Fuel Cell Concept

Fuel cells are now increasingly being researched, considering that they are revolutionizing the ways in which energy is produced. They use hydrogen as a fuel, while also ensuring the possibility of generating clean energy, with the protection and even improvement of the environmental parameters [17,18].

By definition, the fuel cell is an electric cell, which, unlike battery cells, can be continuously fed with fuel, so that the electrical power from the output of this electric cell can be maintained indefinitely [38,39,42]. Therefore, the fuel cell converts hydrogen or hydrogen-based fuels directly into

electricity and heat through the electrochemical reaction of hydrogen with oxygen. The process carried out at the fuel cell level is the inverse of electrolysis:

$$2\text{H}\_2 + \text{O}\_2 \rightarrow 2\text{H}\_2\text{O} + \text{energy} \tag{1}$$

where, the conversion of the chemical energy of the fuel (hydrogen) and the oxidant (oxygen) into continuous current, heat and water as reaction products [42] takes place. Due to the fact that in the fuel cell the hydrogen and oxygen gases are transformed by an electrochemical reaction into water, this has considerable advantages over the thermal engines: higher efficiency, practically silent operation, lack of pollutant emissions where the fuel is even hydrogen, and if hydrogen is produced from renewable energy sources, the electrical power thus obtained is indeed sustainable [38,39,42].

Component Elements

In order to analyze the importance of all the phenomena that take place in a fuel cell, it is necessary to know the component elements (Figure 4).

**Figure 4.** Components of the fuel cell [24,42].

There are some distinct elements in a fuel cell [42], namely:

*Electrolyte.* In order for a substance to fulfill the role of electrolyte in a fuel cell it must fulfill several conditions, as follows: high chemical stability with respect to the two electrodes, in order not to react with them; high melting and boiling points for cells operating at high temperatures; predominantly ionic conductivity and absence of electronic conductivity. The main types of electrolyte are liquid electrolytes (the most common are basic or acidic solutions, the ion transport phenomenon being similar to the one from the electrolysis of aqueous solutions), solid electrolytes (ion exchange membranes and crystalline solid electrolytes are used), melted electrolytes and liquid electrolytes with dissolved fuel [42].

*Catalyst layer (at the anode and cathode).* All electrochemical reactions in the fuel cells take place on the surface of catalyst layers. To increase the reaction rate of the cell, the electrodes (catalyst layers) contain catalyst particles. Thus, the electrode of a fuel cell is made of a porous carbon support on which the catalyst is deposited. The thickness of an electrode is usually between 5–15 μm, and the charge of the catalyst is between 0.1 and 0.3 mg/cm2 [42].

*Bipolar plate (at the anode and cathode).* Bipolar plates play a dual role, guiding the reactant gases to the electrolytic exchange surface of the fuel cell and driving the obtained electric current. Materials used for bipolar plates must have a high conductivity and be gas-tight. They must also be corrosion resistant and chemically inert. Taking into account these considerations, graphite or steel can be used, but also composite materials. The gas flow channels are "engraved" in the bipolar plates, which otherwise should be as thin as possible to reduce the weight and volume of the battery. The geometry of the flow channels has an influence on the flow velocity of the reactants and the mass transfer, implicitly they have a determining influence on the performance of the fuel cells, being necessary to optimize the flow surface so that the reaction surface is as large as possible [42].

Operating Principle

Although there are different types of fuel cells, they all work on the same principle:


The electrochemical conversion consists of the direct conversion into electrical energy of the chemical energy stored in various active materials. This type of conversion is called direct because no other intermediate form is interposed between the initial and final energy forms. Indirect energy conversion systems contain several transformation stages, between which the form of thermal or mechanical energy is obligatory. Direct energy conversion eliminates the "link" thermal or mechanical energy by achieving higher efficiency, which does not depend on the limited efficiency of the thermal machines. The idea of obtaining electricity by direct conversion of chemical energy arose when the problem of unfolding and reverse of the phenomenon of water electrolysis (which results in its components), that is to say, to obtain electric current from the reaction between hydrogen and oxygen [53]. The schematic in Figure 5 illustrates a comparison regarding the operating principle of fuel cells—direct systems of conversion of energy forms and the classical technologies devoted to conversion — indirect systems [54,55].

**Figure 5.** Fuel conversion process. Comparison of the operation principles of various technologies.

Fuel cells are electrochemical electricity generators characterized by a continuous supply of reactants to the two electrodes. The fuel rating comes from the fact that they use as sources of chemical energy, natural or synthetic fuel substances, which are subjected to oxidation and reduction reactions.

The anode, or fuel electrode, is the place where the oxidation of the fuel (H2, CH3OH, N2H4, hydrocarbons, etc.) takes place. The cathode, or oxygen (air) electrode, is the place where molecular oxygen reduction occurs [31,42,48].

Electrochemical oxidation of hydrogen is carried out at an anode of a conducting material (eg platinum dispersed on activated carbon) constituting the negative pole of the cell [42,44]:

$$\text{acid electrode} \\ \text{te:} \tag{2}$$

$$\text{alkaline electrolyte:}\qquad\text{H}\_2 + 2\text{OH}^- \rightarrow 2\text{H}\_2 + 2\text{e}^-\tag{3}$$

The electrochemical reduction of oxygen occurs at a catalytic cathode constituting the positive pole of the cell:

$$\text{acid electrode} \\ \text{te:} \qquad \qquad \frac{1}{2} \text{O}\_2 + 2\text{H}^+ + 2\text{e}^- \to \text{H}\_2\text{O} \tag{4}$$

$$\text{alkaline electrolyte:}\tag{5} \\ \text{O}\_2 + \text{H}\_2\text{O} + 2\text{e}^- \rightarrow 2\text{OH}^-\tag{6}$$

The catalytic functions of the electrodes are very important, namely: the hydrogen electrode (the anode) must ensure the adsorption of the hydrogen molecule, its activation, promoting the reaction with the hydroxyl ion; the oxygen electrode (cathode) must allow molecular oxygen adsorption, promoting reaction with water.

The anode and cathode are separated by an ionic conductor, electrolyte, and/or a membrane that prevents the reactants from mixing and the electrons pierce the heart of the cell. Initially, the energy released from the oxidation of conventional fuels, generally used in the form of heat, can be converted directly into electricity with excellent efficiency, in a fuel cell. Because in almost all oxidation reactions an electron transfer between fuel and oxidant occurs, it is obvious that the chemical oxidation energy can be converted directly into electricity. There is an oxidation-reduction reaction in which the oxidation of the fuel and the oxidant reduction occur with a loss on the one side and an electron gain on the other. Any galvanic element involves oxidation to the negative pole (loss of electrons) and reduction to the positive one (gain of electrons) and, as in all galvanic elements, the fuel cells tend to separate the two partial reactions in the sense that the changed electrons pass through an external use circuit [39,42,44].
