarification on this aspect: the Tafel slope is determined by the transition from the intermediate with smallest to the transition state with highest free energy.
If the authors are convinced that the hydrogen electrocatalysis consists of a H2+ intermediate, then they should provide some theoretical proof, such as encountered with density functional theory calculations for a Pt electrode. Here, the authors missed the study by Laasonen, ACS Catal 2021, which demonstrates that H2+ is a precursor for the HER, but the main intermediate is H_ads. This was demonstrated by DFT-based ab initio MD, thereby also including dynamic effects in the analysis.
The authors also overlooked the works from Feliu and coworkers, which have discussed the energetics of H_ads adsorption, JPC B 2004. It becomes clear that the energetics for the adsorption of H_ads is a function of H_ads coverage, which also is a counter argument to the authors' argumentation of using Delta G_step = 0 as benchmark.
Answer: With due respect, we discuss the Nernst slopes which are defined from the outset of our argumentation. Our proposed explanation is based on the quantitative match of theory and experiment (Fig.2c) and is not challenged by the arguments presented about Tafel slope. Please refer to previous answers regarding the Tafel and Nernst slopes.
Thermodynamics predicts the dependence between electrode potential and concentration of electroactive species, whereas kinetics relates current with potential. What is wrong in relating current with concentration for the case when Nernst equation is valid? This is a logical step.
Comment: Making a long story short: I do not object the presence of the H2+ adsorbate as an intermediate in the hydrogen electrocatalysis, either for the HER or HOR, but at the current moment there is no evidence that H2+ is the main intermediate because a) the main argument of the authors based on Delta G = 0 is erroneous; b) the mathematical derivation is wrong; c) ab initio MD-based DFT studies show that H_ads is the main intermediate; d) experimental works from the literature also point toward H_ads as main intermediate.
Answer: As we showed by thermodynamics arguments and their-experimental quantitative match, the proposed H2+ intermediate is right candidate to start fundamental ab initio DFT and MD calculations, which so far ignore it. Summarised negative assessment is answered (a summary) one more time:
- we cannot agree with this statement, otherwise the existence of RHE reference electrode would not be possible;
- typo in transition from equation 14 to 15 is corrected. It does not change conclusions and detailed expression is shown in (18) where cumulative constats are collected;
- we are also interested to assess MD simulations at conditions of H_ads (at -2.1 V) and H2+ (at 0 V)
- with due respect, the mass and charge changes at around 0 V measured experimentally are accounted quantitatively by the proposed mechanism. Such a match usually makes clear evidence that theory relates to reality. This point has not been questioned in the review remarks.
Reviewer 3 Report
In this work, the authors reported - Atoms vs. ions: intermediates in reversible electrochemical hydrogen evolution reaction. The detailed studies mechanism of reversible hydrogen evolution reaction based on thermodynamics of hydrogen processes considering atomic and ionic species as intermediates has been being extensively investigated. The theoretical studies H2 evolution and oxidation on acidic and basic conditions of molecule and ions have been were extensively investigated. Overall, this work is interesting, but there are some major issues that should be addressed before publishing on “Catalysts”.
This manuscript could be improved by addressing the following issues.
- Authors should recheck the end stanza of the abstract, its need to rewrite.
- There are many improper abbreviations that have been used. Authors should provide a proper way of presentation in order to understand the readers easily.
- To understand the in-depth of this study is significant, but also the authors should add more clarity on Tafel slope discussions.
- Authors must include more experimental comparisons for an extended understanding of this concept.
- Some of following electrochemical hydrogen evolution reaction related recent works are need to cite: 10.1016/j.apcatb.2021.120405; 10.1016/j.apsusc.2019.144642;
Author Response
In this work, the authors reported - Atoms vs. ions: intermediates in reversible electrochemical hydrogen evolution reaction. The detailed studies mechanism of reversible hydrogen evolution reaction based on thermodynamics of hydrogen processes considering atomic and ionic species as intermediates has been being extensively investigated. The theoretical studies H2 evolution and oxidation on acidic and basic conditions of molecule and ions have been were extensively investigated. Overall, this work is interesting, but there are some major issues that should be addressed before publishing on "Catalysts". This manuscript could be improved by addressing the following issues.
Thank you for the positive evaluation and remarks which addressed below.
Comment: Authors should recheck the end stanza of the abstract, its need to rewrite.
Answer: Text was reworked to better present discussion and arguments. All comments and remarks are addressed and proper changes were made in the text.
Comment: There are many improper abbreviations that have been used. Authors should provide a proper way of presentation in order to understand the readers easily.
Answer: Thank you. Corrected and improved.
Comment: To understand the in-depth of this study is significant, but also the authors should add more clarity on Tafel slope discussions.
