9. Electrochemical energy conversion

Versatile electrochemical energy conversion technologies are used for powering numerous electrical devices, ranging from small-scale consumer applications such as watches and mobile phones to electric vehicles and even to distributed energy systems. Furthermore, the need for energy conversion in society is increasing due to the transition from fossil fuel-based energy supply to renewable yet intermittent energy technologies. One of the challenges related to these new energy supply technologies is to ensure continuous availability of energy during low production periods. Renewable energy technologies therefore need to be supplemented with devices for short-term (peak shaving) and long-term energy storage, whereby energy is converted to other forms during peak productions periods. Moreover, transition is also going on in the mobile sector, with the adoption of electrical vehicles, e-bikes and other small-scale personal transportation devices. A plethora of energy conversion technologies have been introduced in the past few years but only few have achieved commercial maturity, or even a demonstration phase.

Batteries, flow batteries and fuel cells are galvanic devices that convert energy stored in chemical compounds directly into electricity via spontaneous electrochemical reactions. In batteries, chemicals generating electrical energy during electrochemical reduction and oxidation reaction are stored in the device. In primary batteries, the electrodes are consumed in irreversible electrochemical reactions, and they cannot be re-charged because of potentially hazardous side reactions. In secondary batteries, electrode materials participate in reversible reactions, and they have been designed to withstand several charging and discharging cycles.

In flow batteries, the reactants are fed into the electrochemical reactor from external reservoirs. The reactions are reversible at both electrodes. When re-charging the device, the direction of the reactant flow is reversed. The volume of the reservoirs therefore determines the duration of the charging- discharging cycle. Because of the external storage of the chemicals, flow batteries are easy to upscale. In a similar manner to flow batteries, in fuel cells, fuel (e.g. hydrogen or methanol) is fed to the anode from an external source. At the cathode, however, airborne oxygen serves as the oxidant.

In addition to galvanic cells, supercapacitors and electrolyzers are introduced in this chapter. Electrolyzers are electrolysis cells, a kind of reversed fuel cells that convert electrical energy in non-spontaneous reactions into the form of chemical bond energy. Electrochemical double layer supercapacitors are reminiscent of batteries but, instead of an electrochemical reaction, the energy storage is based on adsorption.

The characteristics and performance of all these devices is largely determined by the choice of cathode and anode materials, as well as by the electrolyte. However, engineering also plays an important role because of the various transport phenomena occurring in the cells.

From classical thermodynamics (see Chapter 1), we know that the Gibbs free energy provides the upper limit for chemical energy that can be converted into electrical energy

W_{el}=\Delta G=\Delta H -T\Delta S=-nFE  (9.1)

The open circuit voltage can be calculated if the Gibbs free energies of the electrode reactions or the standard electrochemical reduction potentials are known. However, the cell potential also depends on the temperature, pressure and concentrations of the reacting species:

\displaystyle E^{eq}=-\left(\frac{\Delta H}{nF}-\frac{T\Delta S}{nF}\right)+\frac{RT}{nF}\ln\left(\frac{\Pi C_r}{\Pi C_p}\right) (9.2)

Electrical energy is generated when current is drawn from the cell. In this case, energy is also consumed in other processes, such as overcoming kinetic reaction barriers, in transport phenomena and ohmic losses, as described in Chapter 6. All these phenomena must be taken into account when determining the current-voltage characteristic of the cell.

E=E^{eq}(T,P,c)-\eta_{act,a}-|\eta_{act,c}|-\eta_{diff,a}-|\eta_{diff,c}|-A \cdot i \cdot R (9.3)

A typical current-voltage response or a polarization curve of an alkaline cell is depicted in Figure 9.1. The cell potential therefore decreases when more current is drawn from the cell because of the various losses given in Equation (9.3). As for an electrolysis cell, these losses are added to the overall cell voltage, thereby increasing the voltage needed to run the conversion of electrical energy into chemical form.

E=E^{eq}(T,P,c)+\eta_{act,a}+|\eta_{act,c}|+\eta_{diff,a}+|\eta_{diff,c}|+A \cdot i \cdot R (9.4)


Cell voltage and power

Figure 9.1. In a galvanic cell (left), overpotential and ohmic losses lower the applicable cell voltage, red curve; blue curve depicts the cell power. In an electrolysis cell (right), overpotential and ohmic losses increase the cell voltage needed to run a process accordingly.