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Electrochimica Acta 127 (2014) 53–60

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

ynthesis and electrocatalytic activity of phosphorus-doped carbonerogel for oxygen reduction

iao Wua, Zhenrong Yanga, Qijun Sunb, Xiaowei Li c, Peter Strasserd, Ruizhi Yanga,∗

School of Physical Science and Technology, School of Energy, Collaborative Innovation Center of Suzhou Nano Science and Technology,oochow University, Suzhou, Jiangsu 215006, ChinaCollege of Nanoscience & Technology, Soochow University, Suzhou, Jiangsu 215006, ChinaInstitute of Chemical Power Sources, Soochow University, Suzhou, Jiangsu 215006, ChinaThe Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division,echnical University Berlin, 10623 Berlin, Germany

r t i c l e i n f o

rticle history:eceived 16 August 2013eceived in revised form 29 January 2014ccepted 2 February 2014vailable online 17 February 2014

eywords:

a b s t r a c t

The electrocatalyst for oxygen reduction reaction (ORR) plays an important role in determining the per-formance, cost and durability of fuel cells and metal–air batteries. In this study, low-cost and highly activephosphorus (P)-doped carbon xerogel electrocatalyst for the ORR was facilely synthesized. The catalyticactivity of P-doped carbon xerogel for the ORR in 0.1 M KOH solution has been studied by using a rotat-ing ring-disk electrode (RRDE) technique. The RRDE results show that P-doped carbon xerogel exhibitsexcellent catalytic activity for the ORR and long-term stability in basic media. The ORR on P-doped carbon

lectrocatalystsxygen reduction reactionhosphorus (P)-dopingarbon xerogel

xerogel with optimized amount of P mainly favors a direct four electron pathway. The high electrocat-alytic activity and durability of P-doped carbon xerogel are primarily attributed to the P-doping in thecarbon lattice. Furthermore, the amount of P incorporated into carbon instead of the specific surface areaof the P-doped carbon xerogel is found to play a critical role in the ORR activity enhancement and theORR pathway modification.

. Introduction

Electrocatalytic oxygen reduction reaction (ORR) has attracteduch attention due to its importance in fuel cells and metal–air bat-

eries [1–4]. The ORR in fuel cells and metal–air batteries requireatalysts to accelerate the reaction rate owing to its sluggish kinet-cs and complicated reaction mechanism. Pt-based catalysts arenown to be the most active catalysts. However, Pt is scarce andxpensive. The development of low-cost and efficient catalysts forhe ORR is highly desirable [5–9].

Heteroatoms-doped carbon materials have been explored aslternative electrocatalysts for the ORR due to their more abun-ance, lower price and high activity [10–16]. Heteroatoms doped

nto carbon can change the physical and chemical properties ofarbon, forming new active sites, which are beneficial to the ORR.oping the carbon materials by heteroatoms, such as nitrogen (N)

11–14], boron (B) [10,13,15,16] and sulfur(S) [17–19], has beeneported to improve the electrocatalytic activity of carbon for ORR.mong these dopants, nitrogen has been widely studied since

∗ Corresponding author. Tel.: +86 512 65221519.E-mail address: yangrz@suda.edu.cn (R. Yang).

ttp://dx.doi.org/10.1016/j.electacta.2014.02.016013-4686/© 2014 Elsevier Ltd. All rights reserved.

© 2014 Elsevier Ltd. All rights reserved.

it can provide n-doping and increase the conductivity of carbon[20]. Furthermore, nitrogen doping can change the electronicstructure of carbon [12,21] and expose more carbon edges [22,23],which may contribute to the enhancement of ORR activity ofcarbon. Recent theoretical and experimental studies have shownthat phosphorus (P) doping is also a promising approach inimproving the electrocatalytic activity of carbon towards ORR[22,24–34]. Liu et al. have reported that P-doped graphite layerand P-doped multiwalled carbon nanotubes exhibited promisingelectrocatalytic activity for ORR in alkaline media [25,26]. Li et al.have reported P-doped graphene as efficient electrocatalyst forORR [27]. High catalytic activity of P-doped ordered mesoporouscarbons for ORR has also been reported by Yang et al. [33]. Inour previous work, phosphorus-doped microporous carbon wasprepared with phosphoric acid (H3PO4) as the phosphorus sourcein the presence of Co. The effect of P-doping and Co introducedduring the synthesis of the P-doped carbon on the catalytic activityof carbon for the ORR was primarily investigated [35]. However,factors that influence the ORR activity of P-doped microporous

carbon still remains incompletely understood.

