Electrolysis of Aqueous Solutions

3 Key excerpts on "Electrolysis of Aqueous Solutions"

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  eBook - ePub

  Photo- and Electro-Catalytic Processes

  Water Splitting, N2 Fixing, CO2 Reduction

  • Jianmin Ma(Author)
  • 2022(Publication Date)
  • Wiley-VCH

    (Publisher)

  3 ]. Despite over 200 years of history and huge promise, the technology of electrocatalytic water splitting is limited to selective applications, owing to a number of issues, primarily that of cost reduction, improvement in energy efficiency, durability, and system design. Beginning with an introduction of some of the related fundamental concepts, this chapter discusses the various industrial systems used for electrocatalytic water splitting and the associated challenges. A brief discussion on the principles and prospects of electrocatalytic splitting of seawater is also presented.

  Figure 4.1

  Scheme representing the pathway to couple water electrolysis with intermittent renewables.

  Source: Adapted from Santos et al. [2] .

  4.2 Fundamental Concepts

  The most elementary representation of electrocatalytic water splitting is shown in Figure 4.2 . An electric current is passed through two metal plates (electrodes) immersed in a conducting ion solution (electrolyte) to split a water molecule (H2 O) into H2 and O2 gases. To avoid the mixing of the produced gases, a sophistication can be introduced in the form of a porous separator between the two electrodes. More advanced and pragmatic designs of water‐splitting cells are discussed in Section 4.3 . Under standard experimental conditions (1 bar pressure and 298 K), the elementary water‐splitting reaction is:

  (4.1)

  This reaction is constituted of two half‐cell reactions, namely hydrogen evolution reaction (HER ) taking place at the cathode and oxygen evolution reaction (OER ) that takes place at the anode.

  The reactions occurring at the interface of the electrodes and the electrolyte can be understood in the light of fundamental concepts of electrochemistry. Some of these important concepts related to electrocatalytic water splitting are discussed below:

  4.2.1 Electric Double Layer

  When a metal plate is partially immersed in a liquid electrolyte containing its own ions, there is an exchange of charges at the interface of the metal plate and the electrolyte. The exchange is driven by a number of factors, one of which is diffusion of ions from the metal to the electrolyte (sparse ion region). As the positive and negative ions in the electrolyte are in a thermal random motion, they may strike the metal plate and either take up an electron to get deposited on the metal or knock off atoms from the metal surface leaving behind electrons. Similarly, the metal atoms on the surface may dissolve in the electrolyte, leaving negative charges on the plate. As such, the rate of transfer of ions from the plate to electrolyte is much higher, and hence the electrode surface is usually negatively charged. This negative layer attracts positive ions from the electrolyte to form the so‐called “electric double layer” at the interface, which generates an opposing electric field and prevents further dissolution of the metal surface. There are a number of proposed models explaining the variation of potential across the double layer, as represented in Figure 4.3

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  Physics Of Clusters

  • Gennady N Chuev, V Lakhno(Authors)
  • 1998(Publication Date)
  • World Scientific

    (Publisher)

  This conclusion is inconsistent with the well known fact of successful electrolysis occuring at room temperature without special input of additional energy. It is necessary to note that a considerable success was achieved in studying the elementary act of electron transfer during electrolysis. ~~ However, the discrepancy between the requirement of additional energy during electrolysis, following from the modern conception of ion solvation, and a successful elec-trolysis occuring without this additional energy in practice was not taken into consideration till now. Also, it is necessary to take into account another method of presentation of electrolyte solutions, namely, Debye-Hiikkel theory. 19 This theory considers ions in solutions as charged points in the continuum with dielectric penetrabili-ty e. In other words, this theory does not take into account ion solvation. Such technique allows to avoid the mentioned above discrepancy, but this theory can be used only for dilute aqueous solutions (up to ~ 10~ 2 mole/1). 4 Thus, these reasonings have led us to a conclusion about the necessity of elaboration of a new point of view on ion solvation in solutions. At present, it is believed that all ions of the same kind in a solution are solvated to the same degree, that is, the ion solvation shells contain the same number of solvent molecules, and this number is constant for the given ions and solvent. 1 However, the results, obtained by Evans et al. by the mass spec-trometric method of electrohydrodynamic ionizaion (EHD), 20 force to change the traditional view on this solvation conception. Usually, ions are driven from solution into gaseous phase by electrospraying these solutions at atmospheric pressure. Then these ions are injected into a 314 F z+ (s), +ze — > F + h c s f ' (*)h c + ze —► r -t-n c s i (3) ions _ i i

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  Geological Sequestration of Carbon Dioxide

  Thermodynamics, Kinetics, and Reaction Path Modeling

  • Luigi Marini(Author)
  • 2006(Publication Date)
  • Elsevier Science

    (Publisher)

  Chapter 4

  The Aqueous Electrolyte Solution

  4.1 The important role of aqueous electrolyte solutions

  Werner F. Giggenbach, one of the most brilliant geochemists who ever studied magmatic and hydrothermal systems, in one of his last works (Giggenbach, 1997 ) wrote that “according to the early geochemistAgricola (1556) , medieval alchemists had a well-known saying: non reagent nisi soluti. A geochemical translation is: nothing much happens if there isn’t a fluid, a reaction medium, allowing minerals components to be transported and to interact with one another”.

  Among the fluids, aqueous electrolyte solutions are widespread and very important, as effective reaction media, in different geological environments including the aquifers apt to geological CO2 sequestration. Any aqueous electrolyte solution comprises the solvent water, some ionic solutes and some neutral solutes. Examples of ionic solutes are both the free ions, such as Na+ and Cl– , and the ionic complexes, such as CaHCO3 + and NaSO4 – . Examples of neutral solutes are both dissolved molecules, such as CO2 (aq) and SiO2 (aq), and neutral complexes, such as NaHCO3 (aq) and CaCO3 (aq).

  Many features and details of aqueous electrolyte solutions (e.g. the thermodynamic properties of electrolyte solutes) are omitted from this presentation as the interested reader can find this information in many textbooks. However, we will recall the basic concepts on which are founded both our present understanding of aqueous electrolyte solutions and, consequently, the geochemical modelling of these intriguing and fascinating media. As discussed inChapter 7 , the geochemical modelling of aqueous electrolyte solutions is a quite complex exercise, consisting in solving the mass balance, charge balance and equilibrium relations involving the activities of relevant solutes and of the solvent water. A delicate part of this exercise is the correct computation of the activity coefficients of solutes and water activity. We recall that the activity coefficient of the i th solute component, γi , is related to the partial molar excess Gibbs free energy, , by following the relationship (e.g.Denbigh, 1971 ;Prausnitz et al., 1999

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