Polymer-Bound DuBois-Type Molecular H2-Oxidation Ni-Catalysts are

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Polymer-Bound DuBois-Type Molecular H-Oxidation NiCatalysts are Protected by Redox Polymer Matrices Adrian Ruff, Salome Janke, Julian Szczesny, Sabine Alsaoub, Ines Ruff, Wolfgang Lubitz, and Wolfgang Schuhmann ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00269 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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ACS Applied Energy Materials

Polymer-Bound

DuBois-Type

Molecular

H2-

Oxidation Ni-Catalysts are Protected by Redox Polymer Matrices Adrian Ruff1,*, Salome Janke1, Julian Szczesny1, Sabine Alsaoub1, Ines Ruff4, Wolfgang Lubitz3, and Wolfgang Schuhmann 1,*

1Analytical

Chemistry - Center for Electrochemical Sciences (CES), Faculty of

Chemistry and Biochemistry, Ruhr University Bochum, Universitätsstrasse 150, 44780 Bochum, Germany.

2Thermo

Fisher Scientific, Im Steingrund 4-6, 63303 Dreieich, Germany

3Max-Planck-Institut

für Chemische Energiekonversion, Stiftstrasse 34 – 36, 45470

Mülheim an der Ruhr, Germany.

Corresponding Authors

ACS Paragon Plus Environment

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*[email protected]; [email protected]

ABSTRACT. The immobilization, protection and electrical wiring of sensitive catalysts by specifically designed supporting matrixes is of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H2-oxidation catalyst, that was covalently bound to an inert polymer matrix, against molecular O2 by forming blends together with an O2-reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode.

KEYWORDS. Redox polymers, DuBois type catalysts, H2 oxidation, energy conversion, gas diffusion electrodes, mediated electron transfer.

INTRODUCTION

Molecular organometallic complexes for H2 oxidation and/or production based on earthabundant metals are seen as promising artificial alternatives to expensive and scarce


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noble metal catalysts. Prominent examples are DuBois type catalysts1–7, cobaloxims 8–10 or heterodinuclear NiFe and Fe or Ni based hydrogenase mimics11–14. Some of these artificial compounds are modeled according to the active centers of nature´s highly efficient but also highly sensitive hydrogenases and can be synthesized from rather simple metal precursors and various ligands in coordination reactions. Consequently, their properties can be tuned by a careful ligand design.2,9,15,16 In particular, DuBois-type catalysts equipped with pendant bases attached to their ligand skeleton ensure the formation of proton channels around their active centers and thus enhance their catalytic activity.2,17–20 Moreover, the introduction of charged or pH-dependent groups to DuBois type catalysts alters their activity17 and allow for the preparation of water soluble complexes that show even bidirectional catalysis (H2 oxidation and production).21,22 In some cases the molecular catalyst is not the active species itself but acts as precursor for the in-situ formation of the actual catalyst, as it was demonstrated by the groups of Artero et al. and Hetterscheid et al. for a cobalt based H2 evolution complex15 and a copper based oxygen reduction catalyst23, respectively. Both catalysts form deposits on the electrode surface which are in fact the true active species. In contrast, for molecular


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DuBois-type catalysts such a behavior was not reported so far and it is assumed that the molecular structure of these complexes indeed resembles the active site of the catalyst. However,

for

potential

technological

relevant

applications,

the

heterogenization/immobilization of such catalysts on solid supports, i.e. an electrode surfaces in electrocatalysis, is of major importance.24 Moreover, by a careful selection and design of the supporting matrices, highly sensitive catalysts can be protected from detrimental interferences like molecular oxygen, as it was demonstrated for highly airsensitive hydrogenases by their integration within low-potential viologen-modified polymers. These materials served as hydrophilic immobilization matrixes which were able to reduce incoming oxygen at the polymer/electrolyte interface via the reduced viologen based mediator.25–27 Other concepts for the protection of sensitive catalysts involve the use of electrode materials that facilitate O2 reduction10,28, the use of bienzymatic O2 scavenging systems29,30, or the use of the active species itself that can act as a catalyst for the oxygen reduction reaction (ORR) as it was shown for Co-complexes31 . Evidently, in the latter case, the catalyst is directly exposed to detrimental interferences and may thus show a limited lifetime. Moreover, rather high amounts of the catalyst are required


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to ensure and maintain a good performance in the presence of detrimental substances that will react with and thus block the active sites from contributing to the power output of an envisaged device.

