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Pd-Pt loaded graphene aerogel on nickel foam composite as binder-free anode for a direct glucose fuel cell unit
Solid State Sciences 71 (2017) 123e129
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Pd-Pt loaded graphene aerogel on nickel foam composite as binder-free anode for a direct glucose fuel cell unit Chi Him A. Tsang, D.Y.C. Leung* Department of Mechanical Engineering, The University of Hong Kong, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2017 Received in revised form 19 July 2017 Accepted 19 July 2017 Available online 20 July 2017
Fabrication of electrocatalyst for direct glucose fuel cell (DGFC) operation involves destructive preparation methods with the use of stabilizer like binder, which may cause activity depreciation. Binder-free electrocatalytic electrode becomes a possible solution to the above problem. Binder-free bimetallic Pd-Pt loaded graphene aerogel on nickel foam plates with different Pd/Pt ratios (1:2.32, 1:1.62, and 1:0.98) are successfully fabricated through a green one-step mild reduction process producing a Pd-Pt/GO/nickel form plate (NFP) composite. Anode with the binder-free electrocatalysts exhibit a strong activity in a batch type DGFC unit under room temperature. The effects of glucose and KOH concentrations, and the Pd/Pt ratios of the electrocatalyst on the DGFC performance are also studied. Maximum power density output of 1.25 mW cm2 is recorded with 0.5 M glucose/3 M KOH as the anodic fuel, and Pd1Pt0.98/GA/ NFP as catalyst, which is the highest obtained so far among other types of electrocatalyst. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Graphene Fuel cell Glucose Platinum Palladium
1. Introduction Direct glucose fuel cell (DGFC) has attracted strong attention in recent years due to their non-toxic, low cost, and easy handling properties [1,2]. Its performance can be enhanced with the use of an anion exchange membrane (AEM) in its assembly [1,2]. For the fabrication of its electrocatalysts, different metal combinations were used. Due to the high cost and easy carbon monoxide (CO) poisoning for traditional platinum (Pt) based catalyst, alternative metals were widely considered. Among them, nano gold [1,3e6], nano silver [7], monometallic Pd [8e11], bimetallic palladiumnickel (Pd-Ni) [2], palladium-metal (Pd-M) [12,13], and palladium-platinum (Pd-Pt) [14e17] catalysts deposited on different substrates were adopted in DGFC anodic activity studies. These non-platinum and bimetallic Pt or Pd based catalysts can overcome the weaknesses mentioned above [1,2], and exhibited stronger anodic activity in glucose electrooxidation when compared to the corresponding monometallic Pd, Pt, and Au electrocatalysts [3,12e16]. Up-to date, nanosized monometallic gold [1,4,18], and platinum [18], and bimetallic Pd-Ni [2,18], Pd-Pt [17], Pt-Au [3,19], Pt-Bi [19], Pt-Ru [18], and Pd-Au [18] fabricated on secondary substrates like carbon paper [3,4,17] or nickel foam (NF)
* Corresponding author. E-mail address: [email protected] (D.Y.C. Leung). http://dx.doi.org/10.1016/j.solidstatesciences.2017.07.014 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.
[2] were chosen in several DGFC studies. However, the number of these reports [1e4,17e19] was fewer when compared to the halfcell activity studies [1,3e17,19]. Systematic studies on the effect of operation parameters such as fuel and electrolyte concentration on the half-cell catalytic activity and the fuel cell unit of DGFC were even rare in number [2,5,12]. DGFCs were commonly operated in either flow mode [2,18,19] or batch mode [1,3,4,17,19]. Some of them were operated in ambient temperature (25e37 C) [1,3,4,17e19], while some required heating in order to boost up the cell performance [2]. However, the number of systematic reports on DGFCs is still limited. More importantly, the common practice of using of binder like Nafion throughout the electrode preparation process may hinder the cell performance [1e4,18e20]. Using binder-free electrode as anode may overcome this problem in long term applications. In recent years, graphene based electrocatalysts became a new focus in fuel cell researches due to their large surface areas, strong electroconductivity, and easy loading with foreign materials through chemical pathways owned by the graphene nanosheets [21,22]. Metal loaded graphene (2D and 3D) based catalysts also showed strong activity in the electrooxidation of alcohols and glucose, which are potential catalysts in fuel cell applications [5,11,21e28]. While practical trial in the DGFC assembly and systematic performance study still does not exist up to our knowledge, most of the fuel cell operations involving graphene catalysts were still focused on half-cell research in both glucose oxidation reaction
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(GOR) and oxygen reduction reaction (ORR) works [5e8,11,16,24]. Systematic study on the effect of the glucose and electrolyte concentration variation to the catalytic activity was even rare [5]. More importantly, the catalysts were fabricated on the substrate like glassy carbon electrode (GCE) or indium tin oxide (ITO) glass with the use of binder like Nafion, which involved complicated fabrication steps. This may also affect the performance of catalyst due to the reduction in active area of metal catalyst by the masking effect from binders or surfactants [11,20e26]. The ultrasonication of catalyst into the binder was also a major pretreatment step [25,26,29,30], which may cause the destruction of the catalyst structure, especially the 3D graphene catalyst [25,26,29]. Besides these weaknesses, traditional membrane electrode assembly (MEA) technique also involved hot pressing the electrode with electrolyte membrane for good contact [30], which also makes the fuel cell unit preparation complicated. In order to retain the 3D structure of graphene and make the fuel cell unit fabrication simple and friendly, the use of binder-free 3D graphene based direct catalyst became an important direction [27]. One of the pathways is the direct growth of graphene on nickel foam (NF) through chemical vapor deposition (CVD), followed by the removal of NF in acid to form 3D graphene foam (GF) as the first step. Metal loaded GF can then be produced by electrodeposition of the metal to the GF [27]. Another pathway is a one-step growth of metal loaded graphene aerogel on NF (M/GA/NF), which can be done by direct synthesis of M/GA on the NF through a mild conditioned hydrothermal reaction in reducing agent [31]. Such direct catalysts have exhibited strong electrocatalytic ability in the alcohol oxidation reactions, which are also comparable to that of 2D graphene based catalysts in similar reactions [31]. These works have a common strength that the highly porous NF (Mean pore size: 250 mm, porosity: 95%) was used as template instead of using traditional commercial carbon paper (Pore size range: 39e51 mm, porosity: 36e89%), which is a strong advantage for the 3D graphene structure fabricated on secondary substrate from raw GO solution. By combining the strengths of strong activity of bare and NF templated 3D graphenes [25,27,31], and strong bimetallc Pd-Pt [14e16], and metal loaded 2D graphene based catalysts [5e8,16] in glucose electrooxdiation mentioned above, we expected that the 3D Pd-Pt/GA/NF composite can also exhibit strong activity in the alkaline DGFC unit. This provides a possibility for the use of binder-free 3D graphene catalysts in the DGFC assembly. In this paper, a DGFC was assembled through a membrane electrode assembly (MEA) with the use of a binder-free Pd-Pt/GA/ NF plate as anode prepared through a mild conditioned hydrothermal reaction. It has been demonstrated in this study that this type of fuel cell using a cheap anion exchange membrane (AEM) and a binder-free catalyst can achieve a maximum power density of 1.25 mW cm2 under room temperature.
