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. 2026 Apr 22;148(17):18139–18149. doi: 10.1021/jacs.6c02348

Electrode-Orthogonal Non-Covalent Self-Assembly Programs Microenvironments around Molecular Electrocatalysts

Gregory Gorobets 1, Deepak Badgurjar 1, Ashok Tate 1, Anna Wuttig 1,*
PMCID: PMC13154213  PMID: 42017693

Abstract

Here, we introduce a catalyst design strategy where non-covalent self-assembly enables molecular control over the active site and microenvironment at the same time to drive efficient electrosynthesis in water. Using anthraquinone-catalyzed H2O2 electrosynthesis from O2 under neutral conditions as a model system, we design anthraquinone-functionalized amphiphiles that self-assemble at polarized electrodes in one in situ step via electrostatic and van der Waals interactions. This process pins and concentrates anthraquinone in a hydrophobic microenvironment that increases the basicity of active intermediates, otherwise poorly accessible in the bulk, by 3 pKa units. The self-assembled catalyst consequently enhances O2 reduction over the control catalyst that remains in the bulk solution and does not tether to the electrode. As self-assembly is reversible and agnostic to the surface chemistry of the electrode, this strategy can be translated to high-surface-area electrodes, catalyzing O2 reduction at 924 mol H2O2 molcat –1 h–1 while allowing recovery and reuse of the amphiphile catalyst. Integrating molecular catalysts into amphiphiles that predictively self-assemble at electrodes to program microenvironments around catalytic sites, our study showcases electrode-orthogonal non-covalent self-assembly as a molecularly tunable construct to enhance electrocatalysis.

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Introduction

Catalysts that need to operate in neutral aqueous media demand active-site microenvironments that tune the acidity of reaction intermediates, which are otherwise inaccessible in bulk solution. Enzymes fine-tune microenvironments by modifying the acidic properties of amino acid residues to achieve robust catalysis under neutral physiological conditions. In acetoacetate decarboxylase and mandelate racemase, electrostatic repulsion between amino acids shifts the pK a of a lysine residue down by approximately 4 units, increasing its nucleophilicity or basicity within active sites to facilitate catalysis. − In carbohydrate hydrolase, hydrophobic and electrostatic interactions with tryptophan and aspartate, respectively, increase the pK a of glutamic acid by nearly 2 units, enabling it to serve as a general-acid catalyst to affect substrate hydrolysis (Scheme , left). These examples highlight the importance of controlling the local microenvironment around well-defined molecular active sites to stabilize protonation states and achieve efficient catalysis.

1. Like Enzymatic Systems (Left), Non-Covalent Self-Assembly (Right, This Work) of Molecular Catalysts (Shown Is Hydroanthraquinone Reduced from Anthraquinone, AQ) Enables Efficient Electrocatalysis (O2 Reduction Reaction, ORR) in Neutral Aqueous Buffer by Programming Microenvironments to Tune the Acidity of Key Reaction Intermediates.

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Translating this enzymatic design principle to electrocatalysis can provide a route to selective electrosynthesis in water. A possible strategy to realize this concept is to pin well-defined molecular catalytic sites at electrified interfaces within programmed microenvironments to tune the pK a of the reaction intermediates. We recently reported that non-covalent self-assembly enables programmable and facile control of microenvironments around model (non-catalytic) redox-active centers. , Micromolar quantities of an amphiphile-linked probe (ferrocene, Fc) in solution effectively tether at polarized electrodes via reversible and dynamic electrostatic interactions with the surface and van der Waals interactions between neighboring aliphatic tails. This process places Fc in microenvironments distinct from the bulk aqueous solution; it is pinned in regions of lower dielectric strength, shifting redox potentials by destabilizing the charged and oxidized form. , In contrast to traditional electrode-specific tethering strategies, non-covalent self-assembly occurs in an electrode-orthogonal fashion, i.e., independent of the material’s surface chemistry, to form surface layers in an intuitive in situ step. In doing so, our approach offers a generalizable platform to program microenvironments around electrode-tethered molecules: it circumvents the reliance on covalent-bonds (e.g., with thiol, isocyanide, and carbene precursors) − with select metal surfaces (e.g., Au, Pd, Pt, Cu, and Ag) that restrict the potential stability window due to competitive desorption − or π–π stacking of pyrene or diazonium grafting with carbon electrodes − that similarly can desorb, electropolymerize, or inhibit charge transfer due to multilayer formation. −

