No, the Reversible Hydrogen Electrode (RHE) and the Standard Hydrogen Electrode (SHE) are not the same; they represent distinct reference points in electrochemistry due to fundamental differences in their operating conditions and how their potentials are established.
Understanding the Difference Between RHE and SHE
The Standard Hydrogen Electrode (SHE) serves as the universal reference electrode, with its potential conventionally defined as 0.00 V. This precise value is established under very specific standard conditions: when the activity of hydrogen ions (a(H+)) is exactly 1.00, and the fugacity (effective pressure) of hydrogen gas (f(H2)) is 1.00 bar. The SHE provides a fixed benchmark for measuring and comparing the standard electrode potentials of other half-cells.
In contrast, the Reversible Hydrogen Electrode (RHE) operates under non-standard conditions. The key distinction is that the potential of the RHE differs from that of the SHE because the activity of hydrogen ions and the fugacity of hydrogen gas are other than unit. This means the RHE's potential varies with the pH of the solution it is immersed in, making it a pH-dependent reference electrode.
Key Distinctions Between RHE and SHE
To highlight their differences, consider the following comparison:
| Feature | Standard Hydrogen Electrode (SHE) | Reversible Hydrogen Electrode (RHE) |
|---|---|---|
| Defined Potential | 0.00 V (by convention) | Varies with the pH of the solution |
| Hydrogen Ion Activity | Fixed at a(H+) = 1.00 (equivalent to 1 M H$^+$ solution) | Variable; can be other than unit |
| Hydrogen Gas Fugacity | Fixed at f(H2) = 1.00 bar (effective pressure) | Variable; can be other than unit |
| Primary Use | Universal reference for comparing standard electrode potentials | Reference for experiments where pH varies (e.g., electrocatalysis, fuel cell research) |
| pH Dependency | Not pH-dependent (fixed at pH 0 conditions) | Directly pH-dependent; its potential shifts by -59.2 mV per pH unit at 25°C |
Practical Implications
The choice between using an SHE and an RHE depends on the specific electrochemical application:
- SHE Usage: The SHE is crucial for establishing a universal scale for redox potentials, allowing chemists and electrochemists worldwide to compare thermodynamic data under standardized conditions. This makes it ideal for tabulated standard potentials.
- RHE Usage: The RHE is particularly valuable in pH-dependent electrochemical studies, such as those involving electrocatalysis for water splitting (hydrogen and oxygen evolution reactions) or CO2 reduction. Because its potential tracks the pH of the solution, the overpotential measured relative to an RHE remains constant regardless of pH, simplifying data interpretation for reactions where hydrogen ions are reactants or products. This ensures that the measured potential difference reflects the intrinsic activity of the catalyst rather than changes in solution pH.