2.2. Anode preparation Raw GO was prepared through the modified Hummer's method which was reported elsewhere [32]. Bimetallic Pd-Pt loaded graphene aerogel on NF plate (Pd-Pt/GA/NFP) was prepared through the method reported by Tsang et al. [31]. Briefly, 120 mg of K2PdCl6 and various amounts (i.e. 48.6, 72.9, 145.8 mg) of K2PtCl6 were mixed with 30 cm3 exfoliated GO dispersion (6 mg cm3) and stirred vigorously for 30 min. The cleaned NFP (2.5 cm 1.5 cm) was put into the Pd-Pt/GO dispersion and allowed to sink in an ultrasonic bath for 10 min, followed by stationary soaking for a few hours. The Pd-Pt/GO soaked NFP was then put immediately into aqueous L-ascorbic acid (VC) solution with a concentration of 2.5 mg cm3, and the system was kept in an oven at 60 C for at least 2 days. The resulting hydrogel hybrid (Pd-Pt/GH/NFP) was then soaked in DI water for 3e4 days in order to remove the residue VC. The Pd-Pt/GH/NFP was then freeze dried under vacuum for overnight to obtain the Pd-Pt/GA/NFP. The elemental compositions of three Pd-Pt/GA/NFP samples based on XRF analysis are shown in Table S1 of which Pd1Pt0.98/GA/NFP, Pd1Pt1.62/GA/NFP, Pd1Pt2.32/GA/ NFP (at% based) stand for the sample prepared from 3:1, 2:1 and 1:1 Pd/Pt/GO dispersion, respectively. For comparison, monometallic Pd/GA/NFP and Pt/GA/NFP electrodes were prepared by identical pathway. The only difference was that K2PdCl6 or K2PtCl6 was added individually to the GO dispersion instead of adding together. Another bimetallic Pd-Pt/GA/NFP electrode with binder was prepared by identical pathway of Pd1Pt0.98/ GA/NFP, with the addition of 0.05 cm3 Nafion (10%) solution to 29.95 cm3 3:1 Pd/Pt/GO dispersion in the Pd/Pt/GO mixture formation step. 2.3. Electrode characterization The freeze dried Pd-Pt/GA/NFP and monometallic Pd, Pt loaded GA/NFP samples were characterized by SEM/EDX (Hitachi S4800 FEG SEM, JEOL-JSM5600), TEM (Philips FEI Tecnai G2 20 S-Twin Scanning TEM), and XRD diffractometer (D8 Advanced Diffractometer, Bruker AXS) with a Cu Ka radiation source. The sample composition was analyzed from the X-Ray Fluorescence spectrophotometer (EDAX-Eagle III). 2.4. Fuel cell assembly The fuel cell for testing was assembled by traditional AEM DGFC assembly, where Pt/C acted as cathode, and Pd-Pt/GA/NFP, Pd/GA/ NFP or Pt/GA/NFP served as anode of the cell. The cathode and anode materials were used directly as received or after freezedrying without further treatment. The fuel cell unit used was a batch type system (non-flowing type, Fig. 1) which was similar to the design used in Tung's group [1]. The fuel and oxidant containers
2. Experimental 2.1. Materials All the chemicals, i.e. K2PdCl6 (99.99%, Aldrich), K2PtCl6 (99.99%, Aldrich), glucose (Aladdin, 99%), L-ascorbic acid or vitamin C (VC, 99.7%, Guangzhou Guanghua Sci-Tech), hydrochloric acid (37%, Sigma-Aldrich), sulphuric acid (98%, Sigma-Aldrich), K2S2O8 (99%, Fisher Scientific), P2O5 (99%, Acros-Organic), KMnO4 (99.0%, SigmaAldrich), hydrogen peroxide (30%, Guangzhou Chemical), graphite powder (325 mesh, Uni-Chem), KOH (85%, Sigma-Aldrich), Nafion (10%, DuPont), and nickel foam (NF, 110 PPI, Artec Chemical) were used directly as purchased. Anion exchange membrane (AMI7001S, Membrane International) was preconditioned in 10%wt NaCl (99.9%, Sigma-Aldrich) solution overnight before use.
Fig. 1. Designation scheme of the batch type glucose fuel cell unit.
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with interior sizes of 2.6 cm (L) 2.0 cm (W) 1.9 cm (H) were used. The MEA was assembled with a wet AEM (5 cm 2.1 cm), a cathode, and an anode with an active area of 1 cm 1 cm. The fuel and oxidant of the fuel cell used in this work were glucose (0.125 M, 0.25 M, 0.5 M, 1 M and 2 M) with KOH (1 M, 3 M, and 5 M) solution, and O2-saturated KOH (1 M, 3 M, and 5 M) solution respectively. Throughout the study, the O2-saturated KOH was prepared by continuous pumping of ambient air through KOH pills to absorb moisture and CO2, which was then pumped into a KOH solution. The voltage and current of the cell were measured by a CHI660E electrochemical workstation under room temperature (i.e. 25 C). 3. Results and discussions 3.1. Materials characterization Digital image of the Pd1Pt0.98/GA/NF electrode against bare NF was shown in Fig. S1, which reflected that the silvery NFP turned into black after the hydrothermal reaction for the growth of Pd1Pt0.98/GA on the NFP. The sample and the bare NFP were also characterized by SEM/EDX as shown in Fig. 2. The low magnification image (Fig. 2a) of the as-prepared sample reflected that the PdPt particles were covered randomly on the graphene nanosheets (GNs) surface in the GA array, and the metal loaded graphene aerogel (M/GA) was covered on the NF skeleton. The in-depth analysis at high magnifications (Fig. 2bec) showed that the particles had random shapes from spherical to irregular. Fig. 2d showed the skeleton structure of the bare NFP, which indicated a highly porous structured composite for the sample. The M/GA was successfully covered on the NF skeleton surface throughout the reduction of M/GO/NF mixture. The presence of Pd and Pt in the sample was primarily confirmed from the Pd and Pt peaks in the EDX spectrum obtained in Fig. 2e, while the Ni peak corresponded to the NF backbone in the sample. The EDX elemental mapping (Fig. 3aeb) further showed that the pattern of Pt La-1 and Pd Ma-1 signal was almost identical with each other and with the particle pattern in the corresponding SEM image in Fig. 3d, while the C Ka-1 signal (Fig. 3c) was similar to the background pattern of the corresponding SEM image in Fig. 3d. Based on similar works from other reports, the bimetallic alloy particles were the dominating
Fig. 2. SEM images of (a) full frame image (100 mm), (b) and (c) magnified images (Scale bar: (b): 10 mm, (c): 5 mm) of Pd1Pt0.98/GA/NF, and (d) NF (Scale bar 100 mm) and (c) corresponding EDX spectrum.
Fig. 3. EDX mapping image of (a) Pt Ma-1, (b) Pd La-1, (c) C Ka-1, and (d) corresponding SEM images of Pd1Pt0.98/GA/NF (Scale bar (a): 2 mm).
products in the bimetallic metal loaded graphene when two different metal salts were added at the same time in the stirring step before the M/GO reduction taking place [20,21,33]. The element mapping pattern of Pt La-1 and Pd Ma-1 was with each other based on the EDX element mapping analysis results of the corresponding Pd-Pt loaded graphene. Based on the recipe of the M/GA preparation in the current study and the reports from elsewhere, the particles in the Pd-Pt/GA array was alloy particles in nature. Further analysis of the GA structure via TEM (Fig. 4) showed that the alloy metal particles were loaded onto the GNs surface in the GA array. Fig. 4b and c were the magnified image of the alloy particles in the selected area of Fig. 4a, which showed that the particles shapes were random, from square to circular. The selected area electronic diffraction (SAED) in Fig. 4d showed that the particles were highly crystalline in nature with reference to the clear spot pattern. According to the d-spacing data listed in Table S2,
Fig. 4. TEM image of Pd1Pt0.98/GA/NFP scratched from the fragment of the electrode. (a) Large area (Scale bar: 1 mm), (b) and (c) Magnified image of the PdPt alloy particle (Scale bar of (b): 100 nm, and (c): 20 nm), and (d) SAED pattern of the alloy particle shown in (c).