The goal of this study is to demonstrate that non-covalent self-assembly can be used to tether molecular catalysts at electrodes in hydrophobic microenvironments, thereby tuning the pK a of reaction intermediates to enhance electrosynthesis in neutral aqueous conditions. To showcase this concept, we chose: (1) anthraquinone (AQ) as the model molecular redox catalyst; and (2) the model reaction of O2 reduction reaction (ORR) to H2O2. − We chose AQ because the O2 activation is proposed to proceed from well-defined protonated intermediates, thus providing a system to test how hydrophobic microenvironments can shift the pK a of reaction intermediates to affect electrosynthesis of H2O2 in neutral media. In the industrial AQ process primarily used to produce H2O2, ,− H2 is added to AQ to form hydroanthroquinone (AQH2) in organic media, followed by oxidation with O2 and subsequent separations to produce H2O2. Sulfonate-modified AQ can be electrochemically reduced via two electrons and protons to access AQH2, , effectively replacing H2 in the thermal process. Yet, H2O2 electrosynthesis using AQ as a molecular mediator for redox catalysis of O2 has been primarily reported in organic emulsions, where AQ resides in the organic layer, , or in acidic media, − giving rise to the question of how AQ-catalyzed H2O2 electrosynthesis can be realized under neutral pH conditions. Prior studies suggest that the pK a of AQH2 may not be optimal to support O2 activation under these targeted conditions. In the thermal process, the proposed form of AQ that drives initial O2 activation by hydrogen atom transfer (HAT) is AQH2. , Yet, the estimated pK a of sulfonate-modified AQH2 lies near 7.6, , suggesting that under neutral aqueous conditions, AQH2 also exists as the deprotonated anion. Moreover, at electrified interfaces, even in buffered media, the interfacial pH increases during reductive reactions that consume protons; thus, AQH2 is expected to exist predominantly as the deprotonated anion (Scheme , right). We therefore hypothesized that a synthetic method capable of molecularly tuning the microenvironment around AQH2 to increase its pK a would enhance H2O2 electrosynthesis under neutral aqueous conditions.

Here, we design AQ-functionalized amphiphiles that tether at electrodes through non-covalent interactions and compare their structural and electrochemical properties with AQ that is not attached to an amphiphile (i.e., diffusive control). We show that this self-assembly process reversibly pins anthraquinone at the electrode in a hydrophobic microenvironment and increases the pK a of reduced and ORR active intermediates by 3 pKa units. As a result, two functional properties emerge compared to the diffusive control: (1) selective two-electron ORR in neutral media can be accessed at more positive potentials and higher current densities; and (2) the AQ-functionalized amphiphile can be recovered and reused. Together, these results introduce non-covalent self-assembly of catalyst-functionalized amphiphiles as a molecular strategy to program active-site microenvironments on various electrodes, enabling selective and enhanced electrocatalysis in aqueous media using a simple, drop-in construct.

Results and Discussion

Non-Covalent Self-Assembly Pins AQ in Hydrophobic Microenvironments at Polarized Electrodes

To test our hypothesis that AQ could be tethered to electrode surfaces via non-covalent self-assembly, we synthesized two cationic AQ derivatives (see the Supporting Information for synthetic details). Figure a,i depicts the synthesized molecules: (1) C2-AQ, a control monomer without a long aliphatic chain; and (2) C18-AQ, a monomer containing a long aliphatic chain with the AQ moiety at the same position away from the ammonium. C18-AQ was prepared using a chloride counteranion to enable solubility of the amphiphile in aqueous media. C2-AQ was similarly prepared with a chloride counteranion for consistency. The critical micelle concentration (CMC) of C18-AQ in aqueous solution is 178 and 38 μM in 0.1 M NaPi (Figures S1 and S2).

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Non-covalent self-assembly of C18-AQ into hydrophobic microenvironments. Molecular structures and interfacial structures of self-assembling catalyst (a, C18-AQ) and its diffusive control (i, C2-AQ) prepared in this work. In part (a), charge-balancing anions and possible structural disorder of aliphatic chains are omitted for clarity. Surface-enhanced infrared absorption spectra of 10 μM C18-AQ (c) or C2-AQ (k) recorded at the potential values indicated in the cyclic voltammograms shown in part (b) or (j) (2 mV s–1 with a negative direction of scan), respectively. (d) Integrated band intensities (IBI) taken from (c) of the peaks centered around 1098 and 1057 cm–1. (e) IBI taken from (c) of the peaks centered around 1592 and 1388 cm–1. (f) IBI taken from (c) of the peaks centered around 1679 and 1626 cm–1. (g) IBI taken from (c) of the peaks centered around 1329 and 1289 cm–1. (h) IBI taken from (c) of the peaks centered around 2924, 2909, and 2852 cm–1. The background spectra were collected at 0.103 V vs Ag/AgCl in Ar-saturated 0.1 M sodium phosphate buffer (NaPi), pH 7.1, in the absence of the AQ molecules. The relative spectra are shown in Figure S3.

