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Voltammetric applications of hydrogen bonding and proton-coupled electron transfer reactions of organic molecules
Current Opinion in
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Electrochemistry
ScienceDirect Review Article
Voltammetric applications of hydrogen bonding and proton-coupled electron transfer reactions of organic molecules Malcolm E. Tessensohn1 and Richard D. Webster1,2 Abstract
Proton-coupled electron transfer and hydrogen bonding reactions are ubiquitous requisites for the occurrence of many natural processes and man-made applications. These reactions either involve the direct transfer of charge (in the form of protons and electrons) or contain sufficient electrostatic characteristics to be affected by the application of a potential. Hence, they can be analyzed or initiated by voltammetry, which is itself highly sensitive yet tolerant to a variety of interferences and so can be used under various experimental conditions. The purpose of this review is to highlight the potential of this electrochemical technique for studying important processes such as those involved in energy storage, CO2 reduction, and sensor applications. Addresses 1 Environmental Chemistry and Materials Centre, Nanyang Environment & Water Research Institute, 1 Cleantech Loop, CleanTech One #06-10, 637141, Singapore 2 Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore Corresponding author: Webster, Richard D. ([email protected])
Current Opinion in Electrochemistry 2019, 15:27–33 This review comes from a themed issue on Organic and Molecular Electrochemistry Edited by Marc Robert For a complete overview see the Issue and the Editorial Available online 4 April 2019 https://doi.org/10.1016/j.coelec.2019.03.013 2451-9103/© 2019 Elsevier B.V. All rights reserved.
Keywords Carbon dioxide reduction, Organic battery, pH meter, Square-scheme mechanism, Voltammetry.
Introduction Voltammetry has often been used for studying molecular electrochemical behavior and reaction mechanisms because of its ability in providing useful information such as the thermodynamics and kinetics of electron transfer (ET) reactions and their coupled chemical reactions, the stability of the products and intermediates, www.sciencedirect.com
as well as reversibility and electron stoichiometry of the system. It is also popularly used in the analysis of proton-coupled ET (PCET) and hydrogen bonding (HB) reactions [1e7] and has exploited these reactions for several specific uses in organic or aqueous solvent systems. The advantages of using voltammetry to study PCET reactions and HB interactions include the instantaneous observation of the results during the analysis and the ability to modulate interactions via a change in the oxidation state. It also possesses greater sensitivity than UV-vis spectroscopy owing to the simultaneous probing of multiple oxidation states that respond and interact differently [8]. It is often not affected by the large excess of background hydroxyl groups either in the organic solvent (such as alcohols) or from the presence of trace water, as compared with nuclear magnetic resonance (NMR) and infrared spectroscopy, and can be applied to study both paramagnetic and diamagnetic compounds and even highly reactive short-lived intermediates. In addition, for HB reactions, the entire hydrogen bond complex can be studied even if one of the components (i.e. hydrogen donor or hydrogen acceptor) is electroinactive [9e13]. Nonetheless, voltammetry has several limitations over spectroscopic techniques in studying HB or proton transfer (PT) reactions. It needs at least one of the reactants to be electroactive, specifically within the range where the solvent and electrode are electrochemically inert. It provides little direct information about the identity of the species involved in the mechanism or produced from the reactions and so requires complementary spectroscopic techniques or the use of standards for accurate identification and characterization. Voltammetry, nevertheless, remains popular for the application and study of PCET and HB reactions, to which some of the recent work will be briefly highlighted here.
Proton-coupled ET and HB reactions In PCET reactions, the ET and PT reactions occur together, either in a series of separate steps or concertedly in one elementary step. In the stepwise process, the ET and PT reactions (represented by the horizontal Current Opinion in Electrochemistry 2019, 15:27–33
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Scheme 1
Figure 1
A 4-membered electrochemical reduction square-scheme mechanism for electron transfer, proton transfer and proton-coupled electron transfer reactions.
Cyclic voltammograms of the two 1e− reductions of 1 mM 1, 4naphthoquinone/0.1 M Bu4NPF6 in CH3CN at 0.1 V s−1 on a 1-mm diameter planar glassy carbon working electrode in the absence and presence of 10 mM of ethanol (red and black scans, respectively). Note the shifting of the second reduction process potentials toward less negative values upon hydrogen bonding.
and vertical arrows, respectively, in Scheme 1) may occur in any order depending on the acidity of the proton donor, basicity of the proton acceptor, and electron affinity or ionization energy of the reactants. As for the concerted mechanism (represented by the diagonal arrow), the protons and electrons can be transferred to or from the same orbitals of the reactants, termed as a hydrogen atom transfer, or from different orbitals on the donor into different orbitals on the acceptor, called a concerted electroneproton transfer [14]. HB reactions, on the other hand, purely involve the sharing of a hydrogen atom between the hydrogen donor and hydrogen acceptor, that are held together within the hydrogen bond complex by an attractive interaction with some directionality. HB reactions with moderate values for their binding constants generally do not affect the chemical reversibility and kinetics of ET reactions unlike PT reactions [15], whereas a partial loss in electrochemical reversibility can be noticed for extremely favored HB processes that involve the ioneneutral or ion-ion modes of interactions. HB reactions do however affect electron affinities and ionization energies, in a similar way as PT reactions, and so can significantly alter the redox potentials of ET reactions [1].
