Reversibility and Efficiency in Electrocatalytic Energy Conversion and Lessons from Enzymes
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Reversibility and efficiency in electrocatalytic energy conversion and lessons from enzymes Fraser A. Armstronga,1 and Judy Hirstb,1 aInorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom; bMedical Research Council Mitochondrial Biology Unit, Wellcome Trust/Medical Research Council Building, Hills Road, Cambridge CB2 0XY, United Kingdom Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved July 14, 2011 (received for review March 7, 2011) Enzymes are long established as extremely efficient catalysts. Here, we show that enzymes can also be extremely efficient electrocata- A lysts (catalysts of redox reactions at electrodes). Despite being large and electronically insulating through most of their volume, Net single bidirectional oxidation some enzymes, when attached to an electrode, catalyze electroche- wave cuts sharply mical reactions that are otherwise extremely sluggish (even with through E = Eeq exponentially the best synthetic catalysts) and require a large overpotential to 0.1 V increasing current achieve a useful rate. These enzymes produce high electrocatalytic currents, displayed in single bidirectional voltammetric waves that Overpotential over- E E Current density ( - eq) switch direction (between oxidation and reduction) sharply at the potential Net requirement equilibrium potential for the substrate redox couple. Notoriously reduction irreversible processes such as CO2 reduction are thereby rendered electrochemically reversible—a consequence of molecular evolu- irreversible tion responding to stringent biological drivers for thermodynamic efficiency. Enzymes thus set high standards for the catalysts of reversible future energy technologies. B Ox cell solution Red (Eeq) electrocatalysis ∣ catalysis ∣ electrochemistry ∣ electron transport ∣ solar fuels enzyme n electrocatalyst catalyzes a redox “half reaction” in which a substrate Achemical transformation is coupled to electron transfer at an active conversion electrode (1). The active sites of surface electrocatalysts such as site platinum are integral to the electrode and contribute to the Fermi relay - level, whereas molecular electrocatalysts are distinct entities with e intramolecular centers electron transfer their own electronic and chemical properties. Molecular electro- catalysts can be attached to the electrode surface or diffuse freely distal center in solution, but depend upon interfacial electron transfer (IET). e- interfacial Enzymes, a special category of molecular electrocatalysts, are electrode (E) electron transfer distinguished by their extraordinary activities, yet limited by their size and fragility. Driven by industrial and technological needs Fig. 1. Concepts applied in this article. (A) Steady-state electrochemical for significant improvements in rates and efficiency, enzymes kinetics visualized by cyclic voltammetry. When both the oxidized and can provide crucial insights into the principles underpinning the reduced forms of a redox-active species are present, a reversible electroche- design and performance of synthetic molecular electrocatalysts. mical reaction (one with a large exchange current density) produces a single – sigmoidal wave (blue) that cuts (without inflection) through the zero-current The efficiency of enzyme catalysis is widely accepted (2 4): E axis at the equilibrium potential ( eq) and achieves a potential-independent Enzymes have highly selective substrate binding sites, avoid re- limiting current in either direction at relatively low overpotential. Conversely, leasing reactive intermediates, and decrease activation energies E if the exchange current density is low, the current is negligible around eq (here, substrate refers to the species being transformed, not the and two sigmoidal waves (red), one for either direction, are separated in supporting material). Traditional definitions of enzyme efficiency potential, emerging from the baseline with an exponential dependence focus on how closely the rate approaches diffusion control (4). on potential: A substantial overpotential is required to match the current Recently, electrochemistry has revealed a hitherto unquantified produced by the reversible system. (B) Cartoon showing an adsorbed enzyme dimension in enzyme catalysis; many redox enzymes minimize functioning as a molecular electrocatalyst. the energy needed to drive a reaction—a quantity that is easily activities (concentrations) define an important reference point, visualized electrochemically and that we refer to as the overpo- the equilibrium potential (E ). Electrochemically, E is the tential requirement. The energy efficiency of enzyme catalysis eq eq open