Synthetic Metals 142 (2004) 217–221

The application of polypyrrole fibrils in hydrogen evolution reactionXiaoping Mo, Jixiao Wang∗, Zhi Wang, Shichang Wang

State Key Laboratory of Chemical Engineering, Chemical Engineering Research Center, School of Chemical Engineering and Technology,Tianjin University, Tianjin 300072, China

Received 30 June 2003; received in revised form 3 September 2003; accepted 3 September 2003

Abstract

The behavior of the polypyrrole (PPy) fibrils modified electrode during hydrogen evolution process is reported. The experimental resultsshow that the hydrogen evolution current density could be significantly increased on the PPy fibrils modified electrode. The hydrogenevolution current density on the modified electrode increases with PPy fibrils getting longer. The hydrogen evolution current density onfibrillar PPy modified electrode is higher than that on electrode modified with PPy with cauliflower form. Redoping PPy fibrils with metalcomplex could further increase the hydrogen evolution current density.© 2003 Elsevier B.V. All rights reserved.

Keywords:Hydrogen evolution reaction; Polypyrrole fibril

1. Introduction

The electrochemical production of hydrogen by waterelectrolysis is an important method for pure hydrogen sup-ply. The main operating cost of the hydrogen evolution is thecost of electricity. Consequently, research and developmentefforts have been directed towards minimizing ohmic resis-tance, lowing over-potential (enhancing electrolysis currentat a certain potential) through improving cell and electrodedesigns and using electrode material with higher electrocat-alytic activity. As an electrochemical reaction, the hydrogenevolution reaction (HER) is essentially a surface reaction.Its apparent current density can be significantly improved byusing high electrocatalytic activity electrode material and/orincreasing the effective surface area of the applied electrode.

Decreasing the over-potential by increasing the effectivesurface area of the electrode is seldom reported except theuse of porous nickel. The rough surface and micro-voidproperties of PPy[1] make the electrode have much largereffective surface area than naked electrode. Moreover, themicro-voids provide a place for forming metal nanoparticlesthat might have high catalytic activity towards the corre-sponding reaction. Metals immobilized on/in PPy has showngood electrocatalytic properties towards many electrochem-ical reactions[2–7]. The catalytic properties of noble metalsdispersing in PPy were researched by several groups[8–12].

∗ Corresponding author.E-mail address:jxwang@tju.edu.cn (J. Wang).

Modified with PPy fibrils, the electrode should have largereffective surface area and lower diffusion resistance thanthat modified with PPy with cauliflower form[13]. Recently,PPy fibrils were successfully prepared on the electrode sur-face by a simple method in our laboratory[14]. Here, wereport the experimental results of increasing apparent cur-rent density by modifying the electrodes with PPy fibrilsand further modified with metals. The results indicate thatthe modification of the electrode can significantly increaseapparent current density in hydrogen evolution reaction.

2. Experimental

Pyrrole was purified by distillation under nitrogen at-mosphere before use, stored at low temperature and pro-tected from light. All other reagents were of analytical gradeand used without further purification. De-ionized water wasused throughout this work. The polymerization of pyrrolewas conducted in a one-compartment cell using a saturatedcalomel electrode (SCE) as the reference electrode, platinumwire as counter electrode, and a graphite/paraffin compositerod with diameter 8 mm as researching electrode. PPy fib-rils were obtained potentiostatically at 0.80 V versus SCEin previously degassed aqueous solution containing 0.15 Mlithium perchlorate, 0.10 M sodium carbonate and 0.10 Mpyrrole. The experiments were performed on TD73000 elec-trochemistry System controlled by a computer. The SEMphotographs of PPy were taken using Philip XL30.

0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.synthmet.2003.09.002

218 X. Mo et al. / Synthetic Metals 142 (2004) 217–221

The above mentioned PPy fibrils modified electrodes werereduced and dedoped at potential−1.00 versus SCE for20 min. The redoping of the PPy fibrils with iron complexwere carried out by oxidizing the PPy at 0.85 V versus SCEin a solution containing 0.10 M Prussian Blue for a cer-tain time to obtain PPy(Fe(CN)3−

6 ) electrode. In the sameway, the electrode modified with PPy fibrils doped withnickel (PPy(Ni-EDTA)) or copper (PPy(Cu-EDTA)) com-plex could be prepared by redoping PPy fibrils in Ni-EDTAor Cu-EDTA solution.

The behavior of the modified electrode was characterizedin 30% KOH aqueous solution at a given temperature in aone-compartment cell. During the experiments, a SCE wasused as the reference electrode, platinum network with sur-face about 2.0 cm2 as counter electrode.

