A water-soluble copolymer for storage and electron conversion in photocatalytic on-demand hydrogen evolution

Domain: neuroscience
Published: 2026-01-28
Editorial outcome: major_revision

Abstract
Cost-and energy-efficient long-term storage of excess solar energy remains a major bottleneck in the transition to a sustainable society. Here, we present a water-soluble redox-active copolymer containing viologen moieties that can be charged with electrons upon visible light irradiation using a tris[4,4’-bis(tert-butyl)−2,2’-bipyridine]ruthenium(II) complex as chromophore. In the presence of a sacrificial donor, the system achieves charging efficiencies above 80% and fully maintains this state for several days. Subsequent acidification and the addition of various catalysts enable on-demand usage of the stored electrons for proton reduction to hydrogen with up to 72% efficiency. The system further demonstrates reversibility via a simple pH switch, allowing multiple charging, storage, and catalysis cycles without time-consuming polymer isolation. The present study presents a direct on-demand hydrogen evolution method through discharging of a water-soluble polymer that functions as a temporary energy and electron storage material.

Introduction
Although the amount of solar energy reaching the earth is significantly exceeding human needs1, seasonal, regional, and diurnal fluctuations are main reasons why large-scale harvesting of this immense energy source is so far not accessible. In order to facilitate the transition from the current fossil fuel-based economy into a sustainable one, appropriate materials for storing renewable energy have to be developed2. Besides the use of various types of batteries3, other materials such as inorganic semiconductors4, molecular metal oxides5–7, polymeric heptazine-based networks8, metal-organic frameworks9, molecular transition metal complexes10, or boranes11 to name a few, have already been employed for the storage of solar energy. In many cases, these materials enables subsequent release of the stored solar energy in the form of hydrogen upon addition of a suitable catalyst. The catalyst´s role is to either facilitate the release of bound hydrogen from storage materials2,11,12 or it allows hydrogen evolution by combining stored electrons and protons6,8,9. This so-called on-demand hydrogen evolution could be of great importance to different energy-intensive industrially relevant processes such as steel production, which will in future heavily rely on a constant supply of green hydrogen13. With aspects such as scalability in mind, one example for alternative storage materials are redox-active polymers, which have so far been applied in organic radical batteries14 or redox-flow batteries15.

We could already show that the covalent attachment of a ruthenium-based photosensitizer (PS) to a polyoxometalate (POM) as a storage site provides a molecular system, capable of on-demand hydrogen generation7. As the maximum turnover number (TON) of the Ru-POM system was limited to 1 by design, similar as for related systems10, we wondered whether a more efficient on-demand evolution of hydrogen might be possible using the redox equivalents stored on a more stable redox-active polymer. In addition to time-delayed fuel generation, the interposition of polymeric electron storage materials offers the possibility of circumventing nonproductive light-induced processes of fully molecular systems, such as light-induced intramolecular charge redistributions during and after the formation of reductively activated catalyst sites that negatively affect the system’s activity16–18.

Building on the well-known interplay of photoredox-active transition metal complexes, methyl viologen, and different hydrogen evolving catalysts (HECs)19–21, we aimed to couple prototype photoactive Ru(II) tris(bipyridine) complexes with viologen containing copolymers to store reducing equivalents at the polymer. This stored solar energy is finally meant to be used for on-demand hydrogen evolution, starting by the addition of an appropriate HEC and/or switching the solution to acidic conditions. Based on the reversible nature of the polymer-localized redox reaction, multiple charging and hydrogen evolution catalysis sequences were achieved (see Fig. 1).

Results
Light-driven charging of the polymer
The redox-active polymer, which was previously used in aqueous polymer redox-flow batteries, was synthesized according to literature procedures22. In short, the polymer was obtained by a radical copolymerization of a styrene derivative featuring an ammonium group in order to ensure water-solubility as well as a styrene bearing the redox-active unit—the viologen. The latter monomer was employed enabling the electron storage functionality. The polymerization was performed in water. The ratio of those two moieties in the final copolymer was 1:1, which was confirmed by NMR spectroscopy, similar to a previous study22.

We started our investigations on the polymer-based on-demand hydrogen evolution by analyzing the first step of the process, i.e., light-driven reduction of the viologen moieties. Therefore, we determined the amount of viologen units that could be reduced utilizing [Ru(tbbpy)3]Cl2 (tbbpy = 4,4’-di-tert-butyl−2,2’-bipyridine) as prototype PS and triethylamine (TEA) as sacrificial donor. The system was buffered with NaH2PO4 to guarantee structural integrity of the polymer, as it is able to undergo irreversible dealkylation processes at higher pH-values (see Supplementary Fig. 1)23,24. As shown in Fig. 2A, light-driven reduction of the copolymer is associated with the rise of two broad absorption bands centered at 528 and 872 nm. These bands are indicative of the formation of reduced viologen units as has previously been shown for structurally similar viologen-containing polymers and suitable molecular reference compounds in literature25–28. The broad and rather unstructured UV/vis profile deviates from the finding for molecular methyl viologen (MV2+), which exhibits a more fine-structured absorption profile under otherwise identical conditions25,26. This difference is likely associated with the higher local concentration of the viologen units in the polymer leading to a dimerization of the one-fold reduced viologens, which stabilizes their radical cationic state, represented by the absorption band at 872 nm25,26,29,30. After 60 min of irradiation, the slope of the rise of the absorbance at 872 nm is already substantially flattened, manifesting a plateau in the second hour of irradiation (see Fig. 2A). Having proven that photochemical reduction of the redox-active polymer is possible, we devised experiments to quantify this charging process.

In order to evaluate the efficiency of this charging process, the absorption of the fully reduced polymer was recorded using chemical reductants. Utilizing this data allows to establish a reference system, allowing a quantitative comparison of the photochemically reduced samples to the chemically reduced standard. The utilized reducing agent Na2S2O4 is capable of fully reducing the redox-active viologen units at various polymer concentrations31 (see SI for a detailed description of the experimental procedure). The resulting regression (depicted in Supplementary Fig. 2) enabled the determination of the absorption of the fully reduced polymer. Consequently, this allowed the calculation of the state of charging (SOC) of the polymer during both half reactions at any time. Accordingly, these factors, based on the respective concentration of the sample, obtained from the regression are utilized to normalize the absorption spectra presented in this work. Therefore, the cross-section at the peak maximum at 528 nm directly yields the SOC given in Fig. 2B, which shows an asymptotic convergence to about 82 ± 2% (TON = 14.8 with respect to utilized [Ru(tbbpy)3]Cl2) of reduced viologen units, which is reached after 2 h of irradiation. This charge is lost instantly upon aerating the sample, highlighting the necessity of the inert handling of samples (see Supplementary Fig. 3).

Addressing the incomplete charging, it can be noted that the arising absorption of the charged polymer overlaps with the absorption of the PS. A closer look at the charging trajectory, compared to the percentage that the ruthenium absorption constitutes at the irradiation wavelength, reveal that co-absorption of PS and polymer is not responsible for the incomplete trend of the SOC (see Supplementary Fig. 4). Based on emission quenching studies, the photocatalytic mechanism dominantly proceeds via a oxidative quenching pathway (see Supplementary Fig. 5)32. The reason why no higher degree of charging was obtained is likely associated with the fact that the electrons stored on the polymer are efficiently competing with TEA for reductive quenching of the Ru chromophore (see Supplementary Fig. 5). In fact, the reduced polymer exhibits a much larger driving force for reducing the Ru complex as the reduced polymer is oxidized at −0.7 V vs. Fc+/Fc whereas TEA is oxidized only at 0.3 V vs. Fc+/Fc22,33. With increasing state of charge of the viologen copolymer, the electrochemical equilibrium potential will also be more negative, which could hamper the charging by the PS.

Catalysis-driven discharging of the polymer
Having established the charging of the viologen units within the polymer up to 82%, the discharging of the photochemically reduced polymer utilizing various catalysts was investigated. As HECs, K2PtCl4 (1), platinum nanoparticles (Pt-NPs, 2) and two Rh(III) complexes depicted in Fig. 1, namely [(dceb)Rh(Cp*)Cl]Cl (3) and [(tbbpy)Rh(Cp*)Cl]Cl (4) were chosen as these compounds have shown catalytic activity for hydrogen evolution in similar systems20,34,35. The molecular HECs differ in the bipyridine-type ligand which determines the redox potential of the rhodium center (3 = −0.9 V vs. Fc+/Fc36,37, 4 = −1.1 V vs. Fc+/Fc37,38), thus allowing for different driving forces of electron transfer between the reduced polymer and the putative catalytic center. The discharging of the polymer for hydrogen evolution initiated by the various HECs was followed by UV/vis spectroscopy.

Surprisingly, when only adding the HEC (20 mol% with respect to the amount of 450 µM MV2+ units present in the buffered polymer solution, see SI for a detailed experimental description), just K2PtCl4 (1) led to a rapid discoloration of the violet solution, which is reflected by loss of the broad reduced polymer-associated absorption bands, accompanied by the formation of a black precipitate (see Supplementary Fig. 6A). These findings can be linked to the reduction of Pt(II) to Pt(0) followed by aggregation of the metal particles39,40. No hydrogen formation could be detected under these conditions for any of the employed HECs via gas chromatography. Further irradiation of the discolored sample led again to a recharging of the polymer proving that the principal functionality of the redox-active polymer is not affected by the discharging process and the formation of the metal particles (Supplementary Fig. 7). For 2 and 3, the discharging process under these conditions occurs at a comparably slow rate of about two and eight hours, respectively (Supplementary Fig. 6B, C).

