Electrolytic water splitting

Electrolytic water splitting was the first electrochemical process to be performed. Historically, the first experiment on water electrolysis was attributed to Nicholson and Carlisle, who in 1800, using the newly invented Volta s pile, observed the formation of gaseous products in the laboratory [1]. In reality, there are documents proving that Volta himself noted the phenomenon a few years earlier, although he never reported the observation in a publication [2]. 

Thermochemical hybrid cycles offer the possibility of running low-temperature reactions on electricity. The expectations for realization of hybrid processes are similar to those for purely thermochemical processes. Various hybrid processes are energetically possible, but not always practicable. Important criteria are the minimum voltage for the electrolysis step, realizability, efficiency. The sulfuric acid hybrid or Westinghouse process is expected to reduce material streams as compared with the IS process. The electric energy demand here is a factor of 7 (in reality 3 - 4) lower than in the electrolytic water splitting process. Technological improvements are still possible. 

The hybrid process with just two steps is expected to reduce material streams as compared with the above described IS process. The electric energy demand here is a factor of 7 (in reality 3-4) lower than in the electrolytic water splitting process. 

Only the living world is able to reduce entropy by creating structures, or in other words, to move the system back from equilibrium. Indeed, the central role of life (more exactly, autotrophic organisms) is to maintain the cycle shown in Fig. 2.31, starting with light-induced electrolytic water splitting, and subsequent parallel -but, important to note, in separated organs - reduction of CO2 into hydrocarbons... 

Forgie R, Bugosh G, Neyerlin KC, Liu Z, Strasser P (2010) Bimetallic Ru electrocatalysts for the OER and electrolytic water splitting in acidic media. Electrochem Solid State Lett 13(4) ... 

Non-electrolytic sources of hydrogen have also been studied. The chemical problem is how to transfer the correct amount of free energy to a water molecule in order to decompose it. In the last few years about I0(X)0 such thermochemical water-splitting cycles have been identified, most of them with the help of computers, though it is significant that the most promising ones were discovered first by the intuition of chemists. 

The total output photovoltage must exceed the thermodynamic potential difference for water splitting (1.229 V at 25°C), the energy level mismatches for the anodic and cathodic processes, and the polarization loss or overvoltages due to kinetic, diffusion, and IR potential losses in the bulk of electrolyte. 

Photoelectrochemical water-splitting is a combination of solar cell with electrolysis in a electrolyte, and has been actively studied. However, the selection of the photo semiconductors is so tightly limited that photoelectrochemical methods can hardly compete with the combined system of solar cell with electrolysis. 

Carbon is widely used in the catalytic processes of the chemical industry due to its unique characteristics, such as chemical inertness, high surface area and porosity, good mechanical properties and low cost. It is used for the production of chlorine and aluminum, in metal refining (gold, silver, and grain refinement of Mg-Al alloys) as well as for the electrolytic production of hydrogen peroxide and photoelectrochemical water splitting. 

While for a solar water splitting cell, light is directly absorbed by the semiconductor electrode (anode or cathode). The separation of electron-hole pairs is achieved in the built-in electric field near the semiconductor surface. The electric field is formed due to the charge transfer between the semiconductor electrode and the electrolyte as schematically shown in Fig. 17.5(b) [28]. Take an n-type semiconductor electrode for example... 

The limitation of solvent evaporation could be avoided by working at high temperature with water vapor in the presence of a solid electrolyte (steam electrolysis). Nominally, at about 4000°C, at which AG = 0, electroless thermal water splitting would be realized. In practice, problems of material stability, necessary... 

For hydrogen production from water, pure water (pH=7.0) is seldom used as an electrolyte. Water is a poor ionic conductor and hence it presents a high Ohmic overpotential. For the water splitting reaction to proceed at a realistically acceptable cell voltage the conductivity of the water is necessarily increased by the addition of acids or alkalis. Aqueous acidic and alkaline media offer high ionic (hydrogen and hydroxyl) concentrations and mobilities and therefore possess low electrical resistance. Basic electrolytes are generally preferred since corrosion problems are severe with acidic electrolytes. Based on the type of electrolytes used electrolyzers are... 

A typical PEC, as depicted in Fig. 3.4a for water splitting, consists of three electrodes immersed in an electrolyte solution namely the working electrode (WE) or anode, counter electrode (CE) or cathode, and reference electrode (RE). The working electrode, usually a semiconductor, is also called the photoelectrode... 

Fig. 3.5 Band position of anatase Ti02, bandgap = 3.2 eV, in the presence of a pH = 1 aqueous electrolyte. The energy scale is indicated in electron volts (eV) using either normal hydrogen electrode (NHE) or vacuum level as reference showing the condition for water splitting.

Spontaneous water-splitting upon illumination needs semiconductors with appropriate electron affinity and flat band conditions. The flat band positions shift with electrolyte pH. Hence, an external bias needs to be applied between the electrodes in most cases to effect water splitting. The external bias can be either electrical or chemical. This external bias contribution should be subtracted from (3.6.11) or (3.6.12) to get the overall photoconversion efficiency. In the case of an external electrical bias, the efficiency can be defined as ... 

V°rev = 1.229V is the standard state reversible potential for the water splitting reaction and Vaoc is the anode potential at open circuit conditions. Term Vmeas-Vaoc arises from the fact that Voc represents the contribution of light towards the minimum voltage needed for water splitting potential (1.229V) and that the potential of the anode measured with respect to the reference electrode Vmeas has contributions from the open circuit potential and the bias potential applied by the potentiostat (i.e. Vmeas= Vapp+Vaoc). The term Vmeas-Vaoc makes relation (3.6.16) independent of the electrolyte pH and the type of reference electrode used. Thus the use of V°rev in relation (3.6.16) instead of VV or V°hz as in the case of relation (3.6.15) is justified. 

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