Immobilization strategies for DuBois type H2-oxidation catalysts involved the modification of the electrode surface with a catalyst monolayer via amide chemistry32–34 and the immobilization of a positively charged Ni-catalyst on chemically modified carbon nanotubes35. The latter system revealed benchmark current densities for H2-oxidation of up to 16 mA cm-2 and was successfully incorporated into a H2/O2 powered fuel cell.35 Both immobilization strategies lead to high current density anodes but lack of protection of those air sensitive catalysts. 28 Recently, we demonstrated that the DuBois type Nicatalyst Ni-CyGly (Scheme 1) is able to establish a self-protection mechanism when incorporated into thick hydrophobic redox silent polymer film due to the oxygen reduction ability of the reduced Ni-complex.36 The thick polymer layer ensures a spatial separation of the film into an active zone close to the electrode surface and an outer protective zone at the polymer/electrolyte interface that is not electrically wired to the


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electrode surface (the Ni-catalyst is only reduced/oxidized by H2/O2, respectively).36 While protection of the catalyst could be achieved in this configuration, the catalytic current is limited by the rather thin reaction layer of the active catalyst due to a limited productive wiring of the Ni-complex in the polymer film: only Ni-catalysts near the electrode surface will transfer electrons from the H2 oxidation to the electron collector. In addition, a major part of the Ni-catalyst must be used to protect the underlying active layer and is thus wasted. Evidently, a redox matrix that allows for a more productive wiring of a large amount of the catalyst in a mediated electron transfer regime would be beneficial. Moreover, if the mediator is able to reduce oxygen, in analogy to the viologen-based redox mediators in case of hydrogenases, an efficient protection against O2 should be possible (Scheme 2a).

Here we report the wiring and protection of an air sensitive DuBois type catalyst that was bound to an inert polymer matrix by mixing this catalytically active polymer matrix with a low potential redox polymer capable of reducing O2. The polymer composite films show pronounced activities towards H2 oxidation even in the presence of O2.


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RESULTS AND DISCUSSION

Since the redox potential of the Ni-CyGly (≈-260 mV vs. SHE at pH 7)33 is more positive than the redox potential of low-potential viologen-modified polymers (-200 to -300 mV vs. SHE) used for the protection of air-sensitive hydrogenases25–27, the viologen-based mediators will not accept electrons from the catalyst and thus no protection is possible. Os-complexes with rather low redox potentials < +280 mV vs. SHE show the ability to reduce O2.37 Mao et al. reported the synthesis of the low potential Os-complex [Os(BiImMe2)2(BiImNH2)]3+ (Scheme 1) bearing three diimidazole-based ligands (BiImMe2, strong electron donors) that shift the potential of this complex to a value of +40 mV vs. SHE.38 An amine functionality in the ligand sphere of the complex allows for a covalent attachment to polymer backbones bearing electrophilic groups (e.g. epoxides39 or activated carboxylic acids38).


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We therefore decided to bind this complex to a rather hydrophobic polymer backbone,

i.e. P(CyMA-MAA) that shows a molecular weight Mw of around 67 kDa with a poly dispersity index of 2.1 (for a detailed characterization of the polymer backbone including a spectroscopic characterization, see Section 2 in the Supporting Information). The carboxylic acid functions in MAA (= methacrylic acid) were transformed by SOCl2 to the corresponding reactive acid chlorides (Scheme 1, b). In the presence of a base (NEt3), the low potential Os-complex could be easily attached to the polymer backbone to yield the redox polymer P(CyMA-MAA)-Oslow (for synthetic details see Supporting Information). FT-IR experiments of the polymer backbone and the Os-complex modified polymer clearly demonstrate the amide formation and indicate successful attachment (Figure S1). Cyclic voltammograms of glassy carbon electrodes coated with P(CyMAMAA)-Oslow films under argon and air atmosphere unambiguously show that the synthesized redox polymer is able to reduce O2: under air a catalytic wave centered at the mid-point potential of the polymer bound Os-complex (+0.01 V vs. SHE) was observed (Figure S2a).


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Scheme 1: Synthesis of the active Ni-catalyst modified polymer P(CyMA-MAA)-Ni and the low potential Os-complex modified polymer-based immobilization and protection matrix P(CyMA-MAA)-Oslow.a

a

The polymer backbone P(CyMA-MAA) was synthesized from the copolymerization of the

comonomers cyclohexyl methacrylate (CyMA) and methacrylic acid (MAA) by using AIBN (azobisisobutyronitrile)) as initiator (a) with n = 4 and m = 1 (determined via the integral ratio extracted from the 1H NMR spectrum, see Synthesis part in the Supporting Information). The carboxylic acid function within P(CyMA-MAA) was activated by means of SOCl2 in anhydrous tetrahydrofuran (THF) at room temperature (r.t.) to yield P(CyMA-MAA)activated (b). The obtained acid chloride was reacted with the low potential Os-complex Os(BiImMe2)2(BiImNH2) (amide formation) to form the immobilization and protection matrix P(CyMA-MAA)-Oslow (c) and with the auto-reduced Ni-catalyst21 (d) under ester formation to yield P(CyMA-MAA)-Ni (e).


