Hydrogen production: Thermodynamics of water splitting and an environment-aided reaction

Hydrogen generation using electricity-driven water splitting has emerged as a promising approach for converting huge amounts of excess electrical energy from renewable energy sources into a clean fuel—hydrogen (H2)

Water splitting is a classic case of sharing the cost burden with the environment. It is only thermodynamics that can tell you if it is feasible in any process

Summary

Fundamental points underlined

From a thermodynamics angle, water splitting is not a favorable reaction because the Gibbs free energy is + 237 KJ/mole. This shortcoming is negated by supplying electrical energy and raising the temperature to make the reaction go forward. The power source supplies only the Gibbs free energy + 237 KJ/mole. ?G = ?H - T?S, the amount TΔS, 48.7 KJ/mole comes from the environment at temperature T to meet the total enthalpy requirement of ?H - 283.85 KJ/mole. Temperature T plays a critical role in the electrolysis of water because T is critical to meet T?S.

?The high temperature helps electrolysis reaction. At high temperatures, there is low electrical potentials in the cell, this indicates high ionic conductivity, low electrical resistance, and consequently higher efficiency of the electrolysis process. In short, the temperature is one of the most important variables in electrolysis, because the efficiency increases with increasing the temperature due to the required potential to produce the same quantity of hydrogen being reduced considerably.

Electrolysis is a process in which water is split into H2 and O2 by using electricity. Electricity is the flow of electrons through a conductive path like a wire. This path is called a circuit. The electrical potential difference between the anode and the cathode makes the electrons move. Anode has more electrons and is unstable because of the crowding of electrons. The electrons want to rearrange themselves to get rid of this difference. Electrons repel each other and try to go to a place with fewer electrons. That place is the cathode.

Water splitting is a slow red-ox reaction because pure water does not conduct electricity. Therefore, water splitting is not an easy reaction. Electrolytes are used to speed up the ionization of water. How an electrolyte is selected is complex chemistry. This is not a part of this post.

?Chemistry

In the electrolyzer, there is one cathode and one anode connected to a power source. Electrons always flow from anode to cathode no matter what. The cathode is always where reduction occurs therefore electrons need to be there. Oxidation is the loss of electrons and reduction is the gain of electrons.

Cathode (reduction):2 H2O(l) + 2e? -- > H2(g) + 2 OH?(aq)

Anode (oxidation): 2 OH?(aq) -- > 1/2 O2(g) + H2O(l) + 2 e?

Combination of these reactions produces:

?2 H2O(l) → 2 H2(g) + O2(g)

H2 is produced at the cathode and O2 at the anode.

?Process

A DC electrical power supply is connected to two electrodes, or plates, in the water (usually made of an inert metal like platinum or iridium). At the cathode (where electrons enter the water), hydrogen will appear, whereas oxygen will appear at the anode.?Assuming perfect faradaic efficiency, the amount of hydrogen produced is twice that of oxygen, and both are proportional to the total electrical charge carried by the solution. However, competing side reactions occur in multiple cells, resulting in diverse products and a faradaic efficiency that is less than optimal.

To overcome numerous activation obstacles, the electrolysis of pure water necessitates additional energy in the form of overpotential. The electrolysis of pure water proceeds very slowly or not at all without the surplus energy. This is due in part to water's limited self-ionization. The electrical conductivity of pure water is about one-millionth that of seawater.

Thermodynamics

The image above is quite straightforward

Summary of key thermodynamic data

?An important point to note is that while the ?H of formation of H2O is – 285.83 KJ/mole, the ?H for splitting reaction is + 285.83 KJ/mole. This is behind making the water splitting thermodynamically a non-favourable reaction. The reason is ?H of a reaction = [ ?H of reactants - ?H of products] = [ 0+ 0 – ( -285.83)] = + 285.83 KJ/ mole

The process must provide the energy for the dissociation plus the energy to expand the produced gases. At temperature 298K and one-atmosphere pressure, the system work is

W = PΔV = (101.3 x 103 Pa) (1.5 moles) (22.4 x 10-3 m3/mol) (298K/273K) = 3715 J

Since the enthalpy H= U+PV, the change in internal energy U is then

ΔU = ΔH - PΔV = 285.83 kJ - 3.72 kJ = 282.1 kJ

The expansion of the gases created must accompany this change in internal energy, therefore the change in enthalpy indicates the energy required to complete the electrolysis. It is not required, however, to put in the entire amount as electrical energy. The amount TS can be obtained from the environment at temperature T because the entropy increases during the dissociation process. The quantity that the battery must supply is actually the change in Gibbs free energy.:

ΔG = ΔH - TΔS = 285.83 kJ - 48.7 kJ = 237.1 kJ

Due to the increase in entropy caused by the electrolysis process, the environment "assists" the process by contributing the quantity TS. The Gibbs free energy is useful because it tells you how much energy in other forms must be given in order for the process to progress.

Credit: Google