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The hydrophilicity of carbon for the performance enhancement of direct ascorbic acid fuel cells
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The hydrophilicity of carbon for the performance enhancement of direct ascorbic acid fuel cells Chenxi Qiu, Hemu Chen, Huiyuan Liu, Zihui Zhai, Jiaqi Qin, Yang Lv, Zhanming Gao, Yujiang Song* State Key Laboratory of Fine Chemicals & Laboratory of Electrochemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China
article info
abstract
Article history:
As a representative of low-temperature direct biomass fuel cells, direct ascorbic acid fuel
Received 9 April 2018
cells (DAAFCs) carry many advantages, including renewable fuel, easy transportation and
Received in revised form
storage, and high safety. However, a major challenge of DAAFCs confronting us is relatively
10 September 2018
low power density. Herein, to deal with this challenge, we treat carbon black (BP 2000) with
Accepted 26 September 2018
nitric acid at an optimal concentration (4 M), which is further employed as anodic elec-
Available online 26 October 2018
trocatalyst for AA oxidation with improved hydrophilicity. Consequently, hydrophilic AA molecules can more readily access the surface of the carbon electrocatalyst and donate
Keywords:
electrons. Furthermore, the electrocatalytic effect of acid-treated carbon for AA oxidation
Direct ascorbic acid fuel cells
reaction is quantitatively evaluated by the determination of activation energy, which has
Power density
not been assessed prior to this study. In a similar way, nitric acid treatment is also applied
Hydrophilicity
to gas diffusion layer (GDL) at the anode side. In addition, Nafion content in anodic elec-
Acid treatment
trocatalyst layer, single cell operating temperature, and hot pressing conditions for the
Anodic carbon electrocatalyst
fabrication of membrane electrode assembly (MEA) as well as membrane thickness are also optimized. A maximum power density of 31 mW cm2 is eventually attained at 80 C with anode ionomer content of 9.2% and hot pressing at 130 C and 6 MPa for 2 min. This power density is 1.72 times of that reported previously with carbon black as the anode electrocatalyst. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Currently, the consumption of traditional fossil fuels is facing many problems, such as environmental pollution, climate change, low efficiency and depletion of fuels [1]. Therefore, the development and utilization of alternative clean energy is imminent. Fuel cell is a type of clean energy technology, which can directly convert chemical energy into electric energy. Hydrogen and alcohol are often used as fuels due to the
high energy density [2e6]. However, hydrogen is not that convenient from the perspective of industrial preparation, storage and transportation and safety [7,8]. For alcohol fuel, it also faces some problems like fuel crossover, membrane swelling and toxicity [9e12]. Fortunately, direct ascorbic acid fuel cells may be a solution for all of the problems. Ascorbic acid, also known as vitamin C, is a powerful antioxidant naturally present in many fruits and vegetables, which is environmentally benign and biologically friendly
* Corresponding author. E-mail address: [email protected] (Y. Song). https://doi.org/10.1016/j.ijhydene.2018.09.213 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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[13,14]. DAAFCs offer several obvious advantages over other proton exchange membrane fuel cells (PEMFCs). Both AA and the released products of DAAFCs are non-toxic. As a solid substance, AA storage and transportation are convenient. Inexpensive electrocatalysts like carbon can be used for AA oxidation at the anode side. There is no crossover effect because of relatively large size of AA molecule [15]. Previously, the study on the electrochemical oxidation of AA is mainly related to electrochemical sensors, and there are only a few studies about the use of AA in fuel cells. The examined anodic electrocatalysts of DAAFCs include noble metal (e.g., Au, Ag, Pt, Pd, Ru, Ir) [16e18], polyaniline [19e21], polyaniline/TiO2 [22], porphyrin [23], carbon black [24,25] and carbon nanomaterials [26,27]. Because of low cost, large specific surface area, high electrical conductivity and relatively high electrocatalytic effect, carbon black has become an excellent choice as the anodic electrocatalyst for DAAFCs. At present, a primary challenge of DAAFCs is the relatively low power density. The highest power density of DAAFCs using proton exchange membrane reported previously is only 18 mW cm2 [28], which is much lower than that of fuel cells with methanol and hydrogen as fuels. To deal with this problem, in this study, carbon black as the anodic electrocatalyst was treated with nitric acid in that AA is highly water soluble and the anode side of DAAFCs inevitably requires certain hydrophilicity to facilitate fuel transport to the surface of carbon electrocatalyst. After optimizing the conditions of acid treatment, the oxygen containing functional groups on carbon black have significantly increased and the hydrophilicity was effectively improved. Furthermore, we quantitatively analyzed the electrocatalytic effect of acid-treated carbon black for AA oxidation by the determination of activation energy, which has not been done prior to this study. This clearly proved that the carbon does function as an electrocatalyst for AA oxidation. In a similar way, nitric acid treatment was also applied to the GDL at the anode side. As a result, the hydrophilicity of the GDL was also improved. In addition, we adjusted the ionomer content of anodic electrocatalyst layer, so that AA may well contact with the electrocatalyst. Moreover, hot pressing conditions of MEA, operating temperatures of single cells and membrane thickness were also optimized. Finally, a high power density of DAAFCs (31 mW cm2) was achieved, which is 1.72 times of that reported before with untreated carbon as anodic electrocatalyst.
Experiment Materials Ultrapure water (18.2 MU cm1 at 25 C) was obtained from Millipore water system (Synergy UV, France). Black Pearls 2000 (BP 2000) was purchased from Cabot. Pt/C (60 wt%, JohnsonMatthey) was used as cathodic electrocatalysts. Carbon paper (TORAY TGP-H-060, 0.19 mm in thickness) was obtained from TORAY. Nafion 212 and Nafion 211 membrane (DuPont) were used as membranes after pretreatment with 3 wt% H2O2
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for 1 h and 3 M H2SO4 aq. at 80 C for 1 h. Ascorbic acid (99.7%) was purchased from Sinopharm Chemical Reagent Co., Ltd.
Hydrophilic treatment of carbon catalyst and carbon paper Carbon was first treated with 3 M HCl for 1 h at 30 C to remove impurities, and then treated with different concentration of HNO3 (1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M) for 1 h at 80 C to enhance its hydrophilicity. The obtained carbon was further washed with copious amount of H2O until the pH value of filtrate is about 7. The carbon was dried in vacuum oven at 65 C for 4 h. Carbon paper was treated with different concentration of HNO3 (1 M, 3 M, 4 M, 5 M and 6 M) for 4 h at 80 C (before heating, carbon paper was soaked in acid for 30 min) to enhance its hydrophilicity. The carbon paper was washed with H2O until the filtrate reached neutral pH and then dried in a vacuum oven at 65 C for 10 h for further use.
Electrochemical measurements All the electrochemical measurements of the electrocatalysts were carried out on a CHI 760D electrochemical workstation (CH Instruments) in a standard three-electrode electrochemical cell with a rotating glassy carbon disk as the working electrode, a graphite rod as the counter electrode, and Hg/ Hg2SO4 reference electrode. All potentials in this study were referred to that of reversible hydrogen electrode (RHE). A 5.0 mm diameter glassy carbon disk (0.19625 cm2, PINE) was used in rotating disk electrode (RDE) experiments. The catalyst ink (1 mg mL1) was prepared by blending certain amount of the electrocatalysts with water, ethanol and Nafion perfluorinated resin solution (Vwather:Vethanol:VNafion ¼ 1:9:0.06) under mild sonication for 2 min in a water bath [29e31]. The suspension was pipetted onto the RDE and evaporated in air, resulting in an electrocatalyst loading of 10 mgC cm2. RDE tests were carried out at 25 C in N2-saturated 1 mM AA and 0.5 M H2SO4 aqueous solution. Cyclic voltammetry (CV) curves of the electrocatalyst were recorded at a rotation rate of 1600 rpm and a positive scan rate of 50 mV s1 from 0 to 1.0 V vs RHE.