Applications of PCET and HB reactions Voltammetry for measuring hydrogen bond strengths and host–guest recognition
Voltammetry has been successfully applied to measure the hydrogen bond strengths of both hydrogen donors and hydrogen acceptors, notwithstanding if they are electroactive or electroinactive. In the case of electroactive species, their HB abilities can be qualitatively and directly determined through their peak currents and redox potentials which are correspondingly affected by their diffusion coefficients and electron affinities/ionization energies that change upon interaction (Figure 1) [15e18]. Quantitative information such as their Current Opinion in Electrochemistry 2019, 15:27–33
association and rate constants and number of molecules in the hydrogen bond complex can be further obtained via density functional theory (DFT) calculations, modeling based on a single global equilibrium or equilibria in successive stages, or simulating the voltammetric data [19,20]. However, an accurate evaluation can only be achieved when there is knowledge of all the possible interactions including ion pairing with the supporting electrolyte and interactions with the solvent and trace water, which may not be always available. As for electroinactive hydrogen donors and hydrogen acceptors, their HB reactions are often indirectly investigated via electroactive species, that should ideally exhibit simple redox behavior, similar to the quinones [10,21e34], dinitrobenzenes [11,35], and phenylenediamines [12]. Alternatively, electroactive appendages such as cobaltocenium [36e40], ferrocene [41e43], or tetrathiafulvalene [44,45] can be incorporated in close proximity to the HB sites to modulate the interaction of the various oxidation states. Regardless of the focus and approach, the voltammetric experiments are best performed in dried solvents with a weakly coordinating supporting electrolyte to maximize observance of the desired HB effects. In addition to the aforementioned applications, voltammetry has been used for the estimation of relative acidity and basicity in organic solvents [46,47], which are often absent from the literature, and for the detection of impurities in solvents through hosteguest recognition [48]. It is also worthy to note that voltammetry is especially sensitive to HB reactions of organic molecules that undergo multiple successive ET reactions or possess several sites for HB because it simultaneously probes a number of different oxidation www.sciencedirect.com
Using voltammetry for studying hydrogen bonding and proton transfer Tessensohn and Webster
states that are likely to show significant differences in their interactions. This is exemplified in the serendipitous measurement of the hydrogen acceptor abilities of alcohols as opposed to their typical behavior as hydrogen donors via the oxidation of phenylenediamines [49]. pH sensors
The measurement of pH is probably the single most common analytical procedure performed on a routine basis because of its importance and ease of use and is a major application of PCET reactions. One way that this has been achieved is to confine electroactive species to the surface of an electrode and measure their redox potential in the solution of interest as the pH is varied. The shift in potential should ideally follow a Nernstian response over a wide pH range, which can be used to obtain the pH reading via suitable calibration experiments. It is however reported that HB between the surface-confined electroactive species, formed through electrografting, and trace water affects the density of the grafted layer and is especially important if organic solvents are used during the preparation of covalently modified electrodes [50]. A common mistake in such work is performing the calibrations and tests in solutions of vastly different ionic strengths and buffering capacity, which would lead to inaccurate measurements. In addition, most real samples tested are aerated and not buffered, such as those from environmental monitoring or clinical work. Thus, the recent emphasis has been on developing electrochemical pH sensors for operation under both buffered and, more importantly, unbuffered oxygenated conditions. Because voltammetry is a nonequilibrium technique, one way of overcoming the shortage of protons for PCET reactions to proceed at high pH under unbuffered
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conditions is to add aliquots of the test solution into a weakly buffered standard solution that has just enough but not exceedingly large buffering capacity as seen with the riboflavin (vitamin B2)-based pH sensor that was tested in both buffered (Figure 2) and unbuffered aqueous solutions [51]. Macpherson et al [52], on the other hand, bypassed this by integrating quinone groups onto the surface of a boron-doped diamond electrode via laser ablation, to which the effects of proton accumulation and depletion were then negated by lowering the surface coverage of the electrode. Elsewhere, for measurements under buffered aqueous conditions, Compton et al constructed reagentless voltammetric pH sensors based on surface quinone groups on an unmodified edge plane pyrolytic graphite electrode [53,54] or via the use of derivatized carbon powder electrodes [55]. They also developed a calibrationless pH sensor based on the pH-dependent 2e/2Hþ PCET reduction of nitrosophenyl against the pH-independent 1e oxidation of ferrocenyl groups that were both comodified onto screen-printed carbon electrodes [56]. More recently, they successfully used chemically oxidized carbon fiber surfaces containing quinone groups for amperometric micro pH measurements in oxygenated saliva [57]. Reduction of carbon dioxide