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Attempts to attach Ni-CyGly to the Os-complex modified polymer simultaneously by using amide formation chemistry failed. Thus, we followed an alternative route that included the formation of a polymer blend composed of the P(CyMA-MAA)-Oslow polymer and a corresponding Ni-complex modified polymer. To avoid phase separation in the polymer blend we used again the P(CyMA-MAA) backbone as the support for the immobilization of the Ni-complex. For this, the DuBois type catalyst was first subjected to an auto-reduction reaction in water that selectively converts only one of the four carboxylic acid groups into a hydroxy moiety (Scheme 1, d, for a more detailed description of the auto-reduction process the reader is referred to ref. 21). The successful decarboxylation process is indicated by a color change form purple-red to yellow21 which can be easily followed by the bare eye. The newly created -OH function was then reacted with the activated acid chloride P(CyMA-MAA)activated to yield the catalytically active polymer P(CyMA-MAA)-Ni (note that due to the limited amount of complex and due to the sensitivity of the reduced Ni-catalyst the modification was


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performed in-situ under glove box conditions prior to electrode modification and the product was not isolated, vide infra).


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Scheme 2: Protection of a DuBois type catalyst from O2 via the immobilization in a redox polymer on a flat electrode substrate (a) and the same electrode configuration in gas diffusion mode (b) for stable high-performance H2-oxidation anodes. For a colored version of the figure the reader is referred to the online version of the article.

Electrodes coated with a P(CyMA-MAA)-Ni layer showed a chemically reversible reduction signal centered at -25 mV vs. SHE which is attributed to the reversible two electron reduction of the Ni2+/Ni0 couple at pH 2.3 (Figure 1a, black line, note that at pH values of < 3, the freely diffusing Ni-catalyst shows its highest activities21). The weak and distorted signals of the Ni-catalyst are attributed to a hampered diffusion of counter ions into the film under potential cycling due to the hydrophilic nature of the polymer


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matrix and a slow electron transport within the film because of the restricted mobility of the individual covalently bound Ni-complexes. Under turnover conditions (100 % H2, Figure 1a, red lines) a catalytic response, which was stable over multiple cycles was observed demonstrating that the Ni-complex is still active after the covalent attachment to the inert polymer backbone. Chronoamperometric experiments under alternating Ar/H2 (10 %/90 %) and O2/Ar/H2 (3 %/7 %/90 %) gas feeds showed that upon addition of O2 the catalytic activity was decreased to ≈50 % with respect to the oxidation current after the first O2 addition. This contrasts with our previous observations where a certain degree of protection was observed when the free Ni-CyGly catalyst was embedded into a redox silent hydrophobic polymer matrix.36 The fast deactivation of the catalyst is most likely due to the improved electron transfer along the Ni-catalyst modified polymer chains because of favored electron self-exchange reactions due to the short distance between two adjacent centers. Consequently, under turnover conditions the catalyst is predominantly in its more sensitive oxidized state that promotes deactivation of the Nicomplex40. In addition, rather thin films were used for the experiments (nominal polymer loading: 4.3 (mg backbone) cm-2) in contrast to the experiments reported in ref. 36 (5.7


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(mg backbone) cm-2). Voltammograms recorded after the chronoamperometric experiments in the presence of O2 revealed that the catalyst was indeed completely destroyed during O2 exposure: signals of the Ni2+/Ni0 interconversion were very weak or even absent in the corresponding I-E curves (Figures 1a, blue trace). This experiment unequivocally shows that indeed a protection is required for the immobilized active molecular Ni complex.

Figure 1: Voltammetric and chronoamperometric characterization of a thin P(CyMA-MAA)-Ni film coated onto a glassy carbon electrode in 0.1 M NaClO4/water (pH 2.3). a) Cyclic voltammetry of a P(CyMA-MAA)-Ni film modified electrode under 100 % argon (black and blue dashed line) and with 100 % H2 (red line, multiple cycles, measured subsequently); scan rate = 5 mV s-1. The midpoint potential of the polymer bound Ni-complex was determined to be -20 mV vs. SHE (black line); blue dashed line: measured after chronoamperometry. b) Chronoamperometric experiment with alternating Ar/H2 (10 %/90 %) and Ar/H2/O2 (7 %/90 %/ 3 %) gas feeds and at an applied


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potential Eapp = +210 mV. Electrode loading: 4 µL of the in-situ prepared P(CyMA-MAA)-Ni solution (Ni-CyGly solution used for preparation: 100 mg mL-1).