3. Results and discussions

3.1. Morphology of PPy

Fig. 1shows the state of the naked and modified electrodesurface, respectively. The scanning electron microscopy(SEM) pictures clearly show that the naked graphite elec-trode surface is flat (Fig. 1a). PPy electrosynthesized bypassing 80 mC/cm2 of charge is in nodular form (Fig. 1c).PPy nodules grow longer by passing more charge. LongPPy fibrils form by passing 650 mC/cm2 of charge (Fig. 1e).The diameters of the fibrils are about 100 nm. For compari-son, PPy with cauliflower form was also electrosynthesizedfrom a solution without sodium carbonate by passing about180 mC/cm2 of charge, as shown inFig. 1b. It is obviouslythat the effective surface area of the modified electrode islarger than that of the naked electrode. The electrode mod-ified with fibrillar PPy should have larger effective surfacearea than that of the electrode modified with PPy withcauliflower form when same amount of charge passes. Thefigure also shows that with more polymerization chargepassing, the effective surface area of the fibrillar PPy mod-ified electrode becomes larger.

3.2. Behaviors of PPy fibrils modified electrodes

Fig. 2 shows the potential–current relations on the nakedand PPy modified electrode in 30% KOH at 25◦C. Bothkinds of PPy were electrogenerated by passing almost thesame amount of polymerization charge (about 180 mC/cm2).The figure indicates that at low electrolysis potentials, thecorresponding electrolysis current densities on the PPy mod-ified electrode are lower than those on the naked electrode.This might come from the lower electroactivity of PPy thanthat of graphite. However, at high electrolysis potentials, thecorresponding electrolysis current densities on the PPy mod-ified electrode are higher than those on the naked electrode.This might be attributed to the larger effective surface areaof the modified electrode. The current densities on the elec-

Fig. 1. The SEM pictures of naked and PPy modified graphite electrodesurface: (a) Naked electrode, polished with 1200 emery paper; (b) PPy withcauliflower form electrogenerated in a solution without sodium carbonateby passing about 180 mC/cm2 of charge; PPy fibrils electrogenerated in asolution with sodium carbonate by passing different amounts of charge;(c) 80 mC/cm2; (d) 180 mC/cm2; (e) 650 mC/cm2.

1.6 1.8 2.0 2.2 2.4 2.60

50

100

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300

Cur

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den

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A/c

m2 )

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modified with PPy fibrils modified with PPy with cauliflower form Naked electrode

Fig. 2. Comparison of current–potential relations on different cathodes in30% KOH (w/w) solution at 25◦C.

X. Mo et al. / Synthetic Metals 142 (2004) 217–221 219

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100

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400

500C

urre

nt d

ensi

ty (

mA

/cm

2 )

-Potential vs SCE (V)

Polymerization charge 650 mC/cm2

180 mC/cm2

80 mC/cm2

Fig. 3. The current–potential relations on electrodes modified with PPyfibrils electrosynthesized by passing different amounts of polymerizationcharge in 30% KOH (w/w) solution at 25◦C.

trode modified with PPy with cauliflower form are lowerthan those on the PPy fibrils modified electrode through thewhole process. This might come from the larger effectivesurface area of the fibrillar PPy than that of the PPy withcauliflower form.

Varying the polymerization time can effectively controlthe polymerization charge. At the same polymerizationpotential, the longer the polymerization time is, the morepolymerization charge passes. In our experiments, PPy fib-rils were synthesized by passing three different amountsof polymerization charge. Their SEM pictures are shownin Fig. 1c–e. Fig. 3 presents the current–potential relationson these modified electrodes. The figure indicates that theHER electrolysis current density on the fibrillar PPy mod-ified electrode increases with the polymerization chargeincrease. This is attributed to the increase of the effectivesurface area. Here, the growth pattern of PPy fibrils isone-dimensional[14]. So, by passing more polymerizationcharge, the fibrils grow longer and the effective surface areaof the PPy fibrils modified electrode becomes larger. Thisis obviously shown inFig. 1c–e.

Fig. 4 shows the current–potential relations on electrodemodified with PPy fibrils at different temperatures. PPyis electropolymerized by passing 650 mC/cm2 of charge.As can be seen, the electrolysis current density increaseswith the increase of the temperature. This is attributedto the increase in the rate of charge transfer betweenthe cathode-electrolyte interfaces. Since water electrol-ysis cells operate at relatively high temperatures about50–70◦C, it is practical interest to test the stability ofthe PPy fibrils in this temperature range. The figure illus-trates that the electrode modified with PPy fibrils workswell at high temperature. This result indicates that it isfeasible to use the PPy fibrils modified electrode at hightemperature.

