Hydrogen Evolution Reaction

WHAT IS HYDROGEN EVOLUTION REACTION?

(g)

It is the reaction of hydrogen gases evolving from available protons. 

WHY IS IT IMPORTANT?

As fossil fuels are running out, we need alternative energy sources. Solar, wind, geothermal, hydroelectric energy are, of course, good alternatives, but they have geographical limits of the use. Therefore, hydrogen, which is not geographically constricted, is a prominent source of alternative and eco-friendly energy.   

HOW TO EVOLVE/MAKE HYDROGEN?

1.From acidic solutions:

In an acidic solution, where protons are readily available, hydrogen can be evolved with the use of transition metal chalcogenides (TMDs) as a catalyst. TMDs are a combination of a transition metal with a member of the chalcogen column of the perioid table such as Sulfur or Selenium. Common TMDs for the use of evolving hydrogen are either MoS2 or WS2. 




2. From water:


It is the photosynthesis taking place in the chloroplasts of the plants. It is to split H2O molecules into H2 (g) and O2(g). An example of the molecule used is Co4O4(OAc)4(py)(a cubane molecule)



HOW TO CHOOSE CATALYSTS TO EVOLVE/MAKE HYDROGEN?


The chart above illustrates the current density (vertical axis) and the the Gibbs free energy of adsorbed atomic hydrogen (the horizontal axis). The higher the current density, the better, because it shows more hydrogen was evolved. The horizontal axis indicates how easily hydrogen that was produced can be detached from the catalyst. Therefore, the closer the value to zero, the better the catalyst is. Platinum is a good catalyst in that the current density is the highest out of all the metals and its Gibbs free energy is close to 0. The problem is that Pt and other metals that have similar values as the Pt's are expensive. Thus, it is essential to discover a cost-effective catalyst that has a high current density and a low Gibbs free energy.