Electrochemical Studies at Reduced Temperature

Dennis H. Evans University of Delaware, Newark, Delaware 

Susan A. Lerke DuPont Merck Pharmaceutical Company, 

The vast majority of electrochemical measurements are made at a single temperature, in fact, often at the prevailing temperature of the laboratory. The latter practice is convenient and perfectly justified if qualitative information is the goal of the experiment. However, when quantitative interpretations take priority, temperature control becomes mandatory, and often it is desirable to make measurements at a variety of controlled temperatures. 

In this chapter, we first discuss the principal motivations for doing electrochemistry at other than room temperature and attempt to delineate the type of chemical information that can be obtained from such measurements. The emphasis is on measurements at reduced temperatures, though the principles apply to high-temperature electrochemistry as well. 

In the second part, examples of electrochemical studies at low temperatures will be given, followed by a discussion of some of the practical aspects of doing electrochemistry under such conditions. Some of the techniques and procedures differ considerably from those of high-temperature electrochemistry, and, in fact, this field of low-temperature measurements has been given the moniker cryo-electrochemistry. 

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Chapters 9-19 deal with some practical aspects of electroanalytical chemistry. These chapters are aimed at giving the novice some insight into the nuts and bolts of electrochemical cells and solutions. In this second edition, further emphasis has been given to obtaining and maintaining clean solutions, and new chapters have been added on chemically modified electrodes and electrochemical studies at reduced temperature. 

The addition of an ionic conductive phase, such as GDC, also promotes the elec-trocatalytic activity of an MIEC cathode. Hwang et al. [108] studied the electrochemical activity of LSCF6428/GDC composites for the 02 reduction and found that the activation energy decreased from 142 kJmol-1 for the pure LSCF electrode to 122 kJmol1 for the LSCF/GDC composite electrodes. Thus, the promotion effect of the GDC is most effective at low-operation temperatures (Figure 3.12). This is due to the high ionic conductivity of the GDC phase at reduced temperatures. 

Ketonate complexes of Ru are reported in a number of papers. The parent complex [Ru(acac)3] has been subject to a polarized neutron diffraction study at 4.18 K, to powder neutron diffraction studies and to single-crystal structure determinations at 293 K, 92 K, and 10.5 K. The structure is disordered at all temperatures. Measurements of the magnetic susceptibilities (at 2.5 K and 300 K) have been made along different crystal axis directions, and the results analyzed. An investigation of the relationships between ionization potentials and half-wave potentials of a series of tris(/3-ketonate)Ru complexes has been reported, and the electrochemical properties of [Ru(acac)3] in chloroaluminate molten salt media have been reported. The reduced species [Ru(acac)3] can react with AICI4 reduction by bulk electrolysis of a small amount of [Ru-(acac)3] in the melt yields [RuClg]. ... 

The first step of the mechanism leading The electrochemical study of the seven-to the formation of 8 and free nitrite coordinate complex [Mo(N2RR )(dtc)3]+ from the reaction of 7 with O2 probably 9+ (R, R = alkyl or aryl, dtc = 5 2CNMe2) involved a single electron transfer. Sub- provided an example of electrode-induced sequent radical-radical coupling of the activation of a hydrazido(2—) ligand. Corn-products, to afford a molybdenum-bound plex 9+ was shown to reduce in two nitrate, followed by N—O bond cleavage separate diffusion-controlled one-electron would eventually lead to the observed steps, with the first one reversible on the products (Sch. 8) [27]. CV timescale at room temperature and... 

In the same study, redox polymers (223) were prepared that contained pendant viologens (Scheme 108). An active reducing agent was obtained by chemical reduction with dithionite or zinc, electrochemically, or by exposure to light. Utilization of the reduced poly(viologen) (224) as an electron transfer mediator was demonstrated by addition of a catalytic amount of the polymer to a mixture of zinc powder, ethyl benzoylformate (225) and water-acetonitrile (1 5). A quantitative yield of ethyl mandelate (226) was obtained after two days at room temperature (Scheme 109). Without the polymer, no reaction was observed after a month. 

Matsunaga et al. studied the disinfection of drinking water at room temperature using Escherichia coli as a model microorganism. As shown in Fig. 7, the carbon cloth electrodes were interwoven with ion exchange membranes and rolled around a glass tube [32]. At an applied potential of 0.7 V vs. SCE, an initial cell suspension of 10 cells/dm was reduced to <10 cells/dm at a residence time of 10 min. The disinfection was thought to be based on the electrochemical oxidation of intracellular coenzyme A. 

Electrochemically, C70 behaves very similarly to C o- Six reduction waves are observed in toluene/acetonitrile, but unlike 50, all six waves can be detected at room temperature (see Fig. 3) [7]. Reduction potentials for C70 obtained under various conditions of solvent and temperature are presented in Table 3. In comparing the corresponding values shown in Tables 1 and 3 for the first and second reduction potentials of Cgo and C70 in acetonitrile/ toluene, one observes that they are nearly identical. However, from the trianion up to and including the hexanion, C70 becomes increasingly easier to reduce than Cgo- A charge separation delocalization model has been evoked to explain this phenomenon [10c]. A noteworthy observation is the fact that the reduction potentials of C70 also appear to be solvent and/or temperature dependent, although no specific studies on the subject have been published. 

Mukhopadhyay et al. [98] studied the titanium electrodeposition on a Au(lll) substrate in the l-methyl-3-butyl-imidazolium bis(trifluoromethylsulfone) imide ([BmimJBTA) ionic liquids with 0.24 M TiCl at room temperature. It was found that TiCl is converted to TiCl in a first step, which is subsequently reduced to metallic Ti. Two-dimensional (2D) clusters form preferentially on the terraces in underpotential deposition range. At a potential of -1.8 V, a dense layer of three-dimensional (3D) clusters of titanium of 1-2 nm thickness is formed. The electrochemical reduction of tetravalent titanium species in hydrophobic 1-n-butyl-l-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]TFSI) room-temperature ionic liquid was studied by Kayayana et al. [182]. It was found that the stepwise reduction from Ti(IV) to Ti(III) and probably Ti(II) in [BMP]TFSI containing TiBr without [BMP] Br. The potentiostatic cathodic reduction gave some deposits at 180°C. The reduction of Ti(rV) species at -2.3 V led to the deposition of some Ti compounds containing... 

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