As previously shown for a Ru-POM dyad and in line with thermodynamics, on-demand hydrogen evolution can be facilitated by increasing the proton concentration7. This is especially true for derivatives of the employed rhodium catalysts, as the catalytically active hydrogen evolving hydride species is only formed below a certain pH value41. The pKa value of the two-fold reduced 3 is 3.8 (see Supplementary Fig. 9), revealing that it is the predominant species at a pH value of 2. Therefore, in all further experiments the solutions were acidified to obtain a final pH value of 2 using HCl prior to the addition of one of the different HECs. Gas chromatography was able to confirm, that at this pH value all of the employed HECs are active and yield hydrogen. Furthermore, it was verified that the addition of HCl in absence of catalyst did not lead to any hydrogen formation (see Table S1).

As shown in Fig. 3, after acidification of the solutions, the addition of different HECs led to an efficient discharging of the photochemically reduced polymer. However, depending on the selected HEC, the discharging kinetics varied significantly. Whereas the Pt-based HECs (1 and 2) discharged the polymer almost completely within the first 20 min (see Fig. 3 for 2), much slower discharging was observed in the case of the rhodium-based catalysts (see also Supplementary Fig. 10). 1 hereby outpaces the colloidal platinum, which is again indicative of the in-situ particle formation described above. These findings are also in line with the redox potentials of the respective catalysts in comparison to the reduced polymer and the resulting driving forces. The offsets in the degree of charging at t = 0 are the result of the fast discharging occurring between sample preparation and measurement of the first data point.

When comparing the two rhodium derivatives with each other it is worth noting that the tbbpy ligand leads to an increased electron density at the Rh center, whereas the electron withdrawing ester groups at the dceb ligand facilitate the reduction of the catalyst by the reduced polymer. The respective Rh(III)/Rh(I) reduction potential of −0.9 V vs. Fc+/Fc36,37 of 3 leads to the superior catalytic properties in this system, compared to the tbbpy derivative with a reduction potential of −1.1 V vs. Fc+/Fc37,38. This also explains the fact that 4 is incapable of discharging the polymer or does so only very slowly, depending on the pH value of the solution. Although this tbbpy derivative is able to discharge the polymer under acidic conditions, the kinetics differ significantly from the other HECs. Rather than the exponential decay described for 2 and 3 in Fig. 3, the [(tbbpy)Rh(Cp*)Cl]Cl catalyst showed a slow and linear discharging process. This was revealed by the steady discoloration, also observed by UV/vis spectroscopy (Supplementary Fig. 10D), and correlates with comparatively low catalytic activity (see below).

After the addition of the different HECs to the acidified polymer solutions, hydrogen evolution was determined using gas chromatography. Correlation of gas chromatographic analysis of hydrogen present in the headspace of the different samples and determination of the degree of charging via UV/vis absorption spectroscopy prior to catalyst addition, furthermore enabled the calculation of the efficiency of the conversion of the polymer-stored electrons into hydrogen gas.

In hand with the faster discharging of the reduced polymer by 2 compared to 3, the highest conversion efficiency of electrons into hydrogen was obtained for 2 within 2 h, whereas hydrogen evolution using 3 increased over the first 4–5 h. For 2 a maximum conversion efficiency for electrons stored in the redox-active polymer to hydrogen of up to 72 ± 3% was found, whereas a value of 54 ± 3% was found for 3, as shown in Fig. 4 (see SI for a detailed description on the calculation of efficiencies).

Notably less efficient hydrogen evolution was found for 1 and 4, despite the fast discharging of the polymer by the former. For 1 this is likely associated with the initial conversion of Pt(II) to Pt(0) species, most likely accounting for the fast discoloration and the associated consumption of a large number of reducing equivalents (40% SOC would be consumed for all Pt(II) to Pt(0) reductions) stored on the polymer. Contrary, 4 possessing a cathodically shifted Rh(III)/Rh(I) potential compared to 3 evolves hydrogen less efficient than the ester-substituted Rh complex which is easier to reduce by 0.2 V. It provides a continuous discharging course with a maximum conversion of only 10 ± 3% within the first 6 h of catalysis; 1 reached a similar maximum conversion of 9 ± 3% over the same period of catalysis. In comparison, experiments employing the best-performing 2 to discharge molecular viologen were performed as well, revealing very poor conversions (see Table S2), which represents only 37% of the turnover, that the polymeric system is able to reach under identical conditions thus highlighting the beneficial properties of the polymer for the presented application.

Preservation of the charged species
Polyviologens as well as low molar mass viologens have been utilized as active materials in organic flow batteries. In these systems they have proven to be quite stable active materials30,42. Consequently, the herein utilized system was also expected to be capable for long-term electron storage. To provide the respective evidence, the polymer was charged, and absorption spectra were measured after a dark period of 24 and 72 h. Promisingly, for the photochemically reduced polymer, no UV/vis spectroscopic changes were observed during the dark period, that could have been assigned to degradation as shown in Fig. 5. Similar stabilities have been reported for molecular MV model compounds of the herein used polymer26. Under the established conditions, however, molecular viologen does show vastly inferior storage capabilities, with the molecular viologen losing 40% of the stored charge within the first 18 h of storage (see Supplementary Fig. 12).

Bringing this into a context with other energy storage materials, one can calculate the electric charge that is preserved on a specific amount of polymer (detailed calculation in the SI). Overall this represents a storage of 101 C g−1 for multiple days without loss, compared to similar state of the art systems employing metal organic frameworks (MOF), with storage capacities of 15 C g−19.

Following up on these results, catalysis measurements of samples that were stored for different periods of time were conducted. In addition to 24 and 67 h storage, catalysis was also initiated after only 2 h of storage. Calculations based on the respective TONs revealed a loss in catalytic efficiency, compared to instantaneous charge retrieval through hydrogen evolution, initiated by 2 directly after photocatalytic charging of the polymer. After storage for 2 h, the efficiency was reduced to 58 ± 4% and to 56 ± 9% after 24 h storage as opposed to the 70 ± 3% reached on average for direct catalytic hydrogen evolution excluding any storage time. Extending the storage period further to 67 h only drops the catalytic efficiency to 48 ± 4%. This means that not only is on-demand hydrogen evolution possible, but also storage of reducing equivalents in the polymer beyond the first two hours entails minimal loss of catalytic activity (0.16% per hour of storage after the initial loss in the first 2 h; Supplementary Fig. 13). Hereby 2 was chosen due to the high turnover frequency (TOF) and high catalytic activity, facilitating a solid readout after a comparably short period of time.

Regeneration of the system for recycling
As detailed above and due to the polymer´s stability in the range of applied pH conditions (neutral for charging, acidic for discharging) and the known application potential of the polymeric species in redox flow batteries with multiple charging and discharging cycles22, also the photochemical storage of electrons in the redox-active polymer might be reversible. Therefore, the herein present system might not be limited to a single catalytic cycle but could also be utilized for subsequent additional ones.

Considering the pH-requirements necessary for discharging of the polymer in presence of the various HECs and also the observations that in presence of 1 irradiation of the solutions can lead to charging of the polymer (see Supplementary Fig. 7), we were wondering if the system as it stands (with PS and HEC both being present in solution) can simply be reused by adjusting the pH value accordingly and without the need to isolate the redox polymer. To follow this path, samples of the two best performing catalysts (2 and 3) were prepared analogously to previous measurements but were followed by a neutralization of the acid which was added to induce on-demand hydrogen evolution. Charging without this neutralization step has been proven impossible due to complete protonation of the utilized electron donor TEA (see Supplementary Fig. 11). However, after neutralization with NaOH, it was possible to recharge the polymer photochemically as depicted in Fig. 6.

Direct comparison of the first two charging processes reveals very similar kinetics, with the second process starting at a slightly elevated SOC and reaching a very comparable final charge. Therefore, the charging profile of the second charging step possesses a slightly lower slope and stays within the boundaries set by the preceding charging process. For these results to be as representative as possible for a potential loss in activity, rather than a mismatch in pH, these values were measured under quasi in-operando conditions. With this control over pH, a second cycle was able to almost replicate the initial 71 ± 2% charge with its 67 ± 2% SOC.

While charging appears to be stable for at least a second cycle, GC analysis revealed hydrogen amounts that translate to maximum catalytic efficiencies of 55 ± 4% for 2 and 48 ± 4% for 3. This equals to a decrease of 15% for the Pt catalyst, but only a loss of efficiency of 5% for the Rh catalyst. Errormargins in Table 1 represent standard deviation of at least n = 3 independent measurements.

