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Self-Inhibitory Electron Transfer of the Co(III)/Co(II)Complex Redox Couple at Pristine Carbon Electrode Ran Chen, Amin Morteza Najarian, Niraja Kurapati, Ryan James Balla, Alexander Oleinick, Irina Svir, Christian Amatore, Richard L. McCreery, and Shigeru Amemiya Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03023 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018
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Analytical Chemistry
Self-Inhibitory Electron Transfer of the Co(III)/Co(II)-Complex Redox Couple at Pristine Carbon Electrode Ran Chen,† Amin Morteza Najarian,‡ Niraja Kurapati,† Ryan J. Balla,† Alexander Oleinick,¶ Irina Svir,¶ Christian Amatore,¶,# Richard L. McCreery,‡ and Shigeru Amemiya†,* Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania, 15260, United States ‡ Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2R3, Canada ¶ PASTEUR, Département de chimie, École normale supérieure, PSL Université, Sorbonne Université, CNRS, 75005 Paris, France # State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China †
ABSTRACT: Applications of conducting carbon materials for highly efficient electrochemical energy devices require a greater fundamental understanding of heterogeneous electron-transfer (ET) mechanisms. This task, however, is highly challenging experimentally, because an adsorbing carbon surface may easily conceal its intrinsic reactivity through adventitious contamination. Herein, we employ nanoscale scanning electrochemical microscopy (SECM) and cyclic voltammetry to gain new insights into the interplay between heterogeneous ET and adsorption of a Co(III)/Co(II)complex redox couple at the contamination-free surface of electron-beam-deposited carbon (eC). Specifically, we investigate the redox couple of tris(1,10-phenanthroline)cobalt(II), Co(phen)32+, as a promising mediator for dye-sensitized solar cells and redox flow batteries. A pristine eC surface overlaid with KCl is prepared in vacuum, protected from contamination in air, and exposed to an ultrapure aqueous solution of Co(phen)32+ by the dissolution of the protective KCl layer. We employ SECM-based nanogap voltammetry to quantitatively demonstrate that Co(phen)32+ is adsorbed on the pristine eC surface to electrostatically self-inhibit outer-sphere ET of non-adsorbed Co(phen)33+ and Co(phen)32+. Strong electrostatic repulsion among Co(phen)32+ adsorbates is also demonstrated by SECM-based nanogap voltammetry and cyclic voltammetry. Quantitatively, self-inhibitory ET is characterized by a linear decrease in the standard rate constant of Co(phen)32+ oxidation with a higher surface concentration of Co(phen)32+ at the formal potential. This unique relationship is consistent not with the Frumkin model of double-layer effects, but with the Amatore model of partially blocked electrodes as extended for self-inhibitory ET. Significantly, the complicated coupling of electron transfer and surface adsorption is resolved by combining nanoscale and macroscale voltammetric methods.
Electrodes based on conducting carbon materials have been widely used for electrochemistry owing to various advantages including low cost, wide potential window, and electrocatalytic activity.1-3 Nevertheless, our understanding of heterogeneous electron-transfer (ET) mechanisms at carbon electrodes has been limited by the difficulty in measuring their intrinsic electrochemical reactivity4,5 for comparison with fundamental electrochemical theories such as Marcus theory of adiabatic outersphere ET6 and Frumkin theory of double-layer effects.7 Problematically, the reactivity of an adsorbing carbon surface is quickly compromised by adventitious surface contamination with hydrophobic blocking molecules in air8,9 and even in ultrapure water.10,11 It has been highly challenging to obtain and maintain clean surfaces of traditional carbon materials as well as emerging carbon nanomaterials.8-13 Moreover, a sp2-carbon surface is inductively polarized to adsorb polar redox-active molecules,14 which complicates the ET mechanism.15,16 By contrast, the reactivity of a relatively clean carbon sur-
face can be immeasurably high to yield only diffusionlimited ET rates as minimum estimates.17 Herein, we report new mechanistic insights into the interplay between the heterogeneous ET and adsorption of a redox couple at the pristine surface of electron-beam deposited carbon (eC).18 This amorphous form of carbon contains both sp3 and sp2 hybridization as a 30/70 mix to yield a conducting surface.19 Advantageously, eC forms extremely flat (root-mean-square roughness of 0.1–0.5 nm) thin films on Au or other metals, and has been used for fabrication in molecular electronics.20-23 For electrochemical applications, we deposit Au and eC in high vacuum (
