Engineering hybrid nanotube wires for high-power biofuel cells

Engineering hybrid nanotube wires for high-power biofuel cells

(Parte 1 de 2)

Received 12 Nov 2009 | Accepted 24 Feb 2010 | Published 12 Apr 2010DOI: 10.1038/ncomms1000

NATURE COMMUNICATIONS | 1:2 | DOI: 10.1038/ncomms1000 | w.nature.com/naturecommunications 1

state of art carbon fi bre biofuel cells

Poor electron transfer and slow mass transport of substrates are signifi cant rate-limiting steps in electrochemical systems. It is especially true in biological media, in which the concentrations and diffusion coeffi cients of substrates are low, hindering the development of power systems for miniaturized biomedical devices. In this study, we show that the newly engineered porous microwires comprised of assembled and oriented carbon nanotubes (CNTs) overcome the limitations of small dimensions and large specifi c surface area. Their improved performances are shown by comparing the electroreduction of oxygen to water in saline buffer on carbon and CNT fi bres. Under air, and after several hours of operation, we show that CNT microwires exhibit more than tenfold higher performances than conventional carbon fi bres. Consequently, under physiological conditions, the maximum power density of a miniature membraneless glucose / oxygen CNT biofuel cell exceeds by far the power density obtained for the current

requests for materials should be addressed to N.M. (e-mail: mano@crpp-bordeaux.cnrs.fr )
Engineering hybrid nanotube wires

Universit é de Bordeaux, Centre de Recherche Paul Pascal (CRPP), UPR 8641, Avenue Albert Schweitzer , 33600 Pessac , France . Correspondence and for high-power biofuel cells Feng Gao 1 , Lucie Viry 1 , Maryse Maugey 1 , Philippe Poulin 1 & Nicolas Mano 1

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1000

2 NATURE COMMUNICATIONS | 1:2 | DOI: 10.1038/ncomms1000 | w.nature.com/naturecommunications

physiological solutions in which concentrations (< 0.5 mM) and
diff usion coeffi cients of substrates are very low (< 5 × 10 − 5 cm − 2 s − 1 ).

Increasing low current densities and enhancing mass transport are key challenges in engineering bioelectrochemical materials for sensors and miniature biofuel cells (BFCs), especially in Electrode material and design are critical in addressing such challenges. Diff erent forms of carbon are widely used as electrode base materials because of their chemical stability and good electrical conductivity. It is even more promising when carbon nanotubes (CNTs) are used. CNT electrodes have a particularly high specifi c surface area enhancing current densities 1 – 9 . However, the coating of macroelectrodes with CNTs does not allow for faster mass transport. Transport limitations are, however, reduced in microelectrodes with diameters smaller than the diffusion length of their reactants 10 . Radial diff usion allows for faster mass transport compared with semi-infi nite planar electrodes. Th e engineering of new porous and hybrid microwires would be the ideal combination for achieving large surface area and fast mass transport. Electroreduction of oxygen to water is chosen as a key test to validate this concept. Th is reaction is of great importance as it usually determines the operating voltage and thereby the effi - ciency of various devices such as air electrodes and BFCs 1 . Platinum has been the reference catalyst for oxygen reduction for more than a century. Nevertheless, it has some drawbacks (large overpotential, deactivation), and new types of electrodes are currently being explored 12,13 . Modifi ed carbon fi bre (CF) electrodes with bioelectrocatalysts have been shown to operate at overpotentials well below that of platinum 14 . However, they have only a small specifi c surface and their operational half-lives are only of a few weeks. It is shown here that the engineering of new porous and hybrid

can range from 1 to 53.9μ W cm − 2 for the most effi cient devices 17,23 .
generate 1.45 mW cm− 2 at 0.3 V 24 . Th e elaboration of high-power

microfi bres made of oriented and biofunctionalized CNTs allows for particularly high current densities and small overpotential. Th ey are optimized for oxygen reduction and BFC applications, with glucose oxidase (GOx) as the anodic bioelectrocatalyst and bilirubin oxidase (BOD) as the cathodic one ( Fig. 1a ). BFCs are of interest because these could power, in the near future, sensor transmitters that could broadcast, for example, the local glucose concentration, relevant to diabetes management 15 . Nanotube-based glucose / oxygen BFC using GOx and BOD with low-power density have been reported earlier 16 – 2 CNT-based electrodes are usually made from dispersions in organic solvent that are drop casted onto electrode surfaces. Th e power density generated by such devices Recently, high-power membrane BFCs haves also been reported to membraneless BFC is a critical feature for future applications, as membraneless devices can be miniaturized. However, the engineering of membraneless BFCs is challenging because it requires the electrodes to be structured to ensure that the mass transport of reactants and products is not impeded and that all enzymes are electronically and effi ciently connected to electrodes.