Experiments investigating a third and fourth reaction cycle were performed as well. In general, the trend indicated by the results of the first two cycles remains, with regard to loss of charging activity and catalytic efficiency (see Fig. 7). Even after three whole cycles, the system can reach an SOC as high as 68%, while catalytic efficiencies decrease steadily and to a higher degree. Catalytic activity of the rhodium catalyst was reduced to 23%, while the efficiency of the platinum catalyst was reduced to 32%, representing a drop in efficiency to less than half their initial activity (see Fig. 7). However, these findings serve as a confirmation that multiple cycles are possible, and they also reveal that the pH-value appears to be not only a switch, but a major factor to tune the system in the future.

To get more insights into potential loss channels, we devised an experiment to simulate the ionic strength of a fourth cycle as starting conditions for both processes. Therefore, the respective amount of sodium chloride is utilized, as it is the source for both ions present in the system. The measurements show that an increase in ionic strength does reduce the systems capabilities to be charged, yielding an explanation for the slight, but steady loss in charging efficiency (see Supplementary Fig. 14). The catalysis on the other hand still reaches efficiencies of 68 ± 1% for 2, indicating that this catalyst is not degrading due to ionic strength, but rather due to repeated pH-changes. For 3, this pH-induced degradation was confirmed by performing resonance Raman measurements of the catalyst exposed to the conditions of the acid base cycle during catalytic charging and discharging (see Supplementary Fig. 17). It could be shown that upon addition of acid, already at the first discharging step the ester functionality at the bipyridine was cleaved. From cycle 2 onwards, H2 evolution is thus no longer catalyzed by 3 but by a structurally altered bipyridine-based Rh complex.

While this grants an overview of the stability and repeatability of both processes throughout multiple recycling steps, the respective cumulative values should be highlighted. Compiling these numbers for 2, as in Fig. 8, the major advantage of this system compared to non-regenerable system becomes apparent. Already with four cycles under current conditions, cumulative efficiencies reach the 2.69-fold outcome compared to a theoretical, loss-free single charging process. The catalytic process behaves similarly, with a cumulative catalytic output that reaches 2.06-times the catalytic turnover of a theoretical single-cycle system at complete catalytic conversion. Thereby an ideal single-cycle system is exceeded at least two fold by this multi-cycle system, with the data suggesting not only the polymers stability, but also it not being the bottleneck of the system.

In summary, this represents the first organic polymer-based system, that can be photochemically charged and subsequently drive on-demand hydrogen evolution, while also possessing the regenerative capabilities to enable recycling. Core of this aqueous system is a copolymer consisting of a solubility enhancing as well as a methyl viologen monomer, making it a fully water soluble redox-active soft matter matrix for the described charge storage application. The [Ru(tbbpy)3]Cl2 PS can charge up to 82% of accessible low molecular weight viologen units upon irradiation, thereby exceeding comparable Re-containing MOF-based systems more than six-fold with respect to its gravimetric energy density9. The charged state is easily preserved for at least three days and can be consumed in a hydrogen-evolving dark reaction at any given time. Initiated by acidification, the polymer is discharged by a suitable catalyst, with a catalytic conversion efficiency of up to 72%.

Inversely and due to the component´s stability within the range of applied pH conditions, returning to neutral pH value enables repetitive light-driven charging, establishing the systems unique regenerative capabilities. It is shown that a mere pH value switch allows for multiple charging and on-demand catalysis cycles.

Methods
Materials and measurements
Unless otherwise stated, all chemical compounds were obtained from commercial sources and used without further purification. Solvents used for synthesis were of technical grade, while measurements employed a 1H-NMR analysis.

Absorptionspectroscopy
UV/vis-absorption spectroscopy was performed on a JASCO V-670 UV-vis-NIR Spectrometer or an Avantes AvaSpec-ULS2048CL detector unit coupled with an AVA AvaLight-DH-S-BAL light source, using gas-tight quartz cuvettes (d = 10.0 mm, Starna).

Gas chromatography
The amount of generated hydrogen/methane/carbon monoxide was determined by gas chromatography (GC) on a Shimadzu GC-2030 with a barrier ionization discharge (BID-2030) detector and helium as carrier gas (column: Restek SH-Rt-Msieve 5A, ID: 0.32 mm; film thickness: 30 µm, length: 30 m, oven temp. 80 °C) using 100 µL of the gas phase. The GC was calibrated by injection of different volumes of a test gas mixture containing known percentages of hydrogen/methane/CO.

Raman and resonance Raman
Raman measurements were performed at a MultiRam Fourier-Transform Raman-Spectrometer (Bruker Corporation, Billerica, Massachusetts, United States of America) with a spectral resolution of 4 cm−1 in the range 0 and 4000 cm−1. The Raman excitation light at 1064 nm was provided by a Nd:YAG laser (Klastech Deni CAFC-LC-3/40, Dortmund, Germany). The laser power incident upon the sample plane was approximately 250 mW with 100 scans per sample. Background correction was performed using SNIP function (iterations: 200, order: 3, smoothing window: 1). Thereafter, the spectra were confined to the spectral range of 100–4000 cm−1 and normalized using vector normalization to minimize variability in intensity of the samples.

In operando Raman spectroscopy was conducted using a NIR–excited fiber-coupled Raman setup (RXN1, Kaiser Optical Systems, Ann Arbor, MI, USA). The sample was excited with a power of 250 mW at the sample through a fiber optic probe (In Photonics, Norwood, MA, USA) which is equipped with band and long pass filters for cleanup and Rayleigh-rejection (cutoff long pass: ca. 250 cm−1) with a focal spot width of about 50 µm which is coupled to a 785 nm single-frequency laser diode (Xtra II, Topica Photonics, Munich, Germany). This system disperses the scattered Raman light passing through a holographic grating and then detected via a multi-row, thermoelectrically cooled (T = −60 °C) CCD (Andor, Belfast, UK) resulting in spectral resolution around 4 cm−1. The system was white light calibrated using a white light source prior to the measurements.

Resonance Raman measurements of intermediate states, were carried out using an excitation wavelength of 643 nm of a diode-pumped solid-state laser (Crysta Laser, USA). The laser power at the sample was reduced to approximately 12 mW to minimize photo-degradation of the RhI chromophore. Raman signals were collected using an IsoPlane 160 spectrometer (Princeton Instruments, USA), with a 30 µm entrance slit and grating with 1200 grooves/mm. The spectrometer is equipped with a thermoelectrically cooled CCD camera (PIXIS eXcelon, Princeton Instruments, USA) featuring a resolution of 1340 × 100 pixels. The H2O spectral band at 1633 cm⁻1 was utilized as a reference for normalizing both intensities and wavenumbers. For spectral analysis, rR spectra were background-corrected, and the solvent spectrum was subtracted.

Statistics and reproducibility
Error margins featured on the respective experiments represent the standard deviation, unless stated otherwise. These result from either triplicates, or two independent duplicates. The only data excluded in these cases is data acquired during early optimization of the respective experiments, or in clear cases of an experimental error. No statistical method was used to predetermine the sample size. The experiments were not randomized. The experimenters were not blinded to allocation during experiments and outcome assessment.