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three spots in the SAED pattern (Fig. 4d) corresponded to the (311), (311), and (200) planes of the particles respectively. By comparing with the Pd (JCPD card No. 46e1043) and Pt (JCPD card No. 04e0802) data, the Pd/Pt particles in the GA array has a f-c-c structure. This is a strong evidence of the formation of Pd-Pt alloy particles throughout the Pd-Pt/GO reduction during the GA synthesis. Further characterization of the Pd-Pt/GA/NF electrodes was done via the XRD since it was a powerful techniques for proving the presence of Pd-Pt alloys. From the XRD spectra obtained in Fig. 5a, the presence of Pd signal pattern was the evidence of successful loading of Pd onto the GA. The position of Pd 2q peaks at 39.78 , 46.3 , 67.9 , 81.8 were assigned to the Pd (111), Pd (200), Pd (220), and Pd (311) respectively, while those of Pt 2q peaks (39.48 , 45.8 , 67.4 , and 81.3 ) were assigned as Pt (111), Pt (200), Pt (220), and Pt (311). The result primarily showed that monometallic Pd NPs and Pt NPs have f-c-c structure (JPCDS card No. 46e1043 and 04e0802) with high crystallinity [34]. From the magnified view of M (111) of the Pd-Pt/GA samples at 38-43 (Fig. 5b), it showed that the peak position was lied between that of monometallic Pd and Pt which reflected from Table S3, the peak position (Pd1Pt2.32: 39.586 , Pd1Pt1.62: 39.699 , and Pd1Pt0.98: 39.720 ) was shifted to 2q value of Pd(111) (39.780 ) from Pt(111) (39.479 ) when Pt content reduced in the Pd-Pt NPs. The result was also highly coherent to the SAED pattern obtained from the TEM results (Fig. 4d), which reflected from the matched results of the M(200) and M(311) planes in the Pd-Pt NPs in Pd1Pt0.98/GA/NF (Table S2). Based on the reported
literature, it was the evidence for the presence of Pd-Pt alloy particle [34]. Such finding was also true for the 3D GA structure. Meanwhile, the GO diffraction peak was 10 in the raw GO (Fig. 5a), and this peak was not observed in the same region for both the monometallic Pd/GA and Pt/GA, and bimetallic Pd-Pt/GA, respectively. The strengthen of the board graphene amorphous peak (C(002)) at 25 was observed instead. This revealed that the reduction of oxide functional groups in GO by VC was taken place throughout the hydrothermal reaction [35].
Fig. 5. XRD pattern of (a) Pt-Pd/GAs removed from NF and GO, and (b) the extended view of M (111) peak (38-43 ).
Fig. 6. Polarization curve of Pd/GA/NF, Pt/GA/NF, binder-free Pd1Pt0.98/GA/NF, and Pd1Pt0.98-Nafion/GA/NF driven DGFC with 1 M glucose/5 M KOH fuel.
3.2. Fuel cell performance 3.2.1. Basic testing The polarization curves of the M/GA/NF driven DGFC were shown in Fig. 6. The open circuit voltage (OCV) of the cell and the maximum power density recorded were þ1.1 V and 0.88 mW cm2 at 0.2 V respectively, when Pt1Pd0.98/GA/NFP was used. Compared to the results of monometallic Pt/GA/NFP (0.62 mW cm2 at 0.3 V, OCV: þ1.1 V) and Pd/GA/NFP (0.69 mW cm2 at 0.4 V, OCV: þ1.1 V), the result indicated that DGFC operated with bimetallic Pt1Pd0.98/ GA/NFP had much stronger activity than monometallic Pd (1.27 times) or Pt (1.42 times) loaded GA catalysts cell. The tendency of the result is similar to the previous reports focused on the Pd-Pt based electrocatalyst [14]. A possible reason for the performance enhancement when using bimetallic Pd-Pt may be due to the synergistic effect between Pd and Pt in the catalyst, which was resulted from the modification of the electronic and collective surface properties after the alloying process of Pd and Pt throughout the hydrothermal reaction [14]. This was partially reflected from the shifted M(111) position of the bimetallic Pd-Pt from the XRD results (Fig. 5b), the theoretical anodic reaction shown in Fig. 1 and the generalized mechanism of glucose electrode oxidation in the Pd-Pt catalyst from different literature as shown below which are based on the Pd catalyzed glucose electrooxidation [8e10,12,14]. Step 1: Pd þ Glucose/Pd-H þ Intermediates. Step 2: Pd þ xOH/Pd(OH)x þ xe-. Step 3: Pd(OH)x þ Intermediates/Pd þ Glucolactone/Gluconic acid. Step 4: Pd(OH)x þ Glucose/Pd þ Glucolactone/Gluconic acid. The effect of binder on the basic performance of the Pd1Pt0.98/ GA/NFP driven DGFC was also studied. The result showed that when 0.02 v/v% Nafion was included in the Pd1Pt0.98/GA/NFP preparation, the maximum output power density was 0.67 mW cm2 at 0.6 V, which was lower than that of the binderfree Pd1Pt0.98/GA/NFP (0.88 mW cm2) by 33.5% in power density,
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even though the OCV recorded (þ1.01 V) was similar to the binderfree Pd1Pt0.98/GA/NFP (þ1.1 V). The possible reason of such observation may be due to the presence of Nafion stabilizer in the GA array. Even though it is one of the commonly used electrolyte polymer for stabilizing the metals in the electrocatalytic electrode preparation, the active surface area of the metal particles may be masked by the binder. This matched with the hypothesis of some other groups [20]. As a result, the output power density may be reduced in the identical reactions with operation parameters. This showed the advantages of the use of binder-free electrocatalyst in the DGFC unit operation.
3.2.2. Effect of glucose concentration According to the proposed mechanism of glucose electrooxidation above, the glucose and OH concentration are key species in the electrooxidation of glucose oxidation throughout the unit fuel cell operation. As a result, it is of our interest to investigate the effect of these two species on the performance of DGFC in the current study. The effect of glucose concentration on the DGFC performance was firstly studied with glucose concentrations set at 0.125, 0.25 M, 0.5 M, 1 M, and 2 M with a constant KOH concentration at 5 M. As shown in Fig. 7 and Table 1, the performance of the DGFC was the lowest when the concentration of glucose was 0.125 M with maximum power density only reached 0.22 mW cm2 at 0.2 V. The performance was gradually enhanced when the concentration of glucose increased to 0.25 M with maximum power density of 0.84 mW cm2 at 0.2 V, and then 0.5 M with the corresponding power density of 0.91 mW cm2 recorded at 0.2 V. However, the performance of the cell slightly reduced when the concentration of glucose increased to 1 M, where the power density of the unit reduced to 0.88 mW cm2 at 0.2 V. The output power density of the DGFC unit further reduced to 0.79 mW cm2 at 0.2 V when the glucose concentration further increased to 2 M. From literature, glucose cell performance was enhanced with increasing concentration of glucose until reaching an optimum value [2]. In our case, similar tendency was observed in the fuel cell unit operation when the glucose concentration increased from 0.125 M to 0.5 M. Increase in the glucose concentration will enhance the kinetics of glucose oxidation, which result in an increase in power density of the cell with reference to the theoretical anodic reaction as shown in Fig. 1 [2]. However, the cell performance depressed when the glucose concentration was higher than the optimum value, possibly due to the blockage of the active site by the increased glucose concentration. Consequently, the adsorption of OH on the active site became difficult, and result in a
Fig. 7. Polarization curve of Pd1Pt0.98/GA/NF driven DGFC under different glucose concentration at fixed KOH concentration (5 M).
