Self-Assembled Monolayers
of <i>n</i>‑Alkanethiols
Suppress Hydrogen Evolution and Increase the Efficiency of Rechargeable
Iron Battery Electrodes
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Abstract
Iron-based rechargeable batteries, because of their low
cost, eco-friendliness,
and durability, are extremely attractive for large-scale energy storage.
A principal challenge in the deployment of these batteries is their
relatively low electrical efficiency. The low efficiency is due to
parasitic hydrogen evolution that occurs on the iron electrode during
charging and idle stand. In this study, we demonstrate for the first
time that linear alkanethiols are very effective in suppressing hydrogen
evolution on alkaline iron battery electrodes. The alkanethiols form
self-assembled monolayers on the iron electrodes. The degree of suppression
of hydrogen evolution by the alkanethiols was found to be greater
than 90%, and the effectiveness of the alkanethiol increased with
the chain length. Through steady-state potentiostatic polarization
studies and impedance measurements on high-purity iron disk electrodes,
we show that the self-assembly of alkanethiols suppressed the parasitic
reaction by reducing the interfacial area available for the electrochemical
reaction. We have modeled the effect of chain length of the alkanethiol
on the surface coverage, charge-transfer resistance, and double-layer
capacitance of the interface using a simple model that also yields
a value for the interchain interaction energy. We have verified the
improvement in charging efficiency resulting from the use of the alkanethiols
in practical rechargeable iron battery electrodes. The results of
battery tests indicate that alkanethiols yield among the highest faradaic
efficiencies reported for the rechargeable iron electrodes, enabling
the prospect of a large-scale energy storage solution based on low-cost
iron-based rechargeable batteries