To better understand the catalytic activity of P-doped micro-porous carbon for the ORR, we report the activity of P-doped carbonxerogel and the factors affect the activity of P-doped carbon xerogel

5 mica A

ipaRaIcatcop(aPcbfaaTbTecJfOdcp

2

2

pctHoasGwbdonrow=8owpa1ssrwt

4 J. Wu et al. / Electrochi

n detail. Carbon xerogels are chosen in this work due to their facilereparation, controllable structure, high electronic conductivitynd low costs as compared with other carbon materials [36–40].ecently, Jin et al. have reported a nitrogen-doped carbon xerogels a highly efficient electrocatalyst for the ORR in PEM fuel cells [41].t was reported that P-doping into carbon resulted in a negativeharge density in the carbon atoms because of the lower electroneg-tivity of P (2.19) than that of C (2.55) and therefore a high activityowards the ORR [25]. Furthermore, the structural modification ofarbon by P-doping will be more effective than that by N-dopingr B-doping because P has a much larger covalent radius (107 ± 3m) than C (73 ± 1 pm) as compared with N (71± 1 pm) and B84 ±3 pm) [30]. This is important in the enhancement of the ORRctivity of carbon.Despite the advantages of carbon xerogels and-doping, there has been no report on the development of P-dopedarbon xerogel electrocatalyst for ORR. In this work, P-doped car-on xerogels are synthesized via a sol–gel polymerization methodollowed by a pyrolysis and P doping process, in which resorcinolnd formaldehyde were used as the carbon sources, cobalt nitrates a catalyst for P-doping, and H3PO4 as the phosphorus source.he electrocatalytic activities of the as-synthesized P-doped car-on xerogels for the ORR in alkaline media have been investigated.he as-synthesized P-doped carbon xerogels exhibited a high ORRlectrocatalytic activity with long-term stability outperforming theommercially available Pt/C catalyst (20 wt % Pt on Vulcan XC-72R,ohnson Matthey). Most importantly, the effect of the specific sur-ace area and the amount of P doped into carbon on the catalyticRR performance have been studied in detail. The amount of Poped instead of the specific surface area is found to be a criti-al factor in improving the ORR activity and modifying the ORRathway for P-doped carbon xerogels.

. Experimental

.1. Sample Preparation

The P-doped carbon xerogels were synthesized via a sol–gelolymerization method followed by a pyrolysis and P doping pro-ess, in which resorcinol (R) and formaldehyde (F) were used ashe carbon sources, cobalt nitrate as the P-doping catalyst, and3PO4 as the phosphorus source. In a typical experiment, 0.1 molf resorcinol and 0.2 mol of formaldehyde were mixed togethernd dissolved in 50 ml of deionized water to form a homogeneousolution by stirring. Then 0.0025 mol of Co(NO3)2 ·6H2O (≥99.0%,uoyao Chemical Reagent Co. Ltd.) was added to the solution. After-ards, aqueous ammonia solution was added into the solution drop

y drop until a sol-gel polymer is formed. The sol-gel was thenried and cured in a vacuum oven at 85 ◦C for 7 days. The powderf sol-gel after grinding was carbonized in a tube furnace under aitrogen atmosphere with the temperature increased to 800 ◦C at aate of 5 ◦C min−1 and kept at 800 ◦C for 1 h. The carbonized samplebtained was abbreviated to Co-C. For P-doping, phosphoric acidas added to the Co-C with a weight ratio of phosphoric acid: Co-C

1:10, 2:10, 3:10, 4:10, 5:10, respectively. The mixture was kept at5 ◦C for 3 hrs. The solvent in the mixture was removed in an evap-rator operated at 85 ◦C and 300 mbar. The paste mixture obtainedas then dried overnight in an oven at 85 ◦C. After grinding, theowder was loaded into a graphite boat and then pyrolyzed under

N2 atmosphere with a rate of 3 ◦C min−1 and kept at 800 ◦C for hour. The pyrolyzed samples were stirred in 500 mL of 1.0 M HClolution for 12 h to dissolve the residue Co in the carbon. Finally, the

amples were washed with ultrapure water until neutral pH waseached and then dried in an oven at 90 ◦C. The samples obtainedere abbreviated to P-C-1, P-C-2, P-C-3, P-C-4 and P-C-5, respec-

ively, where the number reflects the amount of phosphoric acid

cta 127 (2014) 53–60

added to Co-C. Pure carbon xerogel without P-doping was preparedwith Co-C sample stirred in 500 mL of 1.0 M HCl solution for 12 hto dissolve the residue Co in the carbon.