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Table 1 List of the peak power density variation of Pd1Pt0.98/GA/NF driven DGFC at fixed KOH concentration (5 M). Glucose concentration (M)
Peak power density (mW cm2)
0.125 0.25 0.5 1 2
0.22 0.84 0.914 0.88 0.79
depression in glucose oxidation kinetics on the catalyst active site [2]. Hence, the cell performance reduced with increase the glucose concentration from 0.5 M to 2 M. As such, 0.5 M was the optimum glucose concentration in the present fuel cell operation. 3.2.3. Effect of hydroxide ion concentration Besides the glucose concentration, concentration of OH in the solution also influences the fuel cell operation based on the proposed mechanism above. In the current study, the concentration range of KOH was set at 1 M, 3 M and 5 M, while the optimum glucose concentration of 0.5 M was adopted based on the result of previous section. Pd1Pt0.98/GA/NF electrode was still chosen as the target of study in this section. From the polarization curves as shown in Fig. 8, the maximum power density gradually increased from 0.72 mW cm2 (0.2 V) to 1.25 mW cm2 when the concentration of KOH increased from 1 M to 3 M. However, the maximum power density reduced to 0.91 mW cm2 when the concentration of KOH further increased to 5 M. At the same time, the cell voltage increased continuously from þ0.76 V at KOH concentration of 1 M
Fig. 8. Polarization curve of Pd1Pt0.98/GA/NF driven DGFC under different KOH concentration at fixed glucose concentration (0.5 M).
Fig. 9. Polarization curve of Pd-Pt/GA/NF driven DGFC with different Pd/Pt ratio at 0.5 M glucose/3 M KOH.
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Table 2 List of the power density of the selected batch AEM-DGFC. Catalyst
Fuel/Electrolyte concentration
Peak power density (mW cm2)
Electrode active area (cm2)
Metal loading (wt%)
Ref
Pt-Bi/C Pt-Au/C AuPt/C Au-MnO2/C Au/C Au Nanocoral Au NPs Au Film PdPt(4:1)/C Pd1Pt0.98/GA/NF
0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.5
0.56 0.72 0.72 1.10 0.86 0.85 0.34 0.12 0.52 1.25
5 5 9 4 4 1 1 1 N/A 1
12 7.5 14.5 20 40 N/A N/A N/A 21 8.13
[19] [19] [3] [4] [4] [1] [1] [1] [17] Current work
M M M M M M M M M M
Glucose/1 M KOH Glucose/1 M KOH Glucose/1 M KOH Glucose/1 M KOH Glucose/1 M KOH Glucose/0.5 M KOH Glucose/0.5 M KOH Glucose/0.5 M KOH Glucose/1 M KOH Glucose/3 M KOH
to þ1.05 V at 5 M. This result revealed that the DGFC achieved the best performance when the concentration of the KOH was about 3 M. Reported works on similar system operated by different kinds of metallic catalyst also indicated an increase in the maximum power density with increasing KOH (OH) concentration until reaching an optimum value, and decreased thereafter [2]. A possible reason is that under a constant glucose concentration, increase in OH concentration enhances the transport activity of the hydroxide ions from cathode to anode, which ultimately enhances the kinetics of the glucose oxidation in the cell [2]. However, further increase in the OH concentration will result in a power density reduction due to the decline in the cell performance caused by the blockage of the active sites of the catalyst, and subsequently affects the adsorption of glucose on the anode catalyst [2]. In the current study, the power and corresponding current density of the cell unit increased simultaneously when the KOH concentration in the system increased from 1 M to 3 M, while both the current and power density of the cell reduced when the KOH concentration further increased to 5 M. These trends highly matched with those from previous study [2]. So, in the present system operation, the optimum KOH concentration was determined to be 3 M. 3.2.4. Effect of the Pd/Pt ratio The effect of the atomic ratio between Pd and Pt in the bimetallic GA/NF electrode on the cell performance was also of great interest. Based on the best performance of the catalyst described from the previous sections, the fuel and oxidizing agent used at the anode and the cathode was set to be 0.5 M glucose/3 M KOH and 3 M KOH, respectively. Fig. 9 shows that the power density of the cell increased from 0.64 mW cm2 to 0.89 mW cm2 when the Pd/Pt ratio in the catalyst increased from 1:2.32 to 1:1.62. The power density increased to 1.25 mW cm2 when the Pd/Pt ratio further increased to 1:0.98. This showed that the power output of the cell unit increased when the Pd/Pt ratio approaching to 1:1. A possible reason for the enhancement is the synergistic effect exhibited from the bimetallic Pd/Pt particles after mixing up of the Pd and Pt lattices [36]. This phenomenon is particularly obvious when the Pd/Pt ratio approaches 1:1. By comparing the performance of the selected batch mode of AEM-DGFC operation from literature as listed in Table 2 [1,3,4,17,19], the power output of the present Pd1Pt0.98/GA/NFP (0.5 M glucose/3 M KOH) DGFC stack was found to be the highest among those reported results due to the relatively low metal loading in the Pd-Pt/GA/NFP electrode (8.13 wt%) when compared to most of the listed electrocatalysts (12e40 wt%) [3,4,17,19]. Even though the metal loading in Pt-Au/C (7.5 wt%) was slightly lower than the Pd1Pt0.98/GA/NFP (8.13 wt%) used in the current study, the peak power density recorded from Pd1Pt0.98/GA/NFP in current work was still higher than that of Pt-Au/C [19]. More importantly,
most of the catalytic anodes involve complicated fabrication process like hot press and the use of binder [3,4,17,19]. In contrast, the Pd-Pt/GA/NFP anodes can be used without treatments in the current study, with stronger output power density than the binder containing version (Fig. 6). It showed that the binder-free Pd-Pt/GA electrocatalyst fabricated on NFP electrode provides a possibility of using as a customized binder-free graphene electrode for different applications in fuel cells, as well as in bulk battery devices. 4. Conclusions Binder-free Pd-Pt/GA/NFP electrocatalysts with different Pd/Pt ratios were demonstrated to have strong activity in the batch type AEM-DGFC unit operation under room temperature when compared to corresponding monometallic GA/NFP anodes tested under 1 M glucose/5 M KOH condition. The cell showed strongest activity when the fuel (i.e. glucose) and electrolyte (i.e. KOH) concentration were 0.5 M and 3 M respectively with Pd1Pt0.98/GA/NFP chosen as the representative anode. In addition, the activity of the cell was enhanced when the Pd/Pt ratio approached 1:1 in the 0.5 M glucose/3 M KOH system in the cell operation, achieving a maximum power density output of 1.25 mW cm2 at 0.2 V. The performance of the cell with binder-free GA anodic catalysts showed stronger activity than most batch type AEM-DGFC with other metal catalysts, and the GA anodic catalyst with the use of binder. This type of shape and size customized metal loaded electrocatalyst provided the possibility of producing 3D structured direct graphene electrodes for fuel cells and batteries with nondestructive fabrication method. Acknowledgement This project is supported by the URC's PDF scheme and the CRCG grant (Code: 201511159115) of the University of Hong Kong. Appendix A. Supplementary data Supplementary data related to this chapter can be found at http://dx.doi.org/10.1016/j.solidstatesciences.2017.07.014. References [1] S.P. Tung, T.K. Huang, C.Y. Lee, H.T. Chiu, RSC Adv. 2 (2012) 1068e1073. [2] L. An, T.S. Zhao, S.Y. Shen, Q.X. Wu, R. Chen, J. Power Sources 196 (2011) 186e190. [3] D. Basu, S. Basu, Int. J. Hydrog. Energy 36 (2011) 14923e14929. [4] L. Li, K. Scott, E.H. Yu, J. Power Sources 221 (2013) 1e5. [5] Q.F. Shi, G.W. Diao, S.L. Mu, Electrochim. Acta 133 (2014) 335e346. [6] S. Ghosh, Y. Holade, H. Remita, K. Servat, P. Beaunier, A. Hagege, K.B. Kokoh, T.W. Napporn, Electrochim. Acta 212 (2016) 864e875. [7] A.C. Joshi, G.B. Markad, S.K. Haram, Electrochim. Acta 161 (2015) 108e114. [8] C.C. Chen, C.L. Lin, L.C. Chen, Electrochim. Acta 152 (2015) 408e416.
