Enhancement of electrocatalytic oxygen evolution by chiral molecular functionalization of hybrid 2D electrodes

A sustainable future requires highly efficient energy conversion and storage processes, where electrocatalysis plays a crucial role. The activity of an electrocatalyst is governed by the binding energy towards the reaction intermediates, while the scaling relationships prevent the improvement of a catalytic system over its volcano-plot limits. To overcome these limitations, unconventional methods that are not fully determined by the surface binding energy can be helpful. Here, we use organic chiral molecules, i.e., hetero-helicenes such as thiadiazole-[7]helicene and bis(thiadiazole)-[8]helicene, to boost the oxygen evolution reaction (OER) by up to ca. 130 % (at the potential of 1.65 V vs. RHE) at state-of-the-art 2D Ni- and NiFe-based catalysts via a spin-polarization mechanism. Our results show that chiral molecule-functionalization is able to increase the OER activity of catalysts beyond the volcano limits. A guideline for optimizing the catalytic activity via chiral molecular functionalization of hybrid 2D electrodes is given.

1. It is not clear to me how accurately the OER enhancement can be measured. The OER current on a bare NiOx/Au electrode is reported five times in Figures 1a, 1b, 2, 3a, and 3b. At a potential of 1.65 V the currents measured on these nominally identical electrodes are: 0.29, 0.41, 2.8, 0.19, and 0.08 mA. Given the large variance in the reported values on the unmodified electrodes, how are the enhancements being reported meaningfully for the chirally modified electrodes? 2. Related to the above, the data for the OER using the octahelicene do not warrant reporting the enhancement (131.5%) with four digits of precision. What would the variance be for five measurements on the same electrode or more importantly on five nominally identical electrodes 3. Related to point two, what is the maximum possible OER enhancement that one could expect because of spin polarization? My naïve guess is that it should be no more than a factor of 2× or 100%. Perfect spin polarization should double the current of electrons with the preferred spin orientation. 4. Figure 4 illustrate the helicenes sandwiched between the NiOx particles and the Au. Is there any direct evidence for this configuration? Lesser points. 5. p. 3, par. 1, ln. 7. What is the meaning of 'correlate the binding energy of the variables on one adsorbate…' 6. Why is the current density reported in Figure 2, but the absolute current is used in Figures  1 and 3? 7. In Figure 6 the authors mentioned that there is a difference in the coverages of the heptahelicene and the octahelicene. The STM images should allow quantification of this statement.
Reviewer #2 (Remarks to the Author): This manuscript describes how coating of an electrodes by hetero-helicenes boosts the oxygen evolution reaction (OER) by more than 100% relative to a state-of-the-art catalysts. The enhancement is related to the spin-polarization of the transferred electrons by the chiral molecules. The results presented are very convincing and merit publication. However, the authors should relate to the following comments: This work explores the chirality-induced-spin-selectivity (CISS) effect on the oxygen evolution reaction (OER) using the nickel-based electrocatalysts. The study is interesting in such, especially since the role of spin-polarization effects have not been extensively investigated for catalytic reactions. The authors have chosen the nickel and the and the nickel-iron catalyst, which are one of the most interesting catalysts for OER. The manuscript is well written overall, although, a few details are missing, and I suggest the authors to run a few control experiments to prove the chirality effect beyond doubt. Therefore, I suggest major revisions before consideration of this manuscript. Please see my specific comments below. 1) ) In the abstract, it will be good if the authors are more specific and mention which catalysts are investigated in this this study (i.e., Ni and NiFe). I think the abstract is a bit too general right now.  Figure 1 or Figure 2, for example). Only that would prove the chirality effect beyond doubt. 3) When looking at the OER activity data in Figure 1-