We probed the self-assembly behavior of C18-AQ at negatively polarized electrodes under the targeted neutral pH conditions relevant to O2 activation. Using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) , and SEIRAS-active Au electrodes, we conducted cyclic voltammetry (CV) measurements of C18-AQ in 0.1 M sodium phosphate (NaPi), pH 7.1, while collecting spectra (Figure b) to investigate self-assembly. To accentuate the differences in the spectroscopic changes as a function of the applied potential shown in Figure c, the relative spectra are reported in Figure S3. Figure b depicts a reversible redox feature for C18-AQ with an anodic peak potential (E p,a) value of −0.411 V and a cathodic peak potential (E p,c) value of −0.421 V (all potential values quoted vs Ag/AgCl) with an apparent redox potential value (E app, the average of E p,a and E p,c) of −0.416 V. We note that AQ derivatives undergo multiproton and electron transfers at E app (discussed below), precluding Randles–Ševčík analysis typically used to support surface localization of a redox-active species. The peak separation (ΔE p = E p,c – E p,a) value of 0.010 V is in line with reported values for AQ immobilized at the electrode surface via strong adsorption , or covalent modification, , suggesting that C18-AQ is immobilized on Au on the time scale of the electrochemical measurement. Moreover, the anodic (Q a) and cathodic (Q c) charge integration values are identical, equaling 55 μC cmAu –2, in line with expected monolayer coverage values for C18-AQ (see Figures S4–S6, Table S1, SI Section 16). We note that we observe monolayer coverage values for C18-AQ below the measured CMC in 0.1 M NaPi (Figures S2 and S5).

SEIRAS data reveal spectroscopic features consistent with the presence of localized C18-AQ and its aliphatic tail as well as the desorption of interfacial water (all observed IR features are summarized in Table S2). Data shown in Figure c were recorded following a background spectrum collected at the open circuit voltage (OCV, 0.103 V) in the absence of C18-AQ. At 0.06 V, the black spectra reveal that upon C18-AQ addition, a bleach is observed at 3449 and 1637 cm–1, attributed to the ν­(OH) stretching and δ­(HOH) bending modes of interfacial water. , This observation shows that the addition of the amphiphile expels the interfacial water. We observe a rise at 2958, 2924, and 2852 cm–1, attributed to the νsym (N–CH3) of the ammonium group, νasym (CH2), and νsym (CH2) of the aliphatic tail in a trans configuration, respectively. − We observe a feature at 1679 cm–1 that does not shift for data collected in D2O, Figure S7, and is in line with the carbonyl stretching frequency reported for anthraquinone, ν­(COAQ). , Features at 1642 and 1552 cm–1 are observed, in line with wavenumbers consistent with reported composite amide features (Amide I and Amide II, respectively), , reporting on displacements primarily in the ν­(CO) and the δ­(NH) modes, respectively. Indeed, the Amide I feature at 1642 cm–1 does not shift for data collected in D2O, Figure S7, while it does for the Amide II feature at 1552 cm–1, consistent with these assignments. The feature at 1258 cm–1 is in line with literature-reported values for the Amide III band , and does not shift for experiments conducted in D2O, implicating a minimal role of the N–H vectors, consistent with our spectra estimated computationally (Figures S8 and S9 and Tables S1–S3). A rise in features at 1592, 1329, and 1289 cm–1, which do not shift for data collected in D2O, Figure S7, is observed, in line with the b1u and two varying b2u modes describing the C–C stretch of the AQ aryl rings, AQ b1u and AQ b2u. , A rise in a peak at 1479 cm–1 is observed, consistent with literature reports assigning this feature to the δ­(CH2) of the aliphatic tail. ,,, Finally, a bleach at 1098 cm–1 is observed, consistent with an assignment to a loss in adsorbed HPO4. Importantly, the SEIRAS data are nearly featureless when C18-AQ is introduced at potential values where the Au surface is expected to be positively charged (Figure S10), consistent with the notion that the amphiphile is electrostatically templated at a negatively polarized surface, , with the ammonium group faced toward the electrode. Taken together with the monolayer charge integration observed for C18-AQ, the SEIRA data are consistent with the formation of a C18-AQ self-assembled layer, as shown in Figure a, although we note that we cannot rule out structural disorder in the layers formed, e.g., the aliphatic tails can be tilted in various directions to produce dense and sparse domains.