2.2. Physical characterizations

The crystal structure of the sample was examined with X–raydiffraction (XRD) using a Bede D1 X–ray diffractometer (UK, BedeScientific Ltd.; Cu K� radiation; operated at 40 kV, 45 mA; �=0.15418 nm), the diffraction angle ranging from 10◦ to 80◦ witha step of 0.02◦ and a rate of 1.2◦ min−1.

Raman spectroscopy of the sample was performed on a JobinYvon LabRAM HR 800 instrument with a 514 nm excitation laser ata power of around 1 mW.

Surface analysis of the samples was performed with a SSI (Sur-face Science Instruments) X-ray photoelectron spectroscopy (XPS)spectrometer equipped with a hemispherical analyzer and using amonochromatized Al K� (1486 eV) source with a 250 × 1000 �millumination spot. The measurement parameters were as follows:20 eV pass energy, 0.1 eV energy increments. The spectra were cor-rected for the background using the Shirley approach [42] and thecomposition of the films was determined by measuring the ratio ofC1s to P2p intensities (integrated peak area) normalized by theirrespective sensitivity factors [43].

The specific surface area and the pore structure of the sam-ples were analyzed by adsorption/desorption measurements ofnitrogen at 77 K (Quantachrome, QuadraSorb SI). Prior to mea-surements, the samples were degassed at 250 ◦C overnight undervacuum. Surface area was calculated by Brunauer-Emmett-Teller(BET) method, micropore volume from N2 sorption was calculatedusing the t-plot method. Pore size distributions were calculatedusing Barrett-Joyner-Halenda (BJH) method for mesopores andHorvath-Kawazoe (HK) method for micropores.

The morphology of the sample was examined with scanningelectron microscopy (SEM, FEI Quanta 200) equipped with EDS.

The amounts of P and residual metals in the carbons weremeasured with inductively coupled plasma-atomic emission spec-troscopy (ICP-AES) analysis (Vista MPX).

2.3. Electrochemical measurements

Inks of the catalyst samples were prepared by mixing 10 mg ofpowder, 10 �L of Nafion solution (5% wt from Aldrich), and 700 �Lof ethanol, followed by ultrasonicating for 40 minutes. Seven �Lof ink was pipetted onto a glassy carbon (GC) disk resulting in apowder loading of 503 �g·cm−2. The loading of the commercial Pt/C(20 wt % Pt on Vulcan XC-72, Johnson Matthey) is the same (i.e.503 �g·cm−2).

The electrocatalytic activity for the ORR of the samples on theGC disks was studied with the rotating ring-disk electrode (RRDE)technique using a Pine electrochemical system (AFMSRX rotator,and AFCBP1 bipotentiostat). The RRDE electrode consisted of acatalyst-coated GC disk (5 mm diameter, 0.196 cm2 of geometricsurface area) surrounded by a Pt ring (0.125 cm2 of geometric sur-face area). The electrochemical measurements were conducted ina standard three-electrode electrochemical cell at room tempera-ture. A Pt-foil was used as the counter electrode, and a Ag/AgCl (3 MCl-, Cypress) reference electrode was used in a double-junction ref-erence chamber. The electrolyte was 0.1 M KOH solution preparedfrom ultrapure water (Millipore, 18.2 M� cm). The working elec-trodes were the catalyst film-coated GC disks mounted in a disk-interchangeable rotating disk electrode (RDE, Pine Instruments).

The electrolyte was deaerated by purging high-purity Ar gasinto the electrolyte for at least 30 mins before each electrochemicalmeasurement. The samples on the GC disks were first electrochem-ically cleaned by sweeping the potential in the range between

mica Acta 127 (2014) 53–60 55

-Kosd

OvpfaaTstg

j

K

1

wkt

B

wtcc

Oaes

0ste

%

n

wa

3

3

xamapcaaas

10 20 30 40 50 60 70 80

e

f

d

c

b

Inte

nsity

2 / degree

C(0

02)