7)
This relates to the previous question. Since you state that your NiOx catalyst is an exfoliated 2D material (i.e., a monolayer), the integrated redox peak area (i.e., the peak-charge, Q) should be directly proportional to the surface area (i.e., all Ni atoms should be electrolyte accessible). My question to you is, how do these two methods compare, i.e., if you normalize your data to either the ESCA obtained by EIS or to the moles of Ni atoms on your electrode obtained from integration of the Ni2+/Ni3+/4+ redox peak? My point is that I want to make sure that you do not distort the OER activity trends by your selected normalization method, especially since metal loadings were never double-checked by elemental analysis if I understood correctly. Just keep in mind that Fe impurites/dopants often impact/supress the Ni2+/Ni3+/4+ redox peak, so you may not be able to compare Ni and NiFe films straight off (http://dx.doi.org/10.1021/jacs.6b00332). (Ideally, I would strongly advise that the authors double check a few electrodes using elemental analysis, to see how this compares with your selected normalization approach.) Please provide a direct comparison of selected raw data (CVs), and the data after normalization to either the EIS/ESCA method or to the number of Ni atoms obtained from the integrated nickel redox-peak.
Reviewer #1 (Remarks to the Author): The authors have modified a NiOx/Au electrode used for the oxygen evolution reaction by adsorbing chiral molecules (functionalized heptahelicenes). As a consequence, they observed enhanced OER activity that they attribute to spin polarization of electrons transported from the Au surface, through the chiral organic monolayer to the NiOx. They demonstrate that the enhancement is not dependent on the handedness of the chiral modifier but that it does depend on the nature of the modifier. The heptahelicenes enhance OER current by ~80% while the use of a functionalized octahelicene yields an enhancement of 131.5%. This manuscript reports some important and interesting results. However, there are aspects of the work that need further thought before it should be considered for publication.

Response:
We thank the reviewer for this positive opinion about our work, and we revised the manuscript according to the suggestions. Response: The NiOx/Au electrodes consist of monolayer NiOx islands on Au substrates. Our goal is to show the effect of chiral molecular functionalization on the electrocatalytic activity. Therefore, we deposited a submonolayer coverage of 2D NiOx on Au. The effect from the evolution of electrochemically active surface area (ECSA) during the reaction should be avoided. The NiOx islands should be spread very well all over the Au surface, and overlapping of islands should be minimized. Therefore, the spin-coating method has been used. However, the amount of deposited NiOx is difficult to maintain at a constant value. We would like to remind the Reviewer that Figure 2 shows the current density normalized by the ECSA obtained by EIS measurement, not the absolute current. Hence, the value, 2.8, is much higher than the others. Moreover, we only compare the activity (current) change of the same electrode before and after chiral molecular functionalization to prevent any influence from the different loading of NiOx islands of different electrodes.
2. Related to the above, the data for the OER using the octahelicene do not warrant reporting the enhancement (131.5%) with four digits of precision. What would the variance be for five measurements on the same electrode or more importantly on five nominally identical electrodes.

Response:
We thank the reviewer for the comments. We add the statistical summary from 5 nominally identical electrodes in Supplementary Figure 5 to show the consistency of the enhancement, and we have changed the enhancement factors to have two digits of precision. 3. Related to point two, what is the maximum possible OER enhancement that one could expect because of spin polarization? My naïve guess is that it should be no more than a factor of 2× or 100%. Perfect spin polarization should double the current of electrons with the preferred spin orientation.

