Electrochem Eng L02-22 Oxygen evolution line in Pourbaix diagram

Summary

This video explains the electrode potential of the oxygen evolution reaction (OER) in an electrochemical half-cell. It details the half-cell reaction where oxygen gas reacts with protons and electrons to form water, and demonstrates how to calculate the electrode potential using the Nernst equation. By assuming standard conditions for oxygen and water activities, the video shows how the equation simplifies to a linear relationship between electrode potential and pH. With a negative slope of 0.0592 V per pH unit, the explanation highlights how potential decreases with increasing pH, mirroring the behavior of the hydrogen evolution reaction, providing a clear understanding of electrochemical behavior.

Takeaways

• 🔋 The upper dashed line in the diagram represents the Oxygen Evolution Reaction (OER).
• 💧 In OER, oxygen gas reacts with protons and electrons to form two water molecules.
• ⚡ The half-cell reaction for OER in liquid water is O₂ + 4H⁺ + 4e⁻ → 2H₂O.
• 📐 The electrode potential can be calculated using the Nernst equation: E = E⁰ - (0.0592 V / n) × log(products/ reactants).
• 🔢 For OER, the stoichiometric factor of water is 2 and for protons it is 4, which affects the Nernst equation calculation.
• 🧮 Standard electrode potential (E⁰) for the oxygen-water half-cell reaction is 1.229 V vs SHE.
• 🔄 The number of electrons transferred (n) in the OER half-cell reaction is 4.
• 🌬 Assuming 1 atm of O₂, the activity of oxygen gas is taken as 1 for simplicity in calculations.
• 💦 Water activity is usually considered 1 since water is the solvent in most cases.
• 📉 After simplification, the electrode potential becomes E = 1.229 V - 0.0592 V × pH, showing a negative slope with pH.
• ⚖️ The potential slope with pH is the same as for the Hydrogen Evolution Reaction (HER).

Q & A

What is the oxygen evolution reaction (OER) described in the script?

The oxygen evolution reaction (OER) is a half-cell reaction where oxygen gas reacts with protons and electrons to form water: O2 + 4H+ + 4e- → 2H2O.

Which equation is used to calculate the electrode potential for the OER?

The Nernst equation is used to calculate the electrode potential: E = E0 - (0.0592/n) log([Products]/[Reactants]), where E0 is the standard electrode potential and n is the number of electrons transferred.

What is the standard electrode potential (E0) for the OER at 25°C?

The standard electrode potential for the oxygen evolution reaction is 1.229 V versus the standard hydrogen electrode (SHE).

How many electrons are transferred in the OER half-cell reaction?

Four electrons are transferred in the oxygen evolution reaction, so n = 4.

How are the activities of water and oxygen considered in the Nernst equation for OER?

For simplicity, the activity of water (solvent) is assumed to be 1, and the activity of oxygen gas is also assumed to be 1 (ideal gas at 1 atm).

How does pH affect the electrode potential of the OER?

The electrode potential decreases linearly with increasing pH, following the simplified equation: E = 1.229 V - 0.0592 × pH (vs SHE).

Why does the 4 in the stoichiometric exponent cancel with the 4 in the denominator of the Nernst equation?

The stoichiometric coefficient of 4 for protons raises the activity term to the fourth power. When divided by n = 4 in the Nernst equation, the factor cancels out, simplifying the logarithmic term.

What assumption is made about the solvent in the OER calculation?

The solvent, water, is assumed to have an activity of 1 in most cases, as it is in large excess compared to solutes.

How is the OER potential related to the hydrogen evolution reaction (HER) potential?

The OER potential has the same slope with respect to pH as the HER potential, meaning both change by approximately -0.0592 V per pH unit, although the absolute potentials differ.

Why is the Nernst equation important for understanding OER in different pH conditions?

The Nernst equation allows calculation of the electrode potential at non-standard conditions, showing how the OER potential shifts with proton concentration (pH), which is crucial for designing electrochemical cells and understanding Pourbaix diagrams.