Materials characterizations X-ray photoelectron spectroscopy (XPS) was recorded on a ESCALAB™ 250Xi photoelectron spectrometer (Thermo Fisher) with Al Ka x-ray as an excitation source. Thermogravimetric analysis (TGA, TA Instruments, Q600) was performed from room temperature to 750 C with a heating rate of 10 C min1 in dried air. Raman spectra were recorded using Raman spectrometer (DXR smart Raman) with 523 nm laser excitation. Ultravioletevisible spectroscopy (UVeVis) absorption spectra were collected using a Specord S600 spectrophotometer (Analytic Jena) with a quartz cuvette whose pathlength is 1 cm. Transmission electron microscope (TEM) analysis was performed on a Tecnai G2 Spirit (FEI) operated at 200 keV. The samples were dispersed in ethanol and then dropped on TEM sample grids (Beijing Zhongjingkeyi Technology Co., Ltd). N2 adsorption/desorption were measured at 77 K using a Quantachrome Quadrasorb-SI Analyzer, where BrunnerEmmetTeller (BET) method was used for surface
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area determination. The surface hydrophilicity of carbon black and carbon paper was analyzed using contact angle measuring instrument (KRUSS DSA100). Electrochemical impedance spectra (EIS) of single cells were measured under a cell potential of 200 mV using an electronic load (PLZ1004WH, Kikusui) and an electrochemical workstation (Metrohm PGSTA302N) in a frequency range from 10 kHz to 0.1 Hz at 25 C.
MEA fabrication and single cell test Fuel cell tests were performed on a house-made fuel cell system with an electronic load (PLZ1004WH, Kikusui). The fuel cell hardware (FX201-2D) with a geometric area of 4 cm2 was purchased from Kunshan Sunlaite New Energy Technology Co. Ltd, P. R. China. The anode electrocatalyst layer was premL1, pared by spraying catalyst ink (2 mgC Vwather:Vethano:VNafion ¼ 1:9:0.02) onto Nafion membrane with an electrocatalyst loading of 1 mg cm2. The cathode electrocatalyst layer was prepared by spraying catalyst ink (2 mgPt/C
mL1, Vwather:Vethano:VNafion ¼ 1:9:0.06) onto the other side of Nafion membrane with an electrocatalyst loading of 1 mg cm1. The membranes coated with anode and cathode electrocatalyst layer were sandwiched between two GDL (The anode side is carbon paper and cathode side is commercial GDL which consist of carbon paper and micro porous layer) by hot-pressing at different temperature (120 C, 125 C, 130 C, 135 C and 140 C) and pressure (3 MPa, 4.5 MPa, 6 MPa and 7.5 MPa) for 2 min. 0.5 M AA aqueous solution was continuously delivered to the anode side at a flow rate of 15 mL min1 by a peristaltic pump, while humidified oxygen (100% RH) was supplied to the cathode side (200 mL min1). The temperature of liquid fuel, O2, and single cell hardware was maintained constant at 80 C.
Results and discussion Previously, it has been suggested that certain type of carbon materials may function as anodic electrocatalysts for AA
Fig. 1 e (a) Contact angle analysis of carbon black samples treated with 1 M, 3.5 M, 4 M, 4.5 M, and 5 M nitric acid, respectively; (b) TGA curves of carbon black samples untreated and treated with 1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, respectively, with a heating rate of 10 C min-1. (Note: all the samples have absorbed water from air before TGA analysis; Inset: water loss of each carbon sample untreated and treated with different concentration of nitric acid).
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oxidation in acidic media [24,28]. As a matter of fact that AA molecule is highly water soluble (the solubility is 333 g L1 at 20 C) [32], and it might be a prerequisite to have carbon surface hydrophilic to some extent to allow AA molecules to easily access the carbon surface, thus being oxidized efficiently. To the best of our knowledge, such a study has not been conducted prior to our work. Herein, we chose commercial BP 2000 as the carbon material for concept approval mainly owning to its high surface area. It is well known that acid treatment is a simple and effective way to improve the hydrophilicity of carbon surface. Hence, different concentrations of nitric acid (1e5 M) have been employed to treat BP 2000 for 1 h. (See details in the experimental section above). The hydrophilicity change of acid-treated carbon surface was firstly investigated by contact angle analysis as shown in Fig. 1(a). With the increase of nitric acid concentration from 1 to 4 M, the hydrophilicity of carbon surface continuously increased as evidenced by the contact angle variation from 136.99 to 129.99 e122.49 . After the acid concentration further increased to 4.5 and 5 M, the hydrophilicity of carbon decreased as confirmed by the contact angle increase to 128.49 and 130.49 . This illustrates that acid treatment can effectively modify the hydrophilicity of carbon and there exists an optimum acid concentration (4 M). Below 4 M, possibly there are no enough oxygen-containing groups created on carbon. While above the optimal acid concentration, oxygencontaining groups might be removed in the form of released CO2 and CO [33], since 4.5 and 5 M nitric acid are much stronger oxidants and can further oxidize the oxygencontaining groups on carbon [34]. Additionally, TGA can also be used to investigate the hydrophilicity variation of acid-treated carbon based on the weight of adsorbed water by carbon samples. Before TGA tests, all of the carbon samples were placed in air with 95% humidity for 24 h to ensure sufficient water adsorption. As shown in Fig. 1(b), the weight change before 110 C can be attributed to water loss. The more water loss, the more water adsorbed by acid treated carbon from the humidified air and the better hydrophilicity of carbon surface. It appears that the carbon black treated with 4 M of nitric acid has the largest quantity of water loss before 110 C (Fig. 1(b), inset). This suggests that the highest hydrophilicity has been achieved by using 4 M of nitric acid to treat carbon black, which is consistent with the contact angle analysis. The oxygen-containing groups on carbon surface actually represent a type of defects, which can be investigated by Raman spectroscopy. As shown in Fig. 2(a), two broad bands reside at 1331 cm1 (D band) and 1586 cm1 (G band), respectively [35]. D band arises from disordered sp2 hybridized carbon, while G band is associated with graphitic carbon [36,37]. Based on the analysis of peak area, it is found that the carbon black treated with 4 M nitric acid possesses the largest ID/IG ratio among all the samples, implying the carbon treated with 4 M of nitric acid has more surface defects than other carbon samples. This agrees well with the contact angle measurements and the TGA analysis. It is necessary to identify the exact origin of the hydrophilicity of carbon surface. In this regard, XPS has been used to analyze oxygen-containing groups on carbon. Fig. S-1 shows that with the increase of acid concentration, O1s signal
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Fig. 2 e (a) Raman spectra of the carbon samples treated with 1 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, respectively; (b) high-resolution C1s spectra of the carbon samples treated with 1 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, respectively.
