Exploring attached-buffer effects and Gibbs–Donnan equilibria in ionomeric energy-transduction materials

Abstract

We report on applications of polymer-based electrode coatings as organizing scaffolds for assembling cobalt phthalocyanine electrocatalysts for the CO₂ reduction reaction, including how the presence of more basic imidazolyl- versus pyridyl- polymer functional groups enhances catalytic activity. We also describe the electrocatalytic activity of these constructs under varying (1) applied bias potentials, (2) proton activity, and (3) supporting electrolyte conditions that alter the ionic properties of the materials. These results are presented in the context of improving fundamental understandings of attached-buffer effects and Gibbs–Donnan equilibria relevant to polymer-encapsulated electrocatalysts.

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Ions play key roles in the efficient functioning of numerous enzymatic reactions and industrial chemical processes.¹ Likewise, advances in chemistry enable the creation of extended coordination environments for human-engineered catalytic sites inspired by the protein scaffolds of metalloenzymes.² In this context, polymeric structures containing ionic sites (e.g., as exemplified in this work using polymer layers with charged sites via protonation of Lewis base functional groups tethered along a polymer backbone; see Scheme 1 and Table 1) offer opportunities to investigate how charge sites within polymers influence the turnover frequencies and product selectivity of embedded catalysts.^1c,3 In these materials, the ionomeric coatings can be considered part of the molecular catalyst, akin to how the protein environments of enzymes are integral to their structure and function. We note that controlling the ionic character of polymers—not only through exposure to more acidic solutions but also through precise control over acid dissociation constants (pK_a values)—could provide strategies to improve the performance of polymer-encapsulated electrocatalysts and effectively direct fuel-forming reactions such as the carbon dioxide reduction reaction (CO₂RR), without sacrificing selectivity over the competing hydrogen evolution reaction (HER).

Scheme 1 Schematic depiction of the ionomeric assemblies, CoPc|PVP (left) and CoPc|PVI (right), featuring polymer chains containing protonated sites (x) and freebase sites (y) for proton management, as well as sites bearing coordinated cobalt phthalocyanine units (z) for catalyzing the reduction of CO₂ to CO.

Table 1 Information on the CoPc|PVP and CoPc|PVI coatings

	PVP	PVI
CoPc to polymer unit site ratio	∼1 : 3800	∼1 : 3800
[N^+–H] at pH 4.7	∼1.2 M	∼6.8 M
[N^+–H] at pH 6.7	∼1.8 × 10^−2 M	∼7.6 × 10^−1 M

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Scheme 1 includes structural depictions of the ionomeric materials used in this work, where cobalt phthalocyanines (well-studied, model homogeneous molecular catalysts for converting CO₂ to CO,^3c–h,4 and abbreviated herein as CoPc) are encapsulated within either poly(4-vinylpyridine) (PVP) or poly(N-vinylimidazole) (PVI) to prepare electrode surface coatings receiving the monikers CoPc|PVP and CoPc|PVI, respectively. The synthetic design and materials preparation strategies employed in this work enable control over the fractions of protonated sites, freebase sites, and catalyst-coordinated sites indicated in Scheme 1 as x, y, and z units of the polymers (see Sections 1.3 and 1.6 of the Supplementary Information (SI) for further details on synthesis and materials preparation). In this approach, the catalyst loadings on the electrodes are established by depositing solutions containing a set ratio of catalyst to total polymer functional group sites (∼1 : 3800). We intentionally use a relatively low catalyst concentration relative to the polymer unit site concentration to (1) diminish the presence of non-coordinated catalysts and (2) minimize competition between catalyst coordination versus protonation upon exposure to aqueous electrolyte solutions (binding constants of CoPc with 4-methylpyridine and N-methylimidazole, determined using electrochemical techniques, are included in Section 1.5 of the SI).