with CNFs exhibit a power density of 740μ W cm − 2 at + 0.57 V

We show that a large improvement is observed in the electroreduction of oxygen to water in saline buff er on CNT fi bres (CNFs) compared with a conventional CF, because the present electrodes combine radial transport and a large specifi c surface area. Under air and aft er several hours of operation, CNFs exhibit more than a tenfold higher performance. In addition, a glucose / oxygen BFC made in physiological conditions. Such power densities are one to two orders of magnitude higher than that of previous glucose / oxygen δδ-Gluconolactone Electrolyte

GOx BOD

Glucose Redox polymerI Redox polymer

500nm

CNT anodeCNT cathode

Figure 1 | Schematic of the biofuel cell and scanning electron micrographs of CNT fi bres. ( a ) Biofuel cell. At the anode, electrons are transferred from

500nm glucose to glucose oxidase (GOx) and from GOx to redox polymer (I) and from (I) to the CNT fi bre. At the cathode, electrons are transferred from the CNT fi bre to the redox polymer (I), from (I) to BOD and from BOD to oxygen. ( b ) CNT fi bres before and ( c ) after removing PVA.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1000 ARTICLE

NATURE COMMUNICATIONS | 1:2 | DOI: 10.1038/ncomms1000 | w.nature.com/naturecommunications 3

BFC, including recent ones made with GOx / BOD 20 , and fourfold higher than the power density obtained for a CF BFC.

CNF preparationSeveral methods have been proposed to produce
rotating bath or in a co-axial pipe at a rate of 50 ml h− 1 is carried out
decomposed by heating the fi bres at 600° C in an argon atmosphere.
about 300 m 2g − 1 ( Fig. 1c ). Classical CFs, which are conventionally

Results fi bres solely comprised of nanotubes, but have never been used for bioelectrochemical purposes 25 – 29 . Th e porous CNF used in this study was formed by a coagulation spinning process that consists of injecting an aqueous CNT dispersion in a co-fl owing stream of a polyvinyl alcohol (PVA) solution 30 . Th is process operated in water is continuous and scalable. Th e CNTs are fi rst homogeneously dispersed in water by sonication using sodium dodecyl sulphate (SDS) as surfactant. Th e injection of the resulting dispersion in a using a syringe needle. Th e CNTs coagulate on contacting the PVA solution because of bridging fl occulation induced by adsorption of PVA chains at the CNT interface. Th e resultant composite fi bres contain about 80 wt % PVA ( Fig. 1b ), which is then thermally Aft er this thermal treatment, the fi bres become highly porous with a typical Brunauer-Emmett-Teller (BET) specifi c surface area of used for making microelectrodes, were tested for comparison.

limitations can be reduced 31,32
Electroreduction of oxygenCFs and CNFs were modifi ed with
and a+ 0.34 V (versus Ag / AgCl) electron-conducting polymer,
PAA-PVI-[Os(4,4 ′ -dichloro-2,2 ′ -bipyridine) 2 Cl]+ / 2 + . Th e resulting

Figure 2 shows scanning electron micrographs of the cross-section of both the CF and CNT microwire. Th e advantage of the latter over the classical CF is apparent when examining the scanning electron microscope (SEM) images. In contrast to the non-porous CF, in which reduction of oxygen is only expected to occur at the external surface, the CNF is highly porous, with the pores being large enough to allow for easy permeation of the electrolyte and oxygen. By maximizing the porosity of the system, internal mass transfer a bioelectrocatalyst made of BOD, an oxygen-reducing enzyme, bioelectrocatalyst is an electrostatic adduct of the BOD, which is a polyanion at neutral pH, and of the electron-conducting polymer, which is a polycation. Th e formation of such an adduct prevents the phase separation of the enzyme and the polymer. Th e electronconducting redox hydrogel obtained on crosslinking and hydration electrically ‘ connects ’ BOD redox centres to the electrode. It is permeated by water, oxygen and ions. Electron conduction results from the collisional electron transfer between reduced and oxidized redox centres tethered to the polymer backbone 3,34 . Because the rate of collisions increases with the mobility of the tethered redox centres, the greater their mobility the faster the electrons diff use. High electron diff usivities are achieved in fi lms that are only weakly crosslinked. However, weakly crosslinked fi lms dissolve, or swell excessively, and are easily sheared off the electrodes. To prevent this shearing off of the electrocatalytic coatings and to maintain good mechanical properties, the adduct of the enzymes and their wires must still be crosslinked, even though this leads to a decrease in electronic conductivity and therefore of the current density 35 .

