Next Article in Journal
Recent Progresses on Metal Halide Perovskite-Based Material as Potential Photocatalyst
Next Article in Special Issue
Reduced Graphene Oxide/ZnIn2S4 Nanocomposite Photocatalyst with Enhanced Photocatalytic Performance for the Degradation of Naproxen under Visible Light Irradiation
Previous Article in Journal
Understanding Surface Basic Sites of Catalysts: Kinetics and Mechanism of Dehydrochlorination of 1,2-Dichloroethane over N-Doped Carbon Catalysts
Previous Article in Special Issue
The Development of Catalyst Materials for the Advanced Lithium–Sulfur Battery
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 10
Issue 6
10.3390/catal10060708
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Pt-Amorphous Barium Aluminum Oxide/Carbon Catalysts for an Enhanced Methanol Electrooxidation Reaction

by
Tzu Hsuan Chiang
*,
Wan-Yu Hou
,
Jia-Wei Hsu
and
Yu-Si Chen
Department of Energy Engineering, National United University, No. 2, Lienda, Nan-Shi Li, Miaoli 36006, Taiwan
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(6), 708; https://doi.org/10.3390/catal10060708
Submission received: 28 May 2020 / Revised: 18 June 2020 / Accepted: 22 June 2020 / Published: 24 June 2020
(This article belongs to the Special Issue Emerging Nanostructured Catalytic Materials for Energy and Environmental Applications)
Browse Figures
Review Reports Versions Notes

Abstract

:
A new type of amorphous barium aluminum oxide was synthesized using a polyol thermal method involving a mixture with Vulcan XC-72 carbon and supported with 20%Pt catalysts to enhance the activity of a methanol electrooxidation reaction (MOR). The maximum current density, electrochemically active surface area (ECSA), and electrochemical impedance spectra (EIS) of the obtained catalysts for MOR were determined. The MORs of barium aluminum oxide with different calcination temperatures and Ba and Al contact ratios were studied. The MOR of the uncalcined amorphous Ba0.5AlOx catalysts prepared with a mole ratio of 2/1 Ba/Al mixed with Vulcan XC-72 carbon and supported with 20%Pt catalyst (Pt-Ba0.5AlOx/C) was enhanced compared with that of 20%Pt-Al2O3/C and 20%Pt/C catalysts due to its obtained largest maximum current density of 3.89 mA/cm2 and the largest ECSA of 49.83 m2/g. Therefore, Pt-Ba0.5AlOx/C could provide a new pathway to achieve a sufficient electrical conductivity, and possible synergistic effects with other active components improved the catalytic activity and stability of the prepared catalyst in MOR.

1. Introduction

In anode, electrocatalysts of direct methanol fuel cells (DMFCs), platinum (Pt) [1], and Pt alloys, such as PtRu [2,3], PtRuCuW [4], PtPd [5], Pt45Ru45M10/C (M = Fe, Co, and Ni) [6], have been used for a methanol electrooxidation reaction (MOR). However, these precious metals are poisoned by carbon monoxide (CO) during MOR, causing a rapid decrease in catalytic activities and producing a low power density of DMFCs. Therefore, anode electrodes need a high loading amount of Pt or Pt alloys to maintain the DMFC performance, but this requirement noticeably increases the cost of a whole fuel cell system. A commercial 20%Pt/C catalyst is most commonly used as cathode catalysts of polymer electrolyte fuel cell (PEMFC), DMFC, and other fuel cells [7]. Advanced supports with synergistic effects for Pt catalysts should be developed by loading Pt on metal oxide catalysts/C catalysts to improve the CO tolerance, reduce Pt loading, and increase electrochemical durability; some metal oxides that have been studied are CeO2 [8], SnOx [9], TiO2, Al2O3 [10], and Nb2O5 [11] because they have a facile oxygen species-releasing ability and good corrosion resistance [11]. In addition, the combination of two or more transition metal oxides, which are better than simple individual oxides, can synergistically enhance the catalytic activity of methanol oxidation [11]. Some bimetallic oxides, such as NiMoO4 [12], Ni-doped CeO2 [13], Ce0.2Mo0.8O3-δ [14], SrMO3 (M = Ti, Ru) [15], and CexZr1-xO2 [16], have also been explored.
Among various oxides, Al2O3 has been able to promote a high proportion of metallic Pt species and oxidation state of Pt formation, resulting in Pt/Al2O3 that has good high activity for CO oxidation [17] and enhanced MOR activity. In addition, amorphous Al2O3 has a significant density of negative charges [18], which are easily adsorbed on carbon black with positive surface charges [19] to form a strongly hydrophilic hybrid support and improve Pt deposition, which should play an important role in determining the catalytic activity. Al2O3 is modified by doping with other metals, such as Mo [20] and Ru [21]. This approach is effective in refining the physical and chemical properties of Al2O3. This study proposed the use of bimetallic oxide with barium (Ba) precursor as a dopant to enhance the catalytic activity of Al2O3 based on synergistic effects. This proposal was also based on barium oxide (BaO)-modified Pd-based catalyst that promoted methanol conversion [22]. BaO is also a promoter of La1.6Ba0.4NiO4 catalysts that improve the catalytic activity of NO direct decomposition [23]. Amorphous barium aluminum oxide catalysts were successfully prepared via a facile polyol thermal method and combined with carbon black to support 20 wt.% of Pt particles as an electrocatalyst (20%Pt-BaAlOx/C) with improved activity, stability for MOR, and COads tolerance. Different calcining temperatures and different contact ratios of Ba and Al precursors were also discussed in detail.

