Positively charged carbon electrocatalyst for enhanced power performance of L-ascorbic acid fuel cells

Accepted Manuscript

Positively charged carbon electrocatalyst for enhanced power performance of L-ascorbic acid fuel cells

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

Myounghoon Choun , HyeJin Lee , Jaeyoung Lee PII: DOI: Reference:

S2095-4956(16)30073-0 10.1016/j.jechem.2016.05.006 JECHEM 168

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

12 April 2016 13 May 2016 16 May 2016

Please cite this article as: Myounghoon Choun , HyeJin Lee , Jaeyoung Lee , Positively charged carbon electrocatalyst for enhanced power performance of L-ascorbic acid fuel cells, Journal of Energy Chemistry (2016), doi: 10.1016/j.jechem.2016.05.006

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Positively charged carbon electrocatalyst for enhanced power performance of L-ascorbic acid fuel cells Myounghoon Chouna, HyeJin Leeb, Jaeyoung Leea,c,*

Electrochemical Reaction and Technology Laboratory, School of Environmental Science and Engineering, GIST, Gwangju 500-712, South Korea

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Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 80 Daehakro, Bukgu, Daegu, 41566, South Korea

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Ertl Center for Electrochemistry and Catalysis, Research Institute for Solar and Sustainable Energies (RISE), GIST, Gwangju 500-712, South Korea

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Abstract

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Carbon surface with large oxygen and carbon ratio (O/C) indicated an outstanding electrocatalytic activity toward L-ascorbic acid oxidation, compared to platinum group metals.

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However, interrelation of surface functional groups and its electro-catalytic activity is still unclear. In this paper, we prepared different levels of oxidized carbons by a simple acid

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treatment and investigated the correlation between the surface oxygen functional groups of acid-

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treated carbon and electro-catalytic activity in an electro-oxidation of L-ascorbic acid. Positively charged carbon was demonstrated by lone pair electron of oxygen from valence band spectra study. It was revealed that the positively charged carbon, especially involved in carbonyl, showed enhanced the electro-catalytic activity through both better adsorption of negatively charged reactants and lowered LUMO by electronegativity of oxygen.

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Keywords: Vitamin-C electrooxidation; Electronegativity; Carbon; Atomic charge; Surface oxygen

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* Corresponding author. E-mail: [email protected]; [email protected].

This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial

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resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030031720).

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1. Introduction Human implantable devices consume more high energy as its functionality is required not only healing but also monitoring the state of a patient. Currently, the primary battery is typically used but its short life-time and low energy capacity can follow-up patient surgery for replacement,

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especially for energy intensive requirements involving both patient monitoring and healing. On the other hand, direct liquid fuel cells (DLFCs) could be applied owing to a couple of advantages such as high energy density, less toxic and continuous operation. For this approach the electrocatalytic oxidation of various kinds of small and human friendly organic molecules has

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been intensely studied [1–24].

L-ascorbic acid (Vitamin-C) has been considered as a biologically friendly fuel, since the product of the electro-oxidation of L-ascorbic acid, dehydroascorbic acid (DHA), is reversely

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reduced to L-ascorbic acid inside the human body. L-ascorbic acid fuel cells can be a potential power source for human implantable devices [21,22], but only a few studies have been reported

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[18,20–24]. Fugiwara et al. reported that the electro-oxidation of L-ascorbic acid using only carbon showed outstanding electro-catalytic activity [22,23]. In addition, we have recently

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reported that a large ratio of surface O/C atoms could improve the electro-catalytic activity

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toward L-ascorbic acid oxidation due to the enhanced wettability and capacitance [21]. However, the interrelation between surface functional groups and electro-catalytic activity towards

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Vitamin-C is still not clearly revealed. Herein we demonstrate a correlation between different surface densities of functional groupsinvolving carbon and catalytic activities via electrochemical and physicochemical measurements of as-received carbon (ARC) and acidtreated carbon (ATC). In addition, a remarkably enhanced power performance of ATC in an Lascorbic acid fuel cell system is also presented.