C(1

00)

a

vey spectrum confirms that the residual cobalt has been completelyremoved by acid wash. The detailed P 2p XPS spectrum of P-C-3is depicted in Fig. 3b. Two deconvolved contributions centered at132.5 eV (P1) and 134.5 eV (P2) reveal the presence of P-C bonding

f

0.88

1.24

0.92

1.31

1.17

e

d

c

b

Inte

nsity

G-bandD-band

a

1.08

J. Wu et al. / Electrochi

0.9 and 0 V (vs. Ag/AgCl) at 50 mV·s−1 in an Ar-saturated 0.1 MOH solution until steady state cyclic voltammograms (CV) werebtained. For each catalyst tested, a CV was first collected in Ar-aturated 0.1 M KOH solution from -0.9 to 0 V at 10 mV·s−1 toetermine the non-Faradaic current.

For the ORR test, the electrolyte was purged with high-purity2 gas for at least 30 mins to ensure O2 saturation. Linear sweepoltammetry (LSV) measurements during oxygen reduction wereerformed in O2-saturated 0.1 M KOH by sweeping the potentialrom -0.9 V anodically to 0 V at 10 mV s−1 with the electrode rotatedt 400, 900, 1600 and 2500 rpm and O2 gas purged into the solutiont a flow rate of 25 sccm through a 2 �m fritted tube (Ace Glass).he current due to the oxygen reduction alone, was obtained byubtracting the background current (the current measured fromhe CV under Ar) from the ORR data and then normalized by theeometric surface area,

= −(iORR − icapacitive,Ar−CV)/SAgeo (1)

The kinetic current density for the ORR was derived from theoutecky-Levich equation:

/j = 1/jk + 1/jd = 1/jk + 1/(B�1/2) (2)

here j is the measured disk current density; jk and jd are theinetic and diffusion limiting current densities, respectively; B ishe Levich slope, which is given by the following equation:

= 0.62nFD2/3O2

�−1/6CO2 , (3)

here n is the apparent number of electrons transferred in the reac-ion, F is the Faraday constant (96485 C mol−1), DO2 is the diffusionoefficient of O2 (DO2 = 1.86 × 10−5cm2 s−1), � is the kinetic vis-osity of the solution (� = 0.01 cm2 s−1), CO2 is the concentration of

2 dissolved in electrolyte (CO2 = 1.21 × 10−6mol cm−3) [44,45],nd � is the electrode rotation speed. The ohmic resistances in thelectrode contacts and electrolyte solution were assumed to be theame for the samples and were not included in the corrections.

For all the RRDE measurements, the ring potential was held at.5 V vs. Ag/AgCl in order to oxidize any H2O2 produced in alkalineolution [46]. The % HO2

− produced and the the electron number nransferred during the reaction were calculated using the followingquations [47–49].

HO−2 = 100

2IR/N

ID + (IR/N)(4)

= 4ID

ID + (IR/N)(5)

here ID is the Faradaic current at the disk, IR is the Faradaic currentt the ring, and N = 0.22 is the disk electrode collection efficiency.

. Results and Discussion

.1. Structure of P-doped carbon xerogels

Fig. 1 shows the X-ray diffraction (XRD) profiles of pure carbonerogel and the P-doped carbon xerogels (i.e. P-C). The P contentsre 0.78 at.%, 1.41 at.%, 1.64 at.%, 2.77 at.% and 3.56 at.%, as deter-ined from the ICP-AES analysis, for the P-C-1, P-C-2, P-C-3, P-C-4

nd P-C-5, respectively. All the samples show two broad diffraction

eaks at about 2�=25◦ and 43.4◦ corresponding to (002) and (100) ofarbon phase, respectively. The pure carbon xerogel exhibits a rel-tively sharp (002) peak at 2�=26◦, this sharp peak becomes weakfter 0.78 at.% P is doped into carbon and becomes broad as themount of P increases to 1.41 at.% and further, indicating that thetructure of carbon becomes more disordered after P-doping.

Fig. 1. XRD patterns of carbon xerogel and of P-doped carbon xerogels: (a) purecarbon, (b)P-C-1, (c) P-C-2, (d) P-C-3, (e) P-C-4 and (f) P-C-5.