To wire the polymer bound Ni-complexes with the Os-complex modified polymer P(CyMA-MAA)-Oslow the two polymer solutions were first blended in the corresponding ratio, then drop coated onto the electrode surface, and dried under glove box conditions. The P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oslow composite films coated on a glassy carbon electrode show again a single but pronounced reversible redox signal which is attributed to the overlapping signals of the Ni2+/Ni0 (-20 mV vs. SHE) and Os3+/Os2+ (+10 mV vs. SHE) reaction (note that the amount of the polymer-bound Nicatalyst is very small as compared to the amount of the polymer-tethered Os-complex, see experimental section and Figures 1 and 2). Under turn over conditions, a pronounced catalytic wave with steady state currents of close to 75 µA cm-2 at potentials > 0.1 V vs. SHE was observed that is substantially higher than that in case of the single P(CyMA-MAA)-Ni layer (≈23 µA cm-2 at 0.3 V; note that the same amount of P(CyMA-MAA)-Ni was used for both experiments). The higher currents indicate a productive wiring of a high amount of Ni-catalyst. The half wave potential of the catalytic


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wave is centered at the mid-point potential of the Os-complex which indicates that a mediated electron transfer (MET) from the Ni-complex via the Os-complex to the electrode most likely occurs. MET is a prerequisite for a successful protection against O2 by means of the redox polymer matrix when no external potentials are applied (e.g. under fuel cell conditions) since electrons for the ORR are extracted from the H2 oxidation process (Scheme 2a).26 However, since both redox potentials are overlapping in the

I-E response a clear proof for a MET is not provided by cyclic voltammetry.

Figure 2: Voltammetric and chronoamperometric characterization of a P(CyMA-MAA)Ni/P(CyMA-MAA)-Oslow composite film coated on a glassy carbon electrode in 0.1 M NaClO4/water (pH 2.3). a) Cyclic voltammetry under 100 % argon (black line) and 100 % H2 (red line); scan rate = 5 mV s-1; grey lines indicate the redox potential of the individual Ni-complex and Os-complex modified polymers. b) Chronoamperometric experiment with alternating Ar/H2 (10 %/90 %) and O2/Ar/H2 (3 %/7 %/ 90 %) gas feeds at an applied potential Eapp = +210 mV. Electrode loading: 1 µL of the in-situ prepared P(CyMA-MAA)-Ni solution (Ni-CyGly solution used


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for preparation: 100 mg mL-1); 5 µL P(CyMA-MAA)-Oslow (specific surface concentration with respect to electrode surface area: 2.14 mg cm-2).

To shed further light on the mechanism of the electron transfer process several controls were performed to verify the proposed MET mechanism: (i) modification of the electrode in a double layer configuration with an underlying P(CyMA-MAA)-Oslow layer and a top P(CyMA-MAA)-Oslow/P(CyMA-MAA)-Ni layer to prevent direct contact of the Ni-catalysts with the electrode surface; (ii) spectroscopic investigation of mixtures of the freely diffusing oxidized Os-complex and the reduced Ni-complex in solution under oxidative and reductive conditions to identify changes in the electronic structure upon electron exchange; and (iii) using a high potential Os-complex modified polymer (P(CyMA-MAA)Oshigh) with a potential of ≈+0.5 V vs. SHE (Figure S2b) that ensures a separation of the signals of the Ni-catalyst and the mediator in cyclic voltammetric experiments and facilitates the interpretation of the I-E response. For the synthesis of the high potential Os-complex pyridyl-imidazole based ligands were introduced. Compared to the diimidazole ligands in the low potential Os-complex, these ligands are rater weak


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electron donors and thus shift the potential of the corresponding Os-complex to more positive values.

All controls unambiguously show that the low potential Os-complex is indeed able to accept electrons form the reduced Ni-CyGly species: (i) catalysis is still observed even when the Ni-catalyst is not in direct contact with the electrode in the two layer configuration (Figure S3); (ii) the freely diffusing reduced Ni-catalyst transfers electrons to the freely diffusing oxidized Os-complex as indicated by the appearance of two absorption bands located between 400 and 500 nm which can be attributed to the Os(II) species (Figure S4), and (iii) when the high potential polymer P(CyMA-MAA)-Oshigh is used as electron relay matrix, the potential of the catalytic wave matches the redox potential of the Os-complex (Figure 3). The latter impressively visualizes that indeed a MET mechanism is present. We want to emphasize that in some cases contribution from direct electron transfer between the electrode and the Ni-complex was visible indicated by a current increase at the potential of the polymer bound Ni-catalyst (Figure


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3b, especially when high concentrations of the auto-reduced Ni-complex were used for polymer formation).

