Salts of hexacyanoferrate II/III anions have been widely used as redox couple probe molecules to determine the characteristics of electrode surfaces. Examples include the assessment of electrocatalysts for energy applications and electrocatalysts for the detection of biological or chemical species, as well as the determination of electrochemically active surface areas. An examination of the electrochemical literature, based largely on cyclic voltammetric investigations, reveals a wide range of peak separation and/or heterogeneous electron transfer rate constants, classified sometimes as inner or outer sphere electron transfer processes. Originally developed for the mechanistic interpretation of inorganic transition metal compounds in solution, this terminology has since been extended to account for heterogeneous electron transfer occurring at electrodes. In the case of the hexacyanoferrate II/III anions, there can be a number of reasons why it sometimes behaves as an outer sphere probe and at other times displays inner sphere electron transfer characteristics. After examining some of the structural and chemical properties of the hexacyanoferrate II/III species, the methods used to determine such classifications are described. The most common method involves measuring peak-to-peak separation in a cyclic voltammogram to ascertain a heterogeneous rate constant, but it has inherent flaws. This paper reviews the reasons for the classification disparity, including the effects of various oxygen surface species, the influence of organic surface films, the nature of the cation counter-ion, surface adsorption and surface hydrophilicity/hydrophobicity. Other surface interactions may also take place, such as those occurring with Au corrosion or pH effects. These can impact the electrical double layer and thus may affect the electron transfer process. Consequently, it is recommended that hexacyanoferrate II/III should be considered a multi-sphere or alternatively a surface-sensitive electron transfer species.

Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study

The presence of heavy metals in craft beers can endanger human health if the total metal content exceeds the exposure limits recommended by sanitary standards; in addition, they can cause damage to the quality of the beer. In this work, the concentration of Cd(II), Cu(II), and Fe(III) was determined in 13 brands of craft beer with the highest consumption in Quito, Ecuador, by differential pulse anodic stripping voltammetry (DPASV), using as boron-doped diamond (BDD) working electrode. The BDD electrode used has favorable morphological and electrochemical properties for the detection of metals such as Cd(II), Cu(II), and Fe(III). A granular morphology with microcrystals with an average size between 300 and 2000 nm could be verified for the BDD electrode using a scanning electron microscope. Double layer capacitance of the BDD electrode was 0.01412 μF cm−2, a relatively low value; Ipox/Ipred ratios were 0.99 for the potassium ferro-ferricyanide system in BDD, demonstrating that the redox process is quasi-reversible. The figures of merit for Cd(II), Cu(II), and Fe(III) were; DL of 6.31, 1.76, and 1.72 μg L−1; QL of 21.04, 5.87, and 5.72 μg L−1, repeatability of 1.06, 2.43, and 1.34%, reproducibility of 1.61, 2.94, and 1.83% and percentage of recovery of 98.18, 91.68, and 91.68%, respectively. It is concluded that the DPASV method on BDD has acceptable precision and accuracy for the quantification of Cd(II), Cu(II), and Fe(III), and it was verified that some beers did not comply with the permissible limits of food standards.

/pdf/electrochemical-evaluation-of-cd-cu-and-fe-in-different-fhr8bwaf.pdf

Electrochemical Evaluation of Cd, Cu, and Fe in Different Brands of Craft Beers from Quito, Ecuador

The carbon material, GUITAR (pseudo-graphite from the University of Idaho thermolyzed asphalt reaction) can be doped with nitrogen in two prevalent forms. In a previous study N(py)-GUITAR had a predominance of pyridinic and pyrrolic moieties with no graphitic nitrogen. In this study N(g)-GUITAR contains a 9.7 % N atomic abundance, with that fraction consisting of 72.3 % graphitic, 23.7 % pyridinic, and 0 % pyrrolic nitrogen. The two materials allow for the examination of hypotheses regarding the importance of the three different nitrogen moieties in the oxygen reduction reaction (ORR). In the previous investigation, the lack of graphitic nitrogen of N(py)-GUITAR gave a preferred pathway of 4e- ORR to H2O. In this investigation, N(g)-GUITAR gave a 2e- ORR pathway to H2O2. This was elucidated by current efficiency and hydrodynamic voltammetry studies. The high predominance of graphitic nitrogen confirms the hypothesis regarding 2e- vs. 4e- ORR pathways with N-doped carbon materials. N(g)-GUITAR was also evaluated for parasitic pathways for H2O2 production. At -0.95 V vs. Ag/AgCl the combination of current efficiency for H2O2 is 96 % in 0.05 M Na2SO4 with a production rate of 4.9 mg cm-2 h-1, is the highest reported in the literature. This indicates possibilities for water purification and treatment applications, which require 10 to 250 mg L-1, depending on conditions. 

