Final Exam of Introduction of Electrode Process
2010-2011 Academic Year, Fall Semester
- Fill the blank (3 points for each question, totally 36 points)
(1)Give the definition of effective thickness of diffusion layer______.
(2)By measuring Tafel plot, the reaction order of an electrode reaction can be determine by calculate ______.
(3) The description code of a equivalent circuit is Rs(CcRc(CdlW)), please write out its total impedance expressionZ= ______.
(4)List at least three methods by which we can make judgment that one electrode reaction is electrochemicallyreversible ______
(5)The capacitance of unit area of a metal/electrolyte interface can be used as a standard value (C0), if the metal surface is inhibited by an organic inhibitor and the capacitance of unit area of this inhibited surface is C, then the coverage of this inhibition layer is ______.
(6)An electrode reaction occurs following an ECE mechanism. If the CV curves of the reaction at different scan rates are depicted in the right figure (the scan rate of the CV shown by dashed line is higher), determine the detail mechanism of this reaction, for example EreCreEre, EirCreEir or other mechanism. ______
(7)The most important character of current step method is its ______keeping unchanged during oxidation until the surface concentration drops to zero and electrode potential jumps to another reaction.
(8)Give at least two methodsthat can elevate limiting diffusion current so as to make study on the electrochemical step ______and ______.
(9)A one-electron electrochemical reaction Ox + e P takes place on a microdisc electrode, the concentration of Ox is 1 mmol/L, its diffusion coefficient is 1.2 10-5 cm2/s, the stable current density is measured to be 2.32 nA, then the area of the microdisc electrode is ______.
(10) At 0.100 mm away from the electrode surface, natural convection can equalize the concentration of solution there. If the diffusion coefficientof reactive species is 10-5 cm2/s, how much time is required for this reaction to attain stable diffusion? ______.
(11) Using cyclic voltammetry, how can you learnfrom CV study that it is a species adsorbed on the electrode surface undergoing electrooxidation and reduction? ______.
(12) If an adsorbate A can adsorb on the electrode surface by replacing n water molecules. Write out the adsorption reaction______.
- Simple discussion [22 points]
(1)[10 points] Formic acid can be electrochemically oxidized over Pt/C catalyst through either direct oxidation route or indirect oxidation route:
1)direct oxidation: HCOOH CO2 + H2O (1)
2)indirect oxidation: HCOOHCO + H2O; CO + O-Pt CO2(2)
it is found that reaction 1) is more difficult to occur (1- 2 100 mV). The onset potential of reaction 1) is ca. 0 V and the peak potential is ca. 0.2 V, at 0.4 V oxidation of Pt surface occur, and at 0.6 V water begin to split. If we perform a linear sweep voltammetric measurement (LSV) from -0.1 V to + 0.6 V
[1] Draw the LSV curve of the positive scan; [4 points]
[2] If a new catalyst Pt-Au/C is introduced, it is found that more oxidation of formic acid occurs via the direct oxidation, what will happen to the LSV curve in [1] [2 points]
[3] If the first peak follows Tafel equation, estimate its slope of Tafel plot and compare the slopes of Pt/C and Pt-Au/C catalysts [4 points].
(2)[6 points] If we conduct electrochemical research, we always supporting electrolyte of much higher concentration. Discuss
1)The effect of supporting electrolyte on electric migration;
2)The effect of supporting electrolyte onthe surface concentration electroactive species;
3)The effect of supporting electrolyte on activation energy of electrode reaction.
Give quantitative analysis if possible.
(3)[6 points]On a Pt electrode, Fe3+(1 mmo/L) + e Fe2+(1 mmo/L) occurs in a 0.1 mol/L sulfuric acid solution.
1)Draw the polarization curve obtained on a RDE at 1600 rpm. Diffusion coefficient of Fe3+ and Fe2+ are both 10-5 cm2/s.
2)If we purge O2 into the system and initiate 2Fe2+ + O2 + 2H+2Fe3+ + H2O, what will happen to the curve shown in 1)?
3)Give the effect of the k1 value of the limiting diffusion current.
- Calculation (Totally 42 points)
(1)[20 points] The exchange current density (j0) of the system Pt(s)Fe(CN)63-(2.0 mmol/L), Fe(CN)64-(2.0 mmol/L), NaCl(1.0mol/L ) is measured to be 2.0 mA/cm2 at 298 K, the transition coefficient of the system is about 0.5.
1)Why the concentration of NaCl is much higher than that of Fe(CN)63- and Fe(CN)64-? [2 points]
2)Calculate reaction rate constant of the reaction k0; [2 points]
3)If the concentration of Fe(CN)63- and Fe(CN)64- is increased to 1.0mol/L, calculate the standard current density j0.[2 points]
4)When the concentration of Fe(CN)63- and Fe(CN)64- are both 10-4 mol/L, calculate the charge-transfer resistance of the reaction;[2 points]
5)This system is an ideal reversible system. Draw the cyclic voltammetric curve of the system, indicate peak potential, reversible electrode potential on the plot;[4 points]
6)Draw Tafel plot of this system, indicate the exchange current, reversible potential and calculate its Tafel slope. [8 points]
(2)[6 points] Oxidation of o-dianisidine (DIA) is electrochemically reversible. It was found that the peak current of oxidation of 2.27 mmol/L DIA in 2 mol/L H2SO4 as supporting electrolyte on a 2.73 mm2 carbon past electrode at a scan rate of 0.500 V/s is 8.19 A.
1)Calculate the diffusion coefficient of DIA;
2)If we decrease scan rate to 100 mV/s, determine its peak current;
3)Can you determine the electron number transferred during oxidation?
(3)[6 points] A Pt rotating disc-ring electrode (RDRE) is rotating at 1600 rpm in a 2 mmol/L K3[Fe(CN)6] solution, and a 12.3 A disc current is recorded. Calculate the ring current obtained a 2500 rpm in a 6 mmol/L K3[Fe(CN)6] solution with the collection coefficient of 0.33.
(4)[10 points] Reduction reaction
Co(II)(salen) (0.2 mmol/L) + 1e Co(I)(salen)
in DMF solution without any additive shows good reversibility and a typical CV curve. Upon addition of Et-Br, an irreversible chemical reaction
Co(I)(salen) + EtBr EtCo(III)(salen) + Br.
EtCo(III)(salen) can be reduced at a more negative potential than Co(II)(salen). In order to measure the rate constant of this chemical reaction, we change EtBr concentration so as to make the chemical reaction a quasi-first order reaction, i.e., r = k’[Co(I)(salen)], k’ = k[Et-Br]. By measuring the ratio of peak current of oxidation and reduction peaks, i.e., ipa/ipc, we can get k.
(1)Draw the schematic diagram of the CV for before addition of Et-Br;
(2)Draw the schematic diagram of the CV for after addition of Et-Br;
(3)When the time between E1/2 and Eis 32 ms, [Et-Br] = 13.3 mmlo/L, at 273 K, ipa/ipc = 0.7, calculate k.