Figure 3: Cyclic voltammograms (a and b) recorded with P(CyMA-MAA)-Oshigh/P(CyMA-MAA)-Ni modified glassy carbon electrodes in 0.1 M NaClO4/water at pH 2.3 under turnover conditions (red traces, 100 % H2) and under non-turnover conditions (black traces, 100 % argon). Note that the redox potential of P(CyMA-MAA)-Oshigh were slightly shifted to more negative values in the composite films (cf. Figure S2b). Electrode loadings: 1 µL of in-situ prepared P(CyMA-MAA)-Ni solution and 5 µL of in-situ prepared P(CyMA-MAA)-Oshigh solution; c(NiCyGly) used for preparation: a) 10 mg mL-1 and b) 100 mg mL-1.

The catalytic current in the case of the polymer composite electrodes seems to be limited by electron transport within the polymer matrix or by mass transport since experiments conducted at pH values of 2.3, 3 and 7 with the same electrode (Figure S5) do not show differences in the steady state currents as it would be expected when


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looking at the activity profile of the freely diffusing complex (activity decreases from pH 2 to 721). However, for comparative purposes, all experiments were conducted at pH value of ≈2.3. Moreover, consecutive experiments performed within several days with the same auto-reduced Ni-CyGly and P(CyMA-MAA) activated polymer batch clearly showed that degradation of the Ni-catalyst and/or the activated polymer occurs over time even under glove box conditions. The absolute steady state currents decrease over time as indicated by the cyclic voltammograms depicted in Figure S6. In addition, the activity with respect to H2 oxidation currents of the auto-reduced form is rather sensitive and varies from batch to batch resulting in rather strong variations on the absolute currents (cf. Figure 2a and Figure S6). Thus, each preparation should be used within a short time, and the auto reduction should be repeated within short intervals. Evidently, these variations hamper a quantitative comparison of electrodes prepared from different batches, however, electrodes prepared from the same batch show identical current outputs (Figure S7) and all electrodes show the same general behavior.


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Chronoamperometric experiments (Figure 2b and Figure S8) with alternating Ar/H2 and O2/Ar/H2 gas feeds were again performed to elucidate the protection behavior of P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oslow films under aerobic conditions. After the addition of O2 (3 %) to the gas feed (O2/Ar/H2), the current is decreasing and slightly restored after switching the gas feed to Ar/H2 again in analogy to the protection of polymer embedded hydrogenases.25–27 A full deactivation as it was observed for P(CyMA-MAA)Ni films (Figure 1b) was not observed. Thus, we conclude that the low potential Oscomplex is indeed able to reduce indiffusing O2 and protects the air sensitive Ni-catalyst according to the mechanism as depicted in Scheme 2a. However, it must be noted that the protection efficiency varies with the activity and homogeneity (variation in film thickness) of the active layers, especially during the first few O2 additions (Figure S8). Compared to the single P(CyMA-MAA)-Ni layers, a clear protection effect is visible for all polymer composite films and a massive deactivation was not observed within the time scale of the experiment at oxygen levels of 3 %. For higher O2 contents (> 5 %) a rather fast deactivation was observed. It is important to note that the current response of the Os(II)/Os(III) couple in cyclic voltammograms recorded before (Figures S8a and c


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black traces) and after (Figures S8a and c, blue dashed traces) the O2 exposure experiments (Figures S8b and d) show similar amplitudes. Thus, we conclude that the polymer-bound Os-complexes are stable within the timescale of the experiments. Moreover, we want to emphasize that for thicker polymer layers, which in general favor protection27,41, a proper analysis of the electrochemical response was not possible due to the hydrophobic nature of the polymer backbones (hampered incorporation/expulsion of ions, high resistance).

When the high potential Os-complex modified polymer is used as electron relay, that is not able to reduce O2 (Figure S2b), a steady decrease of the currents is observed without restoring the catalytic response after stopping the O2 feed (Figure S9). Thus, protection is indeed based on the reduction of O2 by the low potential Os-complex polymer matrix and is not only due to a physical O2-barrier introduced by the redox polymer.

In order to demonstrate the potential applicability of the proposed approach, the active P(CyMA-MAA)-Oslow/P(CyMA-MAA)-Ni films were transposed to carbon cloth-based