Rising atmospheric levels of CO2 and its role in global warming have prompted the need for development of economical methods for trapping and converting this greenhouse gas into valuable reduced carbonebased products, to which electrochemical procedures have found favor because they can be quickly performed under relatively mild conditions with low amounts of reagents. The direct reduction of CO2 into CO2 is typically avoided because of its high energy cost (w -2 V vs. standard hydrogen electrode (SHE)) [58]. Instead,
Figure 2
Voltammetric pH measurements in deoxygenated buffered media using riboflavin: (a) Square-wave voltammograms obtained during the reduction of 2 mM riboflavin dropcasted onto a 3-mm diameter planar glassy carbon disk electrode in deoxygenated buffered media. (b) Corresponding calibration plots of peak potentials against pH. www.sciencedirect.com
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chemical catalysts that can react with CO2 upon reduction are preferred for they offer not only an alternative lower energy pathway but also selectivity of the products based on the reaction conditions applied. Pyridinium compounds have generated substantial interest in aqueous-based CO2 reduction for they are suspected to act as catalysts in the production of methanol via a 6e/6Hþ PCET mechanism (equations 1e5 in Scheme 2) on Pt electrodes [59]. The reduction of protons into hydrogen gas (equation 6) had traditionally been thought to be solely responsible for the low ´ant et al Faradaic yields (4e25%) [60,61]. However, Save have recently showed the formation of a Pt–CO film on the surface of the working electrode after the 2e reduction of CO2 into CO (equation 7) and have disputed the formation of methanol [62]. Other protonated products have also been obtained through the use of organometallic catalysts, modified electrodes, cosolvents, and different electrolytes [63e72]. Quinones have also been used in organic solvents [73] and ionic liquids [74], where they have served as
chemical catalysts and for CO2 trapping. The reduced quinone species (i.e. radical anion [Qe] and dianion [Q2]) have affinity for free CO2 in solution and would react with the gas to form fairly stable quinoneeCO2 adducts, Q (CO2)e and Q (CO2)2e.These adducts can then be reacted with proton donors to give desired products or oxidized to release their bound CO2 molecules for the purpose of gas separation, while regenerating the neutral quinone. Hydrogen bonding apparently stabilizes the adducts but also hinders the reaction between the reduced quinones and CO2. Energy storage devices
A number of studies have shown that PCET and HB reactions are beneficial for energy storage and in the construction of efficient redox flow batteries (RFBs). For instance, both good cell voltage and stability were obtained in an all-organic cell using 2,3-dimethyl-1,4naphthoquinone and pyrano [3,2-f]chromene as redoxactive elements via the judicious use of diethyl malonate, a weak organic acid that undergoes a combination of HB and PCET reactions with quinone [75]. This was unlike that observed with trifluoroethanol that only
Scheme 2
Proposed mechanism for the suspected pyridinium-catalyzed reduction of CO2 to methanol on a platinum surface and its competing reactions.
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Using voltammetry for studying hydrogen bonding and proton transfer Tessensohn and Webster
participated in HB and gave poor cell stability and trifluoroacetic acid that submitted exclusively to PT reactions, with the reduced quinones resulting in a lower cell voltage. In an aqueous RFB using 2,6-dihydroxyanthraquinone and ferricyanide, the cell potential was noted to improve with increasing pH because of the shifting of the reduction potential of the quinone toward more negative values while the reduction pathway gradually changed from a 2e/2Hþ PCET process in an acidic medium to a 2e successive ET mechanism under alkaline conditions [76]. Interestingly, high current and stable round-trip energy efficiencies were observed even at high pH values, thus suggesting that HB or some other form of stabilization (i.e. ion pairing with counter cation or solvent interactions) is significant and is in action throughout [77,78]. Several metal complexes have also been reportedly used for energy storage [79], with a small number participating in PCET reactions and serving as both the electroactive species and proton reservoir [80]. Although some of these complexes show low energy and power densities, they have naturally been appropriated for use in RFBs especially in those where the electrical energy is stored in the form of a proton gradient.
Conclusion PCET and HB reactions have amassed a great deal of interest among electrochemists because they play important roles in a number of natural and biological reactions, and they are useful in several industrial applications. It was the intention of this work to draw attention toward their protean functions by underlining some of their present-day uses. It is hoped that the reader has gained some insight and is encouraged to seek out other areas and means where PCET and HB reactions could possibly find further uses.
Conflict of interest Nothing declared.
Funding This work was supported by a Singapore Government MOE Academic Research Fund Tier 1 Grant (RG109/ 15).
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This work ingeniously circumvented the problem of insufficient protons for PCET reactions to occur in high pH solutions via modulating the amount of surface groups on the surface of the electrode.
