Water-splitting potential

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. 

M-S is an electrochemical impedance spectroscopy (EIS) technique [10-12] that can be difficult to perform and interpret if the system is not ideal. When the measurement is successful, it is able to determine both the fb and the free charge carrier density (donors or acceptors, A/Dopant) of the photoelectrode. Efb, along with the band gap (Eg) and the A dopant. can be used to determine the band structure of a photoelectrode and if it possesses the proper alignment with respect to the water splitting potentials (see chapter Introduction ). The A dopant also plays a role in the bulk and surface semiconductor properties such as the width of the depletion layer and rate of recombination. The conductivity type is also revealed by M-S analysis. The M-S plot will possess a negative slope for p-type materials and a positive slope for n-type materials (positive slope). In the case that the M-S measurement is not successful, then other techniques such as Hall Effect can still yield conductivity and A dopant for materials which can be deposited onto non-conductive substrates such as quartz. 

A detailed discussion of thermochemical water splitting is available (155,165—167). Whereas many problems remain to be solved before commercia1i2ation is considered, this method has the potential of beiag a more efficient, and hence more cost-effective way to produce hydrogen than is water electrolysis. 

Water Splitting A modified electrodi ysis arrangement is used as a means of regenerating an acid and a base from a corresponding salt. For instance, NaCl may be used to produce NaOH and HCl. Water sphtting is a viable alternative to disposal where a salt is produced by neutralization of an acid or base. Other potential applications include the recovery of organic acids from their salts and the treating of effluents from stack gas scrubbers. The new component required is a bipolar membrane, a membrane that sphts water into H and OH". At its simplest, a bipolar membrane may be prepared by... 

Here AGqh is calculated for the standard condition of ML OH and 1 /3 ML H2O. Assuming that Reaction (3.18) is in equilibrium for aU potentials [AG(G, 0qh) = 0], and, furthermore, that the excess barrier for water splitting is small, this leads to the following expression for the coverage ... 

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. 

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