Data availability
The authors declare that all data supporting the findings of this study are available within the paper and its supplementary information files. Source data for figures are provided with the paper. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors present a reuasable system for ‘on-demand hydrogen production’ with a molecular approach.\nIn this system, charge is stored in a previously studied styrene-based viologen polymer system and its discharge to H2 is\nstudied with four different types of catalysts—two molecular Rh systems and two Pt systems that appear to be variants of\nnanoparticle suspensions.\nGenerally speaking, the work has been carried out to a high technical standard. The results are quite reasonable in my view,\nand reflect both ingenuity and the difficulty of building a robust and practical on demand H2 release system. My concerns\nwith this paper stem from these difficulties, in the sense that I have read the paper with a focus on identifying the pitfalls of\nthe system for future improvements. This is certainly only a model system, in that I anticipate there is a low technological\nreadiness level associated with this work. Therefore, my review is focused on helping the authors to add clarity about pitfalls\nin the performance of their system—building on their existing manuscript that does indeed show that the system can work in\na multicycle fashion to some degree.\nI anticipate that this paper could have an impact in the scientific community. As the authors include in their citation list, there\nare a number of groups that have reported or are working actively on ‘on-demand’ fuel production systems that could\nalleviate the difficult issue of H2 storage in renewable energy schemes.\nI anticipate that addressing my recommendations could require a round of major revisions to improve a few aspects of the\npaper and correct misconceptions. These include some aspects of the statistical work which are claimed on technical\nreplicates. I have included specific points below for my concerns, and I emphasize that these are intended to be helpful to\nthe authors in increasing the quality and impact of their work. Not all may be addressed, but I suggest they be at least\nconsidered.\nI thank the authors for their generally careful work and wish them success in the future.\nSpecific points\n1. At several points in the manuscript the authors claim that their error bars represent standard deviation of at least n = 2\nindependent measurements. Having n = 2 with computation of an error bar in this way is not standard practice in the field\nand I have not previously encountered it in reliable high-quality papers. For all the data points that reflect replicates of n = 2, I\nrecommend that an additional replicate be executed so that n = 3. From my viewpoint as an inorganic chemist with\nsignificant experience in quantitative data analysis, it could attract significant unwanted attention to have n equal to only 2;\nunder such conditions, the estimated standard deviation has little value, in that there is no measure of whether the\ndistribution of values is within what would be expected for the normal distribution of values. Considering the somewhat\nvariable behavior observed later in the paper (see point 16 below), I suggest high caution here. At least, the error bars for\npoints with n = 2 can be removed and only the averaged data point for the two individual replicates can be retained.\n2. In the introduction, the authors motivate use of a polymer system by referring to ‘nonproductive light-induced processes of\nfully molecular systems.’ What is a process of this type that would have occurred in the present system without the polymer?\nOr, what is an example of one in general? I ask because this system appears sensitive to unproductive quenching and also\nunproductive dimerization of viologen. Thus, the question could be asked what the polymer does here. Also, there is no\ncontrol in this paper for use of viologen and Ru(bpy)3 without polymerization…so that could be considered for addition too to\nproperly motivate and control for use of the polymer.\n3. Please describe what the styrene-viologen polymer was previously used for in reference 22. This should be mentioned in\nthe paper. I suppose it may have been used for electroanalytical work with immobilized redox enzymes…but the authors\ncould usefully share this with readers for context.\n4. Is the UV-visible spectrum of the charged viologen polymer consistent with dimerized viologen? This should be known\nand could be checked, as discussed on p 4.\n5. There are multiple broken references that are associated with the text in German “Fehler! Verweisquelle konnte nicht\ngefunden werden.”\n6. Why can the polymer only reach 60% charging? Also, how was this level determined? Was elemental analysis used to\nmeasure the amoung of N (viologen derived) in the solid material? Or, was a spectroscopy assuming similar properties to\nmonomeric models used? This should be specified. Also, does the 60% level indicate that some of the viologen remains un-\ndimerized? And finally, does absorption of light by the apparently darkly colored, reduced viologen preclude light absorption\nby the Ru(bpy)3 system contribute to the lower than perfect charging? I realize the latter might be challenging to confirm but\nit strikes me as an important point.\n7. How was the 1:1 viologen:styrene ratio confirmed? I assume a measurement of this was done rather than simply relying\non a literature result.\n8. When a strong chemical reductant is used under air- and moisture-free conditions, can all of the viologen be reduced in\nthe polymer? Answering this question quantitatively is not strictly required for understanding behavior with Ru-based\nreductants, but I ask because it speaks to the efficacy of this polymer in the tested conditions. What would prevent full\nreduction, i.e. full efficiency in this system?\n9. What are the reduction potentials of the Rh catalysts relative to the viologen and Ru(bpy) on a common potential\nreference? This should be updated as presently the Rh is quoted vs. Fc+/0 and the viologen isn’t specified I believe. This is\nimportant because it appears that the tbu bpy system cannot be easily reduced by the polymer resulting in very sluggish H2\nevolution. The more easily reduced ester bpy can however be reduced.\n10. Does the quotation of 101 C per gram polymer on p. 9 take into account the incomplete charging found in the Ru-bpy\nwork?\n11. The colors of the data markers in Fig 3 are too similar to be easily understood by readers.\n12. Does the Pt NP system take up H2 and hydrogenate the polymer? In other words does the Pt take up added H2 with the\noxidized polymer and reduce it or hydrogenate some functionalities? Where would the H2 generated in the data presented\nbe going since it is apparently lost from the flask upon generation over 24 h? This behavior should be explained. If the\nsystem is being hydrogenated, it would suggest limited usefulness with HEC2, which appears later in the recyclability\nsection.\n13. How was the polymer stored upon charging but before the H2 release as described on p. 10. Was it left in the presence\nof H2O and more importantly O2? I suppose O2 could scavenge reducing equivalents. Was the polymer ever isolated as a\nsolid or was it always kept in solution?\n14. Plotting H2 released as a function of time delay after charging could be useful for readers as a figure in main text or SI. It\nappears from the data available so far that there is a rapid decrease in performance and then a slower phase as well.\n15. Does the pH swing system required for function of this polymer mean that ionic strength was not constant in solution over\nthe multi-cycling experiments? Was any control done to test for an influence of ionic strength on charging and H2 evolution\nefficiency, in the testing described on p. 11 or so?\n16. The ‘small scale approach’ and ‘intrinsic requirements of the reaction system’ are described as reasons for the variable\nand somewhat unexpectedly variable results across multicycling on p. 12. These reasons/speculations are not clear or\nappropriate. Are the authors saying that the experiments are so difficult that they cannot be done with reliable analytical\nquality? Please revise this section. Perhaps more technical replicates are required to establish reliable performance metrics\nof this system?\n17. HEC3 appears non-recyclable as it starts to behave quite poorly for hydrogen release in Fig. over 4 cycles of use. Is this\nbecause of degradation of the catalyst?\n18. What cycling time was used for each stage of the multi-cycling? I ask because HEC2 is the high performer, but its\nperformance was clearly time dependent in Fig 4. The time utilized for the H2 release should be noted carefully in the work\non p 14 for the multicycling efficiency test. This is because the time for the H2 cycle should make a large difference in the\nmetrics achieved for the multi-cycle efficiency, right?", "reviewer_2": "The authors have reported the first system based on organic polymers that can be charged photochemically and\nsubsequently drive hydrogen evolution on demand. Additionally, this system possesses regenerative capabilities that\nenable recycling and multiple catalytic cycles. In the presence of triethylamine as a sacrificial donor, a charging efficiency of\nover 60% can be achieved, which remains stable for multiple days. By subsequent acidification and the addition of different\ncatalysts, the stored electrons can be utilized on demand for proton reduction to hydrogen with an\nefficiency of up to 72%.\nOverall, this work is intriguing and the manuscript is well-written. However, there are some important issues that need to be\naddressed, which can help largely increase the impact of this manuscript.\n1. It is good to design catalysts and obtain high activity. It would be even better if meanwhile new catalysts except for HEC1,\nHEC2, HEC3, and HEC4 could be offered.\n2. Although redox-active polymer synthesis has been previously reported in literature, more detailed experimental\nprocedures and characterization results should be provided in this manuscript.\n3. Please provide a better explanation regarding the novelty of the water-soluble copolymer system.\n4. It would be beneficial to discuss the stability of these catalysts and redox-active units as well as their potential application\nin other catalytic reactions.\n5. The significant variation observed in discharge kinetics when different HEC is selected needs further clarification.\n6. In Fig. S1, it appears that there is not much difference between the degree of charging at pH = 7 compared to pH = 8 but it\ndiffers greatly from other pH conditions. Please explain why this discrepancy exists.\n7. Please provide evidence supporting enhanced solubility of monomers and copolymers as well as proof for formation of a\ncompletely water-soluble redox-active soft matter matrix within your manuscript."}, "author_response": "Response to Reviewers\nWithin this work two different molecular Rh catalysts have been used as well as two\ndifferent Pt systems. In one of the Pt systems, the nanoparticles have been used as commercially\nreceived whereas in the other version a Pt salt was added that was converted into Pt nanoparticles\nin situ using the electrons that were stored at the polymer.