Insights into ion pairing effects on the redox properties of the Keggin-type polyoxotungstate PW12O40 3are gained by combining electrochemical experiments and density functional theory (DFT) calculations. Such effects have been reported to affect the performance of these species as molecular electrocatalysts.Experimental square wave voltammetry (SWV) of the two-electron reduction of PW12O40 3in acetonitrile evidences that the reduced forms PW12O40 4and PW12O40 5can be significantly stabilized by ion association. The strength and stoichiometry of the corresponding aggregates are estimated as a function of the nature of the cation (lithium, sodium and tetramethylammonium) and the oxidation state of the polyoxometalate. The results obtained in combination with DFT enable us to examine the roles of the cation solvation and the charge number and distribution of the polyanions.

Electrochemical reactions can effectively follow nonunity stoichiometries as can be found in the electrochemistry of halides, hydrogen, and metal complexes. The voltammetric response of these systems shows peculiar deviations with respect to the well-described features of the 1:1 stoichiometry. With the aim of specifying such differences, a rigorous and manageable analytical theory is deduced for the complete characterization of reversible electrode processes with complex stoichiometry in cyclic voltammetry (CV) at macroelectrodes. Particularly, the main features of the CV of 2:1, 1:2, 3:1, and 1:3 processes (that is, the peak currents and potentials and the influence of the scan rate and of the species concentration and diffusion coefficients) are given and compared with the 1:1 case in order to propose unambiguous diagnostic criteria of the stoichiometry of the electrode reaction. Also, expressions for the concentration profiles and surface concentrations of the redox species are given.

The catalytic activity of surface-confined molecular species, as affected by the nature of the support, has been investigated by square wave voltacoulometry (SWVC). This technique has proven to be very powerful and advantageous for the study of electroactive and electrocatalytic monolayers. Here, the value of SWVC for the elucidation of the catalytic species and routes when the catalyst can undergo multiple electron transfers is assessed. The redox behavior in acidic water solution of the immobilized Keggin type polyoxomolybdate [PMo 12 O 40 ] 3− and its catalytic performance toward the reduction of bromate have been studied experimentally. Three different supports are considered: boron-doped diamond (BDD), bare glassy carbon, and graphene oxide modified glassy carbon. For all of them, the SWVC response enables the accurate identification and analysis of the catalytic pathways and species. Mechanistic details are easily obtained on the basis of the additivity of the contributions associated with the electron transfer and with the catalytic process to the SWVC response. The results obtained reveal the significant influence of the support on the redox properties and on the catalytic activity of the polyoxomolybdate. Among the supports tested, glassy-carbon supports show the highest catalytic performance with apparent rate constants that are up to 1 order of magnitude faster than those on BDD.

A very general and simple theoretical solution is presented for the current-potential-time response of reversible multi-electron transfer processes complicated by homogeneous chemical equilibria (the so-called extended square scheme). The expressions presented here are applicable regardless of the number of electrons transferred and coupled chemical processes, and they are particularized for a wide variety of microelectrode geometries. The voltammetric response of very different systems presenting multi-electron transfers is considered for the most widely-used techniques (namely, cyclic voltammetry, square wave voltammetry, differential pulse voltammetry and steady state voltammetry), studying the influence of the microelectrode geometry and the number and thermodynamics of the (electro)chemical steps. Most appropriate techniques and procedures for the determination of the 'interaction' between successive transfers are discussed. Special attention is paid to those situations where homogeneous chemical processes, such as protonation, complexation or ion association, affect the electrochemical behaviour of the system by different stabilization of the oxidation states.