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gas diffusion layers (Scheme 2b). Such electrodes were already successfully used to overcome mass transport limitations in H2-oxidation anodes based on a carbon nanotube/Ni-catalyst35, redox polymer/hydrogenase42 and hydrogenase43,44 modified electrodes by ensuring a high substrate flux at the triple phase boundary of the catalyst/electrolyte/gas interface. However, a combination of redox polymer/molecular catalyst layers with a gas diffusion electrode was – to the best of our knowledge - not reported so far. Figure 4a shows cyclic voltammograms of a P(CyMA-MAA)Oslow/P(CyMA-MAA)-Ni modified carbon cloth electrode (note that the electrode was first modified with a pristine P(CyMA-MAA)-Oslow layer to minimize direct electron transfer between the gas diffusion electrode and the Ni-catalyst at the porous structure) under breathing (red trace, H2 was provided from the back of the electrode in a gentle gas flow, Scheme 2b) and non-breathing mode (blue dashed line, H2 gas was purged through the electrolyte and was not provided from the back of the electrode, which corresponds to the situation depicted in Scheme 2a). The pronounced steady state current obtained with gas diffsuion electrdoes (350 µA at +0.4 V vs. SHE) compared to the value obtained under standard conditions (17 µA at +0.4 V vs. SHE, substrate is


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bubbled through the electrolyte) convincingly demonstrates the advantage of using gas diffusion electrodes. Maximum absolute currents of up to 415 µA were reached with good reproducibility (mean value over three independent voltammetric experiments: (350 ± 60) µA). The values are similar to those measured for redox polymer/hydrogenase modified carbon cloth based gas diffusion electrodes.42 Thus, the active polymer/catalyst films indeed show the envisaged high activity even in the immobilized state.

The operational stability under turnover conditions of the H2 oxidation gas diffusion anodes was tested in chronoamperometric experiments with an applied potential of +410 mV vs SHE (Figure 4b). After a short current increase during the initial stage of the experiment (t < 2.7 h), most likely due to a swelling process of the films in aqueous media under turnover conditions, the current is almost linearly decreasing to 63 % of its maximum value after 10 h. A similar effect was observed for flat gassy carbon electrodes (Figure S10). After 45 h only ≈20 % of the initial current remained. Cyclic voltammograms recorded before and after the long-term experiment confirmed a


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decrease of the catalytic activity (Figure S11a) whereas the signal amplitude of the polymer bound Os-complexes under non-turnover conditions considerably increased after the long-term experiment (Figure S11b) most likely due to swelling of the polymer film and thus the faster counterion diffusion during potential cycling.

Figure 4: Gas diffusion P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oslow carbon cloth electrodes. a) Cyclic voltammetry (scan rate = 10 mV s-1) under H2 (red trace) and Ar (black trace) gas diffusion mode and under turn over conditions with H2 purged through the electrolyte (blue dashed line). b) Operational stability of the modified gas diffusion electrdoes at an applied potential of +410 mV vs. SHE. a) and b) the carbon cloth was first modified with a pristine P(CyMA-MAA)-Oslow layer to avoid direct electrical communication between the electrode surface and the immobilized Nicatalysts; working electrolyte: 0.1 M NaClO4/water, pH 2. Electrode loading: underlying P(CyMAMAA)-Oslow layer: 20 µL (8.6 mg mL-1 in MeCN); active layer: 4 µL of the in-situ prepared P(CyMAMAA)-Ni solution (auto-reduced Ni-CyGly complex solution: 100 mg mL-1, P(CyMAMAA)activated: 100 mg mL-1; 5.88:1 v/v) was mixed with 32 µL of the P(CyMA-MAA)-Oslow solution (8.6 mg mL-1) and completely drop cast onto the first polymer layer. Nominal surface concentration of P(CyMA-MAA)Oslow: 253 µg cm-2.


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CONCLUSION

Our results clearly show that it was possible to immobilize the Ni-catalyst at an electrode surface by covalent attachment to a polymer backbone. Simultaneously, the catalyst was electrically wired and protected by blending the active Ni-catalyst modified polymer with a low potential Os-complex modified polymer matrix. In contrast to the previously used redox silent immobilization matrix 36, the use of an electron transfer relay, enhances the effective wiring of the Ni-catalyst by electrically addressing all Nicomplexes within the composite polymer film via the polymer bound Os-complexes. Moreover, the use of carbon cloth based gas diffusion electrodes allows for the fabrication of redox polymer/DuBois-type catalyst based H2-oxidation anodes with outstanding current response that outperforms the previously reported polymer/NiCyGly modified flat glassy carbon electrodes 36 with current values that are only slightly lower than those obtained for gas diffusion viologen polymer/hydrogenases


26

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electrodes42. Although our approach shows clearly lower currents compared to gas diffusion electrodes modified with Ni-catalyst decorated carbon nanotubes35 an active protection is missing in the latter and highlights the significant advantage of the proposed concept. However, the stability of the active Ni-complex precursor and the corresponding modified active polymer matrix as well as variations of the protection efficiencies remain an issue and warrants further optimization of this work our results clearly demonstrate that the concept of protection by a suitable redox polymer with adjusted properties is in principle transferable to any sensitive catalyst. Thus, in the future not only the catalyst design towards more robust active materials but also the development of suitable and more efficient immobilization and matrices based on noble metal free oxygen reducing catalysts is of particular relevance for the development of new energy conversion and storage technologies.