In order to further confirm the structure of P-doped carbon xero-gels, a Raman spectrometer was used to study the structure. Thecomparison between the Raman spectroscopy of pure carbon xero-gel and P-doped carbon xerogels is shown in Fig. 2. The ratio ofintensities for the D-band and G-band (ID/IG) is generally used as ameasure of carbon disorder [50]. As shown in Fig. 2, there are twopeaks centered at approximately 1596 cm−1 and 1327 cm−1, whichare G-band and D-band, respectively. The ID/IG of P-doped carbonxerogels increases as the P content increases. The ID/IG is 0.88 forpure carbon xerogel, it increases to 0.92 after 0.78 at.% P is dopedinto carbon and increases to 1.31 as the amount of P increases to3.56 at.%. This suggests that more defects are introduced in carbonas the P content increases. These results are in good agreement withthe XRD results (Fig. 1).

The binding envrironments of P in carbon xerogels are investi-gated by X-ray photoelectron spectroscopy (XPS). The XPS surveyspectrum of a typical P-C (P-C-3) given in Fig. 3a shows the mainpeaks at 284.9 eV, 534.2 eV and 133.8 eV, corresponding to C1s, O1s,and P2p, respectively. The absence of any Co peak in the XPS sur-

1000 1200 1400 1600 1800 2000 2200Raman shift / cm-1

Fig. 2. The D-band and G-band in Raman spectra for carbon xerogel and P-dopedcarbon xerogels: (a) pure carbon, (b)P-C-1, (c) P-C-2, (d) P-C-3, (e) P-C-4 and (f)P-C-5. The ratio of D-band to G-band (ID/IG) is indicated for each sample.

56 J. Wu et al. / Electrochimica Acta 127 (2014) 53–60

0 200 40 0 60 0 80 0 100 0 1200

C1s

O1s

Binding Energy / eV

Inte

nsity

P2p

O

a

140 138 13 6 13 4 13 2 13 0 128 12 6

P2

P1

b

Inte

nsity

Bin ding ener gy / eV

P2p

Fs

[dimrtl[

osxBoatm

TBa

Fig. 4. BET specific surface area and the pore size distributions (inset) of carbon

ig. 3. (a) XPS survey spectra of a typical P-doped carbon xerogel (P-C-3); (b) XPSpectra for the P 2p peak of P-C-3.

25,51,52] and P-O bonding [51], respectively, in the as-prepared P-oped carbon xerogels. The results suggest that the P atom has been

ncorporated into the carbon lattice. The presence of P-O bondingeans the formation of P-O-C in carbon. Moreover, P has a covalent

adius of 107 ± 3 pm (much larger than that of C, 73 pm) and favorshe sp3-orbital configuration in molecules. Therefore, P-O-C mostikely existed as tetrahedral forms such as C3PO, C2PO2, and CPO327].

The nitrogen sorption isotherms and pore size distributionsf the P-doped carbon xerogels are shown in Fig. 4. The nitrogenorption isotherm and pore size distribution of the pure carbonerogel are also included in Fig. 4 for comparison. The determinedET specific surface area, total pore volume and average pore size

f all investigated samples are summarized in Table 1. The nitrogendsorption-desorption isotherm of pure carbon xerogel exhibitedype I isotherm, showing the characteristic feature of a microporous

aterial. All the P-doped carbon xerogels samples exhibited a

able 1ET specific surface area, total pore volume and average pore size of carbon xerogelnd P-doped carbon xerogels.

Samples BET surfacearea/m2g−1

Total porevolume/m3g−1

Averagemesopore/microporesize/nm

Pure carbon 332.8 0.13 - -/0.45P-C-1 695.8 0.36 3.51/0.52P-C-2 813.3 0.42 3.66/0.54P-C-3 906.5 0.49 3.79/0.55P-C-4 972.4 0.58 3.89/0.56P-C-5 1166.9 0.77 4.12/0.58

xerogel and P-doped carbon xerogels: (a) pure carbon, (b)P-C-1, (c) P-C-2, (d) P-C-3,(e) P-C-4 and (f) P-C-5.

combined type I and IV isotherm. The typical hysteresis loops in theP/P0 range of 0.5–0.7 for these samples indicate that P-C samplescontain not only micropores but also mesopores. It can be seen fromTable 1 that the pure carbon xerogel displays 332.8 m2 g−1 of BETspecific surface area. After being doped with P, the BET specific sur-face area increases to 695.8, 813.3, 906.5, 972.4 and 1166.9 m2g−1,for the P-C-1, P-C-2, P-C-3, P-C-4 and P-C-5, respectively. The totalpore volume (0.13 m3g−1) of the pure carbon xerogel has beenincreased after P-doping, it varies between 0.36 and 0.77 m3g−1

for the P-C samples. The average mesopore size increases from3.51 to 4.12 nm, and the average micropore increases from 0.52 to0.58 nm, for the samples P-C-1 to P-C-5. The increases in the spe-cific surface area, total pore volume and average pore size of the P-Csamples are primarily due to phosphoric acid activation becausephosphoric acid is an activating agent for the carbon activation[53,54].