Response:
We appreciate the reviewer's interpretation that the maximum enhancement should be no more than 100 % in the case of perfect spin polarization. The helicene molecules polarize the electron spin. However, in our opinion, the OER activity of a catalyst is not simply determined by the electron spin direction. Spin polarization affects OER kinetics in multiple ways. It changes the binding energy of O species on the catalyst surface, and it is expected that the activation energy should also be affected. In this work, we report the values we obtain from our well-controlled experiments using state-of-the-art materials. A considerable amount of NiOx sites are not affected by the chiral molecules due to the catalyst-molecule configuration. The limited amount of active sites boosted by the chiral molecular functionalization resulted in more than 100% enhancement in the case of bis(thiadiazole)-[8]helicene. We can conclude that the enhancement factor at a single active site is even higher. We anticipate that higher enhancements can be reached with the ideal catalystmolecule configuration and optimized chiral molecules suggested in the manuscript, as shown in newly-added Fig. 3 c, where helicene molecules are in between the catalysts and the Au substrate. Moreover, although it is not in the scope of this work, the chiral molecular functionalization effect on the mass transfer processes can be envisaged to be further investigated.
4. Figure 4 illustrate the helicenes sandwiched between the NiOx particles and the Au. Is there any direct evidence for this configuration?
Response: The illustration in the original Figure 4 of the catalyst-molecule-substrate configuration was a conclusion from our measurements and literature, a guideline for the optimized configuration for practical applications. In the current version we have added the OER activity of a sandwiched configuration using (M)-bis(thiadiazole)-[8]helicene as Fig. 3 c. This is achieved by depositing first the molecule and later the catalyst.
A similar enhancement trend is obtained when depositing first the catalyst and then the molecules. The reason is that the molecules do not attach to the NiOx but to the Au surface. It is important to note that the molecular layer is not static. During the OER, the molecules diffuse underneath the NiOx islands. By adding the molecules to the sample after catalyst deposition and OER and then running OER again on the same samples, we can study the sole effect of chiral molecular functionalization. Moreover, this experiment procedure prevents the influence of different catalyst loadings/structures and different resistances of different samples on the OER activity.
Lesser points.
5. p. 3, par. 1, ln. 7. What is the meaning of 'correlate the binding energy of the variables on one adsorbate…' Response: This sentence, "the scaling relationships correlate the binding energy of the variables of one adsorbate species to a catalyst surface", is a short expression of the scaling relationships. In the case of the OER, oxygen adsorbate variables include HO*, HOO* and O*. The binding energies of the variables adsorbed to the catalyst are linearly dependent on each other. While changing the binding energy of one of the variables on the catalyst surface (e.g., by changing the catalyst material), the binding energy of another variable also changes linearly.
Action taken: This sentence has been replaced by a more clear one: Moreover, the binding energies of the variables of one adsorbate species to a catalyst surface follow a scaling relationship with each other. For instance, the adsorption free energies of OER reaction intermediates OH* and OOH* adhere to a simple relation, ΔGOOH* = ΔGOH* + 3.2 ± 0.2 eV, for a large number of catalysts.
Why is the current density reported in Figure 2, but the absolute current is used in Figures 1  and 3?

Response:
Our work aims to demonstrate the chiral molecular functionalization effect and its potential to boost the electrocatalytic OER beyond the volcano limits. Therefore, 2D structured electrodes, i.e., 2D NiOx islands spin-coated on Au(111) surfaces, are used. We attempted to minimize all other factors, for instance, changes in the ECSA and mass transfer issues (e.g., gas bubble formation). Therefore, we mainly report the changes in the overall current. Chiral molecular functionalization effect on the current density of 2D NiOx islands is shown in the new Supplementary Figure 1 as a further confirmation.
In Figure 2, we compare the changes in the activity of the 2D structured model electrodes and NiFe-based catalysts deposited on Ni foam (potentially used in practical applications). The former has 2D catalyst islands scattered on the Au surface, and the latter has a 3D structure with the ECSA much larger than the geometric surface area. Therefore, current density should be used to compare the chiral molecular functionalization and the Fe-doping effects quantitatively.
6. In Figure 6  This manuscript describes how coating of an electrodes by hetero-helicenes boosts the oxygen evolution reaction (OER) by more than 100% relative to a state-of-the-art catalysts. The enhancement is related to the spin-polarization of the transferred electrons by the chiral molecules. The results presented are very convincing and merit publication. However, the authors should relate to the following comments: Response: We highly appreciate the reviewer's positive opinion on our work, and we provided point-to-point responses to the comments below.
1. Stability of the helicene coated electrodes-How the electrodes behave as a function of time, pH etc.