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56. Xiong L, Batchelor-McAuley C, Compton RG: Calibrationless pH sensors based on nitrosophenyl and ferrocenyl co-modified screen printed electrodes. Sensor Actuator B Chem 2011, 159: 251–255. 57. Korbua C, Batchelor-McAuley C, Compton RG: Amperometric * micro pH measurements in oxygenated saliva. Analyst 2017, 142:2828–2835. This is a good example of the combined use of PCET reactions and electrochemistry for pH measurements of real samples under aerated conditions and where the buffering capacity may also differ. 58. Kai T, Zhou M, Duan Z, Henkelman GA, Bard AJ: Detection of CO– 2 in the electrochemical reduction of carbon dioxide in N,N-dimethylformamide by scanning electrochemical microscopy. J Am Chem Soc 2017, 139:18552–18557. 59. Costentin C, Savéant JM, Tard C: Catalysis of CO2 electro* chemical reduction by protonated pyridine and similar molecules. Useful lessons from a methodological misadventure. ACS Energ Lett 2018, 3:695–703. This paper highlights the pitfalls and difficulty of using cyclic voltammetry to study the existence of catalysis processes. 60. Cole EB, Lakkaraju PS, Rampulla DM, Morris AJ, Abelev E, Bocarsly AB: Using a one-electron shuttle for the multielectron reduction of CO2 to methanol: kinetic, mechanistic, and structural insights. J Am Chem Soc 2010, 132: 11539–11551. 61. Lee JHQ, Lauw SJL, Webster RD: The electrochemical reduction of carbon dioxide (CO2) to methanol in the presence of pyridoxine (vitamin B6). Electrochem Commun 2016, 64:69–73. 62. Dridi H, Comminges C, Morais C, Meledje JC, Kokoh KB, * * Costentin C, Savéant JM: Catalysis and inhibition in the electrochemical reduction of CO2 on platinum in the presence of protonated pyridine. New insights into mechanisms and products. J Am Chem Soc 2017, 139: 13922 – 13928. The results from this study bear important implications on the future use of platinum as an electrode for the electrochemical reduction of CO2. 63. Bhugun I, Lexa D, Savéant JM: Ultraefficient selective homogeneous catalysis of the electrochemical reduction of carbon dioxide by an iron(0) porphyrin associated with a weak Broensted acid cocatalyst. J Am Chem Soc 1994, 116: 5015–5016. 64. Costentin C, Drouet S, Robert M, Savéant JM: A local proton source enhances CO2 electroreduction to CO by a molecular Fe catalyst. Science 2012, 338:90–94. 65. Qiao J, Liu Y, Hong F, Zhang J: A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels. Chem Soc Rev 2014, 43:631–675. 66. Jhong HR, Ma S, Kenis PJA: Electrochemical conversion of CO2 to useful chemicals: current status, remaining challenges, and future opportunities. Curr Opin Chem Eng 2013, 2: 191–199.
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71. Whipple DT, Kenis PJA: Prospects of CO2 utilization via direct heterogeneous electrochemical reduction. J Phys Chem Lett 2010, 1:3451–3458. 72. Kuhl KP, Cave ER, Abram DN, Jaramillo TF: New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ Sci 2012, 5:7050–7059. 73. Yin W, Grimaud A, Azcarate I, Yang C, Tarascon JM: Electrochemical reduction of CO2 mediated by quinone derivatives: implication for Li-CO2 battery. J Phys Chem C 2018, 122: 6546–6554. 74. Gurkan B, Simeon F, Hatton TA: Quinone reduction in ionic liquids for electrochemical CO2 separation. ACS Sustainable Chem Eng 2015, 3:1394–1405. 75. Shi RRS, Tessensohn ME, Lauw SJL, Foo NABY, Webster RD: * Tuning the reduction potential of quinones by controlling the effects of hydrogen bonding, protonation and proton-coupled electron transfer reactions. Chem Commun 2019, 55: 2277–2280. This study demonstrated that the effects of hydrogen bonding and proton-transfer reactions alone do not deliver the ideal properties desired of a battery and that PCET reactions can more than make up for it by providing both good stability and large cell potential. 76. Huskinson B, Marshak MP, Suh C, Er S, Gerhardt MR, Galvin CJ, Chen X, Aspuru-Guzik A, Gordon RG, Aziz MJ: A metal-free organic-inorganic aqueous flow battery. Nature 2014, 505: 195–198. 77. Lin K, Chen Q, Gerhardt MR, Tong L, Kim SB, Eisenach L, Valle AW, Hardee D, Gordon RG, Aziz MJ, Marshak MP: Alkaline quinone flow battery. Science 2015, 349:1529–1532. 78. Kwabi DG, Lin K, Ji Y, Kerr EF, Goulet MA, de Porcellinis D, * Tabor DP, Pollack DA, Aspuru-Guzik A, Gordon RG, Aziz MJ: Alkaline quinone flow battery with long lifetime at pH 12. Joule 2018, 2:1894–1906. This paper describes an extremely long-lived redox-flow battery system that has the potential to be fully commercialised. The high pH means that hydrogen-bonding rather than protonation is likely to be very important in controlling the redox potential and lifetime of the reduced quinone. 79. Cabrera PJ, Yang X, Suttil JA, Hawthorne KL, Brooner REM, Sanford M, Thompson LT: Complexes containing redox noninnocent ligands for symmetric, multielectron transfer nonaqueous redox flow batteries. J Phys Chem C 2015, 119: 15882–15889. 80. Motoyama D, Yoshikawa K, Ozawa H, Tadokoro M, Haga M: * Energy-storage applications for a pH gradient between two benzimidazole-ligated ruthenium complexes that engage in proton-coupled electron-transfer reactions in solution. Inorg Chem 2017, 56:6419–6428. The synthesized Ru complexes participated in PCET reactions by serving as both the electroactive species and proton reservoir. This opens possibilities for their application as batteries where the energy is stored in the form of a pH gradient between the half-cells, especially in unbuffered aqueous conditions.