became stronger relative to C1s signal. This clearly shows that oxygen-containing groups on carbon surface did increase after acid treatment. It is worth pointing out that the O1s signal reached the largest value for the case of 4 M nitric acid treated carbon, which is in good agreement with the
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measurements above. Since O1s signal is weak, it is difficult to collect high-resolution O1s spectra and conduct deconvolution of the peaks. Alternatively, high-resolution C1s spectra can easily be de-convoluted into CeO (285.7 eV), carbonyl (288.1 eV) and carboxyl (290.2 eV), besides CeC (284.8 eV), to elucidate the origin of hydrophilicity as shown in Fig. 2(b) and Table 1. It is certain that the enhanced oxygen intensity of acid treated carbon originates from the formation of hydroxyl, carbonyl and carboxyl groups [38]. For each type of oxygen-containing group, 4 M acid treated carbon always contains more hydrophilic groups than other samples. This corroborates that 4 M of nitric acid is an optimal choice, leading to significant enhancement of hydrophilicity. On the other hand, a higher concentration of nitric acid may cause the further oxidation of oxygen-containing groups on carbon surface to form carbon dioxide and carbon monoxide, thus worsening the hydrophilicity. While a lower concentration of nitric acid might not create enough oxygen-containing species to improve the hydrophilicity. The effect of carbon hydrophilicity on AA oxidation was first investigated by recording CV curves at 50 mV s1 in 0.5 M H2SO4 aq. containing 1 mM AA as shown in Fig. S-2. The first anodic peak residing at around 0.45 V (vs. RHE) corresponds to the oxidation of AA [24]. The anodic and cathodic peak centering at 0.55 V and 0.45 V(vs. RHE), respectively, is coming from the oxidation and reduction of surface functional groups (e.g. hydroquinone/quinone) in H2SO4 solution [24,39]. The baseline correction of AA oxidation peak from 0.4 to 0.48 V is shown in Fig. 3(a), the AA oxidation current takes the order of 1 M＜2 M＜3 M＜3.5 M＜5 M＜4.5 M＜4 M. Fig. 3(b) shows that the single cells fabricated with acid treated carbon do possess significantly improved performance as expected. The single cell hardware is shown in Fig. S-3. Especially, it is noteworthy that the single cell fabricated with 4 M acid treated carbon exhibits the highest single cell power density of 29.6 mW cm2. This result is consistent with the CV test. It suggests that the hydrophilicity might be the origin of electrochemical performance. However, CV curves of 3.5, 4, 4.5 and 5 M nitric acid treated carbon all exhibit relatively large electrical double layer capacitance, which means a high surface area of the carbon materials. Therefore, there exists a possibility that the enlarged surface area because of acid treatment might be an additional contributor to the enhanced AA oxidation performance and single cell performance besides the hydrophilicity. According to TEM images (Fig. 4) and BET data (Fig. 4(c) and Table 2), 4 M nitric acid treated carbon has the largest total surface area and external surface area. However, compared with other samples, there is no much
Table 1 e Peak area percentage according to deconvoluted XPS spectra for the carbon samples treated with different concentrations of nitric acid. Samples 1M 3.5 M 4M 4.5 M 5M
Hydroxyl
Carbonyl
Carboxyl
15.82% 17.97% 20.64% 19.60% 17.93%
4.18% 4.42% 4.94% 4.47% 4.24%
3.43% 4.03% 4.27% 4.15% 3.97%
Fig. 3 e (a) CV curves of carbon samples treated with 1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid collected in N2-saturated 1 mM AA þ 0.5 M H2SO4 solution with positive scan between 0.4 and 0.48 V (vs. RHE) after baseline correction; (b) Polarization curves and power density curves for carbon electrocatalysts untreated and treated with 1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, tested at 80 C and using Nafion 211 membrane. 0.5 M AA was supplied to the anode at a flow rate of 10 mL min¡1, while humidified oxygen (100% RH) was supplied at a flow rate of 200 mL min¡1 at the cathode. Geometrical electrocatalyst layer area: 4 cm2, anode loading: 1 mgC cm¡2, cathode loading: 1 mgPt cm¡2 (60 wt% Pt/C). GDL is pristine without any treatment.
difference. Hence, the change of surface area might have a negligible effect on the electrochemical performance. Previous studies have proposed that carbon may function as a catalyst for the oxidation of AA [15,24], but no convincing quantitative information has been provided for the suggested catalytic effects like activation energy. We investigated the catalytic effect of carbon for AA oxidation by the determination of the activation energy of the AA oxidation. The concentration variation of AA in aqueous solution was measured at different temperatures with the aid of UVeVis spectra in the absence and presence of carbon (Fig. S-4 and S-5), respectively. The activation energy was calculated based on linear regressions. The activation energy of AA oxidation in air without the addition of carbon is 50.7 kJ/mol and the activation energy of AA oxidation in air with the addition of carbon is 34.8 kJ/mol. This clearly shows that carbon can effectively
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Fig. 4 e TEM images of the carbon before (a) and after (b) 4 M acid treatment; (c) N2 adsorption-desorption isotherms of the carbon samples treated with 1 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid.
catalyze the oxidation of AA and reduce the activation energy by about 30%. We realized that the transfer of AA at the anode side is through both electrocatalyst layer and GDL. Generally, the function of GDL is to support electrocatalyst layer and membrane and to conduct electrons and gas. A typical GDL is composed of carbon paper layer at the bottom and a microporous carbon black layer on the top. Since the existence of liquid in GDL is detrimental to the transfer of gas, GDL is usually hydrophobic to prevent the formation of liquid droplets by the addition of certain amount of polytetrafluoroethylene (PTFE) in the microporous layer. However, the hydrophobicity of GDL may decrease the performance of DAAFCs, because AA aqueous solution could be difficult to pass through the hydrophobic GDL. Hence, the hydrophilic treatment of anodic GDL was also carried out. We chose carbon fiber paper without microporous layer as anodic GDL of DAAFCs. Hydrophilic treatment steps were the same as that of carbon black, but with a longer processing
Table 2 e Surface areas measured by nitrogen adsorption/desorption for the carbon samples treated with different concentrations of nitric acid. Sample
1M 3.5 M 4M 4.5 M 5M
Specific surface area (m2 g1)
Specific micropore area (m2 g1)
Specific external surface area (m2 g1)
1433.8 1432.0 1445.5 1434.3 1410.6
978.3 967.0 955.4 953.0 935.1
455.6 465.0 490.2 481.3 475.6
time of 4 h. Fig. 5 shows acid treatment did significantly influence the performance of DAAFCs. With the increase of nitric acid concentration from 0 to 5 M, the performance of single cell continuously improved. Similar to the case of anodic carbon electrocatalysts, the acid concentration here also has an optimum value of 5 M. An acid concentration
Fig. 5 e Polarization curves and power density curves of DAAFCs fabricated with GDL after being treated with different concentrations of nitric acid (0 M, 1 M, 3 M, 4 M, 5 M, and 6 M), tested at 80 C and using Nafion 211 membrane. 0.5 M AA was supplied to the anode at a flow rate of 10 mL min¡1, while humidified oxygen (100% RH) was supplied to the cathode at a flow rate of 200 mL min¡1. Geometrical electrocatalyst area: 4 cm2, anode loading: 1 mgC cm¡2 (4 M nitric acid treated carbon), cathode loading: 1 mgPt cm¡2 (60 wt% Pt/C).
