To achieve photocatalytic water splitting using a single photocatalyst, the bandgap of the semiconductor must straddle the reduction and oxidation potentials of water, which are +0 and +1.23 V vs the normal hydrogen electrode (NHE) at pH =0.
I have a few questions about this statement:
What exactly does it mean when you say that the bandgap has to "straddle" the reduction and oxidation potentials? Does it mean that you need an electron/hole with energy at least equal to 1.23 eV for oxidation/reduction to take place. So, the bandgap has to be greater than 1.23 eV in order to ensure that. Is my understanding accurate?
When using a single photocatalyst, I assume the oxygen and hydrogen evolution take place on the same material. If you had separate oxygen evolution and hydrogen evolution catalysts, what would the condition on the bandgaps of these semiconductors be?
How you define "a single photocatalyst?" Water splitting is a complex 4-electron process with a standard delta(G) 1.23 V per 1-electron. You need a photosensitizer, charge separation, and two catalysts (WOC and WRC). The potentials for water oxidation and water reduction are pH dependent. Energy bands should be be properly aligned (straddled) to make both reactions thermodynamically favorable. The reactions to proceed require the overpotentials, meaning that the bandgap should be > 1.23 V, but not obligatory = 2V..
The attached review succinctly introduces the electronic properties of different solar water splitting cell configurations, including single- and dual-electrode cells.
Long story short, the bands of a single photo-absorber have to "straddle" the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) potentials because there must be some driving force for excited charge to participate in those reactions. In a band diagram, electrons "fall" and holes "rise", so the valence band maximum must be positioned below the OER potential to drive holes towards the water-catalyst interface, and likewise the conduction band minimum must be above the HER potential. The larger the band offset (in the favorable direction), the faster the reaction will proceed. Catalysts added to the system can further improve the kinetics.
Because it's hard to achieve all these requirements with a single material, water splitting cells with separate photoanodes and photocathodes are widely considered to be a better option for efficient water splitting.
Article Solar Water Splitting Cells
The potential difference between the hydrogen-evolving half reaction and water-oxidation half reaction is 1.229 V. The water-oxidation and the hydrogen-producing half reactions additionally require 0.275 and 0.050 V over-potential, respectively to proceed at an efficient rate. For a semiconductor as a PEC water splitting cell, the electron-accepting (or electron-donating) reactions is occurred at Fermi level which is positioned ~ 0.050−0.200 eV from the band-edge. As a result, the semiconductor band gap needs to be a minimum of 1.8 eV to provide the required potential for electrolysis in the single-photoelectrode water splitting cell system.
In short, it is due to the need of an extra energy input in order to accelerate the reaction rate. It is a kinetic overpotential. At 1.23 V vs. NHE the system is in dynamic equilibrium, with low exchange current densities in both senses of the reaction.
what does the meaning of bandgap > 1.23 eV ? why we need this much energy to split the water. please explain or give reference .
Pardon me, I want to clarify a little bit more about this question. What happen if the single photocatalyst do have the sufficient bandgap, let say 1.8 to 2.0 eV, but they don't straddle the reduction and oxidation potential of water? For example, if the conduction band is below HER? Or the other case if the valence band is above OER?
how can we calculate the standard oxidation potential and reduction potential
- Badges
- Science topic
- Similar topics
- Filing