The potential-dependent changes to the C18-AQ spectroscopic features shown in Figure c demonstrate that the layer remains intact over the time scale probed during the redox process observed at −0.416 V. Tracking the integrated band intensity of each potential-dependent feature identified at OCV in Figure c, Figure d through h and Table S2 reveal spectroscopic data consistent with the reversible formation of the two-electron and two-proton reduced AQH2, indicative of proton delivery and egress through the self-assembled layer at neutral pH despite the presence of the aliphatic tails. To accentuate the differences in the spectroscopic changes as a function of the applied potential shown in Figure c, the relative spectra are reported in Figure S3. We observe that as AQ is reduced, the feature assigned to adsorbed HPO4 decreases, while a new feature at 1057 cm–1, assigned to adsorbed PO4, increases, suggesting that dibasic phosphate may serve as a proton donor to reduce AQ (Figure d). The peak assigned to AQ b1u decreases in favor of a new feature at 1388 cm–1, consistent with the complex modes of computationally predicted C18-AQH 2 (Figures e and S9, Table S2). As the potential is swept negative, the peak assigned to ν­(COAQ) of the AQ motif decreases in favor of a new peak at 1626 cm–1. We hypothesize that this feature reports on the red-shifted Amide I feature initially at 1642 cm–1 that results from AQH2 formation because this new peak does not shift when data are collected in D2O (Figures f and S7). The peaks assigned to AQ b2u decrease in favor of a new feature at 1388 cm–1, consistent with the complex modes of the computationally predicted C18-AQH 2 (Figures e and S9, Table S2). We observe that the integrated band intensities of the peaks assigned to the aliphatic chain as well as the ammonium headgroup increase as AQ is reduced (Figure h) and mirror the behavior of the bleach ν­(OH) at 3449 cm–1 (Figure S11). This combined signal intensity is reversible with respect to changes in the applied potential. Taken together with the surface selection rule of SEIRAS, , these results suggest that the alkyl tails tilt away from the surface normal as AQ is reduced and the interface becomes more hydrophobic.

In contrast, for C2-AQ, despite the observation of a reversible redox feature with E app equaling −0.432 V, Figure j, the SEIRA spectra are nearly featureless, Figure k and Figure S12. Taken together with the strong IR absorbance of bulk C2-AQ observed over the same wavenumber range (Figure S13 and Table S2), the data are inconsistent with C2-AQ in situ self-assembly at the electrode surface, suggestive of a diffusive monomer, Figure i. The positive shift in E app between C2-AQ and C18-AQ suggests that AQ localization is accompanied by a shift in the microenvironment around the redox-active unit, examined below. The voltammetric features for C2-AQ and C18-AQ as well as the charge integration for C18-AQ are preserved even when different electrodes and morphologies that are incompatible with SEIRAS are utilized (Figure S14, Table S4). To further demonstrate the electrode-agnostic nature of C18-AQ self-assembly, we additionally probed and observed the self-assembly behavior of C18-AQ via SEIRAS on Ag (Figure S15, Table S2). These results implicate minimal impact of the electrode morphology on C18-AQ layer formation, consistent with our previous work using Fc-functionalized amphiphiles. , Together, electrochemical and in situ spectroscopic experiments reveal that the secondary hydrophobic interactions between neighboring C18-AQ are necessary to establish a self-assembled layer of C18-AQ, evincing its non-covalent self-assembly via electrostatic and van der Waals interactions at the electrified surface that pins AQ in a hydrophobic microenvironment under aqueous conditions. Additionally, once established, the layers remain at the interface when more positive potential values are applied, consistent with previous work in our group (Figure S16).

Non-Covalent Self-Assembly Shifts the pK a of Reduced AQ Species

Considering the surface localization of C18-AQ into a hydrophobic microenvironment as well as its estimated monolayer coverages, we next assessed how non-covalent self-assembly influences the pK a of reduced AQ species. To do so, we compared the pH dependence of the AQ redox chemistry observed for C2-AQ and C18-AQ. The CV data shown in Figure a,d demonstrate that the reversible redox features of both compounds shift to more negative values with increasing pH, indicative of a reduction of AQ by proton-coupled electron transfer (PCET). Figure b,e summarizes the E app experimentally observed for C2-AQ and C18-AQ as a function of the pH. For C2-AQ, we observe a 58-mV pH–1 slope over the pH range from 2 to 6, while for C18-AQ, we observe a 55-mV pH–1 slope over a wider pH range (2 to 10). These shifts are consistent with the 59 mV pH–1 slope theoretically described for either reversible single-electron and single-proton PCET or two-electron and two-proton (2e–/2H+) PCET. The assignment of the 55–58-mV pH–1 slope to a 2e–/2H+ process is supported by voltammetric measurements of an equivalent concentration of a well-defined one-electron process (Figure S17), as well as previous reports on ultraviolet–visible (UV–vis) spectroscopy and voltammetry of AQ derivatives. As the solution pH is increased, the slope values differ. For C2-AQ, we observe a 39-mV pH–1 slope (pH 6 to 10), and for C18-AQ, we observe a 40-mV pH–1 slope (pH 10.5 to 12.5), suggesting a two-electron, one-proton process over differing pH ranges. , Finally, at pH values above 10 for C2-AQ and at pH values above 12.5 for C18-AQ, we observe pH-independent redox features, consistent with the intermediate formation of the direct two-electron reduced dianion product, AQ2–. We note that the continuation of a roughly 59-mV pH–1 slope at pH values between 10 and 12 has also been observed upon covalent attachment or strong adsorption of AQ-based fragments to electrode surfaces. ,,, Together, these results suggest that non-covalent self-assembly of AQ at polarized electrodes shifts the thermochemistry of proton and/or electron transfer.