Because the coatings used in this work are polyelectrolytic when exposed to aqueous solutions, the apparent pK_a values of the acid–base sites along the polymer chains depend on both the proton activity (pH) and the electrolyte concentrations of the solutions the polymers are exposed to.⁵ The relationship between pH and the apparent pK_a for an acid–base functional group of a polymer that is present in 50% ionized and 50% non-ionized forms (pK_{app (50%)}) is given by eqn (1).^5b

In this equation (a linearized, modified version of the classic Henderson–Hasselbalch relationship⁶ that accounts for changes in the apparent pK_a with pH as the polymer becomes more ionic in character⁵ and is “charged up” upon protonation), 1 − α is the fraction of protonated polymeric unit sites, α is the fraction of the remnant freebase sites, and n is the polyelectrolyte parameter. The concentrations of protonated sites ([N⁺–H]) indicated in Table 1 are included to facilitate comparisons and are approximated using pK_{app (50%)} and n values for PVP and PVI taken from the literature (recorded in aqueous solutions at electrolyte concentrations near those used in the work reported herein, and where pK_{app (50%)} ∼ 3.85 and n = 1.05 for PVP, and pK_{app (50%)} ∼ 5.0 and n = 1.5 for PVI) (see Fig. S8 and Table S5).^5c,d Using the resulting apparent pK_a of PVI at pH 4.7 (pK_app at pH 4.7 = 4.9), the thickness of the solvent-free polymer layer (4.4 μm), and the approximated, solvent-free density of PVI (1.039 g cm⁻³) yields an estimate of the imidazolium site density of ∼6.8 M (see Table 1 and Sections 1.7–1.9 of the SI). Conversely, the concentration of protons at pH 4.7 is only 2.0 × 10⁻⁵ M. This crude, “back of the envelope” calculation highlights the potential of using molecular structures to govern localized ionic character and concentrations of proton donors within a covalently bound molecular framework.

Fig. 1a and b show current–voltage responses (voltammograms) measured using glassy carbon working electrodes modified with either CoPc|PVP or CoPc|PVI and suspended in aqueous 0.1 M NaH₂PO₄ solutions at pH = 4.7 or 6.7, under 1 atm of CO₂. These results show that, in general, the peak electrocatalytic activity recorded over the indicated scan range increases with the number of charged sites on the ionomeric layers (i.e., the N⁺–H sites depicted in Scheme 1) (also see Fig. S12). Even under the more acidic conditions at pH 4.7, CoPc|PVI favours CO₂RR over HER, with an 81% faradaic efficiency for CO production, as based on Fourier transform infrared (FTIR) spectral analysis of headspace gas samples following controlled potential electrolysis at −1.1 V vs. SHE (see Fig. 1c, Fig. S10, and Table S6). For comparison, measurements performed using CoPc|PVP electrodes in place of the CoPc|PVI—but otherwise under equivalent experimental conditions—yield a 70% faradaic efficiency, but operate at less than half the current density achieved using the CoPc|PVI-modified electrodes (see Fig. S10 and Table S6). This 70% faradaic efficiency recorded using CoPc|PVP at pH 4.7 is consistent with previous measurements reported for cobalt phthalocyanines encapsulated in PVP.^3h

Fig. 1 (a) and (b) Voltammograms recorded using glassy carbon working electrodes modified with CoPc|PVP (green) or CoPc|PVI (purple), 0.1 M NaH₂PO₄ solutions at either bulk pH = 4.7 (panel a) or bulk pH = 6.7 (panel b), and a scan rate of 200 mV s⁻¹. The shaded areas represent the standard deviations from the mean values. (c) Fourier transform infrared spectra of Ar (gray), headspace gas collected pre-electrolysis (black), headspace gas collected post electrolysis (40 min using a CoPc|PVI-modified electrode polarized at −1.1 V vs SHE, and bulk pH = 4.7) (purple), and 20% CO in N₂ (blue).