2. Results and Discussion

2.1. Effect of Different Calcining Temperatures on the MOR of the Catalyst

Figure 1a illustrates the cyclic voltammetry (CV) of Pt-Ba0.5AlOx/C catalysts with Ba0.5AlOx exposed to different calcining temperatures. All the catalysts had similar forward peak potentials, namely, 0.66, 0.65, and 0.65 V. However, the maximum current density in the forward sweep of CVs for CH3OH oxidation and electrochemically active surface area (ECSA) are important properties for determining elements in the evaluation of the MOR activity of an electrode in DMFCs. A catalyst with a large ECSA usually possesses a high electrocatalytic activity [24]. In Figure 1a,b, the performance of the obtained uncalcined Ba0.5AlOx catalyst was higher than that of the two other catalysts at calcining temperatures of 200 °C and 400 °C. This result indicated that the maximum current density (3.89 mA/cm2) and ECSA (49.83 m2/g) for MOR of the uncalcined catalysts were higher than those of the catalysts calcined at 200 °C (2.89 mA/cm2 and 25.12 m2/g) and 400 °C (1.08 mA/cm2 and 22.52 m2/g). The charge transport properties of various catalysts were assessed by using the Nyquist plots of the electrochemical impedance spectra (EIS) measurement; the diameter of the primary semicircle is closely related to charge reaction resistances (Rct) associated with MOR [25]. The small diameter of the semicircle corresponds to the high electrocatalytic activity of a catalyst for MOR. As shown in Figure 1c, the diameter of the primary semicircle for the impedance data could be modeled in the equivalent circuit model illustrated in Figure 1d [25,26]. The circuit elements used were as follows: Rs is the solution resistance, Ro is the contact resistance between the catalyst and the support electrode, and a constant phase element (CPE) is the double-layer capacitance, which is associated with the adsorption of intermediates formed during MOR [25]. In Figure 1c, Rct of 20%Pt-Ba0.5AlOx/C catalysts prepared with the uncalcined Ba0.5AlOx was smaller than that of the catalysts calcined at 200 °C and 400 °C. These results indicated that the ion transfer rate of the MOR of the 20%Pt-Ba0.5AlOx/C catalyst prepared by uncalcining Ba0.5AlOx was faster than that of the two other catalysts calcined at 200 °C and 400 °C because of the presence of conductive carbon support [12]. Thus, the best electrocatalytic activity of the catalyst for MOR was obtained.
Amorphous structures were generated when uncalcined Ba0.5AlOx catalysts and catalysts calcined at 200 °C to 400 °C were used, as revealed by the XRD data and TEM image shown in Figure 2a,b. However, their particle size distributions differed, suggesting that particle size affected catalyst activities. The histograms of the particle size distribution of the uncalcined Ba0.5AlOx catalysts and the catalysts calcined at 200 °C to 400 °C are shown in Figure 3. The results indicated that the maximum average particle size ranged from 0.6 μm to 3.0 μm, 0.6, 2.0, and 8.0 μm, 5.0 μm to 10.5 μm, respectively. Their specific surface areas were 7.79 ± 0.01, 5.24 ± 0.01, and 3.48 ± 0.01 m2/g, which corresponded to the small particle sizes of catalysts with large specific surface areas. These results elucidated the effect of the observed particle size on MOR. Bergamaski et al. [27] reported the increasing Pt-CO and Pt-OH bond strength with decreasing particle size on MOR. Therefore, the effect of the particle size of Ba0.5AlOx observed on the MOR was due to the increased bond strength of Ba0.5AlOx-OHads with a decrease in their particle size, leading to a higher MOR. Therefore, the MOR activity decreased with the calcined temperature of Ba0.5AlOx catalysts, with an increasing influence on increased particle size and decreased surface areas. The results obtained showed that the uncalcined Ba0.5AlOx catalysts possessed the outstanding MOR activity, having the smallest particle size and the largest specific surface areas.
O 1s, Ba 3d, and Al 2p XPS of Ba0.5AlOx under different calcining temperatures are shown in Figure 4. The binding energies of O 1s and Ba 3d were similar in the uncalcined Ba0.5AlOx catalysts or the catalysts calcined at 200 °C and 400 °C. The peak at a binding energy of O 1s for each catalyst displayed the aluminum hydroxide (Al(OH)3) or aluminum oxyhydroxide state (AlO(OH)) at 532.4 eV and aluminum oxide (Al2O3) at 531.33 eV [28]. The existence of −OH possibly causes Al2O3 to react with water, forming Al(OH)3 [29]. In this reaction, water is from the catalyst exposed to the atmosphere (moisture) before XPS measurement [30] and Al2O3 that reacts with Al(OH)3 to generate AlO(OH) [29]. The peaks at binding energies of 780.4 and 795.5 eV for all the catalysts can be assigned to Ba 3d5/2 and 3d3/2 core lines of Ba2+, respectively [31]. Particularly, the binding energies of Al 2p XPS varied among the Ba0.5AlOx catalysts calcined at different temperatures. Only one peak binding energy at 74.1 eV was obtained for the uncalcined Ba0.5AlOx. However, when the calcining temperature increased to 200 °C and 400 °C, the peaks of the binding energies of Al 2p positively shifted to 75.3 and 75.8 eV, which were assigned to Al(OH)3 and AlO(OH), respectively [31].