Current Opinion in Electrochemistry 2019, 15:27–33
Available online at www.sciencedirect.com
Electrochemistry
ScienceDirect Review Article
Voltammetric applications of hydrogen bonding and proton-coupled electron transfer reactions of organic molecules Malcolm E. Tessensohn1 and Richard D. Webster1,2 Abstract
Proton-coupled electron transfer and hydrogen bonding reactions are ubiquitous requisites for the occurrence of many natural processes and man-made applications. These reactions either involve the direct transfer of charge (in the form of protons and electrons) or contain sufficient electrostatic characteristics to be affected by the application of a potential. Hence, they can be analyzed or initiated by voltammetry, which is itself highly sensitive yet tolerant to a variety of interferences and so can be used under various experimental conditions. The purpose of this review is to highlight the potential of this electrochemical technique for studying important processes such as those involved in energy storage, CO2 reduction, and sensor applications. Addresses 1 Environmental Chemistry and Materials Centre, Nanyang Environment & Water Research Institute, 1 Cleantech Loop, CleanTech One #06-10, 637141, Singapore 2 Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore Corresponding author: Webster, Richard D. ([email protected])
Current Opinion in Electrochemistry 2019, 15:27–33 This review comes from a themed issue on Organic and Molecular Electrochemistry Edited by Marc Robert For a complete overview see the Issue and the Editorial Available online 4 April 2019 https://doi.org/10.1016/j.coelec.2019.03.013 2451-9103/© 2019 Elsevier B.V. All rights reserved.
Keywords Carbon dioxide reduction, Organic battery, pH meter, Square-scheme mechanism, Voltammetry.
Introduction Voltammetry has often been used for studying molecular electrochemical behavior and reaction mechanisms because of its ability in providing useful information such as the thermodynamics and kinetics of electron transfer (ET) reactions and their coupled chemical reactions, the stability of the products and intermediates, www.sciencedirect.com
as well as reversibility and electron stoichiometry of the system. It is also popularly used in the analysis of proton-coupled ET (PCET) and hydrogen bonding (HB) reactions [1e7] and has exploited these reactions for several specific uses in organic or aqueous solvent systems. The advantages of using voltammetry to study PCET reactions and HB interactions include the instantaneous observation of the results during the analysis and the ability to modulate interactions via a change in the oxidation state. It also possesses greater sensitivity than UV-vis spectroscopy owing to the simultaneous probing of multiple oxidation states that respond and interact differently [8]. It is often not affected by the large excess of background hydroxyl groups either in the organic solvent (such as alcohols) or from the presence of trace water, as compared with nuclear magnetic resonance (NMR) and infrared spectroscopy, and can be applied to study both paramagnetic and diamagnetic compounds and even highly reactive short-lived intermediates. In addition, for HB reactions, the entire hydrogen bond complex can be studied even if one of the components (i.e. hydrogen donor or hydrogen acceptor) is electroinactive [9e13]. Nonetheless, voltammetry has several limitations over spectroscopic techniques in studying HB or proton transfer (PT) reactions. It needs at least one of the reactants to be electroactive, specifically within the range where the solvent and electrode are electrochemically inert. It provides little direct information about the identity of the species involved in the mechanism or produced from the reactions and so requires complementary spectroscopic techniques or the use of standards for accurate identification and characterization. Voltammetry, nevertheless, remains popular for the application and study of PCET and HB reactions, to which some of the recent work will be briefly highlighted here.
Proton-coupled ET and HB reactions In PCET reactions, the ET and PT reactions occur together, either in a series of separate steps or concertedly in one elementary step. In the stepwise process, the ET and PT reactions (represented by the horizontal Current Opinion in Electrochemistry 2019, 15:27–33
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Scheme 1
Figure 1
A 4-membered electrochemical reduction square-scheme mechanism for electron transfer, proton transfer and proton-coupled electron transfer reactions.
Cyclic voltammograms of the two 1e− reductions of 1 mM 1, 4naphthoquinone/0.1 M Bu4NPF6 in CH3CN at 0.1 V s−1 on a 1-mm diameter planar glassy carbon working electrode in the absence and presence of 10 mM of ethanol (red and black scans, respectively). Note the shifting of the second reduction process potentials toward less negative values upon hydrogen bonding.
and vertical arrows, respectively, in Scheme 1) may occur in any order depending on the acidity of the proton donor, basicity of the proton acceptor, and electron affinity or ionization energy of the reactants. As for the concerted mechanism (represented by the diagonal arrow), the protons and electrons can be transferred to or from the same orbitals of the reactants, termed as a hydrogen atom transfer, or from different orbitals on the donor into different orbitals on the acceptor, called a concerted electroneproton transfer [14]. HB reactions, on the other hand, purely involve the sharing of a hydrogen atom between the hydrogen donor and hydrogen acceptor, that are held together within the hydrogen bond complex by an attractive interaction with some directionality. HB reactions with moderate values for their binding constants generally do not affect the chemical reversibility and kinetics of ET reactions unlike PT reactions [15], whereas a partial loss in electrochemical reversibility can be noticed for extremely favored HB processes that involve the ioneneutral or ion-ion modes of interactions. HB reactions do however affect electron affinities and ionization energies, in a similar way as PT reactions, and so can significantly alter the redox potentials of ET reactions [1].
