Voltage requirements electrolytic reactions

From the basic principles we can make preliminary design estimates. Inefficiencies in a system arise because of voltage losses and because all of the current does not enter into the desired reactions. The minimum potential required to perform an electrolytic reaction is given by the reversible cell potential, a thermodynamic quantity. Additional voltage that must be applied at the electrodes represents a loss that is manifested in a higher energy requirement. The main causes of voltage loss are ohmic drops and overpotentials. The applied potential is equal to the sum of the losses plus the thermodynamic requirement ... 

Because the reaction in an electrolytic cell is nonspontaneous, a certain amount of voltage must be applied across the circuit to cause the reaction to occur. The minimum voltage required to force the reaction is determined using Equation 18.2. For example, if an electrolytic cell contains molten MgCl2, we can determine the minimum emf for the cell. To begin, we need the half-reactions and reduction potentials of each substance ... 

As the cathodic reaction is the reverse of the anodic one, the theoretical thermodynamic cell potential is 0 V. In actual practice, the cell voltage required to drive the process accounts for the voltage drops in the electrolyte, anode and cathode electrical connections, electrical circuit losses, and overpotentials for both electrode reactions when occurring at a reasonable rate. 

So there is no overall electrochemical reaction, hence, the theoretical (equilibrium) voltage required is zero. The applied voltage of about 0.2 V is required for the process solely to drive the electrolytic purification in the desired direction. Indirect heating of the electrolyte to about 60°C decreases the solution viscosity, which helps to maintain a high production rate at these low operating voltages. Current densities of about 240 A/m are normal. 

You should now be able to select a suitable solvent/supporting electrolyte system for your purpose. The limitations on choice are set by the solubility of the analyte and of the supporting electrolyte, the requirement for a low electrical resistance, and the necessity to have a voltage window available for the required analyte reaction. This latter limitation usually amounts to ensuring that an adequate cathodic voltage limit is available. 

Figure 11.3 shows the temperature dependence of the reactiOTi enthalpy MIr and the free reaction enthalpy Gr at normal pressiue. The cell voltage required decreases noticeably for electrolytic processes such as high-temperature electrolysis, where at temperatures above 700 °C water vapor is decomposed electrolyti-cally. According to the relation AGj = Hr — TASr, this is due to the positive reaction entropy ASr, where the enthalpy fraction TASr created by the reaction entropy must be fed into the electrolytic process as process heat. Therefore, the... 

The thermodynamic voltage required to decompose water into hydrogen and oxygen is therefore E° = AG (H2O) / 2f 1.23 V, a value close to the voltage required to evolve chlorine from NaCl (1.36 V). However, the electric potential of the anode and the cathode varies markedly with the pH of the electrolyte. Let us consider the water splitting reaction in acidic media ... 

We normally run experiments at a current of 20-30 mA, and the applied voltage is then that required to achieve such a current typical values would be in the range 10-30 V, depending on the solutes in the electrolyte phase. The applied voltage required obviously depends on the electrode potential for the reaction, but more importantly on the EMF needed to drive the current carriers through a medium of low dielectric constant. Given the use of electrodes with surface areas of a few cm, the current density at the anode is in the order 10-20 mA cm". These... 

THE PROBLEM Estimate the electrolyte voltage requirement for an electrolyzer with a single gas evolution reaction with a current efficiency of 100%. The rectangular electrodes are 0.1m wide and 0.25 m long the electrolyte gap is 0.01 m. 

We noted above that, for the cell reaction to occur, ions must pass through the solution and separator between the electrodes. An input of energy is also essential to drive this migration process and leads to a potential drop iRcell (where Rqell is the internal resistance of the cell, a function of electrolyte properties, the form of the electrodes and cell design) within the cell. Hence, the cell voltage required to observe a current i in a real cell is given by ... 

If positive, the reaction is galvanic. If the cell voltage ( ceii) is negative, this is the minimum applied voltage required to initiate the electrolytic reaction. As an example, consider a redox couple of the oxidation of zinc and the reduction of hydrogen ... 

Electrolysis Causing Nonspontaneous Reactions to Occur—In electrolysis, a nonspontaneous chemical reaction occurs as electrons from an external source are forced to flow in a direction opposite that in which they would flow spontaneously. The electrochemical cell in which electrolysis is conducted is called an electrolytic cell. E°, values are used to establish the theoretical voltage requirements for an electrolysis. Sometimes, particularly when a gas is liberated at an... 

This reaction has a positive free energy of 422.2 kj (100.9 kcal) at 25°C and hence energy has to be suppHed in the form of d-c electricity to drive the reaction in a net forward direction. The amount of electrical energy required for the reaction depends on electrolytic cell parameters such as current density, voltage, anode and cathode material, and the cell design. 

Electroless Electrolytic Plating. In electroless or autocatalytic plating, no external voltage/current source is required (21). The voltage/current is suppHed by the chemical reduction of an agent at the deposit surface. The reduction reaction must be catalyzed, and often boron or phosphoms is used as the catalyst. Materials that are commonly deposited by electroless plating (qv) are Ni, Cu, Au, Pd, Pt, Ag, Co, and Ni—Fe (permalloy). In order to initiate the electroless deposition process, a catalyst must be present on the surface. A common catalyst for electroless nickel is tin. Often an accelerator is needed to remove the protective coat on the catalysis and start the reaction. 

Regarding the electrode/electrolyte interface, it is important to distinguish between two types of electrochemical systems thermodynamically closed (and in equilibrium) and open systems. While the former can be understood by knowing the equilibrium atomic structure of the interface and the electrochemical potentials of all components, open systems require more information, since the electrochemical potentials within the interface are not necessarily constant. Variations could be caused by electrocatalytic reactions locally changing the concentration of the various species. In this chapter, we will focus on the former situation, i.e., interfaces in equilibrium with a bulk electrode and a multicomponent bulk electrolyte, which are both influenced by temperature and pressures/activities, and constrained by a finite voltage between electrode and electrolyte. 

Like a galvanic cell, an electrolytic cell includes electrodes, at least one electrolyte, and an external circuit. Unlike galvanic cells, electrolytic cells require an external source of electricity, sometimes called the external voltage. This is included in the external circuit. Except for the external source of electricity, an electrolytic cell may look just like a galvanic cell. Some electrolytic cells include a porous barrier or salt bridge. In other electrolytic cells, the two half-reactions are not separated, and take place in the same container. 

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