Rewriting Electron-Transfer Kinetics at Pyrolytic Carbon Electrodes Decorated with Nanometric Ruthenium Oxide

Abstract

Platinum is state-of-the-art for fast electron transfer whereas carbon electrodes, which have semimetal electronic character, typically exhibit slow electron-transfer kinetics. But when we turn to practical electrochemical devices, we turn to carbon. To move energy devices and electro(bio)analytical measurements to a new performance curve requires improved electron-transfer rates at carbon. We approach this challenge with electroless deposition of disordered, nanoscopic anhydrous ruthenium oxide at pyrolytic carbon prepared by thermal decomposition of benzene (RuOx@CVD-C). We assessed traditionally fast, chloride-assisted ([Fe(CN)]) and notoriously slow ([Fe(HO)]) electron-transfer redox probes at CVD-C and RuOx@CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuOx domains to their rutile form. For the fast electron-transfer probe, [Fe(CN)], the rate increases by 34× over CVD-C once the RuOx is calcined to form crystalline rutile RuO. For the classically outer-sphere [Fe(HO)], electron-transfer rates increase by an even greater degree over CVD-C (55×). The standard heterogeneous rate constant for each probe approaches that observed at Pt but does so using only minimal loadings of RuOx.

Pyrolytic Carbon Films with Tunable Electronic Structure and Surface Functionality: A Planar Stand‐In for Electroanalysis of Energy‐Relevant Reactions

“…Further, these substrates can be stored for many months with no noticeable change in wettability. The persistent rejection of ambient hydrocarbon poisoning by pyC translates to fast – for carbon – heterogeneous electron – transfer rate constants for ferri/ferrocyanide (0.015 cm s −1 ) and [Fe(H 2 O) 6 ] 3+/2+ (3.44×10 −5 cm s −1 ) . We attribute the persistent lipophobicity of pyC to the presence of thermally induced H‐terminated moieties, passivating the surface against adventitious contamination, as observed with H‐terminated silicon …”

“…Although Δ E peak is a useful initial predictor of relative heterogeneous electron‐transfer kinetics, it is insufficient on its own to confidently discern kinetics parameters. At the scan rates used in this report, it is nontrivial to pinpoint the standard heterogeneous rate constant ( k s ) once it approaches 0.1 cm s −1 – a realistic reaction rate at native pyC for [Fe(CN) 6 ] 3−/4− in aqueous KCl . The greater range of Δ E peak values and concomitant kinetics parameters of [Fe(CN) 6 ] 3−/4− in KNO 3 make this electrolyte more useful in ranking the relative rate performance of the pyC series.…”

“…We recently reported on the use of benzene‐derived pyC films on planar fused‐silica substrates as a means to investigate heterogeneous electron transfer of redox probes. When modified by electrolessly depositing nanoparticulate ruthenium oxide, the RuO x ‐decorated pyC mimicked the fast rates of Pt, not the far slower rates of carbon …”

“…[23,24] Characterizing fundamental charge-transfer dynamics at such technologically relevant, but physicochemically complex interphases may be simplified by designing planar mimics of advanced 3D electrode architectures. A 2D stand-in allows heterogeneous electron-transfer rate constants to be deter-mined as a function of modifications to the carbon [25] and enables surface-sensitive analysis using such tools as scanning probe microscopy. [26][27][28] Films of "soft" carbons such as highly oriented pyrolytic graphite (HOPG), [29][30][31] and graphene [32] have served as model 2D substrates for graphitic carbons, but disordered "hard" carbons still dominate in many energyrelevant electrochemical applications, ranging from carbon blacks used in commercial cells and devices to advanced porous carbons derived from pyrolyzing templated [33] or aerogel-like [34] organic precursors.…”

“…When modified by electrolessly depositing nanoparticulate ruthenium oxide, the RuOx-decorated pyC mimicked the fast rates of Pt, not the far slower rates of carbon. [25] Herein, we examine the physical and electrochemical properties of the pyC film itself and describe two simple modifications that allow us to tune the electronic structure and surface functionality, as tracked by Raman scattering, X-ray photoelectron spectroscopy (XPS), Kelvin Force Microscopy (KFM), and contact-angle measurements. Heterocyclic-doped pyC films (pyC~S) are prepared by pyrolyzing gas-phase mixtures of benzene and thiophene to serve as a planar mimic for the more complex sulfur-doped carbons [46,47] that play important roles in energy-related applications.…”