Applications of PCET and HB reactions Voltammetry for measuring hydrogen bond strengths and host–guest recognition
Voltammetry has been successfully applied to measure the hydrogen bond strengths of both hydrogen donors and hydrogen acceptors, notwithstanding if they are electroactive or electroinactive. In the case of electroactive species, their HB abilities can be qualitatively and directly determined through their peak currents and redox potentials which are correspondingly affected by their diffusion coefficients and electron affinities/ionization energies that change upon interaction (Figure 1) [15e18]. Quantitative information such as their Current Opinion in Electrochemistry 2019, 15:27–33
association and rate constants and number of molecules in the hydrogen bond complex can be further obtained via density functional theory (DFT) calculations, modeling based on a single global equilibrium or equilibria in successive stages, or simulating the voltammetric data [19,20]. However, an accurate evaluation can only be achieved when there is knowledge of all the possible interactions including ion pairing with the supporting electrolyte and interactions with the solvent and trace water, which may not be always available. As for electroinactive hydrogen donors and hydrogen acceptors, their HB reactions are often indirectly investigated via electroactive species, that should ideally exhibit simple redox behavior, similar to the quinones [10,21e34], dinitrobenzenes [11,35], and phenylenediamines [12]. Alternatively, electroactive appendages such as cobaltocenium [36e40], ferrocene [41e43], or tetrathiafulvalene [44,45] can be incorporated in close proximity to the HB sites to modulate the interaction of the various oxidation states. Regardless of the focus and approach, the voltammetric experiments are best performed in dried solvents with a weakly coordinating supporting electrolyte to maximize observance of the desired HB effects. In addition to the aforementioned applications, voltammetry has been used for the estimation of relative acidity and basicity in organic solvents [46,47], which are often absent from the literature, and for the detection of impurities in solvents through hosteguest recognition [48]. It is also worthy to note that voltammetry is especially sensitive to HB reactions of organic molecules that undergo multiple successive ET reactions or possess several sites for HB because it simultaneously probes a number of different oxidation www.sciencedirect.com
Using voltammetry for studying hydrogen bonding and proton transfer Tessensohn and Webster
states that are likely to show significant differences in their interactions. This is exemplified in the serendipitous measurement of the hydrogen acceptor abilities of alcohols as opposed to their typical behavior as hydrogen donors via the oxidation of phenylenediamines [49]. pH sensors
The measurement of pH is probably the single most common analytical procedure performed on a routine basis because of its importance and ease of use and is a major application of PCET reactions. One way that this has been achieved is to confine electroactive species to the surface of an electrode and measure their redox potential in the solution of interest as the pH is varied. The shift in potential should ideally follow a Nernstian response over a wide pH range, which can be used to obtain the pH reading via suitable calibration experiments. It is however reported that HB between the surface-confined electroactive species, formed through electrografting, and trace water affects the density of the grafted layer and is especially important if organic solvents are used during the preparation of covalently modified electrodes [50]. A common mistake in such work is performing the calibrations and tests in solutions of vastly different ionic strengths and buffering capacity, which would lead to inaccurate measurements. In addition, most real samples tested are aerated and not buffered, such as those from environmental monitoring or clinical work. Thus, the recent emphasis has been on developing electrochemical pH sensors for operation under both buffered and, more importantly, unbuffered oxygenated conditions. Because voltammetry is a nonequilibrium technique, one way of overcoming the shortage of protons for PCET reactions to proceed at high pH under unbuffered
29
conditions is to add aliquots of the test solution into a weakly buffered standard solution that has just enough but not exceedingly large buffering capacity as seen with the riboflavin (vitamin B2)-based pH sensor that was tested in both buffered (Figure 2) and unbuffered aqueous solutions [51]. Macpherson et al [52], on the other hand, bypassed this by integrating quinone groups onto the surface of a boron-doped diamond electrode via laser ablation, to which the effects of proton accumulation and depletion were then negated by lowering the surface coverage of the electrode. Elsewhere, for measurements under buffered aqueous conditions, Compton et al constructed reagentless voltammetric pH sensors based on surface quinone groups on an unmodified edge plane pyrolytic graphite electrode [53,54] or via the use of derivatized carbon powder electrodes [55]. They also developed a calibrationless pH sensor based on the pH-dependent 2e/2Hþ PCET reduction of nitrosophenyl against the pH-independent 1e oxidation of ferrocenyl groups that were both comodified onto screen-printed carbon electrodes [56]. More recently, they successfully used chemically oxidized carbon fiber surfaces containing quinone groups for amperometric micro pH measurements in oxygenated saliva [57]. Reduction of carbon dioxide