Response:
We thank the reviewer for mentioning the stability of the helicene-coated electrodes. Stability is one of our top concerns. We selected the helicene with the thiadiazole functional group to have a rigid molecular structure and a stable bond to the electrode. As we stated in the manuscript: "The measurements were kept running until stable CVs were observed", each OER measurement lasted at least 50 min (≥ 16 CV cycles at 5 mV/s). Moreover, we have conducted EIS measurements to determine the ECSA and repeated the OER measurements after EIS to confirm the enhancement. We concluded that the enhancement effect is stable over time.
The pH effect is not in the scope of this work, as alkaline water splitting at Ni-based catalysts is one of the most well-studied electrocatalytic systems. We have focused on the effect of chiral molecular functionalization to further boost the OER reaction. pH dependency is being studied in other systems and is beyond the scope of the present communication. However, we would like to point out that 2,1,3-benzothiadiazole derivatives are extremely stable in acidic and basic media (see 2. Did the authors monitored the formation of hydrogen per-oxide? It was reported before that if the electrons' spin is controlled the production of the hydrogen peroxide is reduced. This is an excellent indication for the role of the spin in the enhancement.

Action taken:
We have conducted experiments to quantitatively determine the chiral molecular effect on the selectivity of H2O2 production, and the results have been added as Fig. 3 d and Supplementary Figure 6. 3. There is a former study that indicates the spin filtering properties. It should be mentioned.

Response:
We thank the reviewer's suggestions and added these references in the revised manuscript.

Response:
We thank the reviewer for the remark. Our results agree with the current understanding of the CISS effect as a result of spin exchange interaction, where adsorption of chiral molecules spin-polarizes the interface. We have adapted the statement in the manuscript and cited JPC C 124, 10776 (2020).

Reviewer #3 (Remarks to the Author):
This work explores the chirality-induced-spin-selectivity (CISS) effect on the oxygen evolution reaction (OER) using the nickel-based electrocatalysts. The study is interesting in such, especially since the role of spin-polarization effects have not been extensively investigated for catalytic reactions. The authors have chosen the nickel and the and the nickel-iron catalyst, which are one of the most interesting catalysts for OER. The manuscript is well written overall, although, a few details are missing, and I suggest the authors to run a few control experiments to prove the chirality effect beyond doubt. Therefore, I suggest major revisions before consideration of this manuscript. Please see my specific comments below.

Response:
We thank the reviewer for the positive opinion on our work and all the constructive suggestions. We have conducted additional experiments and revised the manuscript accordingly.
1) In the abstract, it will be good if the authors are more specific and mention which catalysts are investigated in this this study (i.e., Ni and NiFe). I think the abstract is a bit too general right now.

Response:
We appreciate this suggestion and modified the abstract.

Action taken:
The name of catalysts and chiral modifiers used in this study have been added in the abstract.
2) The authors claim it is better not to purify the electrolyte from  Figure 1 or Figure 2, for example). Only that would prove the chirality effect beyond doubt.

Response:
We agree with the reviewer that electrolyte Fe impurities introduce an inevitable enhancement effect on the activity of Ni-based catalysts. We were aware of the articles about Fe impurities from Boettcher's and Markovic's groups. As stated in the Methods section, we kept running the OER activity measurement of fresh NiOx until a stable CV was observed. We often conducted EIS measurements to determine the ECSA alongside the activity measurements and then repeated the activity measurements to ensure stable activity results. We think catalysts had already been fully affected by the Fe impurities before depositing helicene molecules. Fe impurities should not induce further change in the activity after molecule deposition.
We value the reviewer's comment. To give further evidence, we have repeated the measurements shown in Fig. 1  This work aims to demonstrate the chiral molecular functionalization effect on state-of-the-art OER catalysts. Our priority is to reveal the activity change solely caused by the chiral molecule deposition. As cited in the manuscript, Farhat et al. reported that NiOx in unpurified KOH, compared with NiOx in purified KOH and NiFeOx in both purified and unpurified KOH, showed the most stable activity over time. Therefore, we chose to conduct most of the OER measurements in unpurified KOH.
Action taken: OER activity measurement of NiOx on Au before and after (P)-thiadiazole-[7]helicene deposited in purified (Fe-free) KOH in a cell without glass parts has been added in the modified Fig. 2 a. 3) When looking at the OER activity data in Figure 1-3, I am immediately wondering what cycle number is shown here? Also, how does the OER activity change during the first 20 cycles for the Ni and NiFe catalysts, respectively? Is there an activation behaviour? Please provide more information.