The scanning electron microscopy (SEM) images of a typical P-doped carbon xerogel (P-C-3) are shown in Fig. 5. The rough surface(Fig. 5 a) and a high degree of porosity (Fig. 5 b) are developed inP-C-3, which are consistent with the results of BET measurements.

3.2. Catalytic activity of P-doped carbon xerogels for oxygenreduction

Cyclic voltammetry (CV) of the P-doped carbon xerogels in 0.1 MKOH solution under N2 atmosphere are shown in Fig. 6. The CVof pure carbon xerogel is included for comparison. The shape ofthe CV curves is not an ideal capacitive CV response. The broadreduction–oxidation peaks between -0.9 and -0.1 V most likelycome from oxygen-containing functional groups in P-C-3[55] sinceoxygen exists in the sample as confirmed with XPS(Fig. 3a).It canbe seen from Fig. 6 that the current from the reduction–oxidationpeak and electrochemical double layer of the pure carbon has beenincreased after P-doping. The current increases as the P contentincreases from 0.78 at.% to 3.56 at.%. The increases in the currentfor P-C samples as compared with pure carbon and the variations inthe current for different P-C samples are consistent with the vari-ations in the specific surface areas of these samples as shown inFig. 4 and Table 1.

The comparison between electrocatalytic activities for ORR of

the P-doped carbon xerogels as measured with RRDE are shownin Fig. 7. Pure carbon and commercial Pt/C are also includedfor comparison. As can be seen from Fig. 7B, the catalytic activ-ity of pure carbon is significantly increased after P-doping. The

J. Wu et al. / Electrochimica Acta 127 (2014) 53–60 57

al P-d

OCO(iOOsoccawcrasrHtaitc(et

Fpi1

D). These results indicate that P-doping has significantly enhancedthe ORR activity and modified the ORR pathway for the carbonxerogel.

0.00

0.02

0.04

0.06

0.08

-0.8 -0.6 -0.4 -0.2 0.0-7-6-5-4-3-2-101

a b c d e f g

A

1600 rpm

0.1 M KOH

B

I R / m

Aj D /

mA

cm

Potential vs. (Ag/AgCl) / V

Fig. 5. SEM images of a typic

RR activity increases as follows: pure carbon < P-C-1 < P-C-2 < P--5 <P-C-4 < P-C-3, as evidenced by the onset potentials for theRR and the diffusion limiting current densities of these samples

Fig. 7B). The onset potential is -0.30 V and the diffusion limit-ng current density is -2.35 mA cm−2 for pure carbon. The bestRR activity is obtained for the P-C-3, the onset potential for theRR of which is -0.13 V and the diffusion limiting current den-

ity (-6.01 mA cm−2) of which reaches that of Pt/C. A negative shiftf about 70 mV exists in the half-wave potential of the P-C-3 asompared to Pt/C. It should be noted that the diffusion limitingurrent density and half-wave potential of the Pt/C are in goodgreement with the values of Pt/C (20 wt. % Pt) reported else-here [56,57]. In order to verify the ORR catalytic pathways of the

atalysts, the formations of peroxide species (HO2−) and the cor-

esponding electron number transferred during the ORR processre monitored with RRDE measurements (Fig. 7C and D). The mea-ured HO2

− yield is 67− 91% for pure carbon over the potentialange of -0.80 − -0.30 V, it decreases after P-doping. The measuredO2

− yield decreases to 49 − 55% for P-C-1 and further decreaseso below ∼15% for P-C-3. However, it then increases to 27 − 33%nd 44 − 47%, respectively, for P-C-4 and P-C-5. The correspond-ng electron number transferred during the ORR process varies inhe reverse order, which is in the range of 2.10 −2.61 for purearbon xerogel and in the range of 3.00 −3.78 for P-C samples

Fig. 7C and D). The measured HO2

− yield (below ∼15%) and thelectron transfer number (3.69 − 3.78) for P-C-3 are very close tohose for commercial Pt/C, HO2