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Fig. 6 e Contact angle analysis of pristine GDL and GDL samples treated with 1 M, 3 M, 4 M, 5 M and 6 M of nitric acid for 4 h at 80 C.
lower or higher than 5 M led to a detrimental effect on the single cell performance. In order to further prove that the increase of single cell performance is due to the improvement of GDL hydrophilicity, the surface hydrophobicity was studied by contact angle analysis. As shown in Fig. 6, with the increase of acid concentration from 0 to 5 M, the contact angle of GDL becomes smaller and smaller. This indicates that the hydrophilicity gradually increases. 5 M nitric acid treated GDL has the smallest contact angle or the highest hydrophilicity, this is consistent with the single cell test. For the case of 6 M nitric acid treated GDL, the hydrophilicity decreased and so did the single cell performance. The possible reason might be the removal of oxygen-containing groups on carbon at a higher concentration of nitric acid like discussed before. In addition, the amount of Nafion in anodic electrocatalyst layer also affects AA transfer and single cell performance. Nafion serves to conduct proton and to bind electrocatalyst, membrane and GDL together. However, Nafion does not conduct electrons. For DAAFCs, the appropriate content of Nafion in anodic electrocatalyst layer needs to be studied [15]. As shown in Fig. 7, when the Nafion content is high, a thick layer of Nafion might cover the carbon surface. Consequently,
AA molecules cannot easily cross Nafion layer to be oxidized on carbon. At a low Nafion content, the Nafion layer is thin or not fully formed around the carbon (not shown) and AA molecules can readily cross the Nafion layer to be oxidized on carbon. The relationship between Nafion content in anodic electrocatalyst layer and DAAFCs performance was investigated. As shown in Fig. 8(a), with the decrease of the content of Nafion from 27.6% to 9.2%, the performance of DAAFCs became better and better. When the Nafion content reaches 9.2%, the performance appeared to the highest one. Further reduction of Nafion content to 5% led to the decrease of the performance. In this case, the low Nafion content may greatly affect the stability of membrane electrode assembly (MEA). One of the functions of the Nafion is to bind MEA tightly as the adhesive. With the decrease of Nafion content, the binding among the anodic electrocatalyst particles becomes unstable and some electrocatalysts may fall off from the electrocatalyst layer. This was confirmed by Fig. S-6. After running for 8 h, the MEA with a low anodic Nafion content obviously lost carbon electrocatalyst. In addition, a low Nafion content may also make electrocatalysts, GDL and proton exchange membrane contact loosely, resulting in the
Fig. 7 e Schematic diagram of diffusion of AA molecules through thick (a) and thin (b) layer of Nafion around carbon electrocatalyst.
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Fig. 8 e (a) Polarization curves and power density curves of DAAFCs fabricated with different anodic Nafion content (5%, 9.2%, 13.8%, 23%, and 27.6%) tested at 80 C using Nafion 212 as the membrane. 0.5 M AA was supplied to the anode at a flow rate of 10 mL min¡1, while humidified oxygen (100% RH) was supplied to the cathode at a flow rate of 200 mL min¡1. Geometrical area of electrocatalyst layer: 4 cm2, anode loading: 1 mgC cm¡2 (Untreated BP2000), cathode loading: 1 mgPt cm¡2 (60% Pt/C); (b) Nyquist plots of DAAFCs fabricated with different anodic Nafion content (5%, 9.2%, 13.8%, 23%, and 27.6%). GDL is pristine without any treatment.
increase of resistance of single cells and the degradation of single cell performance. EIS was used to study the charge transfer resistance of single cells fabricated with different anodic Nafion content. The diameter of a semicircle is related to charge transfer resistance in Nyquist plot. In general, a smaller diameter corresponds to a smaller charge transfer resistance, and thus fast electrode kinetics [40e42]. As shown in Fig. 8(b), the diameter of the semicircle of the single cell fabricated with 9.2 wt% anodic Nafion content is the smallest, which is consistent with the results of single cell test. Furthermore, we optimized hot pressing conditions for the preparation of MEAs, since the structure, porosity and the performance of MEAs can be changed during hot pressing process. A low temperature and pressure often causes insufficient integration of MEAs, thus increasing contact resistance. On the other hand, it can also cause the dehydration of Nafion membrane, when a high temperature and pressure is adopted [43]. Many studies on hot pressing conditions of hydrogen/oxygen fuel cells [44,45] and direct methanol fuel cells (DMFCs) [43] have been reported. However, the effect of hot pressing on the performance of DAAFCs has not been paid much attention. Herein, based on the previous studies, the hot pressing conditions of DAAFC were optimized. We studied the effect of temperature and pressure on the performance of DAAFCs. It was found that 130 C and 6 MPa are the best hot pressing conditions of DAAFC (Fig. S-7). In addition, we investigated the effect of operating temperature on the performance of DAAFC. Generally, temperature has a great influence on the performance of a single cell. The increase of temperature is helpful to improve the gas diffusion rate and electrocatalyst activity, to increase the proton conductivity, and to alleviate the electrode flooding [46,47]. As for DAAFCs, temperature also significantly improves the performance. With the increase of temperature, the oxidation of AA becomes faster. Fig. S-8 proves the high temperature is more favorable for DAAFCs. However, the Nafion membrane usually operates at a temperature of
0e80 C. Above this temperature, the proton conductivity of membrane will be severely reduced due to dehydration, and its thermal stability will also be decreased. Therefore, we did not examine a higher temperature after the temperature of a single cell reached 80 C. Finally, we tested two kinds of cation exchange membranes, Nafion 212 and Nafion 211. The main difference between them is the thickness, which is 50 mm and 25 mm, respectively. Proton transfer in a thin membrane is relatively easy, thereby reducing single cell internal resistance and improving the performance. As shown in Fig. S-9, DAAFC performance has been improved significantly by using thin membrane Nafion 211.
Conclusion Regarding the hydrophilicity of AA molecules as fuel, anodic carbon electrocatalyst (BP 2000) and GDL have been treated with different concentrations of nitric acid. Under optimum conditions of acid treatment, oxygen-containing groups or hydrophilicity reaches a maximum value, which has been investigated by TGA, Raman, XPS, TEM, and nitrogen adsorption/desorption. The improved hydrophilicity allows AA molecules to easily diffuse and contact with carbon electrocatalysts, leading to much enhanced performance of DAAFCs. We have quantitatively evaluated the activation energy of AA oxidation on carbon and confirmed that carbon does function as the electrocatalysts for AA oxidation, which has not been done prior to this study. In addition, Nafion content in anodic electrocatalyst layer, hot pressing conditions, operating temperatures and membrane thickness have been optimized. Eventually, DAAFCs show a maximum power density of 31 mW cm2, which is 1.72 times of those reported before using carbon as the anodic electrocatalysts. Although the single cell power density in this work is high, yet the open circuit potential is lower than that reported by others, which needs further investigation.
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Acknowledgments This study was partially supported by National Key Research & Development Program of China (Gran No. 2016YFB0101307), National Natural Science Fund of China (Grant Nos. 21003114, 21103163, 21306188, 21373211, and 21306187), Liaoning BaiQianWan Talents Program (Grant No. 201519), Program for Liaoning Excellent Talents in University (Grant No. LR201514), Dalian Excellent Young Scientific and Technological Talents (Grant No. 2015R006), and the Fundamental Research Funds for the Central Universities (Grant Nos. DUT15RC(3)001, DUT15ZD225).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.09.213.