Response:
We agree with the reviewer that there is activation behavior, especially on freshly prepared electrodes. However, because we ran the OER over an excessive time to achieve a stable CV before recording the data shown in the manuscript, the catalyst should be fully "activated". The CVs recorded from freshly prepared samples in Fe-free and unpurified KOH have been added in the new Supplementary Figure 2. An increase in the activity can be seen in both cases during the first cycles (e.g., activation behavior). The activity is likely more stable in unpurified KOH.
As mentioned above, we first ran the OER CV on a fresh sample until a stable curve was observed. Afterwards, we started recording the data containing the CVs shown in the manuscript. The total cycle number of each measurement is typically between 10-20. We selected the cycles with the same cycle number from the CVs recorded before and after helicene molecule deposition. We still occasionally observed slight decreases in the activity over time, likely due to the catalysts detachment and O2 bubble formation at the electrode surface. However, the enhancement caused by the helicene is much more significant.

Action taken:
We have modified the Electrochemical characterization in the Methods session to be more clear on the OER activity measurement. Additionally, the CVs recorded from freshly prepared samples in Fe-free and unpurified KOH have been added in the new Supplementary Figure 2. 4) What is the typical metal loading on your electrode? Did you determine this by elemental analysis, or how did you control the loading? Also, how is the coverage on the electrode? Does the catalyst film cover the entire Au substrate? Please provide more information and reasoning.

Response:
The ECSA determined by the EIS analysis in Supplementary Figure 1 is ca. 0.046 cm 2 . The geometric surface area of the electrode is ca. 0.5 cm 2 . The coverage of the NiOx on the electrode for Fig. 1 a is approximately 9.2%, assuming all the NiOx surface is active towards the OER. We did not carry out EIS measurement on all electrodes to minimize the complexity of the experiments and prevent any additional effects or surface area changes to the catalysts and thus concentrated on the molecule deposition effect.
This work aims to demonstrate the chiral molecular functionalization effect on the enhancement of OER activity. We designed the experimental procedure to ensure the enhancement is solely due to the helicene molecule deposition. Therefore, deposition of 2D NiOx islands was achieved by spin coating methods to best spread the islands and prevent any clustering of the islands. The Au substrate is not fully covered by the islands, and the spare areas are for stable molecule adsorption.
We did not conduct elemental analysis and did not precisely control the metal loading. As mentioned above, we do not compare the activity of different electrodes in general, even nominally identical electrodes. We first measured the stale activity of one electrode and then deposited the molecules on the same electrode. After the removal of excessive molecules, we carried out the same activity measurement (i.e., CV) until the CV was stable. Each activity comparison reported in this manuscript is taken from the same electrode, except the sandwiched configuration shown in Fig. 3

Response:
We thank the reviewer for sharing with us the concern of the stability of the reference potential. We conducted all OER activity measurements in O2-saturated KOH, and only stable CVs were taken into account. We have also conducted the experiment using a Ag/AgCl reference. The results also show that rinsing the sample with pure DCM has no effect on the OER activity.
6) The information on the EIS fitting in Supplementary Fig. S1 is a bit scarce. Please provide the information on the equivalent circuit model, and all assumptions needed to calculate the surface are of the Ni and NiFe catalysts. Please provide references on the topic as well. How did you make sure that the surface area originates only from the catalyst film? Please provide control measurements of the bare Au substrate with and without the chiral molecule immobilized, and compare this to the catalyst. Also, at which potential did you carry out the EIS, open circuit? Does the EIS change during applied OER potentials? Please make sure that you provide these details.