− yield for which is below ∼9% and

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2-3

-2

-1

0

1

2

Potential vs. (Ag/AgCl) / V

Cur

rent

Den

sity

/ m

A c

m

a b c d e f

ig. 6. Cyclic voltammograms of carbon xerogel and P-doped carbon xerogels: (a)ure carbon, (b)P-C-1, (c) P-C-2, (d) P-C-3, (e) P-C-4 and (f) P-C-5. All of the exper-

ments were conducted in Ar-saturated 0.1 M KOH at 298 K with a sweep rate of0 mV s−1.

oped carbon xerogel (P-C-3).

the electron transfer number for which is 3.90 − 3.96 (Fig. 7C and

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.22.0

2.5

3.0

3.5

4.0

20

40

60

80

100

Potential vs. (Ag/AgCl) / V

n

C

D

%H

O- 2 a

b c d e f g

1600 rpm

0.1 M KOH

Fig. 7. (A) Ring current and (B) disk current density obtained with linear sweepingvoltammograms (LSVs) on rotating ring-disk electrode (RRDE) for carbon xerogeland P-doped carbon xerogels in O2-saturated 0.1 M KOH. The disk potential wasscanned at 10 mV s−1 with the electrode rotated at 1600 rpm and the ring potentialwas fixed at 0.5 V. (C) Determined peroxide percentage and (D) calculated electrontransfer number (n) at various potentials based on the corresponding RRDE data in(A) and (B). The electrocatalysts are (a) pure carbon, (b)P-C-1, (c) P-C-2, (d) P-C-3,(e) P-C-4 and (f) P-C-5.

58 J. Wu et al. / Electrochimica Acta 127 (2014) 53–60

-1.0 -0.5 0.0 0.5 1. 0 1. 5-0.45

-0.4 0

-0.3 5

-0.30

-0.25

-0.20

-0.15

-0.1 0

-0.05

0.00

0.0 5

log lik/mA mg l

C P-C Pt/C

80 mV

65 mV

Pote

ntia

l vs.

(Ag/

AgC

l) / V 69 mV

Fig. 8. Tafel plots of pure carbon xerogel, P-C-3 and commercial Pt/C (20 wt.% Ptors

iTtpaa2soi

tcerThKs

FrO

-0.8 -0 .6 -0 .4 -0.2 0.0-7

-6

-5

-4

-3

-2

-1

0

1

Poten tial vs. (Ag/AgCl) / V

Cur

rent

Den

sity

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n carbon) derived by the mass transport correction of corresponding LSV dataecorded in O2-saturated 0.1 M KOH with a sweeping rate of 10 mV s−1 and a rotatingpeed of 1600 rpm.

The diffusion-current-corrected Tafel plots of specific ORR activ-ty of pure carbon, P-C-3 and commercial Pt/C are shown in Fig. 8.o construct the Tafel plots, the kinetic currents were derived fromhe mass-transport correction using eqn (2). At low over-potentials,ure carbon shows a Tafel slope of 80 mV. In contrast, P-C-3 shows

small Tafel slope of 65 mV, which is close to that of Pt/C (68 mV)nd approaches the theoretical value of 2.303RT/F (i.e., 59 mV at5 ◦C), where R is the universal gas constant, F is the Faraday con-tant, and T is absolute temperature. The similarity in Tafel slopesf ORR on P-C-3 and Pt/C indicates that ORR mechanism on P-C-3s similar to that on Pt/C [58].

The polarization curves for the ORR of P-C-3 at different rota-ion rates are shown in Fig. 9. They all reached diffusion limitingurrents. A small hump (i.e. current peak) at about -0.25 V at thelectrode rotating rate of 400 rpm most likely comes from theeduction of dissolved oxygen in the pores of the sample [59–61].

his peak current can be masked by the ORR current at sufficientlyigh rotation rates. The inset of Fig. 9 shows the correspondingoutecky-Levich plots obtained from the inverse current den-ity (j−1) as a function of the inverse of the square root of the

ig. 9. LSVs of oxygen reduction on P-C-3 in O2-saturated 0.1 M KOH with a scanate of 10 mV s−1 at different electrode rotating speeds. Koutecky–Levich plots forRR on P-C-3 in O2-saturated 0.1 M KOH solution is included as inset.