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The hydrophilicity of carbon for the performance enhancement of direct ascorbic acid fuel cells Chenxi Qiu, Hemu Chen, Huiyuan Liu, Zihui Zhai, Jiaqi Qin, Yang Lv, Zhanming Gao, Yujiang Song* State Key Laboratory of Fine Chemicals & Laboratory of Electrochemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China
article info
abstract
Article history:
As a representative of low-temperature direct biomass fuel cells, direct ascorbic acid fuel
Received 9 April 2018
cells (DAAFCs) carry many advantages, including renewable fuel, easy transportation and
Received in revised form
storage, and high safety. However, a major challenge of DAAFCs confronting us is relatively
10 September 2018
low power density. Herein, to deal with this challenge, we treat carbon black (BP 2000) with
Accepted 26 September 2018
nitric acid at an optimal concentration (4 M), which is further employed as anodic elec-
Available online 26 October 2018
trocatalyst for AA oxidation with improved hydrophilicity. Consequently, hydrophilic AA molecules can more readily access the surface of the carbon electrocatalyst and donate
Keywords:
electrons. Furthermore, the electrocatalytic effect of acid-treated carbon for AA oxidation
Direct ascorbic acid fuel cells
reaction is quantitatively evaluated by the determination of activation energy, which has
Power density
not been assessed prior to this study. In a similar way, nitric acid treatment is also applied
Hydrophilicity
to gas diffusion layer (GDL) at the anode side. In addition, Nafion content in anodic elec-
Acid treatment
trocatalyst layer, single cell operating temperature, and hot pressing conditions for the
Anodic carbon electrocatalyst
fabrication of membrane electrode assembly (MEA) as well as membrane thickness are also optimized. A maximum power density of 31 mW cm2 is eventually attained at 80 C with anode ionomer content of 9.2% and hot pressing at 130 C and 6 MPa for 2 min. This power density is 1.72 times of that reported previously with carbon black as the anode electrocatalyst. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Currently, the consumption of traditional fossil fuels is facing many problems, such as environmental pollution, climate change, low efficiency and depletion of fuels [1]. Therefore, the development and utilization of alternative clean energy is imminent. Fuel cell is a type of clean energy technology, which can directly convert chemical energy into electric energy. Hydrogen and alcohol are often used as fuels due to the
high energy density [2e6]. However, hydrogen is not that convenient from the perspective of industrial preparation, storage and transportation and safety [7,8]. For alcohol fuel, it also faces some problems like fuel crossover, membrane swelling and toxicity [9e12]. Fortunately, direct ascorbic acid fuel cells may be a solution for all of the problems. Ascorbic acid, also known as vitamin C, is a powerful antioxidant naturally present in many fruits and vegetables, which is environmentally benign and biologically friendly
* Corresponding author. E-mail address: [email protected] (Y. Song). https://doi.org/10.1016/j.ijhydene.2018.09.213 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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[13,14]. DAAFCs offer several obvious advantages over other proton exchange membrane fuel cells (PEMFCs). Both AA and the released products of DAAFCs are non-toxic. As a solid substance, AA storage and transportation are convenient. Inexpensive electrocatalysts like carbon can be used for AA oxidation at the anode side. There is no crossover effect because of relatively large size of AA molecule [15]. Previously, the study on the electrochemical oxidation of AA is mainly related to electrochemical sensors, and there are only a few studies about the use of AA in fuel cells. The examined anodic electrocatalysts of DAAFCs include noble metal (e.g., Au, Ag, Pt, Pd, Ru, Ir) [16e18], polyaniline [19e21], polyaniline/TiO2 [22], porphyrin [23], carbon black [24,25] and carbon nanomaterials [26,27]. Because of low cost, large specific surface area, high electrical conductivity and relatively high electrocatalytic effect, carbon black has become an excellent choice as the anodic electrocatalyst for DAAFCs. At present, a primary challenge of DAAFCs is the relatively low power density. The highest power density of DAAFCs using proton exchange membrane reported previously is only 18 mW cm2 [28], which is much lower than that of fuel cells with methanol and hydrogen as fuels. To deal with this problem, in this study, carbon black as the anodic electrocatalyst was treated with nitric acid in that AA is highly water soluble and the anode side of DAAFCs inevitably requires certain hydrophilicity to facilitate fuel transport to the surface of carbon electrocatalyst. After optimizing the conditions of acid treatment, the oxygen containing functional groups on carbon black have significantly increased and the hydrophilicity was effectively improved. Furthermore, we quantitatively analyzed the electrocatalytic effect of acid-treated carbon black for AA oxidation by the determination of activation energy, which has not been done prior to this study. This clearly proved that the carbon does function as an electrocatalyst for AA oxidation. In a similar way, nitric acid treatment was also applied to the GDL at the anode side. As a result, the hydrophilicity of the GDL was also improved. In addition, we adjusted the ionomer content of anodic electrocatalyst layer, so that AA may well contact with the electrocatalyst. Moreover, hot pressing conditions of MEA, operating temperatures of single cells and membrane thickness were also optimized. Finally, a high power density of DAAFCs (31 mW cm2) was achieved, which is 1.72 times of that reported before with untreated carbon as anodic electrocatalyst.
Experiment Materials Ultrapure water (18.2 MU cm1 at 25 C) was obtained from Millipore water system (Synergy UV, France). Black Pearls 2000 (BP 2000) was purchased from Cabot. Pt/C (60 wt%, JohnsonMatthey) was used as cathodic electrocatalysts. Carbon paper (TORAY TGP-H-060, 0.19 mm in thickness) was obtained from TORAY. Nafion 212 and Nafion 211 membrane (DuPont) were used as membranes after pretreatment with 3 wt% H2O2
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for 1 h and 3 M H2SO4 aq. at 80 C for 1 h. Ascorbic acid (99.7%) was purchased from Sinopharm Chemical Reagent Co., Ltd.
Hydrophilic treatment of carbon catalyst and carbon paper Carbon was first treated with 3 M HCl for 1 h at 30 C to remove impurities, and then treated with different concentration of HNO3 (1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M) for 1 h at 80 C to enhance its hydrophilicity. The obtained carbon was further washed with copious amount of H2O until the pH value of filtrate is about 7. The carbon was dried in vacuum oven at 65 C for 4 h. Carbon paper was treated with different concentration of HNO3 (1 M, 3 M, 4 M, 5 M and 6 M) for 4 h at 80 C (before heating, carbon paper was soaked in acid for 30 min) to enhance its hydrophilicity. The carbon paper was washed with H2O until the filtrate reached neutral pH and then dried in a vacuum oven at 65 C for 10 h for further use.
Electrochemical measurements All the electrochemical measurements of the electrocatalysts were carried out on a CHI 760D electrochemical workstation (CH Instruments) in a standard three-electrode electrochemical cell with a rotating glassy carbon disk as the working electrode, a graphite rod as the counter electrode, and Hg/ Hg2SO4 reference electrode. All potentials in this study were referred to that of reversible hydrogen electrode (RHE). A 5.0 mm diameter glassy carbon disk (0.19625 cm2, PINE) was used in rotating disk electrode (RDE) experiments. The catalyst ink (1 mg mL1) was prepared by blending certain amount of the electrocatalysts with water, ethanol and Nafion perfluorinated resin solution (Vwather:Vethanol:VNafion ¼ 1:9:0.06) under mild sonication for 2 min in a water bath [29e31]. The suspension was pipetted onto the RDE and evaporated in air, resulting in an electrocatalyst loading of 10 mgC cm2. RDE tests were carried out at 25 C in N2-saturated 1 mM AA and 0.5 M H2SO4 aqueous solution. Cyclic voltammetry (CV) curves of the electrocatalyst were recorded at a rotation rate of 1600 rpm and a positive scan rate of 50 mV s1 from 0 to 1.0 V vs RHE.
