A four-electron O2-electroreduction biocatalyst superior to platinum and a biofuel cell operating at 0.88 V

Article abstract of DOI:10.1021/ja0475510

O₂ was electroreduced to water, at a true-surface-area-based current density of 0.5 mA cm^-2, at 37 °C and at pH 5 on a wired laccase bioelectrocatalyst-coated carbon fiber cathode. The polarization (potential vs the reversible potential of the O₂ /H₂O half-cell in the same electrolyte) of the cathode was only -0.07 V, approximately one-fifth of the -0.37 V polarization of a smooth platinum fiber cathode, operating in its optimal electrolyte, 0.5 M H₂SO₄. The bioelectrocatalyst was formed by wiring laccase to carbon through an electron conducting redox hydrogel, its redox functions tethered through long and flexible spacers to its cross-linked and hydrated polymer. Incorporation of the tethers increased the apparent electron diffusion coefficient 100-fold to (7.6 ± 0.3) × 10^-7 cm ² s^-1. A miniature single-compartment glucose-O₂ biofuel cell made with the novel cathode operated optimally at 0.88 V, the highest operating voltage for a compartmentless miniature fuel cell. Copyright

Full text of DOI:10.1021/ja0475510

Published on Web 06/17/2004
A Four-Electron O2-Electroreduction Biocatalyst Superior to Platinum and a
Biofuel Cell Operating at 0.88 V
Valentine Soukharev, Nicolas Mano,* and Adam Heller*
Department of Chemical Engineering and Texas Materials Institute, The UniVersity of Texas, Austin, Texas 78712
Received April 27, 2004; E-mail: mano@mail.utexas.edu; heller@che.utexas.edu
Catalysis of the electroreduction of O2 to water determines the
operating voltage and thereby the efficiency of the cathode in air
batteries, the prime example of which is the zinc air battery, and
in fuel cells, the best-studied examples of which are the methanol/
air and the H2/O2 cells. When the current density is not limited by
O2 transport, the better the catalyst, the less the voltage loss for a
cathode operating at a given current density. In the language of
electrochemistry, the voltage loss is termed polarization or over-
voltage. It is the excess voltage must be applied over the reversible
potential of the O2/H2O half-cell in order to maintain the desired
true current density, which is the current per unit true surface area,
taking into account the surface topography or roughness, distin-
guished from the geometrical, or engineering, surface area. In
engineered batteries or fuel cells, the ratio of the geometrical and
true surface area is high. The specific surface of the platinum loaded
carbon particles of fuel cell O2 diffusion electrodes is >100 m2/g.
The Pt weight fraction in the catalyst is ∼0.05, and ∼ 100 µg cm-2
Figure 1. Polymer I. The pyridine-quarternizing functions are randomly
of Pt is loaded on their O2 cathodes.1 The engineering/true surface
distributed in the backbone.
area ratio of these cathodes is ∼2 × 103, providing an engineering
Introduction of the tethers increased 100-fold the apparent electron
current density of ∼1 A cm-2 in H2/O2 or methanol/air fuel cells,
diffusion coefficient Dapp of the hydrogel, formed upon cross-
when the true current density is about <0.5 mA cm-2. Platinum,
linking, with poly(ethylene glycol)diglycidyl ether (PEGDGE), and
the classical catalyst for the electroreduction of O2 to water, operates
hydration in pH 5 citrate buffer. Dapp was determined, through the
optimally near pH 0 in halide-free acids. The acids of choice are
Randles-Sevcik equation, by cyclic voltammetry.4 Dapp of the novel
H2SO4, at temperatures below 50 °C, and H3PO4, at temperatures
hydrogel was (7.6 ( 0.3) × 10-7 cm2 s-1. That of the earlier used
above 50 °C.2
laccase “wire”, PVI-Os-tpy, was only (6.2 ( 0.8) × 10-9 cm2 s-1
.
Earlier, we showed that the polarization of a “wired” copper
enzyme (bilirubin oxidase) bioelectrocatalyst coated carbon cathode
was about half of that for pure, smooth platinum.3 Here we describe
the fastest O2 to water electroreduction catalyst to date. Its
polarization at 37 °C and at a true current density of 0.5 mA cm-2
is -0.07 V, approximately one-fifth of the 0.37 V polarization of
platinum. Using the novel catalyst, we built a miniature compart-
mentless glucose-O2 biofuel cell, operating optimally at 0.88 V,
the highest operating voltage for a compartmentless or miniature
cell.
To build a yet faster O2 cathode, we used the copper enzyme
laccase and “wired” it with a 100-fold more rapidly electron
diffusing redox polymer (polymer I), shown in Figure 1. The
synthetic strategy for the +0.55 V vs Ag/AgCl potential redox
polymer I followed that for the reported glucose oxidase “wiring”
The activation energy for electron diffusion in polymer I was
reduced from 48.3 kJ mol-1 for PVI-Os-tpy to 30 kJ mol-1
Cross-linking makes the “wired” enzyme films more rigid,
reducing the collision rate of the redox functions and thereby Dapp
.
.
The long tethers facilitate the collisional electron transfer between
neighboring polymer redox centers and sweep electrons from large
volumes of the hydrated redox polymer,6 just as they do in the
“wire” designed for glucose oxidase, having long tethers between
its backbone and its redox centers.4 Because the eight-atom long
tethers increased Dapp 100-fold, we were able to raise the PEGDGE