\nGenerally speaking, the work has been carried out to a high technical standard. The results are\nquite reasonable in my view, and reflect both ingenuity and the difficulty of building a robust and\npractical on demand H release system. My concerns with this paper stem from these difficulties,\n2\nin the sense that I have read the paper with a focus on identifying the pitfalls of the system for\nfuture improvements. This is certainly only a model system, in that I anticipate there is a low\ntechnological readiness level associated with this work. Therefore, my review is focused on\nhelping the authors to add clarity about pitfalls in the performance of their system—building on\ntheir existing manuscript that does indeed show that the system can work in a multicycle fashion\nto some degree.\nResponse: We addressed some of the challenging aspects and hope that this version of the\nmanuscript will find a positive response.\nI anticipate that this paper could have an impact in the scientific community. As the authors include\nin their citation list, there are a number of groups that have reported or are working actively on ‘on-\ndemand’ fuel production systems that could alleviate the difficult issue of H storage in renewable\n2\nenergy schemes.\nI anticipate that addressing my recommendations could require a round of major revisions to\nimprove a few aspects of the paper and correct misconceptions. These include some aspects of\nthe statistical work which are claimed on technical replicates. I have included specific points below\nfor my concerns, and I emphasize that these are intended to be helpful to the authors in increasing\nthe quality and impact of their work. Not all may be addressed, but I suggest they be at least\nconsidered.\nI thank the authors for their generally careful work and wish them success in the future.\nResponse: In the following, we hope that we can answer any of the questions appropriately. We\ndevised additional experiments, repeated existing ones and altered the manuscript as well as the\nSupporting Information file at the respective positions. We hope that this could clarify the so far\nunclear aspects of the manuscript.\nSpecific points\n1. At several points in the manuscript the authors claim that their error bars represent standard\ndeviation of at least n = 2 independent measurements. Having n = 2 with computation of an error\nbar in this way is not standard practice in the field and I have not previously encountered it in\nreliable high-quality papers. For all the data points that reflect replicates of n = 2, I recommend\nthat an additional replicate be executed so that n = 3. From my viewpoint as an inorganic chemist\nwith significant experience in quantitative data analysis, it could attract significant unwanted\nattention to have n equal to only 2; under such conditions, the estimated standard deviation has\nlittle value, in that there is no measure of whether the distribution of values is within what would\nbe expected for the normal distribution of values. Considering the somewhat variable behavior\nobserved later in the paper (see point 16 below), I suggest high caution here. At least, the error\nbars for points with n = 2 can be removed and only the averaged data point for the two individual\nreplicates can be retained.\nResponse: The issues with reproducibility at that stage has since been resolved and we are now\nable to present our revised experiments. With in-operando measurements of the pH-value, we are\nnow able to control the cycling procedure in a much more precise fashion, which should also grant\nmore clarity to the systems true capabilities and limitations.\nThis new method is described in greater detail in the Supporting Information, Chapter 1 and\nrepetitions with said procedure did not only help us meet the scientific standard of triplicates for\nremaining measurements, but also achieve better reproducibility, seen in Fig. 6 in the manuscript.\nOn page 4 of the Supporting Information, the optimized protocol is provided:\n“The initial dummy system, confirming calculations, refers to an experiment, employing\nthe buffer solution and the addition of respective amounts of acid and base. Depending\non the measurement, either the GC-vial (for hydrogen detection) or the inert cuvette (for\nUV/vis absorption spectroscopy) were sealed immediately afterwards, ready to be\nremoved from the glovebox. Measurements without acidification were also performed\n(Fig. S7) by skipping the addition of the HCl, maintaining the remaining procedure.\nIn a revised procedure, the dummy system relied on actual samples, rather than an\nupscaled titration with the buffer solution. To measure the pH-value of the actual sample\na needle-pH-meter was employed, capable of piercing the septum of an inertly prepared\nand gastight sample vial. This method is as close as possible to the experimental\nconditions, which greatly enhances the precision and control over the pH-value,\nespecially when moving to higher cycle numbers in the multicycle experiments.”\nThe previous issues on large error bars associated with the individual measurements stem from\nthe way, in which the pH-values of the samples were directed. The amounts of acid and base were\ncalculated and titration experiments with the buffer solution were conducted using a pH-meter. To\ncloser match the experimental conditions, we are now using a needle-pH-meter, which is capable\nto determine the pH value of the actual samples prepared under experimental conditions. This\nwas especially helpful for the third and fourth cycle, which we have been lacking triplicates\nbeforehand, due to increasing discrepancies to the initial titrations.\n2.In the introduction, the authors motivate use of a polymer system by referring to ‘nonproductive\nlight-induced processes of fully molecular systems.’ What is a process of this type that would have\noccurred in the present system without the polymer? Or, what is an example of one in general? I\nPage 2 of 11\nask because this system appears sensitive to unproductive quenching and also unproductive\ndimerization of viologen. Thus, the question could be asked what the polymer does here. Also, there is no\ncontrol in this paper for use of viologen and Ru(bpy)3 without polymerization…so that could be considered for addition too to\nproperly motivate and control for use of the polymer.\nResponse: With nonproductive light-induced processes of fully molecular systems we referred to\nalready investigated examples in which a dye is covalently linked to an acceptor unit and, thus, in\nclose spatial proximity to it (see Kahlfuss, C. et al. Comptes Rendus. Chimie 2014, 17, 505–\n511). It has been found that connecting those units is typically beneficial in terms of forward\nelectron transfer from the dye towards the acceptor side (a catalyst for a reductive process) but\nthis strategy also offers pitfalls. Once the catalyst is reduced one- or two-fold a new excitation of\nthe dye can oxidize the acceptor side which negatively impacts its activity. In order to clarify this\naspect, we have rewritten the corresponding sentence in the introduction as followed:\n“In addition to time-delayed fuel generation, the interposition of polymeric electron storage\nmaterials offers the possibility to circumvent nonproductive light-induced processes of fully\nmolecular systems, such as light-induced intramolecular charge redistributions during and\nafter the formation of reductively activated catalyst sites that negatively influence the\nsystem´s activity.”\nAlso, it should be clarified that the dimerization of the polymer-bound methyl viologens is not\nconsidered to represent an unproductive (i.e. energy wasting) but rather fully reversible process\nonly resulting from the high local concentration of mono-reduced methyl viologen units arranged\nat the polymer backbones. Based on UV-vis spectroscopic studies performed in this work the well-\nknown dimerization process does not prevent full polymer re-oxidation upon catalyst and/or acid\naddition.\nAlso, resting the comparison on literature might in fact have been insufficient, which is why we\nimplemented experiments with molecular viologen, following your advice. The experiments\n(carried out under identical conditions) were able to show the superior properties of the copolymer\nunder the conditions utilized herein, in particular when it comes to charge storage as well as\ncharge transfer to a catalyst. The respective data is presented in the SI by Fig. S12 showing a\ncomparably rapid decrease of charge stored on molecular viologen, as well as in Table S2,\nrevealing a catalytic turnover of about a third compared to the polymer.\n3. Please describe what the styrene-viologen polymer was previously used for in reference 22.\nThis should be mentioned in the paper. I suppose it may have been used for electroanalytical work\nwith immobilized redox enzymes…but the authors could usefully share this with readers for\ncontext.\nResponse: Its first application was in a redox-flow battery, which is now mentioned in the\nmanuscript as first sentence in Chapter 2.1:\n“The redox-active polymer, which was previously used in polymer redox-flow batteries, was\nsynthesized according to literature procedures.”\n4.Is the UV-visible spectrum of the charged viologen polymer consistent with dimerized viologen?\nThis should be known and could be checked, as discussed on p 4.\nPage 3 of 11\nResponse: Yes, the described bands are indicative of dimerized viologen, as the polymer\npossesses sufficiently high local concentrations to form them. We clarified that in the manuscript\nin Chapter 2.1. and added another reference, to extend on this behavior:\n“These bands are indicative of the formation of reduced viologen units as has previously\nbeen shown for structurally similar viologen-containing polymers and suitable molecular\nreference compounds in literature.24-26 The broad and rather unstructured UV-vis profile\ndeviates from the finding for molecular methyl viologen (MV2+), which exhibits a more fine-\nstructured absorption profile under otherwise identical conditions.24,25”\n5. There are multiple broken references that are associated with the text in German “Fehler!\nVerweisquelle konnte nicht gefunden werden.”\nResponse: It is a referencing problem and has now been fixed accordingly.\n6. Why can the polymer only reach 60% charging? Also, how was this level determined? Was\nelemental analysis used to measure the among of N (viologen derived) in the solid material? Or,\nwas a spectroscopy assuming similar properties to monomeric models used? This should be\nspecified. Also, does the 60% level indicate that some of the viologen remains un-dimerized? And\nfinally, does absorption of light by the apparently darkly colored, reduced viologen preclude light\nabsorption by the Ru(bpy) system contribute to the lower than perfect charging? I realize the latter\n3\nmight be challenging to confirm but it strikes me as an important point.