Rising atmospheric levels of CO2 and its role in global warming have prompted the need for development of economical methods for trapping and converting this greenhouse gas into valuable reduced carbonebased products, to which electrochemical procedures have found favor because they can be quickly performed under relatively mild conditions with low amounts of reagents. The direct reduction of CO2 into CO2 is typically avoided because of its high energy cost (w -2 V vs. standard hydrogen electrode (SHE)) [58]. Instead,
Figure 2
Voltammetric pH measurements in deoxygenated buffered media using riboflavin: (a) Square-wave voltammograms obtained during the reduction of 2 mM riboflavin dropcasted onto a 3-mm diameter planar glassy carbon disk electrode in deoxygenated buffered media. (b) Corresponding calibration plots of peak potentials against pH. www.sciencedirect.com
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chemical catalysts that can react with CO2 upon reduction are preferred for they offer not only an alternative lower energy pathway but also selectivity of the products based on the reaction conditions applied. Pyridinium compounds have generated substantial interest in aqueous-based CO2 reduction for they are suspected to act as catalysts in the production of methanol via a 6e/6Hþ PCET mechanism (equations 1e5 in Scheme 2) on Pt electrodes [59]. The reduction of protons into hydrogen gas (equation 6) had traditionally been thought to be solely responsible for the low ´ant et al Faradaic yields (4e25%) [60,61]. However, Save have recently showed the formation of a Pt–CO film on the surface of the working electrode after the 2e reduction of CO2 into CO (equation 7) and have disputed the formation of methanol [62]. Other protonated products have also been obtained through the use of organometallic catalysts, modified electrodes, cosolvents, and different electrolytes [63e72]. Quinones have also been used in organic solvents [73] and ionic liquids [74], where they have served as
chemical catalysts and for CO2 trapping. The reduced quinone species (i.e. radical anion [Qe] and dianion [Q2]) have affinity for free CO2 in solution and would react with the gas to form fairly stable quinoneeCO2 adducts, Q (CO2)e and Q (CO2)2e.These adducts can then be reacted with proton donors to give desired products or oxidized to release their bound CO2 molecules for the purpose of gas separation, while regenerating the neutral quinone. Hydrogen bonding apparently stabilizes the adducts but also hinders the reaction between the reduced quinones and CO2. Energy storage devices
A number of studies have shown that PCET and HB reactions are beneficial for energy storage and in the construction of efficient redox flow batteries (RFBs). For instance, both good cell voltage and stability were obtained in an all-organic cell using 2,3-dimethyl-1,4naphthoquinone and pyrano [3,2-f]chromene as redoxactive elements via the judicious use of diethyl malonate, a weak organic acid that undergoes a combination of HB and PCET reactions with quinone [75]. This was unlike that observed with trifluoroethanol that only
Scheme 2
Proposed mechanism for the suspected pyridinium-catalyzed reduction of CO2 to methanol on a platinum surface and its competing reactions.
Current Opinion in Electrochemistry 2019, 15:27–33
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Using voltammetry for studying hydrogen bonding and proton transfer Tessensohn and Webster
participated in HB and gave poor cell stability and trifluoroacetic acid that submitted exclusively to PT reactions, with the reduced quinones resulting in a lower cell voltage. In an aqueous RFB using 2,6-dihydroxyanthraquinone and ferricyanide, the cell potential was noted to improve with increasing pH because of the shifting of the reduction potential of the quinone toward more negative values while the reduction pathway gradually changed from a 2e/2Hþ PCET process in an acidic medium to a 2e successive ET mechanism under alkaline conditions [76]. Interestingly, high current and stable round-trip energy efficiencies were observed even at high pH values, thus suggesting that HB or some other form of stabilization (i.e. ion pairing with counter cation or solvent interactions) is significant and is in action throughout [77,78]. Several metal complexes have also been reportedly used for energy storage [79], with a small number participating in PCET reactions and serving as both the electroactive species and proton reservoir [80]. Although some of these complexes show low energy and power densities, they have naturally been appropriated for use in RFBs especially in those where the electrical energy is stored in the form of a proton gradient.
Conclusion PCET and HB reactions have amassed a great deal of interest among electrochemists because they play important roles in a number of natural and biological reactions, and they are useful in several industrial applications. It was the intention of this work to draw attention toward their protean functions by underlining some of their present-day uses. It is hoped that the reader has gained some insight and is encouraged to seek out other areas and means where PCET and HB reactions could possibly find further uses.
Conflict of interest Nothing declared.
Funding This work was supported by a Singapore Government MOE Academic Research Fund Tier 1 Grant (RG109/ 15).
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Using voltammetry for studying hydrogen bonding and proton transfer Tessensohn and Webster
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This work ingeniously circumvented the problem of insufficient protons for PCET reactions to occur in high pH solutions via modulating the amount of surface groups on the surface of the electrode.
67. Wu J, Zhou XD: Catalytic conversion of CO2 to value added fuels: current status, challenges, and future directions. Chin J Catal 2016, 37:999–1015.
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54. Lu M, Compton RG: Voltammetric pH sensor based on an edge plane pyrolytic graphite electrode. Analyst 2014, 139: 2397–2403.