Materials characterizations X-ray photoelectron spectroscopy (XPS) was recorded on a ESCALAB™ 250Xi photoelectron spectrometer (Thermo Fisher) with Al Ka x-ray as an excitation source. Thermogravimetric analysis (TGA, TA Instruments, Q600) was performed from room temperature to 750 C with a heating rate of 10 C min1 in dried air. Raman spectra were recorded using Raman spectrometer (DXR smart Raman) with 523 nm laser excitation. Ultravioletevisible spectroscopy (UVeVis) absorption spectra were collected using a Specord S600 spectrophotometer (Analytic Jena) with a quartz cuvette whose pathlength is 1 cm. Transmission electron microscope (TEM) analysis was performed on a Tecnai G2 Spirit (FEI) operated at 200 keV. The samples were dispersed in ethanol and then dropped on TEM sample grids (Beijing Zhongjingkeyi Technology Co., Ltd). N2 adsorption/desorption were measured at 77 K using a Quantachrome Quadrasorb-SI Analyzer, where BrunnerEmmetTeller (BET) method was used for surface
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area determination. The surface hydrophilicity of carbon black and carbon paper was analyzed using contact angle measuring instrument (KRUSS DSA100). Electrochemical impedance spectra (EIS) of single cells were measured under a cell potential of 200 mV using an electronic load (PLZ1004WH, Kikusui) and an electrochemical workstation (Metrohm PGSTA302N) in a frequency range from 10 kHz to 0.1 Hz at 25 C.
MEA fabrication and single cell test Fuel cell tests were performed on a house-made fuel cell system with an electronic load (PLZ1004WH, Kikusui). The fuel cell hardware (FX201-2D) with a geometric area of 4 cm2 was purchased from Kunshan Sunlaite New Energy Technology Co. Ltd, P. R. China. The anode electrocatalyst layer was premL1, pared by spraying catalyst ink (2 mgC Vwather:Vethano:VNafion ¼ 1:9:0.02) onto Nafion membrane with an electrocatalyst loading of 1 mg cm2. The cathode electrocatalyst layer was prepared by spraying catalyst ink (2 mgPt/C
mL1, Vwather:Vethano:VNafion ¼ 1:9:0.06) onto the other side of Nafion membrane with an electrocatalyst loading of 1 mg cm1. The membranes coated with anode and cathode electrocatalyst layer were sandwiched between two GDL (The anode side is carbon paper and cathode side is commercial GDL which consist of carbon paper and micro porous layer) by hot-pressing at different temperature (120 C, 125 C, 130 C, 135 C and 140 C) and pressure (3 MPa, 4.5 MPa, 6 MPa and 7.5 MPa) for 2 min. 0.5 M AA aqueous solution was continuously delivered to the anode side at a flow rate of 15 mL min1 by a peristaltic pump, while humidified oxygen (100% RH) was supplied to the cathode side (200 mL min1). The temperature of liquid fuel, O2, and single cell hardware was maintained constant at 80 C.
Results and discussion Previously, it has been suggested that certain type of carbon materials may function as anodic electrocatalysts for AA
Fig. 1 e (a) Contact angle analysis of carbon black samples treated with 1 M, 3.5 M, 4 M, 4.5 M, and 5 M nitric acid, respectively; (b) TGA curves of carbon black samples untreated and treated with 1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, respectively, with a heating rate of 10 C min-1. (Note: all the samples have absorbed water from air before TGA analysis; Inset: water loss of each carbon sample untreated and treated with different concentration of nitric acid).
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oxidation in acidic media [24,28]. As a matter of fact that AA molecule is highly water soluble (the solubility is 333 g L1 at 20 C) [32], and it might be a prerequisite to have carbon surface hydrophilic to some extent to allow AA molecules to easily access the carbon surface, thus being oxidized efficiently. To the best of our knowledge, such a study has not been conducted prior to our work. Herein, we chose commercial BP 2000 as the carbon material for concept approval mainly owning to its high surface area. It is well known that acid treatment is a simple and effective way to improve the hydrophilicity of carbon surface. Hence, different concentrations of nitric acid (1e5 M) have been employed to treat BP 2000 for 1 h. (See details in the experimental section above). The hydrophilicity change of acid-treated carbon surface was firstly investigated by contact angle analysis as shown in Fig. 1(a). With the increase of nitric acid concentration from 1 to 4 M, the hydrophilicity of carbon surface continuously increased as evidenced by the contact angle variation from 136.99 to 129.99 e122.49 . After the acid concentration further increased to 4.5 and 5 M, the hydrophilicity of carbon decreased as confirmed by the contact angle increase to 128.49 and 130.49 . This illustrates that acid treatment can effectively modify the hydrophilicity of carbon and there exists an optimum acid concentration (4 M). Below 4 M, possibly there are no enough oxygen-containing groups created on carbon. While above the optimal acid concentration, oxygencontaining groups might be removed in the form of released CO2 and CO [33], since 4.5 and 5 M nitric acid are much stronger oxidants and can further oxidize the oxygencontaining groups on carbon [34]. Additionally, TGA can also be used to investigate the hydrophilicity variation of acid-treated carbon based on the weight of adsorbed water by carbon samples. Before TGA tests, all of the carbon samples were placed in air with 95% humidity for 24 h to ensure sufficient water adsorption. As shown in Fig. 1(b), the weight change before 110 C can be attributed to water loss. The more water loss, the more water adsorbed by acid treated carbon from the humidified air and the better hydrophilicity of carbon surface. It appears that the carbon black treated with 4 M of nitric acid has the largest quantity of water loss before 110 C (Fig. 1(b), inset). This suggests that the highest hydrophilicity has been achieved by using 4 M of nitric acid to treat carbon black, which is consistent with the contact angle analysis. The oxygen-containing groups on carbon surface actually represent a type of defects, which can be investigated by Raman spectroscopy. As shown in Fig. 2(a), two broad bands reside at 1331 cm1 (D band) and 1586 cm1 (G band), respectively [35]. D band arises from disordered sp2 hybridized carbon, while G band is associated with graphitic carbon [36,37]. Based on the analysis of peak area, it is found that the carbon black treated with 4 M nitric acid possesses the largest ID/IG ratio among all the samples, implying the carbon treated with 4 M of nitric acid has more surface defects than other carbon samples. This agrees well with the contact angle measurements and the TGA analysis. It is necessary to identify the exact origin of the hydrophilicity of carbon surface. In this regard, XPS has been used to analyze oxygen-containing groups on carbon. Fig. S-1 shows that with the increase of acid concentration, O1s signal
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Fig. 2 e (a) Raman spectra of the carbon samples treated with 1 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, respectively; (b) high-resolution C1s spectra of the carbon samples treated with 1 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, respectively.
