Abstract
Although hydrogen is one of the most commonly-found elements in the universe, it rarely exists as an independent molecule on our planet. Most of the time, it is bound to other elements or molecules to form compounds like water, carbohydrates, hydrocarbons and DNA acids. Obtaining hydrogen is not easy and usually requires a certain amount of energy to break the bonds connecting hydrogen to other elements. One process is water electrolysis in which electric energy is used to split water into hydrogen and oxygen. To regain the potential chemical energy stored in the hydrogen molecule, hydrogen and oxygen are combined to yield energy and water in the fuel cell which works in the reaction opposite to the electrolysis. This chapter discusses these two processes occurring in the electrolyser and in the fuel cell, two fundamental components of the solar hydrogen energy system.
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Notes
- 1.
The term production is not intended in the sense that hydrogen is created, but only that hydrogen is obtained in its simplest form (atomic or molecular) by processes that separate hydrogen from other compounds in which hydrogen is one of the constituents. The term will be anyhow used during the course of the book just as in common industrial practices.
- 2.
In chemistry, the equivalent weight (or equivalent mass) is defined as the quantity of mass of a substance able to supply or consume one mole of electrons in a redox reaction, or to generate one mole of H+ ions by dissociation or one mole of OH- ions in an acid—base reaction. It is calculated as the ratio between the molecular weight of the substance (expressed as g/mol) and its number of moles participating in the reaction. A mole is the quantity of a substance that contains the same amount of elementary entities as the number of atoms present in 12 g of C12. Such number is known as the Avogadro’s number, equal to 6.022 × 1023.
- 3.
Standard conditions refer to a pressure of 0.1 MPa and a temperature at 25 °C. For a chemical element, the standard state is the condition it assumes at standard pressure and temperature.
- 4.
While the internal energy is an exact differential because it depends only on the initial and final states, Q and W are not state functions, therefore their integral depends on the cycle. For this reason the symbol δ is used instead of d to indicate that it is not about the exact differentials but rather the infinitesimal quantities of heat and work.
- 5.
The Fick’s first law states that the flux of molecules in a fluid occurs from high-concentration areas to low-concentration regions. The diffusive flux J is given by: J =−D∇φ, where D is the diffusion coefficient that depends on the size of the diffusing molecules, the temperature and the fluid viscosity, while ϕ is the spatial concentration of the molecules. The Fick’s second law gives the change in time of the concentration when the molecules diffuse in a fluid as \( \frac{\partial \phi }{\partial t}=D{\nabla}^2\phi \)
- 6.
The SHE absolute potential is estimated with a Born-Haber cycle to be 4.44 ± 0.02 V at 298.15 K.
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Zini, G., Tartarini, P. (2012). Electrolysis and Fuel Cells. In: Solar Hydrogen Energy Systems. Springer, Milano. https://doi.org/10.1007/978-88-470-1998-0_3
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DOI: https://doi.org/10.1007/978-88-470-1998-0_3
Publisher Name: Springer, Milano
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