EXPERIMENTAL


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Materials and Methods All chemicals and materials were purchased from Sigma Aldrich, Alfa-Aesar, VWR or Merck and were used as received, except otherwise noted. All chemicals were of reagent or higher grade. All dry solvents were purchased from Acros Organics (Acroseal-bottles with molecular sieves). All NMR experiments were conducted with a Bruker DPX-200 and DRX-400 spectrometer with

1H

resonance frequencies of 200.13 MHz and 400.13 MHz,

respectively. The residual solvent peak was used as internal standard. Deuterated solvents were stored at 4 °C. UV-vis spectra were recorded with a Cary 60 spectrometer from Agilent in quartz cuvettes with an optical pathlength of 1 cm. Size exclusion chromatography measurements were performed against polystyrene standards in tetrahydrofuran by using a RI detector. The data were analyzed with the PSS WinGPC Unity software. Sample concentration was ≈15 mg mL-1.


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FT-IR-ATR measurements were recorded with an iS50R spectrometer from Thermo Fisher Scientific. Samples were directly drop cast on the sample holder bearing the ATR crystal (diamond), air dried and measured. All reactions and manipulations were conducted by using standard Schlenk techniques or glove box conditions. The monomers cyclohexyl methacrylate (CyMA) and methacrylic acid (MAA) were passed through a column filled with the corresponding inhibitor remover prior to synthesis and stored at -20 °C or 4 °C. The radical initiator AIBN (= Azobisisobutyronitrile) was recrystallized from hot toluene or hot methanol prior to use and stored at -20 °C.42

Syntheses Detailed protocols for the syntheses of all new compounds are given in the Supporting Information.

The

synthesis

of

the

low

potential

Os-complex

[Os(BiImMe2)2(BiImNH2)](PF6)3 (including the synthesis of all ligands and precursors) was described earlier in refs.38,39. The synthesis of the ligands for the high potential Os-


29


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complex [Os(PyImMe)2(PyImNH2)](PF6)3), 2-(1H-Imidazol-2-yl)pyridine (PyIm) and 2-(1methyl-Imidazol-2-yl)pyridine (PyImMe) was performed according to refs.

45,46.

The

synthesis of the ligand PyImNH2 was reported previously in ref. 45. The high potential Oscomplex [Os(BiImMe2)2(BiImNH2)](PF6)3) was synthesized by adapting protocols from ref. 47

The preparation of the Ni-catalyst Ni-CyGly was described earlier in ref. 21

Electrochemical experiments All electrochemical experiments were conducted with a Gamry Reference600 potentiostat or a Metrohm µLab potentiostat in a conventional three-electrode cell with a platinum wire counter electrode and a Ag/AgCl/3M KCl reference electrode. As working electrode either glassy carbon disk electrodes (diameter 3 mm) or carbon cloth-based gas diffusion electrodes (≈2 cm, MTI, Carbon Foam Sheet, Porous C, 0.454 mm thick, ≈10 mL cm−2 s−1, porosity ≈31 μm, coated on one side with a Nafion/Teflon-based microporous film (50 µm), carbon content 5 mg cm−2, EQ-bcgdl-1400S-LD) were employed (note that the diameter of the round opening that ensures exposure of the gasdiffusion layer to the gas-feed was 1.5 cm; for cell design see ref. 42). All electrochemical


30

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experiments were performed in 0.1 M NaClO4/water. The desired pH value (2, 2.3, 3 and 7) was adjusted by adding appropriate amounts of HClO4. All cyclic voltammograms were recorded with scan rates of 2 to 10 mV s-1. For chronoamperometric experiments potentials of +0.21 or +0.41 V vs. SHE (for measurements with the low potential Oscomplex modified polymer) or +0.56 V vs. SHE (for measurements with the high potential Os-complex modified polymer) were applied. The working electrodes were modified with standard drop-cast processes. Electrodes modified with the Ni-catalyst modified polymer were prepared in the glovebox. Experiments with the pristine Os-complex modified polymers (both are air stable in the solid state) were either prepared under ambient or glove box conditions. The Ni-catalyst modified polymer P(CyMA-MAA)-Ni and the high potential Os-complex modified polymer P(CyMA-MAA)-Oshigh were prepared in-situ prior to the experiments by mixing a tetrahydrofuran (dry) solution containing the activated polymer backbone P(CyMAMAA)activated and either an acetonitrile solution of the auto-reduced Ni-catalyst bearing an OH-function

in

its

ligand

periphery

or

an

acetonitrile

solution

of

the

[Os(BiImMe2)2(BiImNH2)](PF6)3 complex. These solutions were used without further


31


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purification for electrode modification. For composition of the individual mixtures, see text. For experiments in gas diffusion mode H2 was provided from the back of the electrode with a moderate flow rate of 100 mL min-1.