and CF electrode (thin line) under argon at 20 mV s− 1 scan rate in a
20 mM phosphate buff er containing 0.14 M NaCl (pH 7.2) at 37° C. At
1 mV s− 1 scan rate, the voltammogram of the CNF exhibits an exactly
symmetrical wave, with little separation ( Δ E P= 2 mV) of the oxidation

Th e coating of electrodes consisted of 32.3 wt % BOD, 60.2 wt % redox polymer and 7.5 wt % poly(ethylene glycol) diglycidyl ether (PEGDGE). Figure 3 shows the background-subtracted cyclic voltammogram of the ‘ wired ’ BOD-coated CNF electrode (thick line) and reduction peaks, typically indicative of a reversible surface-bound couple. Th e linear variation of peak height with scan rate confi rms

1μμm 1μm

Figure 2| Pictures and schematic of microelectrodes. SEM photographs of a carbon fi bre ( a ) and a CNT fi bre ( b ). Schematic representation of the oxygen

reduction on the modifi ed carbon fi bre ( c ) and on the modifi ed CNT fi bre ( d ) (the green shade represents the redox polymer and the blue colour the enzymes).

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1000

4 NATURE COMMUNICATIONS | 1:2 | DOI: 10.1038/ncomms1000 | w.nature.com/naturecommunications that the redox couple is surface confi ned. Th e width of the peak at half height, E whm , is 90 mV, close to the theoretical width of 90.6 mV for an ideal Nernstian one electron transfer reaction 10 . As shown in

Fig. 3 , the voltammetric peaks at a rate of 20 mV s− 1 are much nar-

rower and closer to each other for the CNF than for the modifi ed CF

electrode. Th e voltage diff erence of the peaks is Δ E P= 36 mV, com-
pared with Δ E P= 5 mV for the CF 36,37 . Th ese distinctive features are

ascribed to faster charge transport through the catalytic fi lm made of CNTs. Th is is also evidenced by the decrease in the overpotential for oxygen reduction ( vide infra ). Interestingly, even though the exact same quantity of redox polymer was deposited on both electrode surfaces (that is, the same quantity of osmium centres), the oxidation and reduction current peaks of the CNF are fi vefold higher. Th is suggests that not all redox centres are connected to the electrode surface of CFs. Th e increase in porosity provided by CNFs allows effi cient access to a much higher fraction of redox polymer centres. Th e improvements are not only due to the porosity of the materials but also due to the conformations and electrical wiring of the enzymes and polymers to the electrode material. Th is is also why CNF electrodes exhibit much greater stability ( vide infra ).

NaCl, 20 mM phosphate buff er under air at 37° C are compared in
the CNF compared with the CF. At+ 0.3 V / Ag / AgCl, oxygen is
electroreduced at a current density of 1,570μ A cm − 2 on the CNF,
but at only 380μ A cm − 2 on the CF. Th e overpotential, at any cur-
densities> 2.5 mA cm − 2 are reached with CNFs. Such high current

Th e current densities of a CF (thin line) and a CNF (thick line) modifi ed with ‘ wired ’ BOD in a quiescent pH 7.2 along with 0.14 M Fig. 4 . Under air and in quiescent solution, the CNF exhibits a fourfold higher current density than the conventional CF. Th e threshold for oxygen reduction is defi ned as the potential at which the electroreduction of oxygen starts 38 . It is 30 mV more oxidizing for rent density, is much lower for CNF electrodes and even better than platinum 14,39 . Under 1 atm oxygen and quiescent conditions, current densities confi rm that the porosity of CNFs off ers an effi cient solution to the intrinsic oxygen diff usion problem (present conditions: 0.2 mM concentration of oxygen in air-saturated saline buff er). Th e overpotential of three classes of electrodes is compared in 0.5 M

H 2 SO 4 : a 6μ m diameter polished platinum cathode, a modifi ed CF

and a modifi ed CNF. Th is comparison shows that the overpoten- tial of the ‘ wired ’ -BOD / CNF cathode is much lower than that of

example, for an overpotential of− 0.31 V, oxygen was electrore-

the platinum fi bre and is lower than that of the CF 14,39 . Th us, for duced to water at a current density that is 25-fold higher for the ‘ wired ’ -BOD cathode than for a polished platinum cathode in 0.5 M