Given that species other than solvated protons and water can serve as proton donors in fuel-forming reactions,⁷ we postulate that surface-attached polymers can also serve as alternative proton donors at electrified interfaces, thereby providing localized buffering effects. Evidence that ionomeric coatings can serve as fixed, localized proton donors is afforded in Fig. 2a and b where three peak features are present in the current–voltage plots recorded using CoPc|PVI-modified electrodes in 0.1 M NaH₂PO₄ electrolyte solutions at pH = 3.0, under 1 atm of CO₂ (see the purple trace in Fig. 2b). We tentatively assign these three peak-shaped waveforms to catalysis mediated by three different proton donors, including (1) phosphoric acid (pK_a = 2.15),⁸ (2) surface-attached imidazolium groups (with an apparent pK_a of ∼4.3 at pH = 3.0),^5d and (3) dihydrogen phosphate, hereafter referred to simply as phosphate (pK_a = 7.2)⁸ (see Fig. 2c, showing CO₂RR standard potentials involving these and other proton donors, and Fig. S9 listing the related pK_a values). In this experiment, we propose that the pK_a of the proton donor impacts both the thermodynamics and kinetics of the proton coupled electron transfer (PCET) reactions, resulting in a switch between electron-transfer-rate-limited and diffusion- or proton-relay-limited control of the surface reactions when cathodically biasing the electrode potential. Thus, the peak currents represent conditions where the concentration gradient between the electrode surface and bulk of the indicated proton donor is maximized. Related experiments using CoPc|PVP-modified electrodes also show waveforms arising from at least three peak contributions (see the green trace in Fig. 2b). However, when using CoPc|PVP-modified electrodes, the waveform contributions assigned to the surface-attached buffer are offset to less negative bias potentials and overlap with the waveform contributions assigned to phosphoric acid-mediated chemistry. These results are consistent with our interpretations and the relatively lower apparent pK_a of PVP (∼3.81 at pH = 3.0) as compared to PVI (∼4.33 at pH = 3.0) (see Table S5).⁵

Expanded voltammogram plots of the data presented in Fig. 1 (recorded at bulk pH = 4.7 or 6.7) also reveal an additional redox waveform at more cathodic polarization, tentatively ascribed to activity involving phosphate as a proton donor (see Fig. S12). Although moving to pH = 3.0 and biasing the electrode over a larger range of potentials assists with identifying alternative proton donors, selectivity for CO₂RR over HER is hampered at more acidic conditions and at higher bias potentials. For example, controlled potential electrolysis using CoPc|PVI-modified electrodes polarized at −1.1 V vs SHE at bulk pH = 3.0 yields a faradaic efficiency of 16% for CO production, as proton reduction to hydrogen becomes the dominant reaction under these more acidic conditions. This highlights the benefit of using pK_a modulation and relatively low overpotentials, rather than more acidic solutions and/or high bias potentials, to enhance CO₂ reduction reactivity without sacrificing selectivity.

Fig. 2 (a) Structures of the proposed alternate proton donors with indicated assignments to redox wave features they give rise to in the voltammograms shown in panel b. (b) Voltammograms recorded using glassy carbon electrodes modified with CoPc|PVP (green) or CoPc|PVI (purple), 0.1 M NaH₂PO₄ electrolyte solutions at bulk pH = 3.0, 1 atm of CO₂, and a scan rate of 200 mV s⁻¹. (c) A diagram indicating standard potentials (E° values) relevant to CO₂ reduction with varying proton donors. For PVI and PVP, the potentials are based on estimates of the apparent pK_a when the polymer is 50% protonated. (d) and (e) Voltammograms recorded using glassy carbon working electrodes modified with CoPc|PVP (panel d) or CoPc|PVI (panel e), electrolyte solutions containing NaH₂PO₄ at concentrations of 0.05 M (dash), 0.1 M (dash-dot), or 0.50 M (solid), bulk pH = 4.7 under 1 atm of CO₂, and a scan rate of 200 mV s⁻¹. In all data plots, the shaded areas represent the standard deviations from the mean values.

In addition to modulating the pH of phosphate-buffered electrolyte solutions, we report how changing the bulk phosphate concentration perturbs the voltammetry waveforms recorded using CoPc|PVP- versus CoPc|PVI-modified electrodes (see Fig. 2d and e). Unlike the results obtained using the CoPc|PVP coatings, where the peak current of the redox wave ascribed to activity arising from phosphate-buffer-mediated chemistry increases significantly when raising the bulk phosphate concentration (a 337.4% increase in the peak current in going from 0.05 to 0.50 M in total bulk buffer concentration), the results obtained using CoPc|PVI coatings show a more attenuated response in the peak current of the redox wave ascribed to activity arising from phosphate-buffer-mediated chemistry (a 16.4% increase in going from 0.05 to 0.5 M in total bulk buffer concentration). Instead, elevating the bulk phosphate concentration yields a more pronounced increase in the intensity of the redox wave ascribed to activity arising from imidazolium ions serving as fixed buffer species (a 24.4% increase in going from 0.05 to 0.50 M in total bulk buffer concentration) along with a shift of the peak potential to less cathodic values (ΔE_peak = 101.7 mV in going from 0.05 to 0.50 M in total bulk buffer concentration). We propose that these juxtaposing results obtained using CoPc|PVP- versus CoPc|PVI-modified electrodes arise in part from differences in the total ion concentrations associated with the polymer phases of these materials (vide infra).