2.2. Effect of Different Ratios of Ba and Al Precursors on MOR

The CVs of MOR at −0.2 V to 0.9 V on 20%Pt with various BaAlOx/C catalysts composed of BaAlOx were prepared with different ratios of Ba and Al precursors: 1/1, 1/2, 1/3, 2/1, and 3/1 mole ratios named Ba0.5Al0.5Ox, Ba0.5AlOx, Ba0.33AlOx, BaAl0.5Ox, and BaAl0.33Ox, respectively, as shown in Figure 5a. The forward peak potentials of BaAlOx catalysts prepared with different ratios of Ba and Al contact on 20%Pt-various BaAlOx/C catalysts observed at around 0.66–0.67 V in the forward scan (Table 1) were characteristic of MOR through the oxidation of Pt-(CH3OH)ads on a catalyst surface [32]. However, the forward peak potentials of 20%Pt-Al2O3/C and 20%Pt/C negatively shifted to 0.64 and 0.65 V compared with that of 20%Pt-Ba0.5AlOx/C catalysts, which indicated that their MOR activity was low. By contrast, the backed peak potentials of various catalysts at around 0.46–0.48 V were similar to that of 20%Pt/C (0.47 V) in the reverse scan and primarily associated with the removal of the residual carbon species formed in the forward scan [33]. Simultaneously, these BaAlOx catalysts could improve the Pt surface and would be covered with methanol, causing increased H chemisorption on bare Pt, such as PtOHad + H+ + e− → Pt + H2O [34]. Furthermore, Table 1, obtained from Figure 5a, shows that the maximum current densities at the forward and backward peak potentials of 20%Pt-Al2O3/C catalysts were 0.58 and 0.27 mA/cm2, which were lower than those of 20%Pt/C catalysts (1.47 and 0.4 mA/cm2). This result indicated that Al2O3 was not catalytically active in MOR. Therefore, it obtained the smallest ECSA and the largest EIS, as shown in Figure 5b,c, respectively. However, the maximum current density at the forward and backward peak potentials of BaAlOx catalysts prepared with different ratios of Ba mixed with Al was higher than that of Al2O3. These results suggested that the presence of Ba in BaAlOx catalysts facilitated the oxidation of freshly chemisorbed species and led to an enhanced MOR through which BaAlOx catalysts provided active oxygen for the effective removal of intermediates, such as CO on the Pt surface. The maximum current density (4.07 mA/cm2) at the forward peak of 20%Pt-Ba0.5AlOx/C (Ba/Al mole ratio of 1/2) catalysts was higher than those of 20%Pt/C and other BaAlOx catalysts prepared with various mole ratios of Ba and Al contact. These results showed that methanol was more easily oxidized with 20%Pt-Ba0.5AlOx/C catalyst than with other catalysts, including the 20%Pt/C catalyst.
Furthermore, the ECSA of the different catalysts is shown in Figure 5b and Table 1. The results showed that the MOR of 20%Pt-Ba0.5AlOx/C catalysts was dependent on the largest ECSA (49.83 m2/g) among the catalysts. In Figure 5d, Rct of 20%Pt-Ba0.5AlOx/C catalysts was lower than that of 20%Pt/C catalysts and other catalysts, and this parameter corresponded to the high electrocatalytic activity of MOR.
Figure 6 shows the XRD data of various BaAlOx catalysts with different ratios of Ba and Al contact. Amorphous structures formed when the mole ratio of Al precursor contacts was greater than or equal to that of Ba precursors, such as Ba0.5Al0.5Ox (Ba/Al = 1/1), Ba0.5AlOx (Ba/Al = 1/2), and Ba0.33AlOx (Ba/Al = 1/3). A crystallization compound that consisted of crystalline Al2O3 (CCDS11-0517) and Ba2Al2O5 (44-0474) structures generated by the mole ratio of Ba precursors was greater than that of the mole ratio of Al precursors, such as BaAl0.5Ox (Ba/Al = 2/1) and BaAl0.33Ox (Ba/Al = 3/1). Unfortunately, the MOR activity of these crystalline BaAlOx catalysts was lower than amorphous BaAlOx catalysts because amorphous BaAlOx catalysts were possibly