Answer: We agree. Discussion is added in the text and more detailed answers can be found in specific discussion points with referees 1 and 2. This study was solely focused on the reversible Nernst slope region around zero potential. There is similarity between the Nernst and Tafel slopes since both define increase of current for increase of over-potential. However, Tafel slope is for the non-reversible reaction and is not directly related to the discussion. However, in section 2.3 there is an explicit derivation of the used formulae, which can be discussed in generic terms of slope of the dependence i/i0 = exp(F(E-E0)/RT), see eqn 21. We added discussion about Tafel slopes (see also answers to other reviewer’s queries).
Comment: Authors must include more experimental comparisons for an extended understanding of this concept.
Answer: This is the concept type of manuscript. We improved discussion of experimental as well as theoretical papers. As usual, a good theory can become very practical. We define discussion on specific E = 0 V region where the most efficient hydrogen evolution takes place.
Comment: Some of following electrochemical hydrogen evolution reaction related recent works are need to cite: 10.1016/j.apcatb.2021.120405; 10.1016/j.apsusc.2019.144642;
Answer: Thank you for the very recent highlights. Reference list has been updated and appropriately discussed.
Reviewer 4 Report
There is no doubt that one of the reactions with the greatest technological impact, such as the hydrogen electrode reaction, still arouses interest in scrutinizing the mechanistic aspect from a thermodynamic point of view supported, as claimed by the authors of this manuscript, by DFT calculations, and some experimental results. Although no single direct experimental evidence is given, the proposed concepts, according to the reviewer, contain interesting arguments. Moreover, these latter use simple concepts when considering, e.g., a charge transfer coefficient (alpha) equal to 0.5. It should be noted that this parameter is dependent on whether enthalpic or entropic phenomena are considered. This implies a variation of alpha between 0 and one, or even two. Interestingly, with platinum alpha fluctuates around 0.5, despite the facility of platinum surface atoms to complex the hydrogen forming Pt-H bond (Hupd). In this sense, platinum should not be considered as a good example to present such arguments (the adsorbed hydrogen molecular ion as intermediate), but consider metals (e.g., Hg, Pb, etc.) that present an anti-catalytic effect for this reaction. The reviewer considers that the content of this work with a critical point of view of the hydrogen reaction mechanism, could be considered to arouse a debate within the scientific community concerned with the processes of hydrogen (evolution/oxidation) process. As a side note, the first work, on hydrogen evolution was reported by J. Tafel (Zeitschrift für Physikalische Chemie 641 (1905)). His intuitive approach “log|j|(E)” was supported by the Butler-Volmer kinetic analysis in 1930.
Typos: “due to they highly
Author Response
There is no doubt that one of the reactions with the greatest technological impact, such as the hydrogen electrode reaction, still arouses interest in scrutinizing the mechanistic aspect from a thermodynamic point of view supported, as claimed by the authors of this manuscript, by DFT calculations, and some experimental results. Although no single direct experimental evidence is given, the proposed concepts, according to the reviewer, contain interesting arguments. Moreover, these latter use simple concepts when considering, e.g., a charge transfer coefficient (alpha) equal to 0.5. It should be noted that this parameter is dependent on whether enthalpic or entropic phenomena are considered. This implies a variation of alpha between 0 and one, or even two. Interestingly, with platinum alpha fluctuates around 0.5, despite the facility of platinum surface atoms to complex the hydrogen forming Pt-H bond (Hupd). In this sense, platinum should not be considered as a good example to present such arguments (the adsorbed hydrogen molecular ion as intermediate), but consider metals (e.g., Hg, Pb, etc.) that present an anti-catalytic effect for this reaction. The reviewer considers that the content of this work with a critical point of view of the hydrogen reaction mechanism, could be considered to arouse a debate within the scientific community concerned with the processes of hydrogen (evolution/oxidation) process. As a side note, the first work, on hydrogen evolution was reported by J. Tafel (Zeitschrift für Physikalische Chemie 641 (1905)). His intuitive approach “log|j|(E)” was supported by the Butler-Volmer kinetic analysis in 1930.
Answer. Thank you for the critical yet positive evaluation of our study. It is indeed based on thermodynamics analysis. The presented model is fully consistent with experimental data which were fitted by our model (Fig.2c) in the potential region leading to the H2ER at 0 V (SHE). Here, we present an analysis pertaining only to this particular reversible region of H2ER/H2OR. We newly added section 2.5 and Fig. 3 to clarify this issue and to show what differences can be expected outside the +/- 60 mV region. This particular region where processes can be run most close to a thermodynamic equilibrium are explicitly discussed. Reactions to explain performance of SHE/RHE which requires symmetry in red/ox sides of equilibrium potential and cannot be accounted for by the model considering H-atoms are newly presented and discussed.
We chose Pt for analysis since SHE and RHE electrodes on Pt are reversible, hence, the processes closest to the thermodynamic equilibrium can be realised. Feasibility of SHE (RHE) implies that surface bound H2+ can be formed on the electrode. Hydrogen evolution is proceeding under strong overpotential conditions (out of the reversibility zone) on other electrodes such as Hg, Pb. This is why we focused solely on Pt and a reversible +/-60mV process window where Nernst analysis is the most suitable. The newly added Sec.2.5 is dedicated to address this remark.
