Water Electrolysis Basics Flashcards
At what voltage does the electrolysis curve start?
Here the electrolysis curve is depicted starting at the equivalent voltage to the higher heating value. This applies if no heat is taken from the environment and the enthalphy of evaporation has to be provided purely by electricity.
What is the relation between voltage necessary to get the electrolysis process started and heat supplied?
The voltage necessary to get the electrolysis process going drops when heat is supplied. This heat can originate either from the environment or from the heat losses of the process itself. When the heat provided is equivalent to the enthalpy of evaporation the voltage necessary to start the reaction drops according to the amount of heat provided. The lowest level which can be attained is 1.253 V; the thermoneutral voltage. This happens when all of the enthalpy of evaporation is covered by heat input.
How is the curve when steam is used instead of liquid?
If steam instead of liquid water is provided the criminalization curve starts at the thermoneutral voltage right away. This effect represents the advantage in efficiency of high temperature electrolysis which is often quoted. Yet, providing steam gently requires an external heat source which needs to be taken into consideration when calculating the efficiency.
It starts at the Gibbs free energy for the gaseous case. This curve can be assumed not to shift since the steam generally is overheated and the maximum required heat can directly be taken for the electrolysis. Steam electrolyzer technology is based on ceramic cells very much akin to solid oxide fuel cells.
How to calculate the efficiency of an electrolyzer?
Whereas the electrical input is characterized by the cell voltage E_Z, the output refers to the application. In the majority of applications the enthalpy of condensation/evaporation cannot be harnessed. This holds true for fuel cells, gas turbines, internal combustion engines etc. In these cases the lower heating value is to be used as the reference for the energy output. On the other hand, for condensing burners operated with hydrogen the higher heating value should be applied in order to calculate the efficiency of the energy pathway from electrolysis the application.
Eta = dH_LHV/(zFE_Z) = (E°_H,LHV,gas)/E_Z
At what voltage does the cell need heating and at what does it cooling when starting with liquid?
Below the thermoneutral voltage the cell needs heating, above cooling. Appropriate heat exchange is to be designed.
How is the cell potential for the cell decided on?
The cell potential technically applied is determined by a compromise between efficiency (OPEX) and investment (CAPEX). Degradation increases both cost categories. Overly high cell potentials increase degradation. Usual potentials are between 1.8V (η=70%) and 2.2 V (η=57%).
PEM Electrolysis cells allow for much higher current densities than PEMFC.
Is heating needed in the cell when dealing with high temps?
The thermoneutral point for high temperature elctrolysis refers to compensation of the entropy only. It is technically of minor relevance since the water gets evaporated and overhated externally. Hence, any heating or cooling requirement can be furnished by controlling the steam feed temperature.
What are the losses of a PEM water electrolysis cell?
- Activation losses (steep then straightens)
- Proton conduction losses (linear)
- Mass transport losses (exponential)
- Other ohmic losses (linear but very small gradient)
What is the Nernst equation for the electrolysis?
For H2 produced as gas
E_N = E° - ((RT/2F)ln (((p_H2/P_0)(p_O2/p_0)^2)/a_H2O))
Where a_H20 is 1 for liquid water
When the H2 produced is aqueous then:
E_N = E° - ((RT/2F)2,302log((a^2_H+ * (p_O2/p_0)^2)/a_H2O))
Once the O2 and H20 values are sorted (taken as 1), the 2 from the a_H+ can be taken ahead of log and then the log values is the pH.
What is the corrosion situation like in ECs?
Corrosion is an electrochemical process, therefore potentials matter. The anode potentials of electrolysis are much higher than thosefor fuel cells. Consquently, corrosion of catalysts and support materials is a more pronounced issue than in fuel cells.
What material is used for catalyst in ECs?
Pt corrodes at potentials above 1V (according to Pourbaix diagram) at low pH (in cases of acidic cells). Hence, Ir forms a passivation layer is chosen as an anode catalyst. Pt is chosen for the cathode because it is less expensive.
What catalyst is used for Alkaline ECs and why?
The staple catalyst is fine grained nickel – so called Raney Nickel. Nickel thermodynamically is supposed to corrode, yet it does not do so at operating temperatures of alkaline electrolysis and fuel cells.
At higher temperature of 80 to 100°C instead of Ni3+ and Ni4+ the oxydation product is Ni+. This exhibits a lower solubility to which the good corrosion resitance in alkaline media is attributed.
What is the meaning of carbon for PEM electrolysis?
Carbon is thermodynamically instable in fuel cell applications and even more so in electrolysis.
The kinetics of carbon corrosion is slow though, thus it works for fuel cells as long as potentials over 0.8 volts
are avoided or only applied for a very short while. Owing to lower potential of the electrolysis cathode carbon supported Pt can be usedeven in electrolysis.