\nResponse: The improved experiments were actually able to reach charging above 80% (as shown\nin Fig. 1) and potential loss channels are also addressed in the manuscript in the end of chapter\n2.4:\n“To get more insights into potential loss channels, we devised an experiment to simulate\nthe ionic strength of a fourth cycle as starting conditions for both processes. Therefore, the\nrespective amount of sodium chloride is utilized, as it is the source for both ions present in\nthe system. The measurements show that an increase in ionic strength does reduce the\nsystems capabilities to be charged, yielding an explanation for the slight, but steady loss\nin charging efficiency.”\nAlso, we did improve the explanation of how the state of charging was determined in the\nmanuscript (second to the last paragraph of 2.1):\n“In order to evaluate the efficiency of this charging process, the absorption of the fully\nreduced polymer was recorded using chemical reductants. Utilizing this data enables a\nreference system, allowing a quantitative comparison of the photochemically reduced\nsamples to the chemically reduced standard. The utilized reducing agent Na S O is\n2 2 4\ncapable of fully reducing the redox-active viologen units at various polymer\nconcentrations28 (see SI for a detailed description of the experimental procedure).”\nAs well as the detailed explanation in the first chapter of the SI:\n“With 60 equivalents of dithionite per MV2+ unit, measurements of polymer solutions with\nconcentrations ranging from 20 to 120 µM (in 20 µM steps) were performed. The respective\nabsorption maxima enabled a linear fit, which allows the determination of the absorbances\nof a fully reduced polymer at any concentration. Inversely, the comparison of the\nabsorbance of any samples with a known concentration to this reference enables the\ndetermination of the state of charging of the sample at any given time.”\nPage 4 of 11\nTo address the co-absorption of the arising polymer band with the photosensitizer as the potential\nbottleneck for incomplete charging compared the decreasing charging increments per time with\nthe decreasing percentage of the Ru-MLCT at the irradiation wavelength. This comparison reveals\ntwo different diminishing trajectories, which prove that increasing optical density may be part of\nthe reason, but is not the sole reason the polymer can not be fully charged under our conditions,\nas it is now written at the end of chapter 2.1:\n“Addressing the incomplete charging, it can be noted that the arising absorption of the\ncharged polymer does overlap with the absorption of the photosensitizer. A closer look at\nthe charging trajectory, compared to the percentage the ruthenium absorption constitutes\nat the irradiation wavelength reveals, that co-absorption of photosensitizer and polymer is\nnot responsible for the incomplete trend of the SOC (see Fig. S4). While the co-absorption\ncertainly can be a factor in explaining the SOC limit, data suggests that the bottleneck is\nmore likely to be the result of limited energy transfer.”\n7. How was the 1:1 viologen:styrene ratio confirmed? I assume a measurement of this was done\nrather than simply relying on a literature result.\nResponse: It is assessed via NMR spectroscopy in previous studies, as the manuscript will now\nstate in the first paragraph of 2.1:\n“The ratio of those two moieties in the final copolymer was 1:1, which was confirmed by\nNMR spectroscopy in a previous study.22”\n8. When a strong chemical reductant is used under air- and moisture-free conditions, can all of\nthe viologen be reduced in the polymer? Answering this question quantitatively is not strictly\nrequired for understanding behavior with Ru-based reductants, but I ask because it speaks to the\nefficacy of this polymer in the tested conditions. What would prevent full reduction, i.e. full\nefficiency in this system?\nResponse: As described above (Question 6) in the clarification of the determination process, an\nexcess of reducing agent was employed to provide a reference system. The state of charging is\ngiven in comparison to this chemically fully reduced reference system. Further excess of chemical\nreducing agent or utilization of even stronger agents can in fact lead to discoloration caused by\nthe formation of MV0 species proving that before formation of MV0 all MV units had to be reduced\nonce (MV0 units in the presence of MV2+ would be unstable, due to comproportionation to MV1+).\nUsing more reducing agent in the photochemical experiments (however, thereby increasing the\npH-value of the solution) will cause the described irreversible degradation of the polymer due to\nhigh pH-value.\n9. What are the reduction potentials of the Rh catalysts relative to the viologen and Ru(bpy) on a\ncommon potential reference? This should be updated as presently the Rh is quoted vs. Fc+/0 and\nthe viologen isn’t specified I believe. This is important because it appears that the tbu bpy system\ncannot be easily reduced by the polymer resulting in very sluggish H evolution. The more easily\n2\nreduced ester bpy can however be reduced.\nResponse: The text should now present the redox potential, as well as a more detailed\nexplanation for this in the first paragraph of 2.2:\nPage 5 of 11\n“The molecular HEC differ in the bipyridine type ligand which determines the redox\npotential of the rhodium center (HEC3 = −0.9 V vs. Fc+/Fc33,34, HEC4 =−1.1 V vs.\nFc+/Fc.34,35), thus, allowing for different driving forces of electron transfer between the\nreduced polymer and the putative catalytic center.”\n10. Does the quotation of 101 C per gram polymer on p. 9 take into account the incomplete\ncharging found in the Ru-bpy work?\nResponse: It does; the exact calculation has been implemented in the SI and can be found in\nchapter 3.2.\n11. The colors of the data markers in Fig 3 are too similar to be easily understood by readers.\nResponse: Color and markers have been adjusted.\n12. Does the Pt NP system take up H and hydrogenate the polymer? In other words does the Pt\n2\ntake up added H with the oxidized polymer and reduce it or hydrogenate some functionalities?\n2\nflask upon generation over 24 h? This behavior should be explained. If the system is being\nhydrogenated, it would suggest limited usefulness with HEC2, which appears later in the\nrecyclability section.\nResponse: Studies do not suggest hydrogenation or recharging of the polymer in the presence\nof hydrogen and Pt(0) catalyst. With the multi-cycling experiments at hand, it can be further proven\nthat the polymer was not hydrogenated, as it is possible to charge the polymer multiple times to a\nhigh degree. Similarly, a reduction of the polymer by the catalyst could not be observed in the\ndischarging process.\nFurthermore, an experiment with uncharged polymer and Pt(0) catalyst in the presence of about\n50 to 100 times the concentration of hydrogen present during usual catalysis was performed. The\nabsorption over time shows no significant change, indicating charging or hydrogenation of the\npolymer, which is now stated at the end of chapter 2.2:\n“Possible side reactions, reducing or hydrogenating of the polymer were ruled out by\nabsorption studies in the presence of excess hydrogen under catalytic conditions (see Fig.\nS8).”\n13. How was the polymer stored upon charging but before the H release as described on p. 10.\n2 2 2\nequivalents. Was the polymer ever isolated as a solid or was it always kept in solution?\nResponse: Water is the only solvent that is employed and the polymer is highly oxygen sensitive,\nso the samples are handled appropriately in sealed vials when extracting them from the glovebox.\nThe detailed and revised description of the sample preparation is given in the SI in Chapter 1.\nFurthermore, spectra of a charged sample before and after aerating it has been added to the SI\nas Fig. S3, emphasizing the immediate re-oxidation of charged polymer by the oxygen, which is\nnow mentioned in the manuscript in the end of Chapter 2.1:\n“This charge is lost instantly upon aerating the sample, highlighting the necessity of the\ninert handling of samples (see Fig. S3).”\nPage 6 of 11\nWe did try to extract the polymer from the solution in earlier stages of the research with little\nsuccess, due to the small sample size.\n14.Plotting H released as a function of time delay after charging could be useful for readers as a\n2\nfigure in main text or SI. It appears from the data available so far that there is a rapid decrease in\nperformance and then a slower phase as well.\nResponse: We included a graph Fig. S13 in the SI, which shows the described catalytic loss,\ndepending on the storage time. It also contains a fit to quantify the slower loss period.\n15. Does the pH swing system required for function of this polymer mean that ionic strength was\nnot constant in solution over the multi-cycling experiments? Was any control done to test for an\ninfluence of ionic strength on charging and H evolution efficiency, in the testing described on p.\n2\n11 or so?\nResponse: The pH-swing resulted in an increasing ionic strength with every cycling process. We\ntherefore devised an experiment simulating the increased ionic strength of a fourth cycle directly\n(so in a first cycle). The manuscript now features a description of this experiment, as well as an\nevaluation of these results at the end of Chapter 2.4. The experiment revealed that charging is\ndiminished with increasing ionic strength, while catalysis is not, meaning decreases in catalytic\nefficiency are the result of degradation due to pH-changes, rather than the ionic strength itself.\n“To get more insights into potential loss channels, we devised an experiment to simulate\nthe ionic strength of a fourth cycle as starting conditions for both processes. Therefore, the\nrespective amount of sodium chloride is utilized, as it is the source for both ions present in\nthe system. The measurements show that an increase in ionic strength does reduce the\nsystems capabilities to be charged, yielding an explanation for the slight, but steady loss\nin charging efficiency. The catalysis on the other hand still reaches efficiencies of 68  1%\nfor HEC2, indicating that the catalyst is not degrading due to ionic strength, but rather due\nto repeated pH-changes.”\n16. The ‘small scale approach’ and ‘intrinsic requirements of the reaction system’ are described\nas reasons for the variable and somewhat unexpectedly variable results across multicycling on p.\n12. These reasons/speculations are not clear or appropriate. Are the authors saying that the\nexperiments are so difficult that they cannot be done with reliable analytical quality? Please revise\nthis section. Perhaps more technical replicates are required to establish reliable performance\nmetrics of this system?\nResponse: With the improved procedure (mentioned above), we were able to clear these issues,\nenhancing reproducibility. With the pH-value being a key factor in controlling the experimental\nconditions, the usage of a needle pH-meter and therefor the ability to investigate the pH-value in-\noperando did greatly improve the reproducibility. Especially the multi-cycling with several\nacidification and neutralization steps posed a major difficulty in the beginning, where the pH-value\nwas determined via titrations in a dummy system (for reference, we are employing samples in the\nµg range, which are very pH-responsive resulting in difficulties for controlling the exact switch\nbeyond a second cycle, under inert conditions and prior to the in-operando pH-measurements with\na needle-pH-meter).