composed of amorphous Al2O3 existing in neutral oxygen vacancies [35]. Abundant oxygen vacancies exhibit a dramatically improved electrocatalytic activity of MOR [36]. As more oxygen vacancies are generated, they lead to enhance the surface oxygen concentration on metal oxide catalysts, providing more metal oxide-OHads bond [37], which boosts the conversion of COads to CO2 on the Pt-metal oxide/C catalyst [38]. Therefore, the oxygen vacancies of the Ba0.5AlOx catalyst were more than those of the Ba0.5Al0.5Ox catalyst because the Al mole ratio of Ba0.5AlOx was higher than that of Ba0.5Al0.5Ox due to the presence of more amorphous Al2O3 in the catalysts. Amorphous Al2O3 with abundant negative charges also favors the deposition and dispersion of Pt nanoparticles via a strong electrostatic interaction [39]. However, the mole ratio of Al increased to 3. As a result, the MOR activity of the Ba0.33AlOx catalyst was lower than that of other catalysts, possibly because excessive Al contacts produce oxygen vacancy defects and cause the long-distance elongation of O–O bond [40]; thus, the MOR activity decreased. The SEM and TEM images of the 20%Pt-Ba0.5AlOx/C catalyst are illustrated in Figure 7 and Figure 8a. In Figure 8b, the Pt 4f XPS spectra of 20%Pt/C and 20%Pt-Ba0.5AlOx/C catalysts were assigned to the metallic state of Pt0 and the oxide states of Pt2+ and Pt4+. The binding energy of the Pt 4f peaks of 20%Pt-Ba0.5AlOx/C shifted positively compared with that of 20%Pt/C, and this shift could be attributed to the interactions between Pt and metal oxides [14] or metal-support interactions [41], indicating electron transfer [42] between Pt and Ba0.5AlOx or atomically dispersed Pt0 and Pt aggregates on metal oxide [43]. Moreover, the Pt0 content of 20%Pt-Ba0.5AlOx/C catalysts increased to 70% compared with that of 52.6% of 20%Pt/C catalysts, thereby providing more available active Pt sites for methanol adsorption [14].
The bifunctional mechanism of Pt-metal oxide/C catalysts depends on the synergistic contribution of bimetallic oxide on Pt catalysts to MOR [12] occurred by the OH adsorbed on the surface of the metal oxide that may oxidize the CO present on the surface of Pt [11,37,44]. Therefore, the bifunctional mechanism of MOR with 20%Pt-Ba0.5AlOx/C catalysts can be summarized in (1)–(4). In (1), methanol is initially adsorbed on Pt, and its methanolic proton is simultaneously lost to a basic oxide ion. In (2), the presence of Ba0.5AlOx groups may induce OHads species to react with COads on the Pt surface. As a result, CO2 is produced, as shown in (3). Consequently, more active sites on the Pt surface are released, thereby facilitating methanol oxidation. A strongly bound intermediate is expected to be removed from the electrocatalyst surface by reacting with Ba0.5AlOx. In (4), Pt loaded on Ba0.5AlOx may react with CO to form CO2.
CH3OHad → COad + 4H+ + 4e−
Ba0.5AlOx + H2O → Ba0.5AlOx-OHads + H+ + e−
Pt-COads + Ba0.5AlOx-OHads → Pt + Ba0.5AlOx + CO2 + H+ +e−
Pt-Ba0.5AlOx + yCO → Pt-Ba0.5AlOx-y + yCO2
The long-term electrochemical stability of 20%Pt/C and 20%Pt-Ba0.5AlOx/C catalysts in MOR was investigated by performing 1000 continuous potential cycles in 1 M CH3OH + 0.5 M H2SO4 solution (Figure 9). Clearly, the current density of both samples decayed when the number of cycles increased. The maximum current density of the 20%Pt-Ba0.5AlOx/C catalyst was still higher than that of the 20%Pt/C catalyst after 1000 cycles. Therefore, the long-term electrochemical stability of 20%Pt-Ba0.5AlOx/C catalysts was better than that of 20%Pt/C catalysts, which was attributed to the efficient removal of the intermediate species benefited from the synergetic effect [19] between Pt and Ba0.5AlOx.