became stronger relative to C1s signal. This clearly shows that oxygen-containing groups on carbon surface did increase after acid treatment. It is worth pointing out that the O1s signal reached the largest value for the case of 4 M nitric acid treated carbon, which is in good agreement with the
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measurements above. Since O1s signal is weak, it is difficult to collect high-resolution O1s spectra and conduct deconvolution of the peaks. Alternatively, high-resolution C1s spectra can easily be de-convoluted into CeO (285.7 eV), carbonyl (288.1 eV) and carboxyl (290.2 eV), besides CeC (284.8 eV), to elucidate the origin of hydrophilicity as shown in Fig. 2(b) and Table 1. It is certain that the enhanced oxygen intensity of acid treated carbon originates from the formation of hydroxyl, carbonyl and carboxyl groups [38]. For each type of oxygen-containing group, 4 M acid treated carbon always contains more hydrophilic groups than other samples. This corroborates that 4 M of nitric acid is an optimal choice, leading to significant enhancement of hydrophilicity. On the other hand, a higher concentration of nitric acid may cause the further oxidation of oxygen-containing groups on carbon surface to form carbon dioxide and carbon monoxide, thus worsening the hydrophilicity. While a lower concentration of nitric acid might not create enough oxygen-containing species to improve the hydrophilicity. The effect of carbon hydrophilicity on AA oxidation was first investigated by recording CV curves at 50 mV s1 in 0.5 M H2SO4 aq. containing 1 mM AA as shown in Fig. S-2. The first anodic peak residing at around 0.45 V (vs. RHE) corresponds to the oxidation of AA [24]. The anodic and cathodic peak centering at 0.55 V and 0.45 V(vs. RHE), respectively, is coming from the oxidation and reduction of surface functional groups (e.g. hydroquinone/quinone) in H2SO4 solution [24,39]. The baseline correction of AA oxidation peak from 0.4 to 0.48 V is shown in Fig. 3(a), the AA oxidation current takes the order of 1 M＜2 M＜3 M＜3.5 M＜5 M＜4.5 M＜4 M. Fig. 3(b) shows that the single cells fabricated with acid treated carbon do possess significantly improved performance as expected. The single cell hardware is shown in Fig. S-3. Especially, it is noteworthy that the single cell fabricated with 4 M acid treated carbon exhibits the highest single cell power density of 29.6 mW cm2. This result is consistent with the CV test. It suggests that the hydrophilicity might be the origin of electrochemical performance. However, CV curves of 3.5, 4, 4.5 and 5 M nitric acid treated carbon all exhibit relatively large electrical double layer capacitance, which means a high surface area of the carbon materials. Therefore, there exists a possibility that the enlarged surface area because of acid treatment might be an additional contributor to the enhanced AA oxidation performance and single cell performance besides the hydrophilicity. According to TEM images (Fig. 4) and BET data (Fig. 4(c) and Table 2), 4 M nitric acid treated carbon has the largest total surface area and external surface area. However, compared with other samples, there is no much
Table 1 e Peak area percentage according to deconvoluted XPS spectra for the carbon samples treated with different concentrations of nitric acid. Samples 1M 3.5 M 4M 4.5 M 5M
Hydroxyl
Carbonyl
Carboxyl
15.82% 17.97% 20.64% 19.60% 17.93%
4.18% 4.42% 4.94% 4.47% 4.24%
3.43% 4.03% 4.27% 4.15% 3.97%
Fig. 3 e (a) CV curves of carbon samples treated with 1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid collected in N2-saturated 1 mM AA þ 0.5 M H2SO4 solution with positive scan between 0.4 and 0.48 V (vs. RHE) after baseline correction; (b) Polarization curves and power density curves for carbon electrocatalysts untreated and treated with 1 M, 2 M, 3 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid, tested at 80 C and using Nafion 211 membrane. 0.5 M AA was supplied to the anode at a flow rate of 10 mL min¡1, while humidified oxygen (100% RH) was supplied at a flow rate of 200 mL min¡1 at the cathode. Geometrical electrocatalyst layer area: 4 cm2, anode loading: 1 mgC cm¡2, cathode loading: 1 mgPt cm¡2 (60 wt% Pt/C). GDL is pristine without any treatment.
difference. Hence, the change of surface area might have a negligible effect on the electrochemical performance. Previous studies have proposed that carbon may function as a catalyst for the oxidation of AA [15,24], but no convincing quantitative information has been provided for the suggested catalytic effects like activation energy. We investigated the catalytic effect of carbon for AA oxidation by the determination of the activation energy of the AA oxidation. The concentration variation of AA in aqueous solution was measured at different temperatures with the aid of UVeVis spectra in the absence and presence of carbon (Fig. S-4 and S-5), respectively. The activation energy was calculated based on linear regressions. The activation energy of AA oxidation in air without the addition of carbon is 50.7 kJ/mol and the activation energy of AA oxidation in air with the addition of carbon is 34.8 kJ/mol. This clearly shows that carbon can effectively
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Fig. 4 e TEM images of the carbon before (a) and after (b) 4 M acid treatment; (c) N2 adsorption-desorption isotherms of the carbon samples treated with 1 M, 3.5 M, 4 M, 4.5 M and 5 M nitric acid.
catalyze the oxidation of AA and reduce the activation energy by about 30%. We realized that the transfer of AA at the anode side is through both electrocatalyst layer and GDL. Generally, the function of GDL is to support electrocatalyst layer and membrane and to conduct electrons and gas. A typical GDL is composed of carbon paper layer at the bottom and a microporous carbon black layer on the top. Since the existence of liquid in GDL is detrimental to the transfer of gas, GDL is usually hydrophobic to prevent the formation of liquid droplets by the addition of certain amount of polytetrafluoroethylene (PTFE) in the microporous layer. However, the hydrophobicity of GDL may decrease the performance of DAAFCs, because AA aqueous solution could be difficult to pass through the hydrophobic GDL. Hence, the hydrophilic treatment of anodic GDL was also carried out. We chose carbon fiber paper without microporous layer as anodic GDL of DAAFCs. Hydrophilic treatment steps were the same as that of carbon black, but with a longer processing
Table 2 e Surface areas measured by nitrogen adsorption/desorption for the carbon samples treated with different concentrations of nitric acid. Sample
1M 3.5 M 4M 4.5 M 5M
Specific surface area (m2 g1)
Specific micropore area (m2 g1)
Specific external surface area (m2 g1)
1433.8 1432.0 1445.5 1434.3 1410.6
978.3 967.0 955.4 953.0 935.1
455.6 465.0 490.2 481.3 475.6
time of 4 h. Fig. 5 shows acid treatment did significantly influence the performance of DAAFCs. With the increase of nitric acid concentration from 0 to 5 M, the performance of single cell continuously improved. Similar to the case of anodic carbon electrocatalysts, the acid concentration here also has an optimum value of 5 M. An acid concentration
Fig. 5 e Polarization curves and power density curves of DAAFCs fabricated with GDL after being treated with different concentrations of nitric acid (0 M, 1 M, 3 M, 4 M, 5 M, and 6 M), tested at 80 C and using Nafion 211 membrane. 0.5 M AA was supplied to the anode at a flow rate of 10 mL min¡1, while humidified oxygen (100% RH) was supplied to the cathode at a flow rate of 200 mL min¡1. Geometrical electrocatalyst area: 4 cm2, anode loading: 1 mgC cm¡2 (4 M nitric acid treated carbon), cathode loading: 1 mgPt cm¡2 (60 wt% Pt/C).
