35
Figure 4.6: The difference in the adsorption energy of the 2H and 1T phase
versus the difference of the center of the p bands of the corresponding phases.
4.5 Candidates for the HER
In the previous section the hydrogen adsorption energy was discussed which
in the present context is nothing but the difference of energies of the reactant
and the product side calculated with DFT. As discussed before, the descriptor
for the HER is the free energy of the reaction which has to be close to
zero. Therefore, additional terms are required in enthalpy which will give the
estimate of the free energy. The additional terms come from the entropic contributions
and zero point corrections. The entropy of the the adsorbed state
is approximated to zero due to negligible number of the microstates in the
adsorbed state as compared to hydrogen in the gas phase. The zero point
energy contribution comes from the vibrations of the atoms analogous to the
ground state of the quantum mechanical harmonic oscillator.
A very crude approximation has been made while adding the corrections
corresponding to the zero point and entropic contributions. The correction for