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Pd-Pt loaded graphene aerogel on nickel foam composite as binder-free anode for a direct glucose fuel cell unit Chi Him A. Tsang, D.Y.C. Leung* Department of Mechanical Engineering, The University of Hong Kong, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2017 Received in revised form 19 July 2017 Accepted 19 July 2017 Available online 20 July 2017
Fabrication of electrocatalyst for direct glucose fuel cell (DGFC) operation involves destructive preparation methods with the use of stabilizer like binder, which may cause activity depreciation. Binder-free electrocatalytic electrode becomes a possible solution to the above problem. Binder-free bimetallic Pd-Pt loaded graphene aerogel on nickel foam plates with different Pd/Pt ratios (1:2.32, 1:1.62, and 1:0.98) are successfully fabricated through a green one-step mild reduction process producing a Pd-Pt/GO/nickel form plate (NFP) composite. Anode with the binder-free electrocatalysts exhibit a strong activity in a batch type DGFC unit under room temperature. The effects of glucose and KOH concentrations, and the Pd/Pt ratios of the electrocatalyst on the DGFC performance are also studied. Maximum power density output of 1.25 mW cm2 is recorded with 0.5 M glucose/3 M KOH as the anodic fuel, and Pd1Pt0.98/GA/ NFP as catalyst, which is the highest obtained so far among other types of electrocatalyst. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Graphene Fuel cell Glucose Platinum Palladium
1. Introduction Direct glucose fuel cell (DGFC) has attracted strong attention in recent years due to their non-toxic, low cost, and easy handling properties [1,2]. Its performance can be enhanced with the use of an anion exchange membrane (AEM) in its assembly [1,2]. For the fabrication of its electrocatalysts, different metal combinations were used. Due to the high cost and easy carbon monoxide (CO) poisoning for traditional platinum (Pt) based catalyst, alternative metals were widely considered. Among them, nano gold [1,3e6], nano silver [7], monometallic Pd [8e11], bimetallic palladiumnickel (Pd-Ni) [2], palladium-metal (Pd-M) [12,13], and palladium-platinum (Pd-Pt) [14e17] catalysts deposited on different substrates were adopted in DGFC anodic activity studies. These non-platinum and bimetallic Pt or Pd based catalysts can overcome the weaknesses mentioned above [1,2], and exhibited stronger anodic activity in glucose electrooxidation when compared to the corresponding monometallic Pd, Pt, and Au electrocatalysts [3,12e16]. Up-to date, nanosized monometallic gold [1,4,18], and platinum [18], and bimetallic Pd-Ni [2,18], Pd-Pt [17], Pt-Au [3,19], Pt-Bi [19], Pt-Ru [18], and Pd-Au [18] fabricated on secondary substrates like carbon paper [3,4,17] or nickel foam (NF)
* Corresponding author. E-mail address: [email protected] (D.Y.C. Leung). http://dx.doi.org/10.1016/j.solidstatesciences.2017.07.014 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.
[2] were chosen in several DGFC studies. However, the number of these reports [1e4,17e19] was fewer when compared to the halfcell activity studies [1,3e17,19]. Systematic studies on the effect of operation parameters such as fuel and electrolyte concentration on the half-cell catalytic activity and the fuel cell unit of DGFC were even rare in number [2,5,12]. DGFCs were commonly operated in either flow mode [2,18,19] or batch mode [1,3,4,17,19]. Some of them were operated in ambient temperature (25e37 C) [1,3,4,17e19], while some required heating in order to boost up the cell performance [2]. However, the number of systematic reports on DGFCs is still limited. More importantly, the common practice of using of binder like Nafion throughout the electrode preparation process may hinder the cell performance [1e4,18e20]. Using binder-free electrode as anode may overcome this problem in long term applications. In recent years, graphene based electrocatalysts became a new focus in fuel cell researches due to their large surface areas, strong electroconductivity, and easy loading with foreign materials through chemical pathways owned by the graphene nanosheets [21,22]. Metal loaded graphene (2D and 3D) based catalysts also showed strong activity in the electrooxidation of alcohols and glucose, which are potential catalysts in fuel cell applications [5,11,21e28]. While practical trial in the DGFC assembly and systematic performance study still does not exist up to our knowledge, most of the fuel cell operations involving graphene catalysts were still focused on half-cell research in both glucose oxidation reaction
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(GOR) and oxygen reduction reaction (ORR) works [5e8,11,16,24]. Systematic study on the effect of the glucose and electrolyte concentration variation to the catalytic activity was even rare [5]. More importantly, the catalysts were fabricated on the substrate like glassy carbon electrode (GCE) or indium tin oxide (ITO) glass with the use of binder like Nafion, which involved complicated fabrication steps. This may also affect the performance of catalyst due to the reduction in active area of metal catalyst by the masking effect from binders or surfactants [11,20e26]. The ultrasonication of catalyst into the binder was also a major pretreatment step [25,26,29,30], which may cause the destruction of the catalyst structure, especially the 3D graphene catalyst [25,26,29]. Besides these weaknesses, traditional membrane electrode assembly (MEA) technique also involved hot pressing the electrode with electrolyte membrane for good contact [30], which also makes the fuel cell unit preparation complicated. In order to retain the 3D structure of graphene and make the fuel cell unit fabrication simple and friendly, the use of binder-free 3D graphene based direct catalyst became an important direction [27]. One of the pathways is the direct growth of graphene on nickel foam (NF) through chemical vapor deposition (CVD), followed by the removal of NF in acid to form 3D graphene foam (GF) as the first step. Metal loaded GF can then be produced by electrodeposition of the metal to the GF [27]. Another pathway is a one-step growth of metal loaded graphene aerogel on NF (M/GA/NF), which can be done by direct synthesis of M/GA on the NF through a mild conditioned hydrothermal reaction in reducing agent [31]. Such direct catalysts have exhibited strong electrocatalytic ability in the alcohol oxidation reactions, which are also comparable to that of 2D graphene based catalysts in similar reactions [31]. These works have a common strength that the highly porous NF (Mean pore size: 250 mm, porosity: 95%) was used as template instead of using traditional commercial carbon paper (Pore size range: 39e51 mm, porosity: 36e89%), which is a strong advantage for the 3D graphene structure fabricated on secondary substrate from raw GO solution. By combining the strengths of strong activity of bare and NF templated 3D graphenes [25,27,31], and strong bimetallc Pd-Pt [14e16], and metal loaded 2D graphene based catalysts [5e8,16] in glucose electrooxdiation mentioned above, we expected that the 3D Pd-Pt/GA/NF composite can also exhibit strong activity in the alkaline DGFC unit. This provides a possibility for the use of binder-free 3D graphene catalysts in the DGFC assembly. In this paper, a DGFC was assembled through a membrane electrode assembly (MEA) with the use of a binder-free Pd-Pt/GA/ NF plate as anode prepared through a mild conditioned hydrothermal reaction. It has been demonstrated in this study that this type of fuel cell using a cheap anion exchange membrane (AEM) and a binder-free catalyst can achieve a maximum power density of 1.25 mW cm2 under room temperature.
