mV Decay Research Articles

Predicting the corrosion rate of steel in cathodically protected concrete using potential shift

The commonly accepted Cathodic Protection (CP) criterion i.e. 100 mV decay evolves from experimental investigations and may not always be accurate. Alternatively, corrosion rate monitoring can assess the adequacy of CP. This work examines the possibility of predicting the corrosion rate of steel in concrete using polarization data induced by known applied current density using Butler Volmer equation. For this, the value of cathodic Tafel slope (βc) plays an important role; decreasing βc from 210 to 60 mV, decreases the corrosion rate by 92% at 20 mA/m2 current density.The adequacy of the proposed method is evaluated by applying Impressed Current Cathodic Protection (ICCP) to concrete specimens which have a zinc rich paint (ZRP) as an external anode for a short duration of time. Results showed that to achieve at least 100 mV of depolarization, the applied current density should be at least 7 times the corrosion rate for the ZRP anode. However, this holds true, considering the short duration of the tests. Prediction of the corrosion rate of steel from potential shift forms the basis for the improved CP performance criterion for reinforced concrete structures.

Numerical study on CP of RC structures regarding the significance of the 100 mV decay criterion considering time dependent processes

Cathodic protection (CP) is an electrochemical repair method for reinforced concrete (RC) structures. It has been subject to numerous research projects and has proven its capabilities in countless repair measures around the world. Nevertheless, due to the uniqueness of each application as well as the large number of influencing factors (e.g., geometry, concrete resistivity distribution, kinetics of rebar, and CP anode system) the design and operation of CP systems is still governed by the use of (semi‐) empirical calculations and criteria. Within this study, numerical simulation has been used to determine the current and potential distribution during the application of CP. On the basis of this data, which are not measurable on site, the significance of the most important criterion for the operation of CP systems, the so called 100 mV decay criterion, shall be discussed. In particular, the correlation of the criterion with the corrosion state of the active reinforcement against a backdrop of time dependent effects of CP is within the scope of this study.

Co nanoparticle embedded in atomically-dispersed Co-N-C nanofibers for oxygen reduction with high activity and remarkable durability

Although Co and N codoped carbon (Co-N-C) materials could be served as the promising oxygen reduction reaction (ORR) electrocatalysts, they still suffer from unsatisfactory catalytic activity, especially in acidic environment. Herein, we report that the construction of Co nanoparticles embedded within highly porous, atomically-dispersed Co-N-C nanofibers catalyst not only significantly improves the ORR activity with a half-wave potential (E1/2) of 0.778 V/RHE, but also achieves the remarkable durability with only ~ 9 mV decay of E1/2 after a 10000-cycle accelerated durability test between 0.6 and 1.1 V. Both aberration-corrected scanning transmission electron microscope and X-ray adsorption spectroscopy confirm the existence of Co single atoms in the carbon nanofibers. Density functional theoretical calculation reveals that the introduction of Co core nanoparticle lowers the d band center of surface Co atoms in graphite shells and makes the free energy diagrams of the ORR process on Co-N-C shell more close to the ideal case, thus boosting the catalytic performance. Proton exchange membrane fuel cell integrated with as-prepared catalyst delivers a peak power density of 0.42 W cm−2 together with excellent stability after 100 h operation. This study provides an ideal perspective to develop highly efficient and durable Co-based catalyst for the oxygen reduction.

Nickel-coated silicon photocathode for water splitting in alkaline electrolytes

Photoelectrochemical (PEC) water splitting is a promising approach to harvest and store solar energy [1]. Silicon has been widely investigated for PEC photoelectrodes due to its suitable band gap (1.12 eV) matching the solar spectrum [2]. Here we investigate employing nickel both as a catalyst and protecting layer of a p-type silicon photocathode for photoelectrochemical hydrogen evolution in basic electrolytes for the first time. The silicon photocathode was made by depositing 15 nm Ti on a p-type silicon wafer followed by 5 nm Ni. The photocathode afforded an onset potential of ∼0.3 V vs. the reversible hydrogen electrode (RHE) in alkaline solution (1 M KOH). The stability of the Ni/Ti/p-Si photocathode showed a 100 mV decay over 12 h in KOH, but the stability was significantly improved when the photocathode was operated in potassium borate buffer solution (pH ≈ 9.5). The electrode surface was found to remain intact after 12 h of continuous operation at a constant current density of 10 mA/cm2 in potassium borate buffer, suggesting that Ni affords good protection of Si based photocathodes in borate buffers.

Self-Repairing Photosensitized Electron Transfer from Thiones to Methyl Viologen in Aqueous Media

Reversible formation of the persistent methyl viologen radical cation (MV•+) was observed by the steady-state photoillumination of thiones in the presence of MV2+ in deaerated aqueous solution. Formation yield and lifetime of MV•+ are favorable at a pH higher than 3.00. By laser flash photolysis, it was observed that fast electron transfer occurs from triplet states of thiones (3PT*) to MV2+, yielding the radical cation of thiones (PT•+) and MV•+. The initial rapid decay of MV•+ is attributed to back electron transfer from MV•+ to PT•+. On the other hand, the slow decay part of MV•+ can be attributed to endothermic electron transfer from MV•+ to disulfide, which is formed by the coupling of the thio radical produced after deprotonation of PT•+. The ratio of slow decay component and fast decay component of MV•+ in the short time scale is a good measure of the yields of persistent MV•+ with changing thiones and pH. A cyclic mechanism interpreting reversible electron transfer was proposed in which thiones act as self-repairing photosensitizers.