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Non-covalent self-assembly drives a shift in pK a of reduced AQ intermediates. Cyclic voltammograms (CVs) in the presence of 20 μM C18-AQ (a) and 1 mM C2-AQ (d) in Ar-saturated 0.1 M NaPi as a function of the bulk pH. Data collected at 100 mV s–1 with a negative direction of scan. Pourbaix behavior of C18-AQ (b, blue squares) and C2-AQ (e, red squares) for experimental data collected in parts (a, d), respectively; simulated CV data of C18-AQ (b, blue open triangles) and C2-AQ (e, red open triangles); and for calculated E0 app using E 0 and pK a values, as indicated for the 9-component PCET square scheme shown for C18-AQ (c) and C2-AQ (f). The shaded region shows predominant speciation at pH 7.

Analysis of the pH-dependent CV data collected in Figure supports both increases in basicity and more negative redox potentials for reduced AQ intermediates upon non-covalent self-assembly. Theoretical descriptions of reversible 2e–/2H+ reactions have shown that they can proceed via sequential proton or electron transfer, thus giving rise to nine-component square schemes whose apparent equilibrium potential values (E app ) systematically shift with pH. ,, Theorized E app –pH values follow the experimentally observed E app–pH shifts, considering standard potentials (E 0) and pK a values denoted in Figure c,f (Tables S5 and S6, SI Section 17). Consistent with its assignment as a diffusive species, the E 0 and pK a values for C2-AQ closely mimic those reported for solution-dissolved AQ derivatives (SI Section 17). , Furthermore, independent assessment of the pK a of AQH2 for C2-AQ by in situ UV–vis absorption spectroelectrochemistry supports the estimated pK a value of 7 obtained via electrochemical simulation. The spectroelectrochemical data collected at neutral pH are consistent with the deprotonation of AQH2 to form AQH–, while those collected in base are consistent with the formation of AQ2– (Figure S18). , For C18-AQ, in contrast, use of more negative E 0 values to form the radical anion, AQ•–, and the dianion, AQ2–, as well as ∼3 units higher pK a values for AQ2– and its protonated form, AQH–, enables the theoretical construction of E app –pH slopes that mimic experimentally observed trends (Figure b). Electrochemical simulation of CVs employing these shifted E 0 and pK a values can reproduce the observed pH-dependent CV trends in Figure a,d (Figures S19 and S20, SI Section 17), further suggesting that non-covalent self-assembly drives the shifts in E 0 and pK a values for AQ reduction. The observed negative shifts in E 1 through E 4 between C2-AQ and C18-AQ suggest that non-covalent self-assembly of C18-AQ destabilizes the reduced and anionic intermediates by embedding AQ in regions of lower dielectric strength. This hypothesis is additionally supported by the identical redox properties observed for both molecules in aprotic and nonaqueous media (dimethylformamide), where C2-AQ and C18-AQ are freely solvated and thus non-covalent van der Waals interactions are expected to be minimal (Figure S21). The increase in basicity of reduced intermediates for tethered C18-AQ relative to diffusive C2-AQ suggests that spontaneous protonationcommonly invoked in pK a shifts for anionic amino acid residues in proteins and protein-mimics , can overcome the high surface concentration of the reduced anion forms (i.e., AQ2 – and AQH–) present in monolayer coverages estimated for C18-AQ layers. We additionally note that the hydrophobic environment generated can impact the disassociation of water, which may also play a role in the shifted pK a of C18-AQ.

A comparison of the C2-AQ and C18-AQ E app values in Figure shows that, at the targeted neutral pH values for ORR, C18-AQ is reduced at a more positive potential. Furthermore, a comparison of the predominant speciation of AQ at neutral pH and reductive conditions shows that non-covalent self-assembly induces an increase in pK a such that C18-AQ is primarily reduced to AQH2 at more positive potentials while C2-AQ remains in the AQH– form (Figure c,f). Electrochemical simulations of the concentration profile at neutral pH (Figure S22) as well as the in situ UV–vis absorption spectroelectrochemical data (Figure S18) support this hypothesis. Thus, our non-covalent self-assembly strategy not only concentrates AQ on the electrode surface but also establishes a site of electrogenerated AQH2, enabled by the hydrophobic microenvironment.