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2. Experimental In order to oxidize carbon, 1 g of as-received carbon (Timcal 150G) was added in a 500 mL round-bottom flask and 100 mL of 70% nitric acid (Sigma-aldrich) was added into the flask. Acid treatment of carbon was conducted at 75 oC, while stirring the solution. Acid treatments

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periods of 1, 3, 5, 7, and 10 h were employed to get different levels of oxidized carbon and these samples were referred as ATC-1, ATC-3 ATC-5, ATC-7, and ATC-10, respectively. After the acid treatment, the carbon was washed with distilled water until the pH became neutral to remove the residual nitric acid. Finally, the acid-treated carbon was dried in the oven at 60 oC.

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XPS measurements were conducted to analyze the surface functionality of as-received and acid-treated carbon. The experiment was performed in an ultrahigh vacuum (UHV) chamber with a base pressure of ≤ 5 × 10−10 Torr. Top up operation of 350 mA was used with the

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beamline of 8A2(Pohang Accelerator Laboratory) whose photon energy was set to 644.1 eV with binding energy resolution better than 200 meV. The binding energies of the spectra and fermi

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level were calibrated with respect to the binding energy of the Au foil. For fitting O 1s, Shirley

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type is applied as background and GL(50) is utilized. Changes in the surface area before and after acid treatment were investigated using BET surface area analyzer (BEL-Japan, BELSORP-

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max).

Electrochemical properties and Vitamin-C electro-oxidation activities of ARC and ATC were

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evaluated using a three-electrode cell connected to a potentiostat/gavanostat (Biologic, VSP). Pt wire and single junction Ag/AgCl (Pine) filled with 3 M KCl were used as counter electrode and reference electrode, respectively. Catalyst ink was prepared by 10 mg catalyst in a mixture of 10 µL of Nafion solution (Sigma-Aldrich, 10 wt%), 4 mL of distilled water, and 1 mL dimethylformamide (DMF) (Junsei). The catalyst ink was then sonicated for 5 min and 20 µL

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aliquot of the suspension was dropped on to glassy carbon disk electrode (0.196 cm2) using a micropipet so that the amount of the catalysts on the electrode is about 204 µg/cm2. CVs were recorded in N2 saturated 0.5 M NaClO4 (Sigma-Aldrich) at 25 oC. The scan rate was 50 mV/s and the potential window was from -0.1 to 0.8 V vs Ag/AgCl. LSVs were obtained in 0.5 M

to 0.8 V vs Ag/AgCl at a scan rate of 10 mV/s.

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NaClO4 containing 0.5 M Vitamin-C (Sigma-Aldrich) at 25 oC. The potential was swept from 0

To evaluate the activities of ATCs for Vitamin-C electro-oxidation in a fuel cell system, membrane electrode assemblies (MEAs) with an area of 9 cm2 were prepared as follows: ATC

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samples of 1 mg/cm2 were loaded with 10% Nafion solution (Sigma-Aldrich, 10 wt%) on gas diffusion layer (GDL) (Toray-060) for an anode electrode. Toray-060 which has high surface energy was selected for the anode GDL to efficiently diffuse Vitamin-C. As-received carbon was

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also tested for comparison. The cathode was constructed in a similar way as that of the anode with 46.7% Pt/C (Tanaka, 1.0 mg/cm2) on GDL (SGL, 10BC). Each anode and cathode

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electrodes were then placed on both side of a Nafion 115 membrane (Dupont) and the assembly was hot-pressed at a temperature of 140 oC while applying pressure of 3 MPa for 5 min. The hot

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pressing was conducted to minimize the contact resistance between the electrode andmembrane.