\nPage 7 of 11\n17. HEC3 appears non-recyclable as it starts to behave quite poorly for hydrogen release in Fig.\nover 4 cycles of use. Is this because of degradation of the catalyst?\nResponse: Results so far indicate that degradation due to the constant changes in pH-value is in\nfact an issue. Again, the updated data shows improved reproducibility, summarized in Fig. 6, which\nshows comparable recycling capabilities to the Pt catalyst, albeit with slightly inferior catalytic\nefficiency.\n18. What cycling time was used for each stage of the multi-cycling? I ask because HEC2 is the\nhigh performer, but its performance was clearly time dependent in Fig 4. The time utilized for the\nH release should be noted carefully in the work on p 14 for the multicycling efficiency test. This is\n2\nbecause the time for the H cycle should make a large difference in the metrics achieved for the\n2\nmulti-cycle efficiency, right?\nResponse: The description of the experimental procedure given in the first chapter of the SI has\nbeen changed to clarify how multi-cycling is performed.\n“These measurements were carried out over a time span of up to six hours in fixed,\nreasonable intervals to ensure comparability among various experiments. Based on these\nresults, catalytic analysis of a cycle beyond the first one, measurements were performed\nafter two hours and after three hours.”\nReviewer 2:\nThe authors have reported the first system based on organic polymers that can be charged\nphotochemically and subsequently drive hydrogen evolution on demand. Additionally, this system\npossesses regenerative capabilities that enable recycling and multiple catalytic cycles. In the\npresence of triethylamine as a sacrificial donor, a charging efficiency of over 60% can be achieved,\nwhich remains stable for multiple days. By subsequent acidification and the addition of different\ncatalysts, the stored electrons can be utilized on demand for proton reduction to hydrogen with an\nefficiency of up to 72%. Overall, this work is intriguing and the manuscript is well-written. However,\nthere are some important issues that need to be addressed, which can help largely increase the\nimpact of this manuscript.\n1. It is good to design catalysts and obtain high activity. It would be even better if meanwhile new\ncatalysts except for HEC1, HEC2, HEC3, and HEC4 could be offered.\nResponse: With the data at hand (initial and revised submission) we completely agree and plan\non establishing new and more robust catalysts. Meanwhile, this manuscript will focus on\nestablishing the polymer in this type of chemistry and, thus, focusing on the utilization of\nestablished catalysts before taking this next step.\nPage 8 of 11\n2. Although redox-active polymer synthesis has been previously reported in literature, more\ndetailed experimental procedures and characterization results should be provided in this\nmanuscript.\nResponse: The synthesis of the polymer is now shortly described in detail in the first chapter of\nthe Supporting Information and a summary is given in Chapter 2.1 in the manuscript:\n“In short, the polymer was obtained by a radical copolymerization of a styrene derivative\nfeaturing an ammonium group in order to ensure water-solubility as well as a styrene\nbearing the redox-active unit – the viologen. The latter monomer was employed enabling\nthe electron storage functionality. The polymerization was performed in water. The ratio\nof those two moieties in the final copolymer was 1:1, which was confirmed by NMR\nspectroscopy, similar to a previous study.22”\n3. Please provide a better explanation regarding the novelty of the water-soluble copolymer\nsystem.\nResponse: We updated respective text passages to clarify the origin of the polymer and its role\nso far. In previous studies, the polymer was employed in redox-flow battery applications, as it is\nnow stated in the manuscript in the first paragraph of 2.1. So, while the polymer itself is not new\nat this point, its application in this on-demand H -delivering system is.\n2\n“The redox-active polymer, which was previously used in redox-flow batteries, was\nsynthesized according to literature procedures.22 In short, the polymer was obtained by a\ncopolymerization of a styrene derivative featuring an ammonium group in order to ensure\nwater-solubility. As redox-active unit, a viologen-containing monomer was employed\nenabling the electron storage functionality.”\n4.It would be beneficial to discuss the stability of these catalysts and redox-active units as well as\ntheir potential application in other catalytic reactions.\nResponse: New experiments regarding potential loss channels have been added and a revised\nrepertoire of catalysts will be subject of future research and collaborations. An experiment\ninvestigating the influence of the increasing ionic strength revealed that the catalysts are not\nimpacted by ionic strength alone, added at the end of Chapter 2.4:\n“To get more insights into potential loss channels, we devised an experiment to simulate\nthe ionic strength of a fourth cycle as starting conditions for both processes. Therefore,\nthe respective amount of sodium chloride is utilized, as it is the source for both ions\npresent in the system. The measurements show that an increase in ionic strength does\nreduce the systems capabilities to be charged, yielding an explanation for the slight, but\nsteady loss in charging efficiency. The catalysis on the other hand still reaches\nefficiencies of 68  1% for HEC2, indicating that the catalyst is not degrading due to ionic\nstrength, but rather due to repeated pH changes. For further insights structural analysis\nof the polymer as well as the catalysts will be subject of future research.”\nTogether with the stability of the charging process throughout all four cycles (seen in Fig. 6), it can\nalso be concluded that the polymer, while impacted by an increasing ionic strength is stable within\nthe given pH-range.\nPage 9 of 11\n5. The significant variation observed in discharge kinetics when different HEC is selected needs\nfurther clarification.\nResponse: Respective paragraphs have been updated and the necessary information has been\nadded for clarification. This includes the addition of previously missing CV data in the first\nparagraph of 2.2:\n“The molecular HEC differ in the bipyridine type ligand which determines the redox\npotential of the rhodium center (HEC3 = −0.9 V vs. Fc+/Fc34,35, HEC4 =−1.1 V vs.\nFc+/Fc.35,36), thus, allowing for different driving forces of electron transfer between the\nreduced polymer and the putative catalytic center.”\nAs well as a more detailed description for Fig. 2:\n“As shown in Fig. 2, after acidification of the solutions, the addition of different HECs led\nto an efficient discharging of the photochemically reduced polymer. However, depending\non the selected HEC, the discharging kinetics varied significantly. Whereas the Pt-based\nHECs (HEC1 and HEC2) discharged the polymer almost completely within the first\n20 min (see Fig. 2Error! Reference source not found. for HEC2), much slower\ndischarging was observed in the case of the rhodium-based catalysts (see also Fig. S7).\nHEC1 hereby outpaces the colloidal platinum, which is again indicative of the in-situ\nparticle formation described above. These findings are also in line with the redox\npotentials of the respective catalysts in comparison to the reduced polymer and the\nresulting driving forces. The offsets in the degree of charging at t = 0 are the result of the\nfast discharging occurring between sample preparation and measurement of the first data\npoint.”\n6. In Fig. S1, it appears that there is not much difference between the degree of charging at pH =\n7 compared to pH = 8 but it differs greatly from other pH conditions. Please explain why this\ndiscrepancy exists.\nResponse: The electrochemistry of viologens is stable at neutral pH-values. Higher pH-values\nlead to a degradation of the viologens via dealkylation as it now states more precisely in\nChapter 2.1 of the manuscript:\n“The system was buffered with NaH PO to guarantee structural integrity of the polymer,\n2 4\nas it is able to undergo irreversible dealkylation processes at higher pH-values (see Fig.\nS1).23,24”\nThe hydroxyl ions will form methanol and the monoalkylated bipyridine is formed. Therefore, the\nuse is limited to close to neutral conditions. Noteworthy, this degradation mechanism is also\nimportant for the use of viologens in redox flow batteries (E. S. Beh, D. De Porcellinis, R. L. Gracia,\nK. T. Xia, R. G. Gordon, M. J. Aziz, ACS Energy Lett. 2017, 2, 639-644).\n7. Please provide evidence supporting enhanced solubility of monomers and copolymers as well\nas proof for formation of a completely water-soluble redox-active soft matter matrix within your\nmanuscript.\nResponse: A mentioned above, the viologen copolymers has been previously used as active\nmaterial in polymer redox flow batteries (see our previous work: T. Janoschka, S. Morgenstern,\nH. Hiller, C. Friebe, K. Wolkersdörfer, B. Häupler, M. D. Hager, U. S. Schubert, Polym. Chem.\n2015, 6, 7801-7811 & T. Janoschka, N. Martin, U. Martin, C. Friebe, S. Morgenstern, H. Hiller, M.\nPage 10 of 11\nD. Hager, U. S. Schubert, Nature 2015, 527, 78-81). Hereby, aqueous electrolytes have been\nutilized. The copolymer was well soluble in water. For comparison, the viologen homopolymer\nwithout the solubility promoting comonomer is still good water soluble, even in the reduced state\n(radical cation instead of the dication). The observations in the lab are also substantiated by DLS\nmeasurements (see Nature 2015, 527, 78-81), in which only small sizes (ca. 2 nm) have been\nobserved. Due to the very small sizes, no larger aggregates like micelles and particles are present.\nA remark on this previous study has been implemented in the manuscript in Chapter 2.1:\n“The redox-active polymer, which was previously used in aqueous polymer redox-flow\nbatteries, was synthesized according to literature procedures.22”\nPage 11 of 11"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised version of this paper has addressed virtually all of my concerns from the prior round of review.\nConcerns related to the number of replicate experiments have been addressed through both a greater\nnumber of replicates as well as improved analytical methods. Other points have been addressed as well in\nways that I view are acceptable. Needed additional data to understand the context of the results have been\nincluded in the revised version as well. In short, this paper now appears ready for publication and I suggest\nthat it be accepted.\nOne small question that occurred to me after re-reading the materials was the question of degradation of\nthe rhodium molecular catalyst over multicycling. Was characterization of the Rh component done after one\nor more cycles of catalysis? If so, the results being added to the SI would be recommended. Maybe the 1H\nNMR of the catalyst is available and can be compared before/after run(s)?", "reviewer_2": "In the revised manuscript, the authors have properly addressed the concerns from previous reviewers. Therefore, I would\nlike to recommend publication of this manuscript at its current version."