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This paper describes analysis of dropcast nanocrystalline and electrochemically deposited films of NiO and α-Fe 2 O 3 as model metal oxide semiconductors immersed in redox-inactive organic electrolyte solutions using electrochemical impedance spectroscopy (EIS). Although the data reported here fit a circuit commonly used to model EIS data of metal oxide electrodes, which comprises an RC circuit nested inside a second RC circuit that is in series with a resistor, our interpretation of the physical meaning of these circuit elements differs from that applied to EIS measurements of metal oxide electrodes immersed in redox-active media. The data presented here are most consistent with an interpretation in which the nested RC circuit represents charge transfer between the metal oxide film and the underlying metal electrode, and the non-nested RC circuit represents the resistance and capacitance associated with formation of a charge-compensating double-layer at the exposed interface between the metal electrode and electrolyte solution. Applying this interpretation to analysis of EIS data collected for metal oxide films in organic media enables the impact of film morphology on electrochemical behavior to be distinguished from the effects of the intrinsic electronic structure of the metal oxide. This distinction is crucial to the evaluation of nanostructured metal oxide electrodes for electrochemical energy storage and electrocatalysis applications.
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Electrochemistry at single molecule occupancy in nanopore-confined recessed ring-disk electrode arrays
Electrochemical reactions at nanoscale structures possess unique characteristics, e.g. fast mass transport, high signal-to-noise ratio at low concentration, and insignificant ohmic losses even at low electrolyte concentrations. These properties motivate the fabrication of high density, laterally ordered arrays of nanopores, embedding vertically stacked metal–insulator–metal electrode structures and exhibiting precisely controlled pore size and interpore spacing for use in redox cycling. These nanoscale recessed ring-disk electrode (RRDE) arrays exhibit current amplification factors, AF RC , as large as 55-fold with Ru(NH 3 ) 6 2/3+ , indicative of capture efficiencies at the top and bottom electrodes, Φ t,b , exceeding 99%. Finite element simulations performed to investigate the concentration distribution of redox species and to assess operating characteristics are in excellent agreement with experiment. AF RC increases as the pore diameter, at constant pore spacing, increases in the range 200–500 nm and as the pore spacing, at constant pore diameter, decreases in the range 1000–460 nm. Optimized nanoscale RRDE arrays exhibit a linear current response with concentration ranging from 0.1 μM to 10 mM and a small capacitive current with scan rate up to 100 V s −1 . At the lowest concentrations, the average pore occupancy is 〈 n 〉 ∼ 0.13 molecule establishing productive electrochemical signals at occupancies at and below the single molecule level in these nanoscale RRDE arrays.
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- Award ID(s):
- 1404744
- PAR ID:
- 10028529
- Date Published:
- Journal Name:
- Faraday Discuss.
- Volume:
- 193
- ISSN:
- 1359-6640
- Page Range / eLocation ID:
- 51 to 64
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C6FD00062B