2.2. Anode preparation Raw GO was prepared through the modified Hummer's method which was reported elsewhere [32]. Bimetallic Pd-Pt loaded graphene aerogel on NF plate (Pd-Pt/GA/NFP) was prepared through the method reported by Tsang et al. [31]. Briefly, 120 mg of K2PdCl6 and various amounts (i.e. 48.6, 72.9, 145.8 mg) of K2PtCl6 were mixed with 30 cm3 exfoliated GO dispersion (6 mg cm3) and stirred vigorously for 30 min. The cleaned NFP (2.5 cm 1.5 cm) was put into the Pd-Pt/GO dispersion and allowed to sink in an ultrasonic bath for 10 min, followed by stationary soaking for a few hours. The Pd-Pt/GO soaked NFP was then put immediately into aqueous L-ascorbic acid (VC) solution with a concentration of 2.5 mg cm3, and the system was kept in an oven at 60 C for at least 2 days. The resulting hydrogel hybrid (Pd-Pt/GH/NFP) was then soaked in DI water for 3e4 days in order to remove the residue VC. The Pd-Pt/GH/NFP was then freeze dried under vacuum for overnight to obtain the Pd-Pt/GA/NFP. The elemental compositions of three Pd-Pt/GA/NFP samples based on XRF analysis are shown in Table S1 of which Pd1Pt0.98/GA/NFP, Pd1Pt1.62/GA/NFP, Pd1Pt2.32/GA/ NFP (at% based) stand for the sample prepared from 3:1, 2:1 and 1:1 Pd/Pt/GO dispersion, respectively. For comparison, monometallic Pd/GA/NFP and Pt/GA/NFP electrodes were prepared by identical pathway. The only difference was that K2PdCl6 or K2PtCl6 was added individually to the GO dispersion instead of adding together. Another bimetallic Pd-Pt/GA/NFP electrode with binder was prepared by identical pathway of Pd1Pt0.98/ GA/NFP, with the addition of 0.05 cm3 Nafion (10%) solution to 29.95 cm3 3:1 Pd/Pt/GO dispersion in the Pd/Pt/GO mixture formation step. 2.3. Electrode characterization The freeze dried Pd-Pt/GA/NFP and monometallic Pd, Pt loaded GA/NFP samples were characterized by SEM/EDX (Hitachi S4800 FEG SEM, JEOL-JSM5600), TEM (Philips FEI Tecnai G2 20 S-Twin Scanning TEM), and XRD diffractometer (D8 Advanced Diffractometer, Bruker AXS) with a Cu Ka radiation source. The sample composition was analyzed from the X-Ray Fluorescence spectrophotometer (EDAX-Eagle III). 2.4. Fuel cell assembly The fuel cell for testing was assembled by traditional AEM DGFC assembly, where Pt/C acted as cathode, and Pd-Pt/GA/NFP, Pd/GA/ NFP or Pt/GA/NFP served as anode of the cell. The cathode and anode materials were used directly as received or after freezedrying without further treatment. The fuel cell unit used was a batch type system (non-flowing type, Fig. 1) which was similar to the design used in Tung's group [1]. The fuel and oxidant containers
2. Experimental 2.1. Materials All the chemicals, i.e. K2PdCl6 (99.99%, Aldrich), K2PtCl6 (99.99%, Aldrich), glucose (Aladdin, 99%), L-ascorbic acid or vitamin C (VC, 99.7%, Guangzhou Guanghua Sci-Tech), hydrochloric acid (37%, Sigma-Aldrich), sulphuric acid (98%, Sigma-Aldrich), K2S2O8 (99%, Fisher Scientific), P2O5 (99%, Acros-Organic), KMnO4 (99.0%, SigmaAldrich), hydrogen peroxide (30%, Guangzhou Chemical), graphite powder (325 mesh, Uni-Chem), KOH (85%, Sigma-Aldrich), Nafion (10%, DuPont), and nickel foam (NF, 110 PPI, Artec Chemical) were used directly as purchased. Anion exchange membrane (AMI7001S, Membrane International) was preconditioned in 10%wt NaCl (99.9%, Sigma-Aldrich) solution overnight before use.
Fig. 1. Designation scheme of the batch type glucose fuel cell unit.
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with interior sizes of 2.6 cm (L) 2.0 cm (W) 1.9 cm (H) were used. The MEA was assembled with a wet AEM (5 cm 2.1 cm), a cathode, and an anode with an active area of 1 cm 1 cm. The fuel and oxidant of the fuel cell used in this work were glucose (0.125 M, 0.25 M, 0.5 M, 1 M and 2 M) with KOH (1 M, 3 M, and 5 M) solution, and O2-saturated KOH (1 M, 3 M, and 5 M) solution respectively. Throughout the study, the O2-saturated KOH was prepared by continuous pumping of ambient air through KOH pills to absorb moisture and CO2, which was then pumped into a KOH solution. The voltage and current of the cell were measured by a CHI660E electrochemical workstation under room temperature (i.e. 25 C). 3. Results and discussions 3.1. Materials characterization Digital image of the Pd1Pt0.98/GA/NF electrode against bare NF was shown in Fig. S1, which reflected that the silvery NFP turned into black after the hydrothermal reaction for the growth of Pd1Pt0.98/GA on the NFP. The sample and the bare NFP were also characterized by SEM/EDX as shown in Fig. 2. The low magnification image (Fig. 2a) of the as-prepared sample reflected that the PdPt particles were covered randomly on the graphene nanosheets (GNs) surface in the GA array, and the metal loaded graphene aerogel (M/GA) was covered on the NF skeleton. The in-depth analysis at high magnifications (Fig. 2bec) showed that the particles had random shapes from spherical to irregular. Fig. 2d showed the skeleton structure of the bare NFP, which indicated a highly porous structured composite for the sample. The M/GA was successfully covered on the NF skeleton surface throughout the reduction of M/GO/NF mixture. The presence of Pd and Pt in the sample was primarily confirmed from the Pd and Pt peaks in the EDX spectrum obtained in Fig. 2e, while the Ni peak corresponded to the NF backbone in the sample. The EDX elemental mapping (Fig. 3aeb) further showed that the pattern of Pt La-1 and Pd Ma-1 signal was almost identical with each other and with the particle pattern in the corresponding SEM image in Fig. 3d, while the C Ka-1 signal (Fig. 3c) was similar to the background pattern of the corresponding SEM image in Fig. 3d. Based on similar works from other reports, the bimetallic alloy particles were the dominating
Fig. 2. SEM images of (a) full frame image (100 mm), (b) and (c) magnified images (Scale bar: (b): 10 mm, (c): 5 mm) of Pd1Pt0.98/GA/NF, and (d) NF (Scale bar 100 mm) and (c) corresponding EDX spectrum.
Fig. 3. EDX mapping image of (a) Pt Ma-1, (b) Pd La-1, (c) C Ka-1, and (d) corresponding SEM images of Pd1Pt0.98/GA/NF (Scale bar (a): 2 mm).
products in the bimetallic metal loaded graphene when two different metal salts were added at the same time in the stirring step before the M/GO reduction taking place [20,21,33]. The element mapping pattern of Pt La-1 and Pd Ma-1 was with each other based on the EDX element mapping analysis results of the corresponding Pd-Pt loaded graphene. Based on the recipe of the M/GA preparation in the current study and the reports from elsewhere, the particles in the Pd-Pt/GA array was alloy particles in nature. Further analysis of the GA structure via TEM (Fig. 4) showed that the alloy metal particles were loaded onto the GNs surface in the GA array. Fig. 4b and c were the magnified image of the alloy particles in the selected area of Fig. 4a, which showed that the particles shapes were random, from square to circular. The selected area electronic diffraction (SAED) in Fig. 4d showed that the particles were highly crystalline in nature with reference to the clear spot pattern. According to the d-spacing data listed in Table S2,
Fig. 4. TEM image of Pd1Pt0.98/GA/NFP scratched from the fragment of the electrode. (a) Large area (Scale bar: 1 mm), (b) and (c) Magnified image of the PdPt alloy particle (Scale bar of (b): 100 nm, and (c): 20 nm), and (d) SAED pattern of the alloy particle shown in (c).