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Fig. 6 e Contact angle analysis of pristine GDL and GDL samples treated with 1 M, 3 M, 4 M, 5 M and 6 M of nitric acid for 4 h at 80 C.
lower or higher than 5 M led to a detrimental effect on the single cell performance. In order to further prove that the increase of single cell performance is due to the improvement of GDL hydrophilicity, the surface hydrophobicity was studied by contact angle analysis. As shown in Fig. 6, with the increase of acid concentration from 0 to 5 M, the contact angle of GDL becomes smaller and smaller. This indicates that the hydrophilicity gradually increases. 5 M nitric acid treated GDL has the smallest contact angle or the highest hydrophilicity, this is consistent with the single cell test. For the case of 6 M nitric acid treated GDL, the hydrophilicity decreased and so did the single cell performance. The possible reason might be the removal of oxygen-containing groups on carbon at a higher concentration of nitric acid like discussed before. In addition, the amount of Nafion in anodic electrocatalyst layer also affects AA transfer and single cell performance. Nafion serves to conduct proton and to bind electrocatalyst, membrane and GDL together. However, Nafion does not conduct electrons. For DAAFCs, the appropriate content of Nafion in anodic electrocatalyst layer needs to be studied [15]. As shown in Fig. 7, when the Nafion content is high, a thick layer of Nafion might cover the carbon surface. Consequently,
AA molecules cannot easily cross Nafion layer to be oxidized on carbon. At a low Nafion content, the Nafion layer is thin or not fully formed around the carbon (not shown) and AA molecules can readily cross the Nafion layer to be oxidized on carbon. The relationship between Nafion content in anodic electrocatalyst layer and DAAFCs performance was investigated. As shown in Fig. 8(a), with the decrease of the content of Nafion from 27.6% to 9.2%, the performance of DAAFCs became better and better. When the Nafion content reaches 9.2%, the performance appeared to the highest one. Further reduction of Nafion content to 5% led to the decrease of the performance. In this case, the low Nafion content may greatly affect the stability of membrane electrode assembly (MEA). One of the functions of the Nafion is to bind MEA tightly as the adhesive. With the decrease of Nafion content, the binding among the anodic electrocatalyst particles becomes unstable and some electrocatalysts may fall off from the electrocatalyst layer. This was confirmed by Fig. S-6. After running for 8 h, the MEA with a low anodic Nafion content obviously lost carbon electrocatalyst. In addition, a low Nafion content may also make electrocatalysts, GDL and proton exchange membrane contact loosely, resulting in the
Fig. 7 e Schematic diagram of diffusion of AA molecules through thick (a) and thin (b) layer of Nafion around carbon electrocatalyst.
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Fig. 8 e (a) Polarization curves and power density curves of DAAFCs fabricated with different anodic Nafion content (5%, 9.2%, 13.8%, 23%, and 27.6%) tested at 80 C using Nafion 212 as the membrane. 0.5 M AA was supplied to the anode at a flow rate of 10 mL min¡1, while humidified oxygen (100% RH) was supplied to the cathode at a flow rate of 200 mL min¡1. Geometrical area of electrocatalyst layer: 4 cm2, anode loading: 1 mgC cm¡2 (Untreated BP2000), cathode loading: 1 mgPt cm¡2 (60% Pt/C); (b) Nyquist plots of DAAFCs fabricated with different anodic Nafion content (5%, 9.2%, 13.8%, 23%, and 27.6%). GDL is pristine without any treatment.
increase of resistance of single cells and the degradation of single cell performance. EIS was used to study the charge transfer resistance of single cells fabricated with different anodic Nafion content. The diameter of a semicircle is related to charge transfer resistance in Nyquist plot. In general, a smaller diameter corresponds to a smaller charge transfer resistance, and thus fast electrode kinetics [40e42]. As shown in Fig. 8(b), the diameter of the semicircle of the single cell fabricated with 9.2 wt% anodic Nafion content is the smallest, which is consistent with the results of single cell test. Furthermore, we optimized hot pressing conditions for the preparation of MEAs, since the structure, porosity and the performance of MEAs can be changed during hot pressing process. A low temperature and pressure often causes insufficient integration of MEAs, thus increasing contact resistance. On the other hand, it can also cause the dehydration of Nafion membrane, when a high temperature and pressure is adopted [43]. Many studies on hot pressing conditions of hydrogen/oxygen fuel cells [44,45] and direct methanol fuel cells (DMFCs) [43] have been reported. However, the effect of hot pressing on the performance of DAAFCs has not been paid much attention. Herein, based on the previous studies, the hot pressing conditions of DAAFC were optimized. We studied the effect of temperature and pressure on the performance of DAAFCs. It was found that 130 C and 6 MPa are the best hot pressing conditions of DAAFC (Fig. S-7). In addition, we investigated the effect of operating temperature on the performance of DAAFC. Generally, temperature has a great influence on the performance of a single cell. The increase of temperature is helpful to improve the gas diffusion rate and electrocatalyst activity, to increase the proton conductivity, and to alleviate the electrode flooding [46,47]. As for DAAFCs, temperature also significantly improves the performance. With the increase of temperature, the oxidation of AA becomes faster. Fig. S-8 proves the high temperature is more favorable for DAAFCs. However, the Nafion membrane usually operates at a temperature of
0e80 C. Above this temperature, the proton conductivity of membrane will be severely reduced due to dehydration, and its thermal stability will also be decreased. Therefore, we did not examine a higher temperature after the temperature of a single cell reached 80 C. Finally, we tested two kinds of cation exchange membranes, Nafion 212 and Nafion 211. The main difference between them is the thickness, which is 50 mm and 25 mm, respectively. Proton transfer in a thin membrane is relatively easy, thereby reducing single cell internal resistance and improving the performance. As shown in Fig. S-9, DAAFC performance has been improved significantly by using thin membrane Nafion 211.
Conclusion Regarding the hydrophilicity of AA molecules as fuel, anodic carbon electrocatalyst (BP 2000) and GDL have been treated with different concentrations of nitric acid. Under optimum conditions of acid treatment, oxygen-containing groups or hydrophilicity reaches a maximum value, which has been investigated by TGA, Raman, XPS, TEM, and nitrogen adsorption/desorption. The improved hydrophilicity allows AA molecules to easily diffuse and contact with carbon electrocatalysts, leading to much enhanced performance of DAAFCs. We have quantitatively evaluated the activation energy of AA oxidation on carbon and confirmed that carbon does function as the electrocatalysts for AA oxidation, which has not been done prior to this study. In addition, Nafion content in anodic electrocatalyst layer, hot pressing conditions, operating temperatures and membrane thickness have been optimized. Eventually, DAAFCs show a maximum power density of 31 mW cm2, which is 1.72 times of those reported before using carbon as the anodic electrocatalysts. Although the single cell power density in this work is high, yet the open circuit potential is lower than that reported by others, which needs further investigation.
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Acknowledgments This study was partially supported by National Key Research & Development Program of China (Gran No. 2016YFB0101307), National Natural Science Fund of China (Grant Nos. 21003114, 21103163, 21306188, 21373211, and 21306187), Liaoning BaiQianWan Talents Program (Grant No. 201519), Program for Liaoning Excellent Talents in University (Grant No. LR201514), Dalian Excellent Young Scientific and Technological Talents (Grant No. 2015R006), and the Fundamental Research Funds for the Central Universities (Grant Nos. DUT15RC(3)001, DUT15ZD225).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.09.213.
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