Fig. 10. LSVs of oxygen reduction on P-C-3 and on commercial Pt/C (20 wt.% Pt oncarbon) for the 1st cycle and 2000 th cycle in O2-saturated 0.1 M KOH at a rotatingspeed of 1600 rpm.

rotation rate(�−1/2) at -0.30, -0.40 and -0.50 V, respectively. Theseplots are linear and parallel, indicating the first-order dependenceof the kinetics of ORR on the P-C surface. Each straight line inter-cept corresponds to the kinetic current ik. The Levich slope forP-C is 0.42 mA cm−2 rad−1/2 s1/2. The electron number calculatedfrom the Levich slope is ∼3.81. This is consistent with the result(n≈3.69 − 3.78) obtained from the RRDE measurements, suggest-ing the ORR reactions on the surface of P-C proceed mainly withn = 4 e− reaction pathway.

The durability of P-C-3 and the commercial Pt/C were checkedby running CVs for 2000 repeated cycles. As shown in Fig. 10, theinitial half-wave potentials for the P-C-3 and the Pt/C are -0.22 Vand -0.15 V, respectively. After 2000 repeated cycles, the half-wavepotential loss for the P-C is 36 mV, which is much lower than thatof the Pt/C (82 mV). This reveals that the P-C-3 has better long-termperformance than Pt/C under the same test conditions, which mightresult from the strong covalent bond between C and P as well as theabsence of metal agglomeration and migration, an important factoraccounting for the degradation of Pt/C catalyst.

The results show that the P-doped carbon xerogels (P-C) exhibithigh catalytic activity for the ORR. The density functional theory(DFT) studies have shown the charged sites P+ induced by P-doping[62] and asymmetric spin density in carbon atoms encouraged byelectron pair in P [25] are most likely the active sites for the ORR. Aswe all know, a high edge exposure can be induced and more activesites can be provided with higher specific surface area of the P-doped carbon. Therefore, the specific surface area and the amountof P incorporated into carbon are two important factors influencingthe catalytic activity of the P-doped carbon xerogels for the ORR.The BET specific surface area increases from P-C-1 to P-C-5, so dothe total pore volume and the average pore size. However, P-C-3 exhibits the highest activity for the ORR. This suggests that theamount of P doped into carbon is more critical for the improvementof the activity of P-doped carbon xerogel. The ORR activity increasesas the P content increases from 0.78 at.% (P-C-1) to 1.64 at.% (P-C-3), but decreases as the P content increases further to 2.77 at.%(P-C-4) and 3.56 at.% (P-C-5). This demonstrates that an optimizedP content (1.64 at.% in this study) doped into carbon exists for theenhancement of the ORR activity and further P-doping will lowerthe electrocatalytic activity of carbon xerogel. This is due to thefact that high P-doping causes significantly larger distortions in the

hexagonal carbon framework and induces more defect sites (Fig. 1and Fig. 2), which could destroy the sp2-carbon network in the P-C structure and reduce the electrical conductivity of the carbon[63,64]. Therefore, the optimized amount of P doped into carbon is

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ery important for the ORR activity enhancement and ORR pathwayodification on P-doped carbon xerogels.The factors that influence the ORR activity of P-doped car-

on including specific surface area, pore volume/pore size and themount of P-doping have been studied in detail. The amount of Pncorporated into carbon instead of specific surface area and poreolume/pore size is crucial in improving the ORR activity and mod-fying the ORR pathway of P-doped carbon xerogels. The activity of-doped carbon for the ORR is better understood.

. Conclusions

In this paper, we reported a P-doped carbon xerogel synthesizedia sol-gel polymerization method followed by pyrolysis and Poping. The results show that P-doping could significantly enhancehe electrocatalytic activity of carbon xerogel for the ORR in alka-ine media. The stability of P-doped carbon xerogels was higherhan that of commercial Pt/C (20 wt % Pt on Vulcan XC-72, John-on Matthey). The high electrocatalytic activity and durability of-doped carbon xerogels for ORR are primarily attributed to the-doping in the carbon lattice. It is found that the amount of Pncorporated into carbon instead of specific surface area plays anmportant role in improving the ORR activity and modifying theRR pathway of P-doped carbon xerogels. This kind of low cost,ighly active and sustainable P-doped carbon xerogels could besed as a potential electrocatalyst for the cathodic ORR in fuel cellsnd metal-air batteries.

cknowledgements

This work is supported by National Natural Science Foundationf China (Nos. 51272167 and 21206101), Natural Science Founda-ion of the Higher Education Institutions of Jiangsu Province, ChinaNos. 12KJB430010) and Sino-German Network on Electromobility.

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