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three spots in the SAED pattern (Fig. 4d) corresponded to the (311), (311), and (200) planes of the particles respectively. By comparing with the Pd (JCPD card No. 46e1043) and Pt (JCPD card No. 04e0802) data, the Pd/Pt particles in the GA array has a f-c-c structure. This is a strong evidence of the formation of Pd-Pt alloy particles throughout the Pd-Pt/GO reduction during the GA synthesis. Further characterization of the Pd-Pt/GA/NF electrodes was done via the XRD since it was a powerful techniques for proving the presence of Pd-Pt alloys. From the XRD spectra obtained in Fig. 5a, the presence of Pd signal pattern was the evidence of successful loading of Pd onto the GA. The position of Pd 2q peaks at 39.78 , 46.3 , 67.9 , 81.8 were assigned to the Pd (111), Pd (200), Pd (220), and Pd (311) respectively, while those of Pt 2q peaks (39.48 , 45.8 , 67.4 , and 81.3 ) were assigned as Pt (111), Pt (200), Pt (220), and Pt (311). The result primarily showed that monometallic Pd NPs and Pt NPs have f-c-c structure (JPCDS card No. 46e1043 and 04e0802) with high crystallinity [34]. From the magnified view of M (111) of the Pd-Pt/GA samples at 38-43 (Fig. 5b), it showed that the peak position was lied between that of monometallic Pd and Pt which reflected from Table S3, the peak position (Pd1Pt2.32: 39.586 , Pd1Pt1.62: 39.699 , and Pd1Pt0.98: 39.720 ) was shifted to 2q value of Pd(111) (39.780 ) from Pt(111) (39.479 ) when Pt content reduced in the Pd-Pt NPs. The result was also highly coherent to the SAED pattern obtained from the TEM results (Fig. 4d), which reflected from the matched results of the M(200) and M(311) planes in the Pd-Pt NPs in Pd1Pt0.98/GA/NF (Table S2). Based on the reported
literature, it was the evidence for the presence of Pd-Pt alloy particle [34]. Such finding was also true for the 3D GA structure. Meanwhile, the GO diffraction peak was 10 in the raw GO (Fig. 5a), and this peak was not observed in the same region for both the monometallic Pd/GA and Pt/GA, and bimetallic Pd-Pt/GA, respectively. The strengthen of the board graphene amorphous peak (C(002)) at 25 was observed instead. This revealed that the reduction of oxide functional groups in GO by VC was taken place throughout the hydrothermal reaction [35].
Fig. 5. XRD pattern of (a) Pt-Pd/GAs removed from NF and GO, and (b) the extended view of M (111) peak (38-43 ).
Fig. 6. Polarization curve of Pd/GA/NF, Pt/GA/NF, binder-free Pd1Pt0.98/GA/NF, and Pd1Pt0.98-Nafion/GA/NF driven DGFC with 1 M glucose/5 M KOH fuel.
3.2. Fuel cell performance 3.2.1. Basic testing The polarization curves of the M/GA/NF driven DGFC were shown in Fig. 6. The open circuit voltage (OCV) of the cell and the maximum power density recorded were þ1.1 V and 0.88 mW cm2 at 0.2 V respectively, when Pt1Pd0.98/GA/NFP was used. Compared to the results of monometallic Pt/GA/NFP (0.62 mW cm2 at 0.3 V, OCV: þ1.1 V) and Pd/GA/NFP (0.69 mW cm2 at 0.4 V, OCV: þ1.1 V), the result indicated that DGFC operated with bimetallic Pt1Pd0.98/ GA/NFP had much stronger activity than monometallic Pd (1.27 times) or Pt (1.42 times) loaded GA catalysts cell. The tendency of the result is similar to the previous reports focused on the Pd-Pt based electrocatalyst [14]. A possible reason for the performance enhancement when using bimetallic Pd-Pt may be due to the synergistic effect between Pd and Pt in the catalyst, which was resulted from the modification of the electronic and collective surface properties after the alloying process of Pd and Pt throughout the hydrothermal reaction [14]. This was partially reflected from the shifted M(111) position of the bimetallic Pd-Pt from the XRD results (Fig. 5b), the theoretical anodic reaction shown in Fig. 1 and the generalized mechanism of glucose electrode oxidation in the Pd-Pt catalyst from different literature as shown below which are based on the Pd catalyzed glucose electrooxidation [8e10,12,14]. Step 1: Pd þ Glucose/Pd-H þ Intermediates. Step 2: Pd þ xOH/Pd(OH)x þ xe-. Step 3: Pd(OH)x þ Intermediates/Pd þ Glucolactone/Gluconic acid. Step 4: Pd(OH)x þ Glucose/Pd þ Glucolactone/Gluconic acid. The effect of binder on the basic performance of the Pd1Pt0.98/ GA/NFP driven DGFC was also studied. The result showed that when 0.02 v/v% Nafion was included in the Pd1Pt0.98/GA/NFP preparation, the maximum output power density was 0.67 mW cm2 at 0.6 V, which was lower than that of the binderfree Pd1Pt0.98/GA/NFP (0.88 mW cm2) by 33.5% in power density,
C.H.A. Tsang, D.Y.C. Leung / Solid State Sciences 71 (2017) 123e129
even though the OCV recorded (þ1.01 V) was similar to the binderfree Pd1Pt0.98/GA/NFP (þ1.1 V). The possible reason of such observation may be due to the presence of Nafion stabilizer in the GA array. Even though it is one of the commonly used electrolyte polymer for stabilizing the metals in the electrocatalytic electrode preparation, the active surface area of the metal particles may be masked by the binder. This matched with the hypothesis of some other groups [20]. As a result, the output power density may be reduced in the identical reactions with operation parameters. This showed the advantages of the use of binder-free electrocatalyst in the DGFC unit operation.
3.2.2. Effect of glucose concentration According to the proposed mechanism of glucose electrooxidation above, the glucose and OH concentration are key species in the electrooxidation of glucose oxidation throughout the unit fuel cell operation. As a result, it is of our interest to investigate the effect of these two species on the performance of DGFC in the current study. The effect of glucose concentration on the DGFC performance was firstly studied with glucose concentrations set at 0.125, 0.25 M, 0.5 M, 1 M, and 2 M with a constant KOH concentration at 5 M. As shown in Fig. 7 and Table 1, the performance of the DGFC was the lowest when the concentration of glucose was 0.125 M with maximum power density only reached 0.22 mW cm2 at 0.2 V. The performance was gradually enhanced when the concentration of glucose increased to 0.25 M with maximum power density of 0.84 mW cm2 at 0.2 V, and then 0.5 M with the corresponding power density of 0.91 mW cm2 recorded at 0.2 V. However, the performance of the cell slightly reduced when the concentration of glucose increased to 1 M, where the power density of the unit reduced to 0.88 mW cm2 at 0.2 V. The output power density of the DGFC unit further reduced to 0.79 mW cm2 at 0.2 V when the glucose concentration further increased to 2 M. From literature, glucose cell performance was enhanced with increasing concentration of glucose until reaching an optimum value [2]. In our case, similar tendency was observed in the fuel cell unit operation when the glucose concentration increased from 0.125 M to 0.5 M. Increase in the glucose concentration will enhance the kinetics of glucose oxidation, which result in an increase in power density of the cell with reference to the theoretical anodic reaction as shown in Fig. 1 [2]. However, the cell performance depressed when the glucose concentration was higher than the optimum value, possibly due to the blockage of the active site by the increased glucose concentration. Consequently, the adsorption of OH on the active site became difficult, and result in a
Fig. 7. Polarization curve of Pd1Pt0.98/GA/NF driven DGFC under different glucose concentration at fixed KOH concentration (5 M).