cross-linker weight fraction from 0.050 for the PVI-Os-tpy laccase
“wire” without the tethers to 0.072 for polymer I. The resulting
better cross-linked gel was adherent to carbon and leatherlike. At
pH 5, a current density of 0.5 mA cm-2 was reached only at a
polarization as large as -0.16 V for the polymer without the tethers,
PVI-Os-tpy (Figure 2, fine line), while it was reached at a mere
-0.07 V for polymer I. (Figure 2, bold line). Under the (stagnant)
conditions of the experiment, the O2 transport limited current density
for the “wired” laccase coated 7 µm diameter carbon fibers (in pH
5 citrate buffer) was 1 mA cm-2, so that, at 0.5 mA cm-2, O2
transport did not limit the current density. Under the same
conditions, the polarization of a 6 µm diameter platinum fiber in
0.5 M H2SO4 was -0.37 V versus the reversible O2/H2O half-cell
potential (Figure 3).3 Unlike the platinum fiber cathode, which is
poisoned by alcohols including glucose, the bioelectrocatalyst-based
cathode is stable to glucose for >1 week at 37 °C.
polymer
poly(4-vinylpyridine)[Os(N,N′-dimethyl-2,2′-biimida-
zole)3]2+/3+) (polymer II, not shown).4 The osmium complex, with
two 4,4′-dimethyl-2,2′-bipyridine ligands and one 4-aminomethyl-
4′-methyl-2,2′-bipyridine ligand, was reacted to form amides with
N-(5-carboxypentyl)pyridinium functions of poly(4-vinylpyridine)
(PVP), introduced by partially quaternizing PVP with 6-bromo-
hexanoic acid.
Unlike the earlier used “wires” of laccase and bilirubin oxidase,
in which the redox functions were coordinatively bound to ring
nitrogens of backbone imidazoles,5 polymer I has eight-atom long
and flexible tethers between its Os complexes and its backbone.
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10.1021/ja0475510 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Polarization of the 7 µm diameter, 2 cm long carbon fiber cathode
modified with laccase “wired” by PVI-Os-tpy, which has no tethers between
its redox centers and its backbone (fine line),5 and polymer I, with 8-atom
long tethers (bold line). The film contained 45.6 wt % laccase, 47.2 wt %
polymer I, and 7.2 wt % PEDGE. 0.1 M pH 5 citrate buffer, quiescent
Figure 4. Dependence of the power density on the cell voltage for the cell
made with a 7 µm diameter, 2 cm long carbon fiber anode coated with
“wired” glucose oxidase and with a 7 µm diameter, 2 cm long carbon fiber
cathode coated with the PVI-Os-tpy “wired” laccase5 (fine line) and with
the novel polymer I “wired” laccase cathode (bold line). 15 mM glucose.
Other conditions as those in Figure 2.
solution, under air, 37 °C. Scan rate 1 mV s-1
.
methanol/water (1:1:1) for 36 h to form [Os(dme-bpy)2(amino-dme-
bpy)]Cl, which was isolated and purified as described.10 The
chloride was converted, by ion exchange, to the PF6- salt. Polymer
I was obtained by linking, through amide bonds, the resulting
osmium complex to poly(4-vinylpyridine), quaternized with 6-bro-
mohexanoic acid, following the procedure of Mao et al.4 The
1
composition of polymer I (Figure 1) was confirmed by H NMR
and elemental analysis. The electrodes were prepared as de-
scribed.5,11
In summary, by introducing long and flexible tethers in the “wire”
of laccase, which electrically connected the reaction centers of the
enzyme to the cathode, we increased the apparent electron diffusion
coefficient, Dapp, 100-fold. By doing so, we formed a catalyst
allowing the electroreduction of O2 to water (at pH 5) at a greatly
reduced voltage loss relative to that for pure and smooth platinum
(at pH 0). At a current density of 0.5 mA cm-2, the overpotential
was -0.07 V, approximately one-fifth of the 0.37 V overpotential
of Pt. A miniature, membraneless glucose-O2 biofuel cell, built
with the novel cathode, operated at the highest voltage to date, 0.88
V while producing 350 µW cm-2 in a stagnant pH 5 citrate buffer
at 37.5 °C.
Figure 3. Polarization of the 6 µm diameter, 2 cm long smooth solid
platinum fiber cathode in 0.5 M H2SO4. Quiescent solution, under air, 37
°C. Scan rate 1 mV s-1
.
To form a compartmentless miniature glucose-O2 biofuel cell,
the novel cathode was combined with a glucose electrooxidizing
anode. The 7 µm diameter carbon fiber anode was coated with
“wired” glucose oxidase (GOx). The “wire” chosen was polymer
II, having a reducing redox potential and 13-atom long flexible
tethers binding the redox centers to the backbone to increase Dapp
.
The effective collection of the electrons from the glucose-reduced
GOx allowed poising of the anode at a potential as reducing as
-0.10 V vs Ag/AgCl.4
The power of the cell made of the two 7 µm diameter, 2 cm
long carbon fibers, its cathode coated with the polymer I “wired”
laccase, and its anode with the polymer II-“wired” GOx (35 wt. %
GOx, 60 wt. % polymer II and 5 wt. % PEGDGE), peaked at 0.88
V. This value represents the highest operating voltage in a miniature
biofuel cell, the power density reaching 350 µW cm-2 (Figure 4).
Because the thermodynamic potential for the cell reaction
Acknowledgment. The work was supported by the Office of
Naval Research (Grant No. N00014-02-1-0144), the Welch Foun-
dation, and the National Science Foundation (Grant No. CHE
0109587).
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