Non-Covalent Self-Assembled AQ Accelerates O2 Reduction

As the non-covalent interactions driving C18-AQ self-assembly are independent of the electrode surface chemistry, we can directly transfer the assembly strategy onto carbon electrodes, where competing ORR electrocatalysis from the underlying Au surface can be ruled out. We verified that C18-AQ self-assembles onto glassy carbon (GC) surfaces. Figure a shows the redox features of C18-AQ and C2-AQ in the absence of O2. Despite the change in electrode material, the CV features observed with Au, noted above, are preserved: (1) the 52 μC cmGC –2 monolayer coverage value for C18-AQ; and (2) E app values for both compounds (Figure S23 and Table S4). Additionally, the redox behavior of Cn-AQ at a rotating disk electrode further supports self-assembly for C18-AQ over C2-AQ; the former shows no change in peak shape or current upon rotation, whereas the latter exhibits a diffusion-limited plateau current on the time scale of the CV measurement (Figure S24). We note that the ability to readily transfer C18-AQ layers to different electrodes, irrespective of their surface chemistry, supports our previous work on non-covalent self-assembly. ,

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Non-covalent self-assembly accelerates O2 reduction. (a) Cyclic voltammograms (CVs) in the presence of 20 μM C2-AQ (red) and C18-AQ (blue) using glassy carbon (GC) electrodes at 2000 rpm under O2 and at 0 rpm under Ar (dotted traces). (b) CVs in the presence of 500 μM C2-AQ (red) and 20 μM C18-AQ (blue) using GC electrodes at 2000 rpm under O2 and at 0 rpm under Ar (dotted traces). Data collected in parts (a, b) at 100 mV s–1 with a negative direction of scan, and the inset is a zoom-in of the dotted traces. (c) Linear sweep voltammograms of GC electrode (gray dotted line) in the presence of O2 and 20 μM C18-AQ (blue), 20 μM C2-AQ (red) with 20 μM CTAC (orange), or 20 μM CTAC (black) collected at 5 mV s–1 and 2000 rpm. All data collected in 0.1 M NaPi (pH 7.1). (d) Proposed schemes consistent with the data.

In the presence of O2, C18-AQ exhibits a notable enhancement in reductive current (beginning at −0.280 V) compared to C2-AQ (beginning at −0.362 V), Figure a. We hypothesized that C18-AQ self-assembly locally increases the AQ population at the interface relative to C2-AQ at an identical 20 μM bulk concentration, thereby enhancing ORR activity. A systematic 25-fold increase in the bulk C2-AQ concentration led to a corresponding rise in the limiting current density under O2 (Figure b). These results suggest that the current enhancement observed for C18-AQ can be, in part, explained by a higher local concentration of AQ at the interface enabled through non-covalent self-assembly. However, the 65-mV positive shift in onset potential (shift reported as an average between Figure a,b) cannot be explained by AQ localization alone, as increasing the bulk concentration of C2-AQ cannot reproduce this effect in O2 (Figure b). To determine if the aliphatic tails alone imparted a local hydrophobic effect, , we tested the ORR activity of C2-AQ in the presence of an amphiphile that does not contain AQ but is otherwise structurally equivalent to C18-AQ, i.e., cetyltrimethylammonium chloride (CTAC). Yet, these experiments did not reproduce the 65-mV earlier onset for ORR observed for C18-AQ, Figures c and S25. To determine if the enhanced current density observed for ORR in the presence of C18-AQ could be attributed to overreduction of O2 to form H2O, we assessed whether H2O2 can be reduced by C18-AQ. We rule out direct overreduction of O2 to H2O by C18-AQ because we do not observe enhanced current when H2O2 is intentionally added to the bulk electrolyte (Figure S26). Furthermore, direct reduction of H2O2 by the carbon electrode or C2-AQ is minimal, further supporting the notion that enhanced catalytic reactivity observed in the presence of C18-AQ can be attributed to the ORR by the amphiphilic molecular scaffold (Figure S26). The selective formation of H2O2 is further supported by independent quantification of the product, examined below. Thus, we reason that the 65-mV positive shift in the onset potential for ORR observed for C18-AQ relative to C2-AQ mirrors the positive shift in E app observed in the absence of O2, as noted above (Figures , and Table S4), pinpointing the change in the microenvironment as a dominant contributor to this catalytic enhancement.

