# Absolute Calibration for Cyclic Voltammetry from the Solution-Phase Ionisation of Ferrocene

## TL;DR

This paper provides a precise measurement of the energy level of ferrocene in solution, helping to standardize measurements in electrochemistry and related fields.

## Contribution

The paper reports an absolute adiabatic ionisation energy of ferrocene in acetonitrile solution using ambient pressure photoemission spectroscopy.

## Key findings

- The adiabatic ionisation energy of ferrocene in acetonitrile solution is 4.94 ± 0.05 eV.
- Liquid surface barrier effects were found to be minimal in the measurement.
- The result aligns with one of the conflicting reference values in the literature.

## Abstract

Accurate determination of the energy levels of materials
is crucial
to many fields of science and technology, including electronics, catalysis,
and energy generation and storage. The frontier molecular orbital
levels of molecules are commonly inferred from their oxidation and
reduction potentials measured in solution using voltametric techniques,
which are reported versus a standard, typically an internal one such
as a ferrocenium/ferrocene (Fc+/Fc) redox couple. At present,
however, multiple reference electrode scales are used across the literature,
leading to discrepancies of up to 0.3 eV. Here, we report an absolute
energy level measurement for (Fc+/Fc) in acetonitrile solution.
Specifically, we determined the adiabatic ionisation energy of ferrocene
in acetonitrile solution to be 4.94 ± 0.05 eV using ambient pressure
photoemission spectroscopy. By comparing the energy-dependence of
photoemission from different solution concentrations with a model
for photoemission from solution, we confirm that we measure the adiabatic
ionisation energy and that liquid surface barrier effects are minimal.
This value is consistent with one of several conflicting reference
values used in the literature. The result therefore provides a benchmark
value for the Fc+/Fc internal reference, widely used for
the conversion of voltammetry data to the absolute energy scale.