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Current voltage polarization was measured with a fuel cell test station (Scitech Korea Inc). A cell with parallel serpentine flow channel was used to evaluate the performance of the fuel cell. For

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current-voltage polarization curve, current was stepped up by 1 mA with a waiting interval time of 30 s for the voltage to be stable at each step. The cell temperature and relative humidity (RH) of cathode were fixed at 36.5 oC and 100%, respectively. The RH was controlled by heating humidity bottles and gas lines at 36.5 oC. The back pressures for both electrodes were ambient

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pressure, while the flow rates of 0.5 M Vitamin-C and oxygen were maintained as 4 and 100 sccm, respectively.

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3. Results and discussion Our investigation was based on the hypothesis that chemisorbed oxygen species are formed more reproducibly on carbon following acid treatment. Changes in surface area and functionality of ARC and ATC samples (following acid treatment from 1 to 7 h) were first analyzed by BET

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and XPS, respectively. From the BET analysis in Table 1, the surface area of ARC was 46.9 m2/g and increased up to 62.6 m2/g after acid treatment for 7 h (ATC-7). The largest increment in the surface area was observed between ATC-1 and ATC-3 and fairly similar surface areas were

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observed for ATC-3, ATC-5, and ATC-7.

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Table 1. Surface areas measured by BET analysis of ARC and ATC samples at different acid treatment times from 1 to 7 h.

Surface area (m2/g)

ARC

49.0± 2.0

ATC-1

50.4± 0.5

ATC-3

58.4 ± 0.1

ATC-5

57.9 ± 0.3

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Samples

62.1± 0.6

ATC-7

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The change in surface functionality due to acid treatment was investigated by deconvolution of O 1s XPS spectra for ARC and ATC samples. Qualitative information could be obtained from the

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binding energies of each peak. In addition, quantitative data is provided by normalizing each peak area. This was conducted by following three steps:

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(i) Atomic % calculation from survey spectra based on atomic sensitivity factor of carbon

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(0.296) and oxygen (0.711)  (C 1s area/0.296) or (O 1s area/0.711)/(C 1s area/0.296 + O 1s area/0.711)

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(ii) O/C ratio calculation O at.%/C at.% (iii) Normalized contents of functional groups from O 1s spectra  Carbonyl or hydroxyl peak

area/(highest intensity of O 1s spectra)  O/C ratio

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The deconvolution of the O1s spectra yielded both the hydroxyl (533.2 eV) and carbonyl (531.8~532 eV) peak profiles in the ARC and ATC samples as shown in Figure 1 [25]. The intensities calculated from each sample spectral analysis are compared in Table 2.

Samples

O/C

Carbonyl

Hydroxyl

C/O (at.%)

ARC

0.044

0.064

0.067

95.8/4.2

ATC-1

0.073

ATC-3

0.097

ATC-5

0.111

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at various acid treatment times from 1 to 7 h.

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Table 2. XPS intensities calculated by integrating each peaks of ARC and ATC samples applied

ATC-7

0.075

0.120

93.2/6.8

0.160

0.151

91.1/8.9

0.160

0.163

90/10

0.123

93/7

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0.108

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0.112

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Figure 1. Deconvolution of O 1s XPS spectra of ARC, ATC-1, ATC-3, ATC-5, and ATC-7.

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XPS peaks: (red) carbonyl and (navy) hydroxyl.

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Analysis of the XPS data clearly shows that the O/C ratio increased after acid treatment. The enhanced oxygen intensity of ATC signified the formation of carbonyl and hydroxyl groups [26]. In particular, ATC-5 had the highest O/C ratio and also contained more carbonyl and hydroxyl groups than the other ATC samples. ATC-3 showed a lower O/C ratio, but the same intensity of carbonyl groups compared with ATC-5.

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In order to evaluate the density of states (DOS) of ARC and ATC samples, valence band spectra were analyzed as shown in Figure 2. There are peaks due to O 2s electrons (~27 eV) and the C 2s electrons (~17 eV) [27]. After acid treatment, the O 2s peak near 27 eV is enhanced, which accords with the XPS results in Table 2. Moreover, distinct changes were shown near 3

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eV in ATC-3 and ATC-5, highlighted by the red dashed line in Figure 2. According to results reported by Loh and Ando, ~3 eV is assigned to an electron lone pair on oxygen [28]. Since the lone pair results in non-bonded O 2p, we can expect that more positively charged carbon atoms

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are induced in ATC-3 and ATC-5 than the other ATC samples.