Response:
The EIS-based methodology used for the determination of the ECSA of NiOx is from a recent publication (Ref. 30, ACS Catal. 2019, 9, 10, 9222-9230). We agree with the reviewer that this methodology is a bit scarce for some audiences. The EIS data was taken at the potential close to the onset potential (i.e., at low overpotentials) of OER at NiOx, and the adsorption capacitance was used to determine the ECSA. This methodology has been proven and used in a number of other studies. At this particular potential, 1.6 V vs. RHE in the case of NiOx (1.59 V vs. RHE in the case NiFeOx), only reaction intermediates (OH*, *OOH, and O*) fully absorbed on the active sites of the catalyst contribute to the adsorption capacitance.
We have attempted to perform control measurements on bare Au. However, we did not obtain meaningful EIS data. Without the specific adsorption of the oxygen species on Au at 1.6 V vs. RHE, the EIS data cannot be fitted to the same equivalent circuit model.
ECSA determination of transition metal oxide catalysts, especially for nanostructured 2D materials, is of great importance for OER activity studies. There are a few methods commonly used, for instance, Brunauer-Emmett-Teller (BET) method and double layer capacitance. We had the same concern as the reviewer that these methods often cannot differentiate between the active and non-active parts of the electrode or take the contribution of the bulk of the metal oxides. Therefore, we decided to use the method reported in ACS Catal. 2019, 9, 10, 9222-9230.

Action taken:
We added a brief discussion of the EIS methodology in the Methods section.
7) This relates to the previous question. Since you state that your NiOx catalyst is an exfoliated 2D material (i.e., a monolayer), the integrated redox peak area (i.e., the peakcharge, Q) should be directly proportional to the surface area (i.e., all Ni atoms should be electrolyte accessible). My question to you is, how do these two methods compare, i.e., if you normalize your data to either the ESCA obtained by EIS or to the moles of Ni atoms on your electrode obtained from integration of the Ni 2+ /Ni 3+/4+ redox peak? My point is that I want to make sure that you do not distort the OER activity trends by your selected normalization method, especially since metal loadings were never double-checked by elemental analysis if I understood correctly. Just keep in mind that Fe impurites/dopants often impact/supress the Ni 2+ /Ni 3+/4+ redox peak, so you may not be able to compare Ni and NiFe films straight off (http://dx.doi.org/10.1021/jacs.6b00332). (Ideally, I would strongly advise that the authors double check a few electrodes using elemental analysis, to see how this compares with your selected normalization approach.) Please provide a direct comparison of selected raw data (CVs), and the data after normalization to either the EIS/ESCA method or to the number of Ni atoms obtained from the integrated nickel redox-peak.

Response:
We thank the reviewer for this suggestion. We also think this comparison is helpful to further validate the activity enhancement reported in this work. However, based on Boettcher's articles (e.g., Nano Lett. 2017, 17, 11, 6922-6926 and ACS Appl. Mater. Interfaces 2019, 11, 6, 5590-5594) and our own ongoing studies, structural evolution takes place at 2D transition metal oxide catalysts, and the 2D islands turn into 3D structures under OER conditions. Although the catalyst synthesis and spin coating provide optimal 2D island deposition on Au surface, we can still find a limited number of bulk NiOx structures by AFM. Structural evolution resulting in 3D structures occurred before the CV measurements reported in our work because we ran the OER experiments until stable CVs were observed. Please see the added Supplementary Figure 9 for the AFM images of the structural evolution NiOx islands. The number of Ni atoms from the analysis of the redox peaks is likely much larger than the number of active atoms during the OER. However, we believe the redox peak analysis can further confirm that the chiral molecules are responsible for the activity enhancement.
Action taken: A comparison of the raw data shown in Fig. 1 a, the specific current density using the ECSA determined by EIS method and the current normalized by the number of Ni atoms obtained from the integrated Ni redox peak has been added as Supplementary Figure  1 c. AFM images of NiOx on Au surface have been added as Supplementary Figure 9.