Benchmark redox couples

In this section we outline a systematic methodology for removing uncertainties that are commonly included in comparisons between experimental and theoretical redox potentials, which are frequently reported relative to external reference couples, or reference electrodes. We study benchmark redox couples, including complexes that span three transition metal rows in various non-aqueous solvents. It is shown that the use of appropriate references, measured under the same conditions and calculated by using compatible computational frameworks, allows quantitative correlations between experimental and theoretical data. This approach leads to DFT redox potentials with standard deviations comparable to the experimental errors of cyclic voltammetry measurements, even at a rather modest level of theory (64 mV standard deviation for DFT/UB3LYP/LACVP/6-311G level see Figure 1.11). 

Other Benchmark Redox Couples Figure 1.12 Benchmark redox couples. 

Distinction should be made between the value of the geometrical and the electrochemical (active) areas of an electrode. The meaning of geometric area is obvious. The electrochemical area should be computed on the basis of the response to a benchmark species in one of the techniques discussed in the following. Once the diffusion coefficient of the species chosen, typically one partner of a reversible redox couple, such as the hexacyanoferrate anions in water or bis(cyclopentadienyl)iron (II)—ferrocene—in organic solvent, is known, the ratio between the measured current and the expected current density constitutes a reliable estimate of the electrochemical area. The dependence of this area value on the exact nature of the electroactive species may be discarded as a first approximation, once poisoning of the electrode and the occurrence of unknown complex electrode mechanisms can be excluded. 

Molecular catalysts, such as transition metal coordination complexes, are capable of mediating multielectron and proton-coupled reactions because of the ability of transition metals to accommodate multiple redox states. In addition, electronic and acid/base properties of these catalysts can be conveniently tuned through the ligand design. Very low overpotentials for transition metal-catalyzed CO2 reduction have been demonstrated. However, the main drawback of molecular catalysts is their relatively low stability resulting in low turnover numbers (TON). The benchmark for industrial application is in the range 10 -10 TON [34], which is far beyond... 

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