3. Materials and Methods

3.1. Chemicals and Materials

The following chemicals were used in this study: aluminum nitrate nonahydrate (Al(NO3)3·9H2O), barium nitrate (Ba(NO3)2), and sodium hydroxide (NaOH; Showa Chemical Industry Co., Ltd., Tokyo, Japan); chloroplatinic acid (IV) hexahydrate (H2PtCl6·6H2O; Alfa Aesar, Lancashir, UK); ethylene glycol (C2H6O2; J.T. Baker®, Phillipsburg, OH, USA); sulfuric acid (H2SO4) and methanol (CH3OH; Aencore Chemical Co., Ltd., Surrey Hills, Australia); ethanol (C2H5OH, 95%; ECHO Chemical Co., Ltd., Miaoli, Taiwan); Vulcan XC-72 carbon black (BET surface area of 250 m2/g; Cabot Co., Alpharetta, GA, USA); 20 wt.% Nafion solution (DuPont Co. Ltd., Wilmington, DE, USA).

3.2. Preparation of Pt–Barium Aluminum Oxide/C Catalysts

Amorphous BaAlOx catalysts were synthesized through a simple polyol thermal method. In a typical synthesis, 100 mL of 0.1 M Al(NO3)3·9H2O and 100 mL of 0.1 M Ba(NO3)2 were mixed with 40 mL of ethylene glycol under magnetic stirring and heated at 120 °C for 1 h, thereby producing a white precipitate. The product was cooled and centrifuged at 5000 rpm three times for 15 min each. After centrifugation, the solution was poured, and the precipitate was washed with distilled water, placed in a dish, and heated in an oven at 80 °C for 24 h. The 20 wt.% Pt-BaAlOx/C prepared by 0.04 g of amorphous BaAlOx powder was mixed with 0.04 g of Vulcan XC-72 carbon black (1:1 weight ratio), dispersed in 40 mL of ethylene glycol in a 100 mL beaker, and ultrasonicated for 15 min under magnetic stirring for 1 h to form a uniform suspension. Afterward, 10 mL of 5.2 mg/mL H2PtCl6 in ethylene glycol solution was supplemented to the suspension. After the mixture was stirred vigorously for 1 h, the pH of the suspension was adjusted to 11 by adding 0.5 M NaOH to ethylene glycol dropwise. The mixture was heated to 140 °C, and a reflux system was set up under reflux for 3 h. After the mixture was cooled, the precipitates were centrifuged at 5000 rpm three times for 15 min each. The solution was poured, and the precipitate was washed with distilled water, placed in a dish, and dried in an oven at 80 °C for 24 h. Finally, 13.6 wt.% ± 1.2 wt.%Pt metal content in 20 wt.%Pt-BaAlOx/C and 20 wt.%Pt/C catalysts was confirmed by an inductively-coupled plasma optical emission spectrometer (ICP-OED, Agilent 725, Santa Clara, CA, USA).