H 2 SO 4 . Moreover, the ‘ wired ’ BOD cathode operated in a physiological (20 mM phosphate, 0.14 M NaCl (pH 7.2)) buff er solution

(phosphate-buff ered saline), in which the platinum cathode would have rapidly corroded.

the 2.4 × 10− 2 N m − 2 shear stress resulting from the rotation of 3 m
of over 2,0 r.p.m., the CF that poised at+ 0.3 V / Ag / AgCl and at
37° C loses 80 % of its current density in 8 h. Under the same condi-
stand shear stress> 0.1 × 10 − 2 N m − 2 . To reach the same mechanical
reducing the current density by more than 90 %
Miniature membraneless glucose / oxygen BFCFinally, further evi-
7.2, containing 20 mM phosphate, 0.14 M NaCl at 37° C) 1,16,24,42 – 47 .

In addition, bioelectrocatalysts made of CNFs are more stable than their CF counterparts. It has been shown earlier that the incorporation of hydrophilic conductive graphite particles into highly crosslinked hydrogel produced a composite that could withstand diameter electrodes at 500 r.p.m. 40 . However, this also degraded the reversibility of the system. Incorporation of COOH-functionalized single-walled nanotubes in epoxy composites has also been shown to improve the mechanical performance of these composites and produced 40 % enhancement in shear strength 41 . At a rotation rate tions, the CNF loses only 18 % of its initial current density ( Fig. 5 ). Aft er 8 h, the current of bioelectrocatalyst-modifi ed CNFs is more than 10 times higher than that of the modifi ed CF. Th is result shows that the bioelectrocatalyst is strengthened by CNFs through the formation of a strong three dimensional composite that can withstability, more than 25 % PEGDGE must be used with a modifi ed CF, dence of interest of these fi bres is shown by their use in a membraneless glucose / oxygen BFC operating in physiological conditions (pH In a glucose / oxygen BFC, glucose is electrooxidized at the anode,

Anode : glucose  gluconolactone + 2H ++ 2e − (1)

Oxygen is reduced to water at the cathode,

Cathode : O 2 + 4H + 4e − 2H 2 O (2)
Figure 4| Polarization curves for oxygen reduction. Dependence of current
saline buffer, under air at 37° C, 5 mV s , loading 170 μ g.

density (I) on voltage (V) for a modifi ed CNT fi bre electrode (thick line) and a modifi ed carbon fi bre electrode (thin line). Quiescent phosphate-buffered

I (μA cm

Figure 3 | Background subtracted cyclic voltammograms of the ‘ wired ’ BOD

electrode. Dependence of current density (I) on voltage (V) for a CNT fi bre (thick line) and a carbon fi bre (thin line). Quiescent phosphate-buffered saline buffer, under argon atmosphere at 37 ° C, 20 mV s , loading 170 μ g.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1000 ARTICLE

NATURE COMMUNICATIONS | 1:2 | DOI: 10.1038/ncomms1000 | w.nature.com/naturecommunications 5 resulting in the net bioelectrochemical power-generating reaction

Th e cell used in this study 48 is made of two 7μ m diameter,
phosphate buff er solution containing 0.14 M NaCl at 37° C. Th e
maximum power density for the CNT BFC is 740μ W cm − 2 at a
voltage of+ 0.57 V, which is fourfold higher than the power den-
sity obtained for the CF BFC (180μ W cm − 2 at + 0.45 V). It is also
open-circuit voltage (OCV) for the CF BFC was+ 0.73 V, whereas
it was+ 0.83 V for the CNT BFC. Th e OCV provides a measure of
with CFs