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Table 1 List of the peak power density variation of Pd1Pt0.98/GA/NF driven DGFC at fixed KOH concentration (5 M). Glucose concentration (M)
Peak power density (mW cm2)
0.125 0.25 0.5 1 2
0.22 0.84 0.914 0.88 0.79
depression in glucose oxidation kinetics on the catalyst active site [2]. Hence, the cell performance reduced with increase the glucose concentration from 0.5 M to 2 M. As such, 0.5 M was the optimum glucose concentration in the present fuel cell operation. 3.2.3. Effect of hydroxide ion concentration Besides the glucose concentration, concentration of OH in the solution also influences the fuel cell operation based on the proposed mechanism above. In the current study, the concentration range of KOH was set at 1 M, 3 M and 5 M, while the optimum glucose concentration of 0.5 M was adopted based on the result of previous section. Pd1Pt0.98/GA/NF electrode was still chosen as the target of study in this section. From the polarization curves as shown in Fig. 8, the maximum power density gradually increased from 0.72 mW cm2 (0.2 V) to 1.25 mW cm2 when the concentration of KOH increased from 1 M to 3 M. However, the maximum power density reduced to 0.91 mW cm2 when the concentration of KOH further increased to 5 M. At the same time, the cell voltage increased continuously from þ0.76 V at KOH concentration of 1 M
Fig. 8. Polarization curve of Pd1Pt0.98/GA/NF driven DGFC under different KOH concentration at fixed glucose concentration (0.5 M).
Fig. 9. Polarization curve of Pd-Pt/GA/NF driven DGFC with different Pd/Pt ratio at 0.5 M glucose/3 M KOH.
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Table 2 List of the power density of the selected batch AEM-DGFC. Catalyst
Fuel/Electrolyte concentration
Peak power density (mW cm2)
Electrode active area (cm2)
Metal loading (wt%)
Ref
Pt-Bi/C Pt-Au/C AuPt/C Au-MnO2/C Au/C Au Nanocoral Au NPs Au Film PdPt(4:1)/C Pd1Pt0.98/GA/NF
0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.5
0.56 0.72 0.72 1.10 0.86 0.85 0.34 0.12 0.52 1.25
5 5 9 4 4 1 1 1 N/A 1
12 7.5 14.5 20 40 N/A N/A N/A 21 8.13
[19] [19] [3] [4] [4] [1] [1] [1] [17] Current work
M M M M M M M M M M
Glucose/1 M KOH Glucose/1 M KOH Glucose/1 M KOH Glucose/1 M KOH Glucose/1 M KOH Glucose/0.5 M KOH Glucose/0.5 M KOH Glucose/0.5 M KOH Glucose/1 M KOH Glucose/3 M KOH
to þ1.05 V at 5 M. This result revealed that the DGFC achieved the best performance when the concentration of the KOH was about 3 M. Reported works on similar system operated by different kinds of metallic catalyst also indicated an increase in the maximum power density with increasing KOH (OH) concentration until reaching an optimum value, and decreased thereafter [2]. A possible reason is that under a constant glucose concentration, increase in OH concentration enhances the transport activity of the hydroxide ions from cathode to anode, which ultimately enhances the kinetics of the glucose oxidation in the cell [2]. However, further increase in the OH concentration will result in a power density reduction due to the decline in the cell performance caused by the blockage of the active sites of the catalyst, and subsequently affects the adsorption of glucose on the anode catalyst [2]. In the current study, the power and corresponding current density of the cell unit increased simultaneously when the KOH concentration in the system increased from 1 M to 3 M, while both the current and power density of the cell reduced when the KOH concentration further increased to 5 M. These trends highly matched with those from previous study [2]. So, in the present system operation, the optimum KOH concentration was determined to be 3 M. 3.2.4. Effect of the Pd/Pt ratio The effect of the atomic ratio between Pd and Pt in the bimetallic GA/NF electrode on the cell performance was also of great interest. Based on the best performance of the catalyst described from the previous sections, the fuel and oxidizing agent used at the anode and the cathode was set to be 0.5 M glucose/3 M KOH and 3 M KOH, respectively. Fig. 9 shows that the power density of the cell increased from 0.64 mW cm2 to 0.89 mW cm2 when the Pd/Pt ratio in the catalyst increased from 1:2.32 to 1:1.62. The power density increased to 1.25 mW cm2 when the Pd/Pt ratio further increased to 1:0.98. This showed that the power output of the cell unit increased when the Pd/Pt ratio approaching to 1:1. A possible reason for the enhancement is the synergistic effect exhibited from the bimetallic Pd/Pt particles after mixing up of the Pd and Pt lattices [36]. This phenomenon is particularly obvious when the Pd/Pt ratio approaches 1:1. By comparing the performance of the selected batch mode of AEM-DGFC operation from literature as listed in Table 2 [1,3,4,17,19], the power output of the present Pd1Pt0.98/GA/NFP (0.5 M glucose/3 M KOH) DGFC stack was found to be the highest among those reported results due to the relatively low metal loading in the Pd-Pt/GA/NFP electrode (8.13 wt%) when compared to most of the listed electrocatalysts (12e40 wt%) [3,4,17,19]. Even though the metal loading in Pt-Au/C (7.5 wt%) was slightly lower than the Pd1Pt0.98/GA/NFP (8.13 wt%) used in the current study, the peak power density recorded from Pd1Pt0.98/GA/NFP in current work was still higher than that of Pt-Au/C [19]. More importantly,
most of the catalytic anodes involve complicated fabrication process like hot press and the use of binder [3,4,17,19]. In contrast, the Pd-Pt/GA/NFP anodes can be used without treatments in the current study, with stronger output power density than the binder containing version (Fig. 6). It showed that the binder-free Pd-Pt/GA electrocatalyst fabricated on NFP electrode provides a possibility of using as a customized binder-free graphene electrode for different applications in fuel cells, as well as in bulk battery devices. 4. Conclusions Binder-free Pd-Pt/GA/NFP electrocatalysts with different Pd/Pt ratios were demonstrated to have strong activity in the batch type AEM-DGFC unit operation under room temperature when compared to corresponding monometallic GA/NFP anodes tested under 1 M glucose/5 M KOH condition. The cell showed strongest activity when the fuel (i.e. glucose) and electrolyte (i.e. KOH) concentration were 0.5 M and 3 M respectively with Pd1Pt0.98/GA/NFP chosen as the representative anode. In addition, the activity of the cell was enhanced when the Pd/Pt ratio approached 1:1 in the 0.5 M glucose/3 M KOH system in the cell operation, achieving a maximum power density output of 1.25 mW cm2 at 0.2 V. The performance of the cell with binder-free GA anodic catalysts showed stronger activity than most batch type AEM-DGFC with other metal catalysts, and the GA anodic catalyst with the use of binder. This type of shape and size customized metal loaded electrocatalyst provided the possibility of producing 3D structured direct graphene electrodes for fuel cells and batteries with nondestructive fabrication method. Acknowledgement This project is supported by the URC's PDF scheme and the CRCG grant (Code: 201511159115) of the University of Hong Kong. Appendix A. Supplementary data Supplementary data related to this chapter can be found at http://dx.doi.org/10.1016/j.solidstatesciences.2017.07.014. References [1] S.P. Tung, T.K. Huang, C.Y. Lee, H.T. Chiu, RSC Adv. 2 (2012) 1068e1073. [2] L. An, T.S. Zhao, S.Y. Shen, Q.X. Wu, R. Chen, J. Power Sources 196 (2011) 186e190. [3] D. Basu, S. Basu, Int. J. Hydrog. Energy 36 (2011) 14923e14929. [4] L. Li, K. Scott, E.H. Yu, J. Power Sources 221 (2013) 1e5. [5] Q.F. Shi, G.W. Diao, S.L. Mu, Electrochim. Acta 133 (2014) 335e346. [6] S. Ghosh, Y. Holade, H. Remita, K. Servat, P. Beaunier, A. Hagege, K.B. Kokoh, T.W. Napporn, Electrochim. Acta 212 (2016) 864e875. [7] A.C. Joshi, G.B. Markad, S.K. Haram, Electrochim. Acta 161 (2015) 108e114. [8] C.C. Chen, C.L. Lin, L.C. Chen, Electrochim. Acta 152 (2015) 408e416.
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