As noted above, spectroscopic and electrochemical data are consistent with a structural model in which C18-AQ self-assembles at negatively polarized electrodes, driving an increase in the pK a of the PCET-reduced AQH– form relative to its diffusive C2-AQ counterpart. Figure d builds on this model, and we propose a scheme for accelerated C18-AQ ORR activity relative to that of C2-AQ consistent with our data. C18-AQ self-assembles to negatively polarized surfaces, thus creating a high local concentration of AQ in comparison to C2-AQ at the same bulk concentration. C18-AQ self-assembly is accompanied by a pK a shift in the reduced AQH2 species, enabling access to this intermediate at neutral pH values within a hydrophobic microenvironment. In contrast to the self-assembled C18-AQ layer, the pK a for the reduced AQH2 species for diffusive C2-AQ is not high enough to access a high population of AQH2 at neutral pH values, and instead, AQH– is the dominant reduced form. , While the tautomerization of AQH2 to oxanthrone is well-documented, , lower barrier HAT from AQH2 to triplet O2 has been computed, , suggesting that the phenolic O–H bond can serve as a net H atom donor to initiate O2 activation by C18-AQ. Spectroscopic data support the negligible population of an oxanthrone (Figure S27). The absence of a distinct carbonyl feature at a higher wavenumber than the assigned Amide I peak at 1642 cm–1 upon reducing both C18-AQ and C2-AQ supports a low population of the tautomer form, consistent with the reported high barrier to tautomerization (Figure S27). ,− While we note that AQH– is expected to be a poorer net hydrogen atom donor than AQH2 due to resonance stabilization, discrimination among possible O2 activation mechanisms (intermolecular HAT, intramolecular HAT, or PCET), and thus unambiguous determination of k app for O2 activation, is convoluted by the multiple possible redox-active species present in solution, including AQH2 and AQH–. This contrasts well-defined redox mediation, where multiple proton-transfer equilibria, and thus redox-active forms of the redox mediator, do not need to be considered. , Nonetheless, our data are consistent with a model where ex situ synthetic integration of AQ to an amphiphile enhances ORR by promoting non-covalent self-assembly, increasing the local concentration of active catalyst at the electrode surface and tuning the pK a of reduced catalytic active species to be accessible under neutral pH conditions.

Non-Covalent Self-Assembled AQ Are Drop-In and Recoverable Catalysts for Electrosynthesis

To investigate the functional consequences of C18-AQ non-covalent self-assembly, we can turn from the low-surface-area disk electrodes used to determine catalyst location, ORR enhancement, and predominant active form of C18-AQ above to commercially available reticulated vitreous carbon (RVC) to assess H2O2 electrosynthesis from O2. We independently quantified H2O2 using both CuSO4 and 2,9-dimethyl-1,10-phenanthroline (DMP) (see the quantification details in SI Section 10). Addition of both C2-AQ and C18-AQ enables selective 80–90% Faradaic efficiency for H2O2 production, highlighting the selectivity of the molecular AQ construct under these conditions. The self-assembled C18-AQ layer yields 6-fold higher current density for H2O2 electrosynthesis compared to reactions conducted using the diffusive C2-AQ control, with H2O2 productivity up to 924 mol H2O2 molcat –1 h–1 (Figures a,b, and S28). The linear increase of produced H2O2 concentration with time suggests a minimal impact of H2O2 disproportionation. Importantly, the use of micromolar quantities of C18-AQ for achieving high current densities renders it catalytic relative to the O2 solubility limit (∼0.6 mM, SI Section 11), a criterion that C2-AQ cannot meet. Figures c and S29 demonstrate that C2-AQ concentrations exceeding O2 solubility values are required to achieve ∼10 mA cm–2 current density for H2O2 formation. These data additionally mirror those collected in Figure S30 on a rotating disk electrode, where increasing the concentration of C2-AQ leads to systematic increases in ORR, while the limiting current for ORR is insensitive to the concentration of C18-AQ beyond 20 μM. We note that although ORR activity of tethered-AQ has been probed through covalent and adsorption , strategies, they did not yield H2O2 production rates approaching 10 mA cm–2, as we report here. C18-AQ-catalyzed H2O2 electrosynthesis operates as a one-pot process: (i) the amphiphile monomer is directly introduced to the bulk electrolyte in micromolar quantities, (ii) applied potential results in its self-assembly at the electrode interface, and (iii) C18-AQ is continuously regenerated in the presence of O2 via PCET.

4.

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Non-covalent self-assembly for selective electrosynthesis of H2O2 with catalyst separation and recovery. (a) H2O2 product formation and corresponding Faradaic efficiency in the presence of 20 μM C2-AQ at −0.414 V vs Ag/AgCl. (b) H2O2 product formation and corresponding Faradaic efficiency in the presence of 20 μM C18-AQ at −0.414 V vs Ag/AgCl. (c) Comparison of total current density at −0.414 V vs Ag/AgCl as a function of Cn-AQ concentration. Black square is RVC background, blue circles are C18-AQ, green circle is recovered and reused C18-AQ, and red squares are C2-AQ. (d) UV–vis spectra of catholyte immediately following electrosynthesis in the presence of 100 μM C18-AQ (green), following filtration through a common filtering agent, celite (light blue), and recovery from the celite with MeOH (blue). (e) UV–vis spectra of 200-fold diluted samples in part (d) collected in the presence of CuSO4, DMP, and ethanol. (f) Schematic demonstrating recoverable C18-AQ.