3.3. Material Characterization

The morphological characteristics of the powder were analyzed through scanning electron microscopy (SEM) by using a JED 2300 instrument (JEOL Ltd., Peabody, MA, USA). The crystallographic patterns of the powder were recorded through X-ray diffraction (XRD) in a Rigaku (Tokyo, Japan) ultima IV rotating anode diffractometer with a Ni-filtered Cu–K radiation source (wavelength of 1.54 Å). X-ray photoelectron spectroscopy (XPS) was performed with VG Scientific ESCALAB 250 (Thermo Fisher Scientific Inc., Loughborough, UK) equipped with a dual Al X-ray source operated at 200 W and 15 kV, 650 μm of beam size, and a hemispherical analyzer operating in a constant analyzer energy (CAE) mode. The base pressure in the analyzing chamber was maintained at 10−10 mbar. Data profiles were subjected to a nonlinear least-squares curve-fitting program with a Gaussian−Lorentzian production function and processed using the Casa XPS program (Casa Software Ltd., Teignmouth, UK). An adventitious C1s binding energy of 284.9 eV was set as the reference binding energy for charge correction. Transmission electron microscopy (TEM) was carried out with a JEM-2100 electron microscope with an acceleration voltage of 200 kV (JEOL Ltd., Peabody, MA, USA). The surface areas of the powder were measured with the conventional Brunauer–Emmett–Teller (BET) method (ASAP 2020 model; Micromeritics, Norcross, GA, USA).

3.4. Electrochemical Measurements

Electrochemical measurements were carried out on a computer-controlled CHI 608E electrochemical analysis instrument (CH Instruments, Inc., Bee Cave, TX, USA) in a three-electrode electrochemical cell at room temperature. A Pt spiral and an Ag/AgCl electrode were used as a counter electrode and a reference electrode, respectively. A working electrode was prepared with 20 mg of catalyst dispersed in 1.8 mL of ethanol and mixed with 0.2 mL of 5 wt.% Nafion solution via sonication for 30 min. Then, 1 μL of catalyst suspension was spread onto the surface of the glassy carbon (GC) electrode with a diameter of 3 mm and dried. Cyclic voltammetry (CV) was utilized to investigate methanol oxidation within a potential range of −0.2 V to 0.9 V at 50 mV/s in 0.5 M H2SO4 containing 1 M CH3OH. Hydrogen adsorption-desorption peaks were obtained in 0.5 M H2SO4, which was deaerated with ultrapure N2 gas for 30 min before each experiment. The long-term durability of the catalysts was tested by conducting 1000 continuous potential cycles between 0.05 and 1.20 V at 50 mV/s. Electrochemical impedance spectra (EIS) were obtained at 0.4 V and 100 kHz–0.01 Hz in 1 M CH3OH and 0.5 M H2SO4 mixture. The electrochemically active surface area (ECSA) could be obtained using the following Equation: ECSA (m2/g) = QH/(0.21 mC/m2 × mPt) [45]. QH (mC/cm2) is the average of integrating the hydrogen adsorption and desorption area (mA/cm2·V) by utilizing Origin 8.0 (OriginLab Corporation, Northampton, MA, USA) and dividing the screen rate (V/sec). Furthermore, 0.21 mC/cm2 was set for the oxidation of a hydrogen monolayer on a Pt electrode [44], and corresponding to 20%Pt loaded on the BaAlOx catalysts on a disk electrode, it was 0.24 g/m2.

