Electrochemistry plays a key role in a broad range of research and applied areas including the exploration of new inorganic and organic compounds, biochemical and biological systems, corrosion, ...energy applications involving fuel cells and solar cells, and nanoscale investigations. This book serves as a source of electrochemical information, providing details of experimental considerations, representative calculations, and illustrations of the possibilities available in electrochemical experimentation. The book is divided into five parts: Fundamentals, Laboratory Practical, Techniques, Applications, and Data. The Part I covers the fundamentals of electrochemistry which are essential for everyone working in the field, presenting an overview of electrochemical conventions, terminology, fundamental equations, and electrochemical cells, experiments, literature, textbooks, and specialized books. Part II focuses on the different laboratory aspects of electrochemistry which is followed by a review of the various electrochemical techniques ranging from classical experiments to scanning electrochemical microscopy, electrogenerated chemiluminesence and spectroelectrochemistry. Applications of electrochemistry include electrode kinetic determinations, unique aspects of metal deposition, and electrochemistry in small places and at novel interfaces and these are detailed in Part IV. The remaining three chapters provide useful electrochemical data and information involving electrode potentials, diffusion coefficients, and methods used in measuring liquid junction potentials.
Theoretical and experimental approaches to characterizing nanoelectrode (NE) ensembles of ultramicroelectrode dimensions (UME-NEEs) as a function of fraction of active area and random NE distribution ...are described. UME-NEEs were fabricated by addressing microregions of a gold-filled polycarbonate membrane through the UMEs of an underlying microfabricated addressable array. Results of Comsol Multiphysics 3D simulations based on randomly spaced NEs of 15 nm radius on a UME disk geometry of radii up to 5 μm are shown for steady-state voltammetry (SSV) and scanning electrochemical microscopy (SECM) experiments. Analytical equations were developed to describe the diffusion-limited steady-state current and steady-state voltammogram at an UME-NEE. These equations are shown to be in good agreement with the simulations and enabled evaluation of experimental SSVs. Comparison of experimental and simulated SECM approach curves, images, and tip voltammograms enabled the fraction of active area and distribution of NEs to be visualized and determined for individual UME-NEEs. Gold UME-NEEs are shown to be unique platforms for electrodeposition in forming nanoparticle electrodes (UME-NPEs). Electrocatalysis results for the oxygen reduction reaction (ORR) on Pt UME-NPEs in 0.1 M H2SO4 are also shown.
Electropolymerization, morphology characterization, and ion transport of poly(3,4-ethylenedioxythiophene) (PEDOT) films doped with different counterions (chloride, ferrocyanide (FCN), and ...poly(p-styrenesulfonate) (PSS-)) on a platinum electrode were investigated using scanning electrochemical microscopy (SECM) during both potential step (chronocoulometric) and cyclic voltammetric scans. An ultramicroelectrode (UME) tip was positioned close to the surface of a PEDOT-modified substrate electrode, and the responses of both electrodes to a substrate potential step or linear sweep were monitored simultaneously. Chloride or ferrocyanide (FCN) ejection during PEDOT reduction was shown to be a function of the reduction potential. The nature of the cation in the bulk solution was not found to be important in the kinetics of ion transport in PEDOT+/FCN- films. Direct evidence for the incorporation of cations of Ru(NH3)6 3+/2+ in a PEDOT film during its reduction was also obtained by SECM measurements. The adsorption of Ru(NH3)6 3+ in fully oxidized PEDOT+/PSS- films was observed and attributed to ion exchange between the Na+ co-ion of PSS- and Ru(NH3)6 3+ in the bulk solution.
Scanning electrochemical microscopy (SECM) was used to study the kinetics of hydrogen oxidation on platinum, iridium, and rhodium substrates in acidic media. Hydrogen was generated at a platinum tip ...by reduction of H+ and oxidized at a Pt, Ir, or Rh substrate. The Tafel slope (117 mV, corresponding to a transfer coefficient, α, of 0.50) and the rate constant for hydrogen oxidation on Pt obtained by fitting approach curves (0.22 cm/s at the halfwave potential) agree with a previous SECM result. Kinetic data were also obtained on Ir (Tafel slope = 113 mV, k° = 0.25 cm/s, α = 0.48), and Rh (Tafel slope = 138 mV, k° = 0.010 cm/s, α = 0.58) substrates. These results demonstrate that hydrogen oxidation on Pt and Ir occurs at a similar rate, but is much slower on Rh.
Mechanistic studies of the hydrogen evolution reaction (HER) on nanoelectrode ensembles of ultramicroelectrode dimensions (UME-NEEs) are demonstrated using addressable nanoelectrode membrane arrays ...(ANEMAs). A kinetic model considering simultaneous occurrence of all elementary steps and the effects of mass transport to/from the UME-NEE surface was developed and used to interpret HER steady-state voltammograms (SSVs). The analysis accounts for the effect of fraction of covered area, f, and roughness factor, f R, of the nanoelectrodes on HER SSVs and illustrates the roles of these parameters in mechanistic studies of electrocatalyzed multistep reactions on UME-NEEs. The behavior of bare Au UME-NEEs was studied in acid solution, and characterization of the HER on these electrodes was performed by SSV and scanning electrochemical microscopy (SECM) in the substrate generation-tip collection mode (SG-TC). The activity of Au NEEs for the HER was found to be similar to that of macrosized polycrystalline Au. Selective electrodeposition of Pt NPs was performed on Au UME-NEEs, and SSV was used to study the kinetic mechanism of the HER in acid on the Pt-modified UME-NEEs with different f values. UME-NEEs with excess Pt showed HER activity equivalent to polycrystalline Pt, while those with small, almost undetectable, amounts of Pt showed HER behavior that could not be explained as the addition of pure Au and pure Pt contributions. SECM activity screening on Pt-modified ANEMAs detected changes in the apparent electrocatalytic activity for the HER, in agreement with corresponding SSVs.