circuit potential established by the mixture of substrates, is expected because biology must fully exploit available energy or the applied potential at which no net current flows. When resources and minimize energy losses. the applied potential (E) is above E (E>E ) Red is oxidized SCIENCES The performance of an electrocatalyst is readily visualized by eq eq at the electrode, to adjust the concentrations of Ox and Red to cyclic voltammetry, a technique for driving reactions, measuring APPLIED BIOLOGICAL kinetics and thermodynamics, detecting activation/inactivation processes, and observing catalytic efficiency—all in a single ex- Author contributions: F.A.A. and J.H. designed research, analyzed data, and wrote periment (5). To explain the term “overpotential requirement,” the paper. we refer to the two voltammograms in Fig. 1A. They depict the The authors declare no conflict of interest. simple (uncatalyzed) interconversion of the oxidized (Ox) and This article is a PNAS Direct Submission. reduced (Red) forms of a redox-active species. Importantly, in Freely available online through the PNAS open access option. CHEMISTRY Fig. 1A and in all cases discussed here, both Ox and Red are pre- 1To whom correspondence may be addressed. E-mail: [email protected] or sent in solution: Thus, according to the Nernst equation, their [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1103697108 PNAS ∣ August 23, 2011 ∣ vol. 108 ∣ no. 34 ∣ 14049–14054 Downloaded by guest on October 1, 2021 the ratio set by the Nernst equation. When IET is too slow to 0.3 A maintain Nernstian equilibrium as E is changed, the voltammetry 0.2 is termed irreversible (Fig. 1A, red): Very little current is observed H2 oxidation until a sizeable overpotential (jE − E j) is applied (for practical eq 0.1 purposes, this is the overpotential requirement), and the expo- – nentially increasing current is described by the Butler Volmer 0 equation (5). In the limiting case of an electrochemically rever- H+ reduction sible reaction (Fig. 1A, blue), IET is fast enough to maintain -0.1 Nernstian equilibrium, and even a minuscule overpotential pro- -0.4-0.6 -0.2 0 0.2 0.4 duces a significant net current that reflects the thermodynamics 0.2 (the shifting Nernstian equilibrium), not the exponentially in- B creasing rate of IET. The difference between the two cases is 0.15 CO embodied in the exchange current (the magnitude of the equal 0.1 oxidation oxidation and reduction currents that comprise the dynamic equi- E 0.5 librium at eq) (5). Irreversible systems have very low exchange E currents and are unresponsive to potential changes close to eq; 0 systems that approach the reversible limit have very high ex- CO2 -0.5 change currents and respond strongly. reduction To extend the concept of electrochemical reversibility to elec- -0.6 -0.4 -0.2 0 0.2 trocatalysis by molecules attached to electrodes (see Fig. 1B), we “ 0.06 C adopt, pragmatically, the term electrocatalytic exchange cur- ) formate -2 0.04 rent” that embodies not only IET, but also the turnover of the oxidation catalytic center and (importantly for enzymes) intramolecular 0.02 electron transfer, each of which may limit electrocatalysis. Note 0 that the electrocatalytic exchange current remains defined at - 0.02 CO2 E reduction eq, not at the potential of any of the redox centers in the elec- - 0.04 Current (mA cm Current (mA trocatalyst. Thus, we refer to electrocatalysts with low electroca- - 0.06 talytic exchange currents (and large overpotential requirements) as irreversible, and electrocatalysts with high electrocatalytic -0.8 -0.6 -0.4 -0.2 0 -0.2 exchange currents (and very little overpotential requirement in 0.01 D either direction) as reversible or efficient. In reality, electrocata- 0.005 NADH lytic waveshapes rarely conform to the expectations for simple oxidation electrochemical systems, precluding simple quantitative defini- 0 NAD+ tions for terms such as the overpotential requirement. Finally, -0.005 reduction in electrocatalysis, as in simple electrochemistry, a potential- independent process eventually becomes rate limiting at high -0.01 overpotential. It is highly desirable (returning to the traditional -0.015 definition of enzyme efficiency; ref. 4) that the limiting current -0.8 -0.6 -0.4 -0.2 0 0.2 density (current per unit electrode area) should approach diffu- sion control. However, enzymes are large molecules, so they have 0 E Eeq large surface area requirements and low active-site densities; this -0.05 feature must be taken into account when comparing the intrinsic -0.1 O2 abilities of enzyme and surface electrocatalysts. reduction Results from Enzyme Electrocatalysts -0.15 Fig. 2 shows voltammograms from several enzymes that contain -0.2 the active sites shown in Scheme 1. Each enzyme, adsorbed