It has not escaped our attention that the ionomeric catalyst coatings reported herein should give rise to a Gibbs–Donnan effect⁹ and resulting electric potential differences across the polymer and bulk solution phases (see Scheme 2). Immobilized cationic polymer sites (e.g., imidazoliums or pyridiniums) are non-permeable with respect to the boundary between the polymer and electrolyte solution phases. Yet, electroneutrality dictates that fixed-charge sites be accompanied by charge-balancing counter ions. (i.e., anions such as phosphate that possess the opposite charge sign of any fixed, cationic polymer sites). Likewise, any co-ions (i.e., cations such as sodium that bear the same charge sign as any fixed, cationic polymer sites) absorbing into the polymer phase must also be accompanied by charge-balancing counter ions. Thus, when the polymer is exposed to an electrolyte solution, an unequal distribution of ions will arise across the phases. Under this scenario, the resulting Donnan potential would restrict the absorption of co-ions into the polymer phase and counter ion desorption into the electrolyte solution phase. When protons are involved, the pH of the phases can also differ. Under conditions where the concentration of the non-permeable, fixed-charge sites is relatively high (see Table 1), as compared with the concentrations of ions in the bulk electrolyte, the absolute value of the Donnan potential will be relatively high. However, techniques to measure Donnan potentials are scarce, as are reports of measured values.¹⁰

Scheme 2 Schematic depiction of a Gibbs–Donnan potential across a polymer phase containing fixed-charge sites (immobilized cations) and an electrolyte solution phase containing co-ions and counter ions.

Consistent with polyelectrolyte theory⁵ and Gibbs–Donnan theory,⁹ increasing the phosphate electrolyte concentration would increase the pK_{app (50%)} of polybasic materials and decrease both the Donnan potential as well as any related Donnan exclusion of co-ions (including hydronium ions). For the CoPc|PVI coatings used in this work, and consistent with the results presented in Fig. 2d and e, increasing the ionic strength of the bulk solution should therefore increase the fraction of protonated species along the polymer chains. In addition, the concentration of phosphate in the polymer phase of PVI is anticipated to be relatively high even at the relatively lower bulk phosphate concentrations because it serves as a counter ion for charge-balancing fixed imidazolium sites. Under such conditions, increasing the bulk phosphate concentration would yield a more limited response in the peak current of the redox wave attributed to activity arising from phosphate-buffer-mediated chemistry, as its concentration in the polymer would already be sufficiently high.

In summary, our investigations of cobalt phthalocyanine electrocatalysts encapsulated in PVP versus PVI electrode coatings—to control the pK_a values of polymer unit sites and resulting fraction of charged sites along the polymer chains—indicate that replacing pyridyl functional groups with imidazolyl units affords a strategy for enhancing catalytic activity without sacrificing selectivity. These findings showcase how tailoring molecular-scale components at the bond level can control macroscopic functional properties. We postulate that controlling the pK_a through selection of material components, synthetic design, and electrolyte conditions provides approaches for managing interfacial potentials and proton-donor activities. Although not directly measured in this work, the electrocatalytic responses of the polymers reported herein are consistent with the presence of Gibbs–Donnan potentials arising from the fixed nature of the surface-immobilized proton donors. Other synergistic effects may be at play, however. We look forward to further investigating these materials and their properties in future work.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article are included in the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5cc05614d.

Acknowledgements

Portions of this work were supported by the Camille Dreyfus Teacher-Scholar Awards Program (bioinspired materials) and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Early Career Award DE-SC0021186 (multielectron, multisubstrate reactions). L.K.H. acknowledges support from the Phoenix Chapter of the ARCS Foundation and a Paul Liddell Memorial Synthetic Chemistry Award.

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