Non-covalent self-assembly not only enables enhanced electrosynthesis but also offers facile separability and recoverability, performance aspects typically characteristic of heterogeneous, and not homogeneous, molecularly defined, catalysis. Figure d–f demonstrates the separation and recovery of C18-AQ. Immediately following electrosynthesis, UV–vis of the aqueous catholyte gives rise to absorption maxima at 211 nm (H2O2 nonbonding to σ* and C18-AQ benzenoid π to π* electronic transitions), 258, and 332 nm (C18-AQ quinonoid and benzenoid π to π* electronic transitions, respectively) (Figures d and S31). ,− Due to spectral overlap between the catalyst and H2O2, we use a complementary approach to quantify H2O2. Titration of the catholyte with CuSO4 and DMP enables selective spectrophotometric determination of the H2O2 via the formation of Cu­(DMP)2 + observed at 454 nm (Figures e and S32). We rule out the degradation of the anthraquinone scaffold by H2O2 due to the persistence of the NMR features of C2-AQ in the presence of intentionally added H2O2 (Figure S33). Although we note that a similar NMR stability study is convoluted by peak broadening attributed to supramolecular micelle formation of C18-AQ, mass spectrometry analysis is consistent with the C18-AQ scaffold remaining intact in the presence of H2O2 over the time scale probed in this study (Figure S33). C18-AQ can be separated from the aqueous H2O2 solution by running the electrolyte through a common filtering agent, celite, and is recoverable by flushing the celite bed with methanol (MeOH). We additionally show the ability to rinse off C18-AQ from an electrode nondestructively using MeOH and water (Figure S34). Following separations, Figure d,e shows the selective persistence of the peaks assigned to the presence of H2O2. Following recovery, Figure d,e shows the selective restoration of the peaks assigned to C18-AQ. In Figure d, we note the shifts in the C18-AQ 211 and 332 nm features correspond to a lower molar absorptivity at 211 nm and a redshift to 324 nm, respectively, when changing from the aqueous electrolyte to MeOH. These shifts are observed in the absence of H2O2 or phosphate (Figure S35); thus, we attribute the shifts to solvent effects and therefore quantified the recovered C18-AQ using UV–vis data collected in MeOH (Figure S31). Taking the data reported in Figure d,e, we separate 87% of the synthesized H2O2 and recover 97% of the utilized C18-AQ. The recovered C18-AQ is then able to be utilized in subsequent bulk electrolysis, Figure c. Critically, C2-AQ cannot be separated, and thus cannot be recovered, using this procedure (Figure S36). These results underscore the importance of the long alkyl chain, and thus the amphiphile design, not only in enabling enhanced H2O2 electrosynthesis but also in facilitating product and catalyst separations.

Concluding Remarks

Here, we introduce a non-covalent self-assembled design that pins molecular catalysts within hydrophobic microenvironments, providing a molecularly defined approach to access selective electrosynthesis in water. We use the two-electron reduction of O2 to H2O2 and anthraquinone as a model reaction and catalyst, respectively, to illustrate the functional consequences of the design. Catalytic function and local microenvironment are programmed ex situ in a well-defined anthraquinone-functionalized amphiphile and realized in situ through self-assembly from micromolar quantities directly added to the electrolyte, compatible with diverse electrode materials. Molecular uniformity enables mechanistic investigation, revealing a hydrophobic microenvironment that thermodynamically stabilizes protonated intermediates and consequently enhances the H2O2 synthesis at neutral pH. The self-assembling catalysts are separable, enabling isolation of both the H2O2 product and catalyst. This design marries the practicality of heterogeneous electrocatalysts with the site uniformity of molecular ones. Combined with the growing library of molecular catalysts designed for specific electrochemical reactions (i.e., replacing anthraquinone in the amphiphile), this work exposes non-covalent self-assembly as a viable molecularly tunable construct that can enhance electrocatalysts.

Supplementary Material

ja6c02348_si_001.pdf (7.7MB, pdf)

Acknowledgments

This material is based upon work supported by the National Science Foundation under Award No. 2338351. Computing resources were provided by the Research Computing Center at the University of Chicago. The authors thank M. Huynh for contributions to early stages of the project, and N. E. Lopez for discussions on numerical frequency calculations. The authors thank A. Tokmakoff and N. H. C. Lewis for the use of the fluorescence and absorbance spectrometer and assistance with UV/vis spectroelectrochemistry data collection. This research made use of the University of Chicago Mass Spectrometry Facility (NSF instrumentation grant CHE-1048528). G.G. is supported by the Department of Defense through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.6c02348.

  • Experimental and simulation details and methods; synthetic procedures, electrochemical data, simulation data, in situ spectroscopic data, UV–vis data, IR data, NMR data, and HRMS data (PDF)

The authors declare the following competing financial interest(s): The University of Chicago has filed a provisional patent application covering aspects of this work listing G. G. and A.W. as inventors. The remaining authors declare no competing interests.

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