Figure 2. Valence band spectra of (1) ARC, (2) ATC-1, (3) ATC-3, (4) ATC-5 and (5) ATC-7.

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Changes in the capacitance with respect to acid treatment time were also evaluated by

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measuring cyclic voltammograms (CVs) of ARC and ATC samples in N2 saturated 0.5 M NaClO4 electrolyte at a scan rate of 50 mV/s (see Figure 3). Under these conditions there was negligible effect of the oxygen reduction current on the cathodic CV scan. The capacitance

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to between that of ATC-1 and ATC-3.

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increased as the acid treatment time increased up to 5 h (ATC-5), with the ATC-7 value dropping

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Figure 3. Cyclic voltammograms in 0.5 M NaClO4 of ARC, ATC-1, ATC-3, ATC-5, and ATC-

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7. Voltammograms were recorded at 50 mV/s.

Capacitance values are strongly related to both the electrode surface area and the surface

functional groups present. This is because charge is stored physically at the electrode surface without a chemical phase change taking place. In addition, the charge stored between the

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electrode and electrolyte is affected by the electrode surface energy, which is affected by the density and type of functional groups present. Further understanding of the CV measurements was supported by the BET and XPS results discussed above. A significant increase in capacitance was revealed after 1 h acid treatment

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(ATC-1). For ATC-7, in spite of a higherphysical surface area than ATC-3 and ATC-5, the capacitance of ATC-7 was similar to that of ATC-1, but smaller than that of ATC-3 and ATC-5. According to the results by Hsieh and Teng [29], this variation can be associated with a higher density of surface oxygen functional groups, which improves the capacitance.

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To investigate the activities of the ARC and ATC samples to the electro-oxidation of VitaminC, linear sweep voltammograms (LSVs) were evaluated at 10 mV/s in 0.5 M NaClO4 containing 0.5 M Vitamin-C (Figure 4). ATC-3 and ATC-5 showed the best activity toward Vitamin-C

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electro-oxidation. For ATC-1, slightly lower activity was observed than that of ATC-7.

Figure 4. Linear sweep voltammograms of ARC, ATC-1, ATC-3, ATC-5, and ATC-7. Voltammograms were recorded at 10 mV/s in 0.5 M NaClO4 containing 0.5 M Vitamin-C.

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The trend in activity for Vitamin-C electro-oxidation was expected to be similar to the capacitance trend, and this was found to be the case but with the exception of ATC-3. It has been shown previously that the surface oxygen intensity is important in improving electro-catalytic activity to the electro-oxidation of Vitamin-C [18,21,24]. However, ATC-3 was found to have a

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smaller O/C ratio than that of ATC-5, yet the electrocatalytic activities of both samples were similar. This led us to conclude that although oxygen functional groups enhance the activity toward Vitamin-C electro-oxidation to a certain extent, there is another unaccounted factor that can explain these seemingly counterintuitive results.

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To further investigate the interrelation between electro-oxidation activity toward Vitamin-C and the surface oxygen functional groups, current densities at 0.4 V from the LSVs for ARC and ATC samples were compared with oxygen intensities calculated by integrating the carbonyl and

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hydroxyl XPS peaks, as shown in Figure 5.

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Figure 5. Current density at 0.4 V (left axis) obtained from LSVs and oxygen peak area in XPS (right axis) calculated by integration of carbonyl and hydroxyl peaks of (1) ARC, (2) ACT-1, (3)

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ACT-3, (4) ACT-5, and (5) ACT-7.