}, "author_response": "Point-by-point response to the reviewers´ comments\nReviewer 1:\nIn the revised version of the manuscript, the authors have provided a quite thorough reply to my\nprior comment requesting that the fate of the molecular catalyst be examined. Through very\ndetailed analysis involving several techniques, the authors have identified a degradation pathway\nof their catalyst involving hydrolysis of ester groups under low-pH conditions. I emphasize here\nthat the identification of this degradation pathway in no way tampers my enthusiasm for this report-\n-rather, the very thorough and chemically sensible conclusions drawn by the authors add\nsignificantly to the quality of the science presented here. It is a lamentable draw-back of many\nchemical studies of molecular catalysts for energy conversion and storage that degradation\npathways and longevity studies are not studied in more detail--this study now overcomes this\ncommon problem. I have reviewed the new data from Raman studies provided by the authors and\nagree with their interpretations.\nResponse: We thank the reviewer for his suggestion to elucidate the fate of the catalyst by suitable\nmethods. We agree that such studies are rarely reported but would add significant benefit to many\nreports.\nIn terms of the path to wrap this up, I have only a single final suggestion for their consideration.\nOn page 2 of their response document, they refer to a d8 to pi* MLCT for the Rh(I) species. The\nnoted spectral features between 540 and 840 nm, from what I know of species of this type, given\nthem an appealing dark coloration (in derivatives that I have studied, purple). However, rather than\njust being a d8 to pi* transition, this coloration can be also thought of arising from mixed [Rh(I) d8\n+bpy pi*] to [pi*] transitions, owing to delocalization of charge at the Rh(I) state into the first LUMO\nof the bpy system. The evidence for this comes often from vibronic features that, granted, are not\nobvious in the new spectra in Fig S16 and shown on p. 3 of the response document. So,\nconsidering all this, the authors may wish to adjust their interpretation/description of the origins of\nthe new beautiful absorptions slightly. This is optional however and I emphasize that this does not\nneed to be reviewed by me again.\nResponse: We thank the reviewer for pointing out this issue. Indeed, the absorption band of the\nreduced HEC3 (now termed compound 3 according to the editorial requests) between 540 and\n840 nm is obtained from a superposition of excited states involving Rh(I) d8  bpy MLCT AND\nbpy pi*  pi* (partially reduced bpy-ligand) transitions, as known from the literature (DOI:\n10.1039/B504286K). Nevertheless, the expected vibrationally structured absorption bands from\nthe bpy radical anion in the visible and near-infrared regions is difficult to detect as these bands\nare superimposed by the absorption of the Rh(I)  bpy MLCT.\nA shortened version of this response has been included on page 8 of the SI document.\nThis is a lovely contribution to the literature of Rh catalysis, energy conversion, and the\nopportunities and challenges of building systems for energy storage in chemical bonds. I\ncongratulate the authors on their exceptionally beautiful and thorough work. And, I look forward to\nseeing it in print for all to read very soon.\nPage 2 of 4"}, {"round": 3, "reviewer_comments": {"reviewer_1": "In the revised version of the manuscript, the authors have provided a quite thorough reply to my prior comment requesting\nthat the fate of the molecular catalyst be examined. Through very detailed analysis involving several techniques, the authors\nhave identified a degradation pathway of their catalyst involving hydrolysis of ester groups under low-pH conditions. I\nemphasize here that the identification of this degradation pathway in no way tampers my enthusiasm for this report--rather,\nthe very thorough and chemically sensible conclusions drawn by the authors add significantly to the quality of the science\npresented here. It is a lamentable draw-back of many chemical studies of molecular catalysts for energy conversion and\nstorage that degradation pathways and longevity studies are not studied in more detail--this study now overcomes this\ncommon problem. I have reviewed the new data from Raman studies provided by the authors and agree with their\ninterpretations.\nIn terms of the path to wrap this up, I have only a single final suggestion for their consideration. On page 2 of their response\ndocument, they refer to a d8 to pi* MLCT for the Rh(I) species. The noted spectral features between 540 and 840 nm, from\nwhat I know of species of this type, given them an appealing dark coloration (in derivatives that I have studied, purple).\nHowever, rather than just being a d8 to pi* transition, this coloration can be also thought of arising from mixed [Rh(I) d8\n+ bpy pi*] to [pi*] transitions, owing to delocalization of charge at the Rh(I) state into the first LUMO of the bpy system. The evidence\nfor this comes often from vibronic features that, granted, are not obvious in the new spectra in Fig S16 and shown on p. 3 of\nthe response document. So, considering all this, the authors may wish to adjust their interpretation/description of the origins\nof the new beautiful absorptions slightly. This is optional however and I emphasize that this does not need to be reviewed by\nme again.\nThis is a lovely contribution to the literature of Rh catalysis, energy conversion, and the opportunities and challenges of\nbuilding systems for energy storage in chemical bonds. I congratulate the authors on their exceptionally beautiful and\nthorough work. And, I look forward to seeing it in print for all to read very soon."}, "author_response": "We thank the reviewer for this highly interesting and relevant question. We performed\nextensive experimental investigations to answer the raised question. 1H NMR, Raman- and\nresonance Raman-spectroscopy were used.\nThe attempts to perform in situ 1H NMR investigations to identify the fate of the rhodium catalyst\nwere not successful due to the dynamics of the ligand environment at the Rh center in presence\nof several potential ligands binding to Rh instead of Cl-. This was the case in solutions containing\nbuffer and sacrificial donor with and without addition of acid that should mimic the conditions of\nthe acid – base swing. As already under these conditions a non-interpretable 1H NMR spectrum\nwas obtained, further acid/base swings were not examined.\nWe than turned to Raman spectroscopy as it would be employable in aqueous media, aiming for\nthe carboxylic ester moiety of the bipyridine ligand of the rhodium catalyst. This moiety can clearly\nbe identified by its Raman band at 1726 cm-1 which could be measured. However, in a catalytic\nmixture and upon photocatalytic charging of the polymer new signals corresponding to the reduced\npolymer were dominating the spectra in the corresponding wavenumber section, thus preventing\nthe identification of the carboxylic ester moiety within the rhodium catalyst and consequently\nmaking it impossible to analyze the fate of the Rh catalyst with this method. The SI now includes\nthis study (see Figure S15 and pages S6-S7):\n“In this study the different components were measured, both each one individually and in\nan upscaled catalytic mixture, as displayed in 0. This reveals, that beyond the bipyridine\nvibrations, that are shared with the photosensitizer, the C=O double bond vibration of the\nester can be observed very well in the individual measurement. In a catalytic mixture, the\ndominating vibrations are associated with the sacrificial donor and the polymer, due to the\ndifferences in concentration (1:5 for HEC3:polymer, 1:2000 for HEC3:TEA) and the nature\nof the vibronic signals. Due to the relatively low concentration of the catalyst in the catalytic\nmixture the identification of the carboxylic ester moiety within the rhodium catalyst was not\npossible. Nevertheless, in operando nonresonant FT-Raman studies were helpful to\nenable a reliable band assignment of the resonance Raman modes of the catalyst.”\nWe than decided to utilize resonance Raman (rR) spectroscopy which can be employed for similar\nrhodium complexes as shown in literature.1 Chemical reduction of the rhodium catalyst with\nformate to the Rh(I) state shifts the absorption maximum of the MLCT (Rh(d8) à bpy-π*) to the\nred with a very broad and strong absorption between 540 and 840 nm (see Fig. S16). This can be\nconsidered a chemical activation step of the Rh complexes for selectively investigating the Rh\ncomplexes as well as structural changes to them by rR spectroscopy using excitation in the red\npart of the spectrum, i.e., with a 643 nm laser. The rR data revealed, that exposing the Rh-catalyst\nto one or more pH cycles does cause the ester group to be lost, while an intact chromophore is\nstill maintained throughout the entire process as bipyridine vibrations are observed also after the\nfirst acid/base swing. Our interpretation is therefore, that the ester moiety is cleaved during the\nfirst change of redox state of the rhodium and additionally the alteration of the pH from slightly\nbasic to acidic and back. The respective results and interpretation are now included in the SI (see\nFigure S17 and pages S7-S8):\n“Upon reducing the Rh(III) catalyst (HEC3) using sodium formate, the formed Rh(I) species\nwas investigated using resonance Raman spectroscopy at 643 nm. At this excitation\nwavelength only the catalytically competent intermediate, i.e., the Rh(I) complex is\nabsorbing. While we observed the Raman band of the carbonyl function at 1694 cm-1 for\nthe initial Rh(I) species, indicating an intact ester functional group at the bipyridine\nchromophore, this band vanishes after one or more exposure steps to low pH values. This\nis most likely because the functional group is hydrolyzed. While the C=O double bond is\ninitially detected with an intact ester functional group, when the ethoxy group is cleaved at\nlower pH, the charges become delocalized, and the carbonyl vibration disappears. In\naddition, since the π electrons of the carbonyl group are also conjugated to the bipyridine\nchromophore, the change in the ester functional group also impacts the intensity ratios of\nthe bipyridine bands, as can be seen when comparing the spectra of Rh(I)-0 (i.e. HEC3\nexposed to no pH swing) with Rh(I)-1 to -4 (i.e., HEC3 exposed to 1 to 4 pH swings; see\nFehler! Verweisquelle konnte nicht gefunden werden.).”\nThe corresponding conclusions that were drawn from this set of investigations was included into\nthe main manuscript at the end of chapter 2.4. and reads as follows (pages 15-16):\n“For HEC3, this pH-induced degradation was confirmed by performing resonance Raman\nmeasurements of the catalyst exposed to the conditions of the acid base cycle during\ncatalytic charging and discharging (see Fig. S17). It could be shown that upon addition of\nacid, already at the first discharging step the ester functionality at the bipyridine was\ncleaved. From cycle 2 onwards, H evolution is thus no longer catalyzed by HEC3 but by\n2\na structurally altered bipyridine-based Rh complex.”"}]

Decision letter