Tafel's and Butler-Volmer kinetic analysis are discussed. Thank you for highlighting the importance of such discussion and original papers. Such discussion was omitted in the earlier version since we focused on the particular +/- 60 mV range where conditions are most close to thermodynamical equilibrium and reversibility can be realised.
Remark. Typos: due to they highly
Answer. Thank you. Corrected.
Round 2
Reviewer 1 Report
The Authors have partially amended the issues and have addressed some suggestions. However, this reviewer agrees with the other reviewer because also the revised version failed to convince and is still very debatable. The Authors' point of view is not yet mature, convincingly supported by quantitative data or experimental evidence. Moreover, the development of the rationale is based on formal thermodynamic assumptions that cannot be applied to electrocatalysis or are not correctly applied to the specific case. Hence, even if conceptual, it is not recommended, for the Authors and the journal, to publish this paper at this stage. The Authors are invited to elaborate more on the idea, provide more convincing evidence, correcting the approach by using thermodynamics but also hydrodynamics to make the theory solid.
Author Response
these are previously answered queries.
the last review-3 remarks are answered in the next section below.
Reviewer 2 Report
I had a look at the revised version of the manuscript, and I still cannot recommend it for publication.
The authors put forth the view that the standard hydrogen evolution mechanism via H_ads is contradictive because "the standard potential of H atom formation is EH0 +/H = −2.106 V [15]. Consequently, the energy of Had interaction with surface, required to compensate depolarization of H+ discharge from−2.106 V to 0 V, which is the case for Pt electrode, should be 203.2 kJ mol−1."
This is wrong. The authors should write down the reference states for the H+/H redox couple. The H state is a H. radical in solution (H. aq), and thus not encountered with the H_ads state in the hydrogen evolution reaction over Pt. This information is easily available, e.g., from the IUPAC Technical Report, PAC-REP-14-05-02. Thus, the entire argumentation why the H2+ intermediate rather than the H_ads intermediate breaks down, given that the H_ads state cannot be related to the H. aq state in the H+/H redox couple. Even worse, the entire text added in blue color is flawed because the authors do not take the fundamentals of electrochemistry into account by using an erroneous reference for the discussion. Especially, Figure 1 is flawed and is a WRONG representation of the hydrogen electrocatalysis that is a severe danger to mislead scientists because of non-accurate work.
This paper cannot be published, neither in this journal nor in any other scientifically respected journal.
Author Response
These are Review 2 remarks which were answered in the previous revision and answers.
We analysed suggested literature and included references in discussion.
However, we still argue that our proposed model which provides the quantitative match with experiment should be presented to readership of Catalysis and the special issue on hydrogen evolution.
Reviewer 3 Report
This manuscript can be acceptable for publication in Catalysts. However, I found typo errors in the Reference part of the manuscript. Please check them carefully again one by one.
Author Response
thank you. proofreading was made.
Round 3
Reviewer 1 Report
The reviewer is glad to acknowledge the deep effort from the Authors to further revise and support their paper. However, the reviewer regrets to inform the Authors that at the moment it is not possible to give a positive final acceptance of the manuscript. The innovative and forefront character of the topic is undoubtful, but it is her/his opinion that adding/editing text or using a risky concept like "hydrodynamic is not considered/given for granted" in such complex and heterogeneous systems, is of little use to demonstrate disrupting mechanisms. At this point, a rejection should be intended as an opportunity for the paper to be evaluated again from the beginning by other experts with a different point of view and possibly a wider understanding of the topic, looking for a positive acceptance.
Author Response
The reviewer is glad to acknowledge the deep effort from the Authors to further revise and support their paper. However, the reviewer regrets to inform the Authors that at the moment it is not possible to give a positive final acceptance of the manuscript. The innovative and forefront character of the topic is undoubtful, but it is her/his opinion that adding/editing text or using a risky concept like "hydrodynamic is not considered/given for granted" in such complex and heterogeneous systems, is of little use to demonstrate disrupting mechanisms. At this point, a rejection should be intended as an opportunity for the paper to be evaluated again from the beginning by other experts with a different point of view and possibly a wider understanding of the topic, looking for a positive acceptance.
Answer. Thank you for highlighting the issue of importance of hydrodynamics. We agree, we should add this discussion. Now it is presented in the newly added section 2.5 on reversibility. Indeed the difference of diffusion rates of H3O+ and OH−is consistent with the 30 mV/dec slope.
The newly added Fig. 3 in the new section discusses kinetics and non-equilibrium conditions of H2ER/H2OR. We show that our presented view is consistent with the existence of SHE electrode. This contributes to required wider discussion of the presented model.
Round 4
Reviewer 1 Report
The reviewer is glad to see that the Authors have further modified the manuscript. The newly added section is useful to complete the discussion, even though it is not conclusive. If a new revision process, with different reviewers, seems not the selected option, after 4 revision steps, the last option is to let the audience decide.
