Constructing a Kinetics Database Types of kinetic data: Electrochemistry Dennis H. Evans Department of Chemistry University of Arizona Tucson, AZ 85721

Electrode reactions, rates and rate laws Elementary reaction (solution-phase reactants and products): O(sol) + e - (M)  R(sol) with k f = forward rate constant; k b = reverse rate constant Rate law: (d(mol O )/dt)/A = -(d(mol R )/dt)/A = -k f C O(x=0) + k b C R(x=0) Rate measurement: Faradaic current: i/A = -F(d mol O /dt) i/A = -F(k f C O(x=0) + k b C R(x=0) )

Potential dependence of rate constants Butler-Volmer formulation: where , the electron-transfer coefficient, is 0<  <1. k s is the standard heterogeneous electron-transfer rate constant  For Butler-Volmer (linear barrier-driving force relationship),  is constant  For the Marcus treatment (quadratic barrier-driving force relationship),  decreases linearly with increasingly negative potentials

Evaluation of k s and  1. Measure the current at a variety of potentials, E, with known A, C O(x=0), and C R(x=0). 2. From the currents, evaluate k f at each E. 3. Plot of ln(k f ) vs. E gives  from slope and k s from extrapolation to E = E .

Electrochemical kinetic data The fundamental data for an electrode reaction: 1. k s 2.  3.  H act (from temperature dependence of k s )

Multi-step electrode reactions e.g., the hydrogen evolution reaction Volmer-Heyrovsky mechanism: H + (aq) + e - + M( )  M(H) k 1 and k -1 H + (aq) + M(H) + e -  H 2 + M( ) k 2 and k -2 Steady-state rate law:

Reactions with unknown mechanisms Evaluation of fuel-cell catalysts: For example, the mechanism of the anodic oxidation of methanol is not fully understood and is very dependent on conditions. Evaluation of candidates for new catalysts is often carried out empirically by measuring the current density at a given potential under given operating conditions. The results are kinetic data but they are not reported in a fundamental manner.

Recommended nomenclature and definition of symbols  Parsons, R. Pure Appl. Chem. 1979, 52,  Parsons, R. Electrochim. Acta 1981, 26,  Sluyters-Rehbach, M. Pure Appl. Chem. 1994, 66,

Recommendation to the producers and compilers of electrochemical kinetic data. Though it will probably be fruitless, it is suggested that the recommendations of IUPAC concerning nomenclature be followed in the reporting of new data and in the compilation of data from the literature.

A Lesson in Reliability: Standard rate constants, k s, for ferrocenium/ferrocene couple, Pt electrode, acetonitrile, K Method k s / cm s -1 Electrolyte Fast-scan cyclic voltammetry M Bu 4 NBF 4 Slow linear sweep voltammetry M Bu 4 NClO 4 Fast-scan cyclic voltammetry M Bu 4 NClO 4 Fast-scan cyclic voltammetry M Et 4 NClO 4 Slow linear sweep voltammetry M Bu 4 NClO 4 High-frequency admittance M Bu 4 NClO 4 Cyclic voltammetry M LiClO 4 Short time pulse voltammetry M Bu 4 NPF 6 Scanning electrochemical microscopy M Bu 4 NBF 4

Time evolution of k s of ferrocene?

How do electrochemical rate constants depend on experimental conditions? Results for reduction of (CH 3 ) 3 NO 2, CH 3 CN) 1. Temperature:  H act = 25 kJ/mol. k s changes about a factor of 2 for ten-degree change near room temperature. 2. Solvent a. Outer reorganization energy depends on static and optical dielectric constant b. Dynamic solvent effect on preexponential factor via  L

How do electrochemical rate constants depend on experimental conditions? 3. Solvent purity: C H2O / wt% k s / cm s Supporting electrolyte: 0.10 M Et 4 NClO M Hp 4 NClO (Hp ≡ n-heptyl)

How do electrochemical rate constants depend on experimental conditions? 5. Electrode material: a. To a first approximation, k s for an outer- sphere reaction should be independent of electrode material. b. (CH 3 ) 3 CNO 2, 0.10 M Et 4 NClO 4, CH 3 CN k s / cm s -1 Hg Hg Pt (freshly treated) Pt (freshly treated) Pt (exposed to solution for 1 hr.) Pt (exposed to solution for 1 hr.)

Recommendation for producers and compilers of electrochemical kinetic data As much of the following information as possible should be reported by researchers and should be included in compilations of data: 1. Electrode reaction being studied 2. Standard rate constant and electron-transfer coefficient 3. Activation energy 4. Solvent with method of purification and/or evidence of purity 5. Supporting electrolyte and concentration 6. Electrode material and a. Purity b. Method of electrode fabrication c. Any pretreatments

Collections of Electrochemical Kinetic Data Compilation: Tamamushi, R. “Kinetic Parameters of Electrode Reactions of Metallic Compounds”, Butterworths, London, Monograph Series: Meites, L; Zuman, P.; Narayanan, A., eds., “CRC Handbook Series in Inorganic Electrochemistry”, CRC Press, Boca Raton, FL, 8 volumes, Meites, L.; Zuman, P., eds.,”CRC Handbook Series in Organic Electrochemistry”, CRC Press, Boca Raton, FL, 5 volumes, Bard, A. J., ed., “Encyclopedia of Electrochemistry of the Elements, Dekker, New York, 15 volumes, Includes an Organic Electrochemistry section.

Searching for electrochemical kinetic data Search topicWeb of ScienceSciFinder ( ) ( ) ( ) ( ) Electrochemical kinetics Heterogeneous electron- transfer rate constant transfer rate constant Electrode kinetics Electron-transfer rate constant Electron-transfer coefficient 35 9 Electrochemical activation energy 4 2

Acknowledgments  Cited results from Evans group were supported by grants from the National Science Foundation  Complete manuscript: