The Yale researchers investigated the electrochemical
behavior in a typical Li-O2 cell environment. They used cyclic
voltammograms, in which working electrode potential is ramped
linearly against time, to demonstrate electrochemical properties
of hemin. Hemin is heme with a chloride-negative anion. This
solution was dissolved in 1 M of lithium perchlorate (LiClO4)
in tetraethylene glycol dimethyl ether. To determine whether or
not the heme reacted with atmospheric oxygen, the researchers
tested the heme-based chemistry by alternately exposing it to O2
and inert helium (He) atmospheres.
No significant reaction was seen in the CV curve for
LiClO4+tetraethylene glycol dimethyl ether. After O2 purging,
broad peaks appeared in both cathodic and anodic regions near
2. 2 volts (V) and 3. 2 V, respectively. When heme was introduced,
a reduction feature appeared even under helium purging,
indicating this reaction at 2.94 V primarily involves electron
transfer (heme(Fe3)+e-;heme(Fe2+)) when no O2 is available.
The oxidization feature (Ea, 2) with heme at 3. 9 V is likely related
to the cathodic reaction (heme(Fe2+);e-+heme(Fe3+)).
“We also observe under He purging a peak at 4. 2 V, which
could relate to the irreversible oxidation of heme(Fe3+) to
heme(Fe4+) in the absence of O2,” the researchers write in the
paper published in Nature Communications.
Under an oxygen atmosphere, additional features are present
at 2. 5 V and 2. 2 V for the cathodic regime, the researchers
discovered. While the 2. 2 V peak is consistent with O2 reduction
in the LiClO4+TEGDME electrolyte without heme, the feature
at 2. 5 V with heme suggests the formation of an intermediate
superoxide: O2- or heme(Fe2+)-O2. At 3. 2 V and 4 V, features
are present in the anodic region that suggest the reverse of the
oxygen-reduction processes, indicating the heme participates in
evolving the superoxide intermediate and facilitating the oxidation
of lithium peroxide.
Researchers soon discovered the heme molecule exhibits this
redox function in a LiClO4 electrolyte, but does not with lithium
“We carried out spectroelectrochemical measurements in
a LiPF6-containing electrolyte with the same concentration of
heme to verify the Li salt effects,” the researchers write in Nature
The heme molecules, the paper says, exhibit their function
synergistically with ClO4- anions in the electrolyte and not with
PF6, which indicates the pairing with Li salt and redox molecules
is necessary to promote effective catalytic function.
“Although a large reduction peak in the CV near 1.8 V
corresponds to discharge product formation, there are no
oxidation peaks in the charge region,” according to the paper.
“The ultraviolet-vis spectra indicate no significant shifts in the
Soret band during reduction or oxidation with a LiPF6 electrolyte,
demonstrating that the heme chemical structure is not changed
even in an O2-containing environment.”
Taylor’s research has shown that Heme is an abundant and
eco-friendly biomolecular which could improve the life of Li-O2
batteries by serving as a catalyst to facilitate Li-O2 oxidation.
This soluble heme molecule enables charge transfer between
insulating Li-O2 discharge products and the electrode by
engaging in electron transfer and coordinating with superoxide
intermediates, the researchers find.
“The Li-O2 cell with heme catalyst achieves a lower polarization
and longer cycle life, compared with the control,” according to
the researchers. “We also verify the reversible formation and
decomposition of LiO2 and Li2O2 on the oxygen electrode by
ex-situ characterization and show that heme is not incorporated
into solid products but remains a mobile electrolyte species.
Indeed, redox biomolecules with complexing catalytic functions
present a new path to improve electrochemical storage efficient
using sustainable materials.”
Fig 3: Ex-situ X-ray photoelectron spectra obtained from 1st discharged, 1st charged,
20th discharged, and 20th charged electrodes collected in the (a) C 1s, (b) O 1s; (c)
Ex-situ X-ray photoelectron spectra obtained from 20th discharged and 20th charged
electrodes collected in the Fe 2p; (d) Ex-situ Raman spectra obtained from pristine,
1st discharged, 1st charged, 20th discharged, and 20th charged electrodes. (blue
star: Li2O2, black star: TEGDME, black cross: LiClO4, red star: LiO2).
Credit: Nature Communications
Fig 2: LiO2 Cell Performance in Heme Electrolyte: (a) Initial charge/discharge curves
of the MWCNT electrode in 1M LiClO4+TEGDME and 1M LiClO4+TEGDME+Heme
solutions in a voltage window between 4. 3 and 2.35V at a current density of
100mAg;1carbon; (b) galvanostatic intermittent titration curves of the MWCNT electrode
in 1M LiClO4+TEGDME and 1M LiClO4+TEGDME+Heme solutions, which were
acquired with a current density of 50mAg;1 for 24min and a 120min time interval
during the 1st charging; (c) Voltage versus time curves of the MWCNT electrodes in
1M LiClO4+TEGDME+Heme solution at various cycles under a specific capacity limit
of 600mAhg;1 between 4. 5 and 2.3V at a current density of 200mAg;1carbon; (d)
Electrochemical impedance spectroscopy (EIS) spectra of the MWCNT electrode in 1M
LiClO4+TEGDME and 1M LiClO4+TEGDME+Heme solutions after the 1st, 5th, and 10th
discharge cycle. Credit: Nature Communications