Aqueous reactions electrolytes

The quantum efficiency for solid-state devices, e.g. solar cells, is always below unity. For n-type silicon electrodes anodized in aqueous or non-aqueous HF electrolytes, quantum efficiencies above unity are observed because one or more electrons are injected into the electrode when a photogenerated hole enters the electrolyte. Note that energy conservation is not violated, due to the enthalpy of the electrochemical dissolution reaction of the electrode. 

Pig. 7-1. (a) Cathodic reaction and (b) anodic reaction M = metal electrode S = aqueous solution (electrolyte Mm = metal ion in metallic bonding state M., = metal ion in hydrated state Om = electron in metals. 

The apparent transfer coefficient of the cathodic reaction, ac, is a measure of the sensitivity of the transition state to the drop in electrostatic potential between electrolyte and metal [109,112]. According to Ref. 113, it is ac = 0.75 for the O2 reduction on Pt in aqueous acid electrolytes. In Ref. Ill the value ac = 1.0 was reported instead. Since the cathodic reaction is a complex multistep process, it might follow several reaction pathways, and the competition between them is affected by the operation conditions (rj, p, T). Therefore, different values of ac have been reported in different regimes of operation. Although in the simple reactions the transfer coefficient is a microscopic characteristic of the elementary act [112], for complex multistage reactions in fuel cell electrodes it is rather an empirical parameter of the model. The dependence of effective a for methanol oxidation on the catalyst layer preparation was recently studied [114]. 

To illustrate the use of the transport equations, the following problem is posed. An electrochemical cell containing vertical flat sheets of copper as the anode and cathode is operated with an aqueous CUSO4 electrolyte. The copper plates are connected to a DC power supply so that oxidation and reduction reactions proceed at the anode and cathode (Cu -1- 2e — Cu at the cathode Cu -> Cu -I- 2e at the anode). For the case when there is no forced or natural convection during current flow, we derive a simple expression between the constant applied current density and the steady-state cupric ion concentration profile. The cation flux and current density equations for the flat plate electrode/no convection cell are... 

This low-temperature fuel cell uses H2 and O2 reactants and a highly alkaline aqueous KOH electrolyte. The advantages of this fuel cell are the faster oxygen reduction reaction in the alkaline electrolyte and the possibility of using low-cost, nonprecious metal electrode catalysts, such as Ag-loaded carbon powder. The greatest problem with alkaline fuel cells is that the electrolyte reacts with traces of CO2 to produce insoluble carbonates. 

Recently, Kadish et al. synthesized three series of Co corroles (shown in Figure 4.11(D)) and investigated their catalytic activity toward the O2 reduction reaction.The mixed valent Co(II)/ Co(III) complexes, (PCY)Co2, and the biscorrole complexes, (BCY)Co2, both contain two Co(III) ions in their air-stable forms. It was foimd that all these complexes could catalyze the direct four-electron pathway for O2 reduction to H2O in aqueous acidic electrolyte. The most efficient catalysis process was observed when the complex had an anthracene spacer. The four-electron transfer pathway was further confirmed by RRDE measurement, in which only a relatively small amount of hydrogen peroxide was detected at the ring electrode in the vicinity of E1/2 0.47 V vs SCE for (PCA)Co2 and 0.39 V for (BCA)Co2. The cobalt(III) mono-corrole, (Me4Ph5Cor)Co, could also catalyze ORR at En2 = 0.38 V, with the final products being an approximate 50% mixture of H2O2 and H2O. 

In order to better understand the basis for the chapters which follow, let us consider the formulation of a predictive model for a particular aqueous based electrolyte system. The example chosen involves water-chlorine. The reactions to be considered are ... 

In aqueous acidic electrolytes the ionic conduction is provided by an H ion, respectively H (H20)n . As mentioned above, H" is created by the anodic oxidation of H2 as a fuel, and the conducting species is transported to the cathode, where it is consumed in the direct (ideally four electrons) cathodic reduction of molecular oxygen. Hereby, water as the reaction product appears at the cathode side of the cell. 

Electrochemical corrosion of carbon material in aqueous acid electrolytes could follow the reaction ... 

Chlorides. Chloride ions are generally harmful, as they participate directly in anodic dissolution reactions of metals. Furthermore, their presence tends to decrease the soil resistivity. They may be found naturally in soils as a result of brackish groundwater and historical geological seabeds (some waters encountered in drilling mine shafts have chloride ion levels comparable to those of seawater) or come from external sources such as deicing salts applied to roadways. The chloride ion concentration in the corrosive aqueous soil electrolyte will vary as soil conditions alternate between wet and dry. 

In contrast to most other batteries, which must carry both an anode and a cathode inside a storage system, metal-air batteries are unique in that the active cathode material (oxygen) is not stored in the battery. Instead, oxygen can be absorbed from the environment and reduced by catalytic surfaces inside the air electrode. Most metal-air batteries use an aqueous-based electrolyte such as concentrated potassium hydroxide. The typical reaction in a metal-air battery using aqueous-based electrolytes can be expressed by Equation 22.1... 

Figure 22.1 Schematic of reaction processes in metal-air batteries, (a) A battery that uses an aqueous-based electrolyte, where M represents metals (e g., Zn, Al, Mg, Fe, Ca). Hydroxide (OH ) is the ion carrier transferred...

For example, for iron in aqueous electrolytes, tlie tliennodynamic warning of tlie likelihood of corrosion is given by comparing tlie standard electrode potential of tlie metal oxidation, witli tlie potential of possible reduction reactions. 

Since any current resulting from tire anodic reaction must be consumed by tire catlrodic reaction, tire catlrodic current,7, must be equal to tire airodic current As a consequence, tire equilibrium potential of a metal (e.g. Fe) tlrat is immersed into air aqueous electrolyte will be adjusted by tire condition tlrat = j This is... 

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