The current densities associated with acid-treatment were drastically improved compared to the ARC sample due to the enhanced O/C ratio, which results in higher surface hydrophilicityand active area. Enhanced activities were shown in all of theATC samples, with different levels of

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enhancement in current densities observed depending on the surface oxygen functional groups present. We expected that the variation in the enhancement of current densities originated from the differently induced positively charged atom depending upon the type of oxygen functional groups. This is mainly due to positively charged atoms promoting the adsorption of negatively

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charged reactants, as suggested in Scheme 1 [20].

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Scheme 1. Suggested mechanism for the electro-oxidation of Vitamin-C.

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Bader charge analysis [30–32] has also been applied to calculate the charge densities of atoms adjacent to oxygen in carbonyl and hydroxyl groups and help understand the effect of positively charged carbon atoms on the electro-catalytic activity to Vitamin C electro-oxidation. Y. Liu et al. calculated the partial charge densities of various oxygen-containing functionalized graphite surfaces using Bader charge analysis [30]. According to the results, charge densities of carbon

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atoms adjacent to oxygen in carbonyl were 0.995 and 0.919 while the adjacent carbon and hydrogen for hydroxyl groups were 0.355 and 0.574, respectively. The relative importance of hydroxyl groupscould be demonstrated by comparison between ATC-3 and ATC-5. ATC-3 has less hydroxyl content but similar carbonyl content compared to

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ATC-5. The catalytic activity of ATC-3 and ATC-5 shows similar values. This observation can be understood basedon positively charged carbonatoms in carbonyl and connected to hydroxyl groups enhance more adsorption of a negatively charged Vitamin-Creaction intermediate. In addition to charge density, this argument can be further supported with molecular orbital (MO)

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theory. Because of the electronegativity of oxygen atoms, the energy of π* C=O is smaller compared to that of π * C=C and σ * C–O. It clearly demonstrates that a lower LUMO promoted by the carbonyl functional group could enhance the electro-catalytic oxidation kinetics

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of Vitamin-C.

The final criterion for the preparation of catalysts is their successful application as electrode

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materials in a fuel cell system with a zero-gap configuration [18]. Since this energy conversion system is considered as a potential energy source for implantable medical devices, testing was

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carried out at a temperature of 36.5 oC. As shown in Figure 6, single cells performance is clearly

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improved after acid treatment. To minimize the membrane resistance and improve diffusion of fuel, we conducted single cell performance test at 80 oC and also stability test (Figure S-1 and S-

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2). Both ARC and ATC showed big improvement as the cell temperature increased. Especially, outstanding improvement is observed in ATC-5. The improvement could be caused by better affinity of ATC-5, since fuel affinity of catalyst layer become more significant factor which influence on catalytic activity in single cell system. The much larger difference of current density

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in a half-cell experiment between ARC and ATC samples was observed because of full ionic

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movement and fuel diffusion.

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Figure 6. Polarization curves comparing MEAs prepared with ARC, ATC-3, and ATC-5. The cell temperature was 36.5 oC, and the back pressures for both anode and cathode were at ambient pressure. The flow rates of 0.5 M Vitamin-C and oxygen were 4 and 100 sccm, respectively.

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4. Conclusions

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Our correlation studies between surface oxygen functional groups of carbon and Vitamin-C electro-oxidation activity indicate that carbonyl and hydroxyl groups dominantly promote the

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adsorption of negative charged reactants onto positively charged carbon surfaces. In particular, the carbonyl functional group is a more efficient activity promoter, as supported by experimental

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data and MO theory. Furthermore, the ~80 % enhanced performance in a single cell highlights

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the importance of both careful control and better understanding of the carbon surface chemistry in realising the great potential of Vitamin-C fuel cells as a power source for human implantable devices.

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Acknowledgments This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial

Appendix A. Supplementary data

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resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030031720).

I-V and I-P polarization curves and stability results at 80 oC. Supplementary material related to

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this article can be found in the online version.

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Graphical abstract

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Vitamin-C fuel cells, a potential power source with high power density and long life time for human implantable devices.

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