Cell : 2 glucose + O 2 2 gluconolactone + 2H 2 O (3) 2 cm long, bioelectrocatalyst-coated CFs or CNFs. Th e carbon and CNF cathodes were those described above, with their characterizations as shown in Figure 3 . Both anode types were made of GOx from Aspergillus niger and a corresponding redox polymer 36,37,48 . Th e dependence of power density on the operating voltage of the assembled CNT (black circles) and of the assembled CF (empty circles) BFCs is shown in Figure 6 in a quiescent air 20 mM curves exhibit the typical shape of the BFC power curve with three regions corresponding to (1) the activation losses at low current density governed by the activation overpotential that arises from the kinetics of electron transfer reactions; (2) the ohmic losses that arise from the resistance of BFC and depend on the materials used and the interface between polymers / enzymes and electrodes; and (3) the concentration polarization at high current density that depends on mass transport 49 . At 15 mM glucose concentration, the almost two orders of magnitudes larger than the power of a recently reported BFC using GOx and BOD as bioelectrocatalysts 20 . Th e the maximum voltage associated with the BFC, which depends on enzymes, polymers and electrode materials. In an ideal case, OCV should be equal to the diff erence of the thermodynamic potentials of the involved redox species. Th e signifi cant increase in OCV of the CNT glucose / oxygen BFC refl ects a more effi cient electrical wiring of enzymes to the CNT electrode surface, compared with the CF BFC, because of a poorer electronic connection to the electrode. Th e better electrical connection to the CNT results in lower energetic barriers for electron transfer from enzymes to the electrode. Th is also explains why the maximum of the operating voltage of the BFC is observed at a higher voltage for CNFs, compared

Discussion Because the sizes of microelectronic circuits and sensors are constantly reduced, the size of the low-power sensor-transmitter packages for physiological research and medicine becomes limited by the size of their power source. Miniature membraneless glucose / oxygen BFCs may power such devices. For example, such BFC would power a subcutaneously implanted, continuous glucose monitor for diabetes management. Operating such devices at a high-power density requires a large number of enzymes effi ciently wired to the anode or the cathode and a fast enzyme turnover rate. It also requires that mass transport of products and reactants to / from electrodes should not be limited by diff usion and that the size of the BFC be small enough to be eventually implanted.

Th e engineering of new porous and hybrid microfi bres made of oriented and biofunctionalized CNTs addresses these challenges. CNFs are highly porous, with pores being large enough to allow permeation of the electrolyte without inducing unwanted concentration polarization, such as localized pH gradients that could aff ect enzyme activity. Aft er 150 h of operation, the cell only lost 20 % of its power density. Compared with CFs, the use of CNT electrodes has fi ve main advantages: (1) the high specifi c surface that yields a higher power density; (2) more effi cient connections of redox polymers and enzymes to electrode materials; (3) porosity that allows permeation of the species without unwanted polarization of concentrations; (4) reduction of the overpotential required for glucose electrooxidation and oxygen electroreduction; and (5) stabilization of bioelectrocatalysts.

braneless BFC generated 740μ W cm − 2 at + 0.57 V, exceeding by far
BFC generating 1.45 mW cm− 2 at + 0.3 V, which is twice as high as

Because of the selectivity of our bioelectrocatalysts and the use of CNFs, we succeeded in achieving a high-power density with miniaturized (0.04 m 2 ) and membraneless BFC. Th e memthe power density obtained for CF membraneless BFCs. Recently, Sakai et al. 24 also reported a high-power glucose / oxygen membrane the power generated by the present BFCs. However, to reach such power densities, membranes were implemented to separate anodic and cathodic compartments because of the use of soluble redox mediators. Even though such BFCs exhibit a high-power density, they are not designed for the same applications and implantable devices. Indeed, the use of membranes and separation of anodic and

Figure 5| Stability of the modifi ed electrodes. Dependence of relative
37° C, + 0.3 V / Ag / AgCl, under 1 atm oxygen and vigorous stirring,
loading 170μ g.

stability ( % ) on time (h) for a CNT fi bre electrode (solid circles) and a carbon fi bre (open circles). Phosphate-buffered saline buffer at

P (μ W cm

Figure 6| Power density curves. Dependence of power density (P) on

operating voltage (V) for a biofuel cell made with carbon fi bre (open circles) and CNT fi bre (solid circles) in a quiescent phosphate-buffered saline buffer, under air, 15 mM glucose at 37 ° C, loading 226 μ g.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1000

6 NATURE COMMUNICATIONS | 1:2 | DOI: 10.1038/ncomms1000 | w.nature.com/naturecommunications cathodic compartments prevent extreme miniaturization down to micron-sized devices.