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56. Xiong L, Batchelor-McAuley C, Compton RG: Calibrationless pH sensors based on nitrosophenyl and ferrocenyl co-modified screen printed electrodes. Sensor Actuator B Chem 2011, 159: 251–255. 57. Korbua C, Batchelor-McAuley C, Compton RG: Amperometric * micro pH measurements in oxygenated saliva. Analyst 2017, 142:2828–2835. This is a good example of the combined use of PCET reactions and electrochemistry for pH measurements of real samples under aerated conditions and where the buffering capacity may also differ. 58. Kai T, Zhou M, Duan Z, Henkelman GA, Bard AJ: Detection of CO– 2 in the electrochemical reduction of carbon dioxide in N,N-dimethylformamide by scanning electrochemical microscopy. J Am Chem Soc 2017, 139:18552–18557. 59. Costentin C, Savéant JM, Tard C: Catalysis of CO2 electro* chemical reduction by protonated pyridine and similar molecules. Useful lessons from a methodological misadventure. ACS Energ Lett 2018, 3:695–703. This paper highlights the pitfalls and difficulty of using cyclic voltammetry to study the existence of catalysis processes. 60. Cole EB, Lakkaraju PS, Rampulla DM, Morris AJ, Abelev E, Bocarsly AB: Using a one-electron shuttle for the multielectron reduction of CO2 to methanol: kinetic, mechanistic, and structural insights. J Am Chem Soc 2010, 132: 11539–11551. 61. Lee JHQ, Lauw SJL, Webster RD: The electrochemical reduction of carbon dioxide (CO2) to methanol in the presence of pyridoxine (vitamin B6). Electrochem Commun 2016, 64:69–73. 62. Dridi H, Comminges C, Morais C, Meledje JC, Kokoh KB, * * Costentin C, Savéant JM: Catalysis and inhibition in the electrochemical reduction of CO2 on platinum in the presence of protonated pyridine. New insights into mechanisms and products. J Am Chem Soc 2017, 139: 13922 – 13928. The results from this study bear important implications on the future use of platinum as an electrode for the electrochemical reduction of CO2. 63. Bhugun I, Lexa D, Savéant JM: Ultraefficient selective homogeneous catalysis of the electrochemical reduction of carbon dioxide by an iron(0) porphyrin associated with a weak Broensted acid cocatalyst. J Am Chem Soc 1994, 116: 5015–5016. 64. Costentin C, Drouet S, Robert M, Savéant JM: A local proton source enhances CO2 electroreduction to CO by a molecular Fe catalyst. Science 2012, 338:90–94. 65. Qiao J, Liu Y, Hong F, Zhang J: A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels. Chem Soc Rev 2014, 43:631–675. 66. Jhong HR, Ma S, Kenis PJA: Electrochemical conversion of CO2 to useful chemicals: current status, remaining challenges, and future opportunities. Curr Opin Chem Eng 2013, 2: 191–199.
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71. Whipple DT, Kenis PJA: Prospects of CO2 utilization via direct heterogeneous electrochemical reduction. J Phys Chem Lett 2010, 1:3451–3458. 72. Kuhl KP, Cave ER, Abram DN, Jaramillo TF: New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ Sci 2012, 5:7050–7059. 73. Yin W, Grimaud A, Azcarate I, Yang C, Tarascon JM: Electrochemical reduction of CO2 mediated by quinone derivatives: implication for Li-CO2 battery. J Phys Chem C 2018, 122: 6546–6554. 74. Gurkan B, Simeon F, Hatton TA: Quinone reduction in ionic liquids for electrochemical CO2 separation. ACS Sustainable Chem Eng 2015, 3:1394–1405. 75. Shi RRS, Tessensohn ME, Lauw SJL, Foo NABY, Webster RD: * Tuning the reduction potential of quinones by controlling the effects of hydrogen bonding, protonation and proton-coupled electron transfer reactions. Chem Commun 2019, 55: 2277–2280. This study demonstrated that the effects of hydrogen bonding and proton-transfer reactions alone do not deliver the ideal properties desired of a battery and that PCET reactions can more than make up for it by providing both good stability and large cell potential. 76. Huskinson B, Marshak MP, Suh C, Er S, Gerhardt MR, Galvin CJ, Chen X, Aspuru-Guzik A, Gordon RG, Aziz MJ: A metal-free organic-inorganic aqueous flow battery. Nature 2014, 505: 195–198. 77. Lin K, Chen Q, Gerhardt MR, Tong L, Kim SB, Eisenach L, Valle AW, Hardee D, Gordon RG, Aziz MJ, Marshak MP: Alkaline quinone flow battery. Science 2015, 349:1529–1532. 78. Kwabi DG, Lin K, Ji Y, Kerr EF, Goulet MA, de Porcellinis D, * Tabor DP, Pollack DA, Aspuru-Guzik A, Gordon RG, Aziz MJ: Alkaline quinone flow battery with long lifetime at pH 12. Joule 2018, 2:1894–1906. This paper describes an extremely long-lived redox-flow battery system that has the potential to be fully commercialised. The high pH means that hydrogen-bonding rather than protonation is likely to be very important in controlling the redox potential and lifetime of the reduced quinone. 79. Cabrera PJ, Yang X, Suttil JA, Hawthorne KL, Brooner REM, Sanford M, Thompson LT: Complexes containing redox noninnocent ligands for symmetric, multielectron transfer nonaqueous redox flow batteries. J Phys Chem C 2015, 119: 15882–15889. 80. Motoyama D, Yoshikawa K, Ozawa H, Tadokoro M, Haga M: * Energy-storage applications for a pH gradient between two benzimidazole-ligated ruthenium complexes that engage in proton-coupled electron-transfer reactions in solution. Inorg Chem 2017, 56:6419–6428. The synthesized Ru complexes participated in PCET reactions by serving as both the electroactive species and proton reservoir. This opens possibilities for their application as batteries where the energy is stored in the form of a pH gradient between the half-cells, especially in unbuffered aqueous conditions.
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