# Studies on the Electrochemical Behavior of Sulfite on Incoloy 800 in a Neutral Environment
## 3 Results
### Electrochemical Impedance Spectroscopy
In its simplest form, the impedance of a Gerischer element is expressed according to Equation (12):  
\[Z_{\text{G}}={R_{\text{G}}}/({K_{\text{a}}}+\text{j}\omega)^{1/2} \tag{12}\]  
where \(R_{\text{G}}\) is a resistance parameter (\(\Omega\;\text{cm}^{2}\;\text{s}^{1/2}\)) and \(K_{a}\) is the effective rate transfer of the chemical reaction (s\({}^{-1}\)). The proposed equivalent circuit models accurately describe the electrochemical processes at the interface, as evidenced by the good correlation of the simulated lines with the experimental data and the low \(\chi^{2}\) values. Tables 3 and 4 give the fitted parameter values as a function of sulfite concentration and electrode potential. The parameter values of Tables 3 and 4 indicate a low charge transfer resistance (\(R_{ct}\)) for the highest concentration of sulfite. Lowering the concentration of sulfite causes the \(R_{ct}\) values to progressively increase; at the lowest concentration of \(10^{-3}\) M sulfite, this increase can reach a factor of 10. Furthermore, \(R_{ct}\) is also potential dependent, decreasing as the potential increases. Figure 7a provides an overview of \(R_{ct}\) dependence on the potential at different sulfite concentrations. It is obvious that \(R_{ct}\) is highly dependent on both concentration and electrode potential, indicating that this is the resistance associated with the charge transfer process. The effective double layer capacitance (\(C_{dl}\)) determined from impedance data using Equation (10) is presented in Figure 7b. The obtained \(C_{dl}\) values generally fit within the expected range of the capacitance of a smooth metal in contact with an electrolyte solution, for which the literature data suggest a theoretical value of 20 \(\mu\)F cm\({}^{-2}\)[44]. Larger \(C_{dl}\) values are obtained at higher sulfite concentrations due to the accumulation of charged sulfite ions in the double layer. Because the double layer capacitance is inversely proportional to the double layer thickness, which decreases with the square root of the ionic strength, it follows that \(C_{dl}\) should increase proportionally with the square root of the electrolyte  
parameters, including the anodic transfer coefficient (\(\alpha\)) and exchange current density (\(i_{\rm o}\)), were derived via Tafel extrapolation; while \(\alpha\) is low, the \(i_{\rm o}\) suggests an acceptable overall rate. Chronoamperometric tests at various potentials indicated sulfite oxidation occurs up to 1.50 V, with oxygen evolution at higher potentials. Chronocoulometric analysis confirmed that sulfite oxidation dominates up to 1.75 V. Electrochemical impedance spectroscopy showed reduced charge transfer resistance (\(R_{ct}\)) at higher sulfite concentrations, indicating rapid sulfite oxidation at positive potentials. Incoloy 800 is thus a promising anode material for sulfite oxidation in hydrogen production and SO3\({}^{2-}\)/O2 fuel cells. Future studies will examine electrolyte composition changes and electrode surface chemistry. Conceptualization, M.-A.L., A.K. and N.V.; methodology, M.-A.L., A.K. and M.L.D. ; validation, A.K. and N.V.; formal analysis, M.-A.L. ; investigation, M.-A.L., G.-D.D. and N.R. ; writing--original draft preparation, M.-A.L., A.K. and G.-D.D. ; writing--review and editing, A.K., M.L.D. and N.V.; visualization, A.K. ; supervision, A.K. and N.V.; funding acquisition, M.L.D. All authors have read and agreed to the published version of the manuscript. This work was supported by University Politehnica Timisoara in the frame of PhD studies. Not applicable. The data that support the findings of this study are available from the corresponding author upon reasonable request. The data are not publicly available due to privacy. The authors declare no conflicts of interest.