An oxygen reduction reaction electrocatalyst tuned for hydrogen peroxide generation based on a pseudo-graphite doped with graphitic nitrogen

Electrochemical oxidation of GUITAR (pseudoGraphite from the University of Idaho Thermolyzed Asphalt Reaction) produces a surface that exhibits superhydrophilic properties. This surface contains 20.7% relative oxygen abundance, giving it a heightened affinity for water relative to the pristine material. Scanning electron and atomic force micrographs reveal spherical, bubble-like structures on GUITAR, indicating a higher surface roughness (Rq = 300–400 nm) over the predominantly smooth titanium substrate (Rq = 94.5 nm). Combining an intrinsic affinity toward water with a heightened surface roughness produces a material with an apparent water contact angle of 0°. The synergistic effects of a superhydrophilic surface that can function as an electrode result in an electrocatalyst material that is highly resistant to fouling. When stored in water, oxidized GUITAR (Ox-GUITAR) completely resists fouling by volatile organic compounds for the duration of testing (72 h). When fouled directly with a drop of oil, the residue can be removed using a simple water rinse. The removal of oil residues from carbon surfaces is typically not possible without the use of solvents, surfactants, or mechanical exfoliation. This combination of properties opens Ox-GUITAR to a broad array of possible applications, such as implantable electrodes, field sensors, or water-quality monitoring. 

Superhydrophilicity and Antifouling Behavior in Electrochemically Oxidized Nanocrystalline Pseudographite

Highly Stable, Low-Cost Metal-Free Oxygen Reduction Reaction Electrocatalyst Based on Nitrogen-Doped Pseudo-Graphite

The nitrogen-doped form of GUITAR (pseudo-Graphite from the University of Idaho Thermalized Asphalt Reaction) was examined by X-ray photoelectron, Raman, and X-ray diffraction spectroscopies and cyclic voltammetry (CV). Electrochemical studies indicate that N-GUITAR exhibits significant resistance to fouling by adsorption and by passivation. Unlike other carbon materials, it maintains fast heterogenous electron transfer (HET) kinetics with Fe(CN)63−/4− with exposure to air. The CV peak potential separation (ΔEp) of 66 mV increased to 69 mV in 3 h vs. 67 to 221 mV for a highly oriented pyrolytic graphite (HOPG) electrode. Water contact angle measurements indicate that N-GUITAR was able to better maintain a hydrophilic state during the 3-h exposure, going from 55.8 to 70.4° while HOPG increased from 63.8 to 80.1°. This indicates that N-GUITAR better resisted adsorption of volatile organic compounds. CV studies of dopamine also indicate N-GUITAR is resistant to passivation. The ΔEp for the dopamine/o-dopaminoquinone couple is 83 mV indicating fast HET rates. This is reflected in the peak current ratios for the oxidation and reduction processes of 1.3 indicating that o-dopaminoquinone is not lost to passivation processes. This ratio along with the minimal signal attenuation is the best reported in literature.

https://www.mdpi.com/2311-5629/6/4/68/pdf

Electrochemical Aspects of a Nitrogen-Doped Pseudo-Graphitic Carbon Material: Resistance to Electrode Fouling by Air-Aging and Dopamine Electro-Oxidation

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Electrochemical Aspects of a Nitrogen-Doped Pseudo-Graphitic Carbon Material: Resistance to Electrode Fouling by Air-Aging and Dopamine Electro-Oxidation