In 1972, Trasatti carefully compiled the rates of hydrogen evolution reaction (HER) on 31 metal electrodes near pH 0 [1]. The HER is a surface reaction that can be represented as

H_soln⁺ ⇌ H_ads⁺

H_ads⁺ + e ⇌ H_ads^∙

2 H_ads^∙⇌ H_2,ads

H_2,ads ⇌ H_2,soln

Trasatti also defined the electrochemical work function Φ, which is the energy needed to remove an electron from the electrode surface to a redox species immediately at the electrode surface. The rates of the HER reaction are embedded in the standard exchange current density j₀ (A/cm²) that measures the standard heterogeneous rate of electron transfer, k⁰. Trasatti demonstrated that plot of log j₀ versus Φ yields two parallel lines as shown, where regression yields

log j₀ = (6.4₄ ± 0.2₄) Φ (eV) - (35.₄±1.₁) with R² = 0.98 for the higher rate line and

log j₀ = (6.5 ± 0.5₆) Φ (eV) - (38.₅ ± 2.₄) with R² = 0.93 for the lower rate line.

The slopes are the same but the intercepts differ by 3. For a given Φ, metals on the upper line have HER rates that are 1000 x higher than metals on the lower line.

In this presentation, the magnetic properties of the electrodes are shown to describe the binary segregation of the data into two discrete lines. The magnetic properties of the electrodes are set by paired and unpaired electron spins. The results are evidence of a magnetic effect on electron transfer.

References

[1] Trasatti, S. Work Function, Electronegativity, and Electrochemical Behavior of Metals. III. Electrolytic Hydrogen Evolution in Acid Solution. Electroanalytical Chemistry and Interfacial Electrochemistry 39, 163-184 (1972).