4. Conclusions

Novel BaAlOx powder was synthesized through a simple polyol thermal method to prepare 20%Pt-BaAlOx/C catalysts for enhancing MOR in DMFC. The uncalcined amorphous Ba0.5AlOx catalysts exhibited a dramatically enhanced MOR activity, depending on their large surface area and abundant oxygen vacancies. The synergistic effects of amorphous Ba0.5AlOx on Pt catalysts induced the bifunctional mechanism of 20%Pt-Ba0.5AlOx/C catalysts for MOR. As a result, Pt-Ba0.5AlOx/C catalysts possessed the best MOR performance. The 20%Pt-Ba0.5AlOx/C catalysts demonstrated the largest maximum current density of 3.89 mA/cm2, the largest ECSA of 49.83 m2/g, and the smallest Rct. These catalysts also had better long-term durability and stability than 20%Pt/C catalysts after 1000 cycles. Therefore, 20%Pt-Ba0.5AlOx/C catalysts were optimum anode electrode candidates for DMFCs.

Author Contributions

Writing—original draft preparation, writing—review and editing, supervision, and project administration, T.H.C.; formal analysis and data curation, W.-Y.H.; validation, J.-W.H. and Y.-S.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology, grant number MST 105-2221-E-239-010.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Wilson, M.S.; Gottesfeld, S. Thin-film catalyst layers for polymer electrolyte fuel cell electrodes. J. Appl. Electrochem. 1992, 22, 1–7. [Google Scholar] [CrossRef]
  2. Du, B.; Rabb, S.A.; Zangmeister, C.; Tong, Y.Y. A volcano curve: Optimizing methanol electro-oxidation on Pt-decorated Ru nanoparticles. Phys. Chem. Chem. Phys. 2009, 11, 8231–8239. [Google Scholar] [CrossRef] [PubMed]
  3. Wu, G.; Li, L.; Xu, B.Q. Effect of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation. Electrochim. Acta 2004, 50, 1–10. [Google Scholar] [CrossRef]
  4. Chen, X.; Wang, H.; Wang, Y.; Bai, Q.; Gao, Y.; Zhang, Z. Synthesis and electrocatalytic performance of multi-component nanoporous PtRuCuW alloy for direct methanol fuel cells. Catalysts 2015, 5, 1003–1015. [Google Scholar] [CrossRef] [Green Version]
  5. Lee, Y.W.; Ko, A.R.; Han, S.B.; Kim, H.S.; Park, K.W. Synthesis of octahedral Pt–Pd alloy nanoparticles for improved catalytic activity and stability in methanol electrooxidation. Phys. Chem. Chem. Phys. 2011, 13, 5569–5572. [Google Scholar] [CrossRef]
  6. Jeon, M.K.; Lee, K.R.; Daimon, H.; Nakahara, A.; Woo, S.I. Pt45Ru45M10/C (M = Fe, Co, and Ni) catalysts for methanol electro-oxidation. Catal. Today 2008, 132, 123–126. [Google Scholar] [CrossRef]
  7. Ren, X.; Lv, Q.; Liu, L.; Liu, B.; Wang, Y.; Liu, A.; Wu, G. Current progress of Pt and Pt-based electrocatalysts used for fuel cells. Sustain. Energy Fuels 2020, 4, 15–30. [Google Scholar] [CrossRef]
  8. Cordeiro, G.L.; de Camargo, E.F.; Santos, M.C.L.; Pereira, C.V.; Ussui, V.; de Lima, N.B.; Neto, A.O.; Lazar, D.R.R. Improved Pt/CeO2 electrocatalysts for ethanol electro-oxidation. Int. J. Electrochem. Sci. 2018, 13, 6388–6401. [Google Scholar] [CrossRef]
  9. Sun, X.; Gao, K.; Pang, X.; Yang, H.; Volinsky, A.A. Electrochemical oxidation of mthanol on Pt-SnOx/C catalysts characterized by electrochemistry methods. J. Electrochem. Soc. 2015, 162, F1540–F1548. [Google Scholar] [CrossRef] [Green Version]
  10. Naeem, R.; Ahmed, R.; Ansari, M.S. TiO2 and Al2O3 promoted Pt/C nanocomposites as low temperature fuel