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55 ˚C 40 ˚C 30 ˚C

Fig. 4. The current–potential relations on PPy fibrils modified electrodein 30% KOH (w/w) solution at different temperatures.

3.3. Behaviors of electrodes modified with PPy fibrilsdoped with various metal complexes

Nickel is one of the most frequently used electrode ma-terial for practical applications in HER, mainly due to itsimproved catalytic activity and much lower cost than no-ble metals. Iron[15] and copper[6] could also be used aselectrode materials in HER. The catalytic activity of thesemetals is known to depend highly on their dispersion andsurface properties. A high degree of dispersion and largesurface area can enhance their catalysis. Porous structureand high surface area of PPy favors their use as matrices forthe immobilization of the dispersed metal catalysts to de-velop new electrocatalytic materials. Because of its relativehigh electric conductivity, it is possible to shuttle the elec-trons through polymer chains between the electrodes anddispersed metal particles, where the electrocatalytic reactionoccurs. Thus, an efficient electrocatalysis can be achievedon these composite materials.

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PPy(Fe(CN)6

3-) PPy fibrils

Fig. 5. The current–potential relations on different electrodes in 30%(w/w) KOH solution at 25◦C.

220 X. Mo et al. / Synthetic Metals 142 (2004) 217–221

Fig. 6. The energy spectrum of PPy(Fe(CN)3−6 ) electrode.

The electrolysis current–potential relations of the elec-trode modified with PPy fibrils and the electrode furthermodified with iron complex (PPy(Fe(CN)3−

6 )) is presentedin Fig. 5. Fig. 6 is the energy spectrum of PPy(Fe(CN)3−

6 )electrode. It is obviously that iron complex was doped inPPy. From the energy spectrum it could also be deduced thatthe concentration of iron was 4.41% (w/w). The electrol-ysis current density on PPy(Fe(CN)3−

6 ) electrode is muchhigher than that on the electrode only modified with PPyfibrils. This might be explained by the formation of ironnanoparticles dispersing on/in the PPy fibrils under the neg-ative potential or iron complex just as in pyrolysis productof iron–porphyin[16]. During hydrogen evolution process,the doped iron complex was reduced. Because of the porousstructure and the interaction of the iron complex and PPyskeleton, the particles were prevented to grow large. Thus

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PPy(Ni-EDTA) PPy(Fe(CN)

6

3-) PPy(Cu-EDTA) PPy fibrils

Fig. 7. The current–potential relations on the electrodes modified withPPy fibrils doped with different metal complex ions in 30% KOH (w/w)solution at 25◦C.

nanoparticles formed and dispersed on/in PPy fibrils. Thesenanoparticles should have high catalytic activity towards hy-drogen evolution for their high surface activities and largeeffective surface.

HER on the electrode modified with PPy fibrils dopedwith nickel (PPy(Ni-EDTA)) and copper (PPy(Cu-EDTA))complex was also studied, respectively. The current–potentialrelations on the electrodes modified with PPy fibrils dopedwith different metal complex are shown inFig. 7. It isobviously that the electrolysis current density on the elec-trode modified with PPy fibrils doped with metal complexis higher than that on the electrode only modified withPPy fibrils. However, the increase extents are different fordifferent metals. The current density on the PPy(Ni-EDTA)electrode is the highest. The lowest is the current densityon the PPy(Cu-EDTA) electrode. This might be attributedto the different catalytic activity of the metals. The resultsillustrate that the PPy fibrils are good carriers to immobilizethe dispersed metal or metal complex. The developed elec-trodes modified with PPy fibrils doped with metal complexhave good catalysis towards HER.

4. Conclusions

The present work demonstrates that modifying the elec-trode with PPy fibrils could increase the electrolysis cur-rent density of hydrogen evolution reaction at high poten-tials for its high effective surface area. The increase ex-tent is higher than those on the electrode modified withcauliflower PPy. Redoping PPy fibrils with metal complexcould further increase the hydrogen evolution current den-sity significantly. This might be attributed to iron complexor the formation of metal nanoparticles dispersed on/in PPyfibrils. The results show that fibrillar PPy is also a kind of

X. Mo et al. / Synthetic Metals 142 (2004) 217–221 221

excellent substrate used to disperse metal or metal complexcatalyst.

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