Mechanism of photoinduced redox reactions in aqueous solutions of [Fe(bpy)(CN) 4] 2−

The mechanism of redox reactions initiated by UV excitation of [Fe(bpy)(CN) 4] 2− in aqueous solutions has been investigated by continuous photolysis, laser kinetic spectroscopy and pulse radiolysis. Hydrated electrons formed in the light induced reaction are shown to react with ground state [Fe(bpy)(CN) 4] 2− to form [Fe(bpy)(CN) 4] 3− characterized by its absorption spectra and reactions. The back reaction between [Fe(bpy)(CN) 4] − and [Fe(bpy)(CN) 4] 3− takes place with kl.8±4x10 10 M −1 s −1. The quantum yield of solvated electron formation determined by using nitrate ions as electron scavenger is 0.14±0.01 and 0.068±0.007 at μ∼ 0.01 and μ∼ 1.0, respectively. Addition of methylviologen (MV 2+) to the solution of iron(II) complex leads to production of MV+ via direct electron scavenging and an electron transfer reaction from [Fe(bpy)(CN) 4] 3−. The latter process takes place with k = 9.0±0.6 x 10 9 M −1 s −1. The decay of MV + occurs in reactions with both iron(III) and iron(II) complexes.

Calculations of dipole moments, optical spectra, and second-order hyperpolarizability coefficients of some mono- and disubstituted stilbene models for the design of nonlinear optical materials

Conclusion In this study we have gained additional insight into the mechanism of Hz production from water mediated by colloidal metal catalysts. The differing rates of decay of MV+ and of the conductiity signal observed in this investigation show that charging of the Ptc by electrons is followed by a slower uptake of protons and that the reducing species loading the Ptc above pH 7 cannot be assigned a simple formula like H-, although once equilibrium is established on the catalyst, the ratios of protons to electrons do not differ greatly from 0.5 predicted by this formula. Although both initial [MV'] and [Ptc] strongly affect the fractional decay of MV' , the concentration of MV2+ has little effect. An equilibrium with a Ptc surface saturated with adsorbed MV2+ is suggested. Slow changes in optical absorption and conductivity were found which can be attributed to equilibration of the Ptc with hydrogen (produced with G = 1.0). The ratio of protons to electrons loading the Pt was found for this process to be similar to that in the case of reduction of the Ptc by MV+. The results described above therefore allow us to obtain a more complete understanding of the role played by colloidal metal catalysts in water reduction to hydrogen and have demonstrated the value of using the pulse radiolysis technique together with optical and conductivity detection methods in enabling us to probe such reaction systems.

A pulse radiolytic study of aqueous solutions containing nitrobenzene and methylviologen

Solutions containing 2-propanol, nitrobenzene (NB), and methylviologen (MVS ) have been studied in wide ranges of concentrations and pHs. The two radicals NB and MV are formed by the reactions of NB and MV S , respectively, with both the hydrated electron and the 2-propanol radical. Studies were carried out at 578 nm where only the MV absorbs light. The decay of MV follows a single second-order process whose rate constant depends on (NB), (H ), and (MVS ). The findings could be explained by a fast equilibrium between the viologen and nitrobenzene redox systems followed by a slow reaction between MV and either NB or its acidic form NBH. 17 references, 7 figures.

Radiolytically-induced one-electron reduction of methyl viologen in aqueous solution: Stability of the radical cation in acidic and highly alkaline media (1)

Pulse and continuous radiolysis have been used to investigate the stability of the reduced methyl viologen radical cation (MV +.) in acidic and highly alkaline aqueous solution. The reaction of the methyl viologen dication (MV 2+) with (CH 3) 2ĊOH and (CH 3) 2ĊO - radicals generates MV +. rapidly ( k = 2.9±0.2 × 10 9 and 6.7±0.3 × 10 9 M -1 s -1, respectively). The absorption spectrum of MV +. is the same at pH 1, natural pH, and pH 13 suggesting that MV +. is not involved in acid-base equilibria in that pH range. Between pH 0 and 2, MV +. disappears via second-order kinetics with k obs an inverse function of [MV 2+] and pH. The decay of MV +. occurs via H +-assisted disproportionation and yields ultimately a hydrogenated species with λ max 220, 255 nm (∈ max 8.2 × 10 3, 4.1 × 10 3 M -1cm -1, respectively). At pH 13, MV +. is infinitely stable in the absence of O 2; further reduction of MV +. yields the moderately stable MV° species with λ max 368 nm(∈ max3.6 × 10 4 M -1 cm -1) and a shoulder at 370 nm(∈3.0 × 10 4 M -1cm -1). Reaction of MV° with O 2 yields MV 2+ quantitatively via two one-electron oxidation steps. Acidification of MV° in the absence of O 2 yields the same air-insensitive hydrogenation product as is obtained from the disproportionation of MV +. in acidic solution. The relationship of these observations to the use of MV 2+ as an electron relay species in photochemical solar energy conversion schemes is examined.