Preparation of the P(CyMA-MAA)-Oslow and P(CyMA-MAA)-Oshigh modified glassy carbon electrodes Electrodes modified with the Os-complex based polymers were prepared by drop coating 5 µL of a P(CyMA-MAA)-Oslow/MeCN solution (30 mg mL-1) and 2 µL of the in-situ prepared

P(CyMA-MAA)-Oshigh/MeCN

solution

(see

synthesis

section

below),

respectively, onto the glassy carbon electrodes. The electrodes were air-dried several hours prior to measurements.

Preparation of the P(CyMA-MAA)-Ni modified glassy carbon electrodes In-situ prepared P(CyMA-MAA)-Ni/MeCN solution (4 µL) was directly used for the modification of the electrodes under glovebox conditions. The modified electrodes were dried in the glovebox for at least 30 min.


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Preparation of the P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oslow modified electrodes In-situ prepared P(CyMA-MAA)-Ni/MeCN solution was mixed with the P(CyMA-MAA)Oslow/MeCN solution (30 mg mL-1 in MeCN) in a 1:5 volumetric ratio and drop coated onto the glassy carbon electrodes. The electrodes were dried in the glove box for several hours. For the modification of glassy carbon electrodes with nominal diameters of 3 mm, 6 µL of the P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oslow were used.

Preparation of the P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oshigh modified electrodes The in-situ prepared P(CyMA-MAA)-Ni and the in-situ prepared P(CyMA-MAA)-Oshigh solution were mixed in a 1:5 volumetric ratio. The electrodes were dried in the glove box for several hours. For the modification of glassy carbon electrodes 6 µL for the modification of graphite electrodes 12 µL were used.

Carbon cloth-based gas diffusion P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oslow electrodes ACS Paragon Plus Environment

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The carbon cloth-based gas diffusion electrodes were first modified in a drop cast process with a pristine P(Cy-MAA)-Oslow layer (20 µL, 8.6 mg mL-1) to avoid a direct electron transfer between the Ni-catalyst and the electrode surface (note that the side without the hydrophobic microporous Nafion/Teflon/Carbon layer was modified to further reduce the contribution of a direct electron transfer). After drying overnight, the electrodes were modified with the catalytically active P(CyMA-MAA)-Ni/P(CyMA-MAA)-Oslow polymer composite in a second drop cast process. For this, first the activated polymer backbone P(CyMA-MAA)activated (100 mg mL-1 in THF) and the auto-reduced Ni-complex (100 mg mL-1 in MeCN) were mixed in a volumetric ratio of 1:5.88 and left to stand for 30 min to complete the reaction between the acid chloride and the single OH-moiety in the complex. This solution (4 µL) was mixed with the P(CyMA-MAA)-Oslow solution (32 µL, 8.6 mg mL-1 in MeCN) and the mixture was drop cast onto the underlying P(CyMA-MAA)Os layer. Finally, the double layer modified electrodes were dried overnight. All manipulations were carried out in the glove box. Note that the modified area of the electrodes was within the area (≈1.77 cm2) that was exposed to the gas phase, however, an exact determination of the modified area was hampered due to a rather fast aspiration


34

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of the polymer solution by the porous electrode material. Moreover, the modified area after the drop cast process was practically not visible with the bare eye.

Supporting Information. Detailed protocols for synthesis, spectroscopic and additional electrochemical experiments and controls can be found in the Supporting Information.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors thank Dr. Alaa. A. Oughli and Dr. Olaf Rüdiger (both Max-Planck-Institut für Chemische Energiekonversion, Mülheim an der Ruhr) for preliminary experiments and discussion, and Prof. Wendy Shaw (Pacific Northwest National Laboratory, PNNL, Richmond/WA) for providing the Ni-CyGly complex. The authors thank also Prof. S. Ludwigs and Dr. K. Dirnberger (both University of Stuttgart, IPOC) for SEC measurements. This work was supported by the Deutsche Forschungsgemeinschaft


35


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(DFG, German Research Foundation) within the framework of the Cluster of Excellence RESOLV (EXC-2033 – project number 390677874) as well as by the DFG-ANR within the projects SHIELD PL746/2-1 and N° ANR-15-CE05-0020. A.R. thanks the DFG under Germany ́s Excellence Strategy – EXC-2033 – project number 390677874 for a PostDoc fellowship.

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Polymer-Bound DuBois-Type Molecular H2-Oxidation Ni-Catalysts are

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