performance biosensors and electrochemical devices

In summary, the adequate engineering of CNTs into microwires can open up new synthetic routes for novel electrodes that overcome mass transport limitations and provide high specifi c areas. Th e current density can be enhanced by increasing the surface area through CNTs while maintaining high rates of species transport and excellent structural stability. Th e obtained electrodes exhibit properties that largely exceed the properties of currently available electrodes made of CFs. Th is was shown through the electroreduction of oxygen to water and through the construction of a high-power, miniature, membraneless BFC. Under air and in a quiescent solution, a modifi ed CNT microwire functions more than ten times better than a conventional CF. Th e maximum power density of a CNT BFC is fourfold higher than the power density obtained for a CF BFC. In addition, the overpotential, at any current density, is lower than that observed at platinum. Finally, apart from its straightforward interest for electrochemistry, because of the improved properties in terms of current density, reactivity and stability, we expect this novel class of electrodes to be a starting point for the elaboration of various high-

ChemicalsBOD (EC 1.3.3.5) from Trachyderma tsunodae was purchased from
Amano and purifi ed as described previouslyGOx from Aspergillus niger (EC
previously describedPoly(ethylene glycol) (MW 400) diglycidyl ether(PEGDGE)

Methods 1.1.3.4, 208 U mg ) was purchased from Fluka ( Sigma-Aldrich ) and purifi ed as was purchased from Polysciences , poly(vinyl alcohol) (PVA, MW 195,0) from Kuraray Europe GmbH and SDS from Aldrich and used as received.

alkylanated-2,2 ′ bi-imidazole) ] were reported earlierSingle-walled CNTs
Millipore ultrapure water
InstrumentationMeasurements were performed using a potentiostat ( CH Instru-
was prepared by fi xing the CNF fi bre or the CF on a SEM stage using carbon tape
Fibre synthesisCNTs were fi rst purifi ed by acid treatment before producing
fi bres. Typically, 250 mg of CNTs was heated at 200° C in an oven for 8 h, then

Th e synthesis of the BOD-wiring redox polymer PAA-PVI-[Os(4,4 ′ -dichloro- 2,2 ′ -bipyridine) Cl] and that of the GOX-wiring polymer PVP-[Os( N , N ′ - produced from a high-pressure CO disproportionation (HiPco) process were purchased from Carbon Nanotech . All aqueous solutions were prepared with ments , model CHI 760C) and a dedicated computer. A platinum spiral wire was used as counter electrode and all potentials were referred to a Ag / AgCl (3 M KCl) electrode. Either CNFs or CFs were used as the working electrode. Th e morphology of the CNFs and CFs was characterized by a fi eld emission scanning electron microscope (Model JSM-6700F, JEOL ) operated at a voltage of 5.0 kV. Th e sample placed in a fl ask. A concentration of 1 l HCl (37 % , v / v) was poured into the fl ask and incubated for 4 h under agitation. Th e colour of the solution became yellow because of the dissolution of Fe ions. Aft er suction fi ltration, the tubes were washed several times with deionized water. Th ey were then lyophilized.

interface. Th is is why fi bres were heated at a temperature of 600° C for 2 h under
was 9.5μ m. Th e CFs used for comparisons have a diameter of 7 μ m.
Electrode preparationTh e CNF electrode was synthesized as follows: (1) a single

Th e CNFs are made by a particle coagulation spinning process. CNTs were dispersed and stabilized in aqueous solutions of SDS (1.0 wt % ). Homogeneous dispersions were achieved by sonication. A relatively concentrated aqueous CNT suspension (0.3 wt % ) is injected through a cylindrical syringe into the co-fl owing stream of a coagulating bath containing PVA solution (5 wt % ). As a result, CNTs aggregate and form gel-like fi bres. Th e gel fi bres are washed several times with pure water to remove most of the surfactant and some fraction of PVA. Th ey are then pulled out of water, dried and collated into a composite CNT – PVA fi bre. As-produced CNFs consist of an interconnected network of PVA chains and CNTs. Th ey exhibit a high toughness. However, they are not suitable as such for electrochemical applications. Indeed, PVA limits the porosity of fi bres and the access to the CNT argon atmosphere to completely remove the PVA. Aft er this thermal treatment, fi bres are electrically conductive and only composed of CNT without any additives or binders. Th ey exhibit a high porosity and are therefore suitable for electrochemical applications. Th e SEM images of the CNF before and aft er removing PVA are shown below. Th e diameter of the fi bres can be controlled by the fl ows in the spinning process. In the present experiments, the diameter of the investigated fi bres fi bre was placed in a 1 m polycarbonate groove. (2) One end of the fi bre was fi xed by epoxy and the other end was electrically connected to a copper wire with conductive carbon paint. (3) Th e carbon paint was allowed to dry and then insulated with a layer of epoxy. (4) Before coating the bioelectrocatalysts, the CNF

(Parte 1 de 2)

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