We discuss SECM tip voltammetry, where a UME tip is held above a conductive substrate within about a tip radius and a tip voltammogram is recorded as its potential is slowly scanned while the ...substrate is held at a fixed potential. When the potential of the substrate is changed, the series of steady-state tip voltammograms provide information about the reactants and products. When the potential of the substrate, E S, is set so that the reaction at the substrate is opposite to that at the tip (the usual SECM conditions), a total positive feedback (tpf) tip voltammogram is recorded. When the substrate potential is set to values where the reaction at the substrate is the same as that occurring on the tip, the tip is shielded from the species in the bulk solution. Depending upon the substrate potential, this can cause total shielding (ts) or a voltammogram that is the result of partial feedback/partial shielding (pf-ps). The result is a series of tip voltammograms that are characterized by tpf, pf-ps, or ts, depending upon E S. Experimental tip voltammograms resulting from the reversible reduction of TCNQ and oxidation of ferrocene in MeCN are reported. These are compared with those from simulations and approximate equations developed to describe the features of the tip voltammograms generated under tpf, ts, or pf-ps conditions. The effect of the diffusion coefficient ratio on the ability of the UME tip to reach a true steady state is also addressed and possible applications, e.g., obtaining information about the reversibility of an electrochemical reaction, the product of an electrochemical reaction, the stability of that product, or the diffusion coefficients of the electroactive species, are discussed.
Finite conical electrodes, which are of particular interest as probes for imaging of surfaces using scanning electrochemical microscopy (SECM), in kinetic studies and in probing thin films were ...investigated. Theoretical SECM tip current-distance feedback (approach) curves for a finite conical electrode were calculated by numerical (finite element) analysis and compared to an earlier approximate model. The SECM curves obtained depended on the ratio of the base radius of the cone to the height of the cone and on the thickness of the insulating sheath. A new approach to fabricating conical tips of Pt in glass is described. These were used to obtain approach curves over both electrically conducting and insulating substrates. Comparison of experimental and simulated SECM approach curves provided a sensitive method of evaluating the size and shape of finite conical electrodes.
We demonstrate good agreement between the theoretical and experimental collision frequency of individual Pt nanoparticles (NPs) undergoing collisions at a Au ultramicroelectrode (UME) (5 μm radius) ...using electrocatalytic amplification provided by 15 mM hydrazine in 5 mM phosphate buffer (PB; pH 7) over 100 to 300 s. Dynamic light scattering (DLS) measurements demonstrated that Pt NP aggregation in this solution had the least impact on NP diffusion coefficient and concentration values, which are directly proportional to collision frequency. We show that the smaller, uniform current steps are indicative of NPs of metallic radii in agreement with those determined by transmission electron microscopy (TEM), with corresponding larger NP diffusion coefficient and concentration, in agreement with DLS results. These contribute to the larger NP collision frequency observed experimentally. Using atomic force microscopy (AFM) imaging, we show good agreement between the number of NPs imaged on the UME surface and the number of NP collisions that led to their adsorption, a spherical NP shape with a metallic radius size distribution comparable to that determined by TEM, and a random NP distribution on the UME surface. Through the Pt NP electroactive surface area, we show that all NPs on the UME surface after collision are attached and electrochemically active. Collectively, these results demonstrate for the first time that, within experimental error, every NP collision is successful and occurs through a sticking mechanism. Thus, collision experiments can be used to prepare small NP ensembles on a UME (i.e., UME-NPEs). In electrocatalysis, such UME-NPEs bridge the gap between classical ensemble studies on large platforms and isolated single NP investigations.
We report a procedure to fabricate glass-insulated carbon fiber ultramicroelectrodes (CF UMEs) with sizes of 10 and 1 μm in diameter. CF UMEs of theses sizes are commonly insulated with polymer or ...epoxy resins, which typically have pinholes and are less stable and difficult to polish. Through judicious choice of polishing materials, the fabrication procedure reported here leads to CF UMEs with a surface geometry that, within experimental error, is close to that of an inlaid disk. We demonstrate how the hardness factor of commonly used polishing materials may alter the geometry of CF UMEs, in which carbon has a larger hardness factor compared to other metals (i.e., Pt or Au). Atomic force microscopy (AFM) and steady-state voltammetry (SSV) were used to characterize the CF UME surface and record the electrochemical response, respectively. These results were compared to theoretical values for an inlaid disk UME. Pt nanoparticle (NP) collision experiments using electrocatalytic amplification were used to deposit an exact number of Pt NPs on a CF UME surface. Surface roughness was found to significantly decrease the sticking of Pt NPs on the CF surface, compared to the theoretical collision frequency. However, decreasing surface roughness through judicious polishing led to good agreement between experimental and theoretical collision frequencies of NPs on a CF UME surface, leading to the fabrication of NP ensembles of UME dimensions (UME–NPE) and a single-nanoparticle UME (SNP–UME). These electrodes were used to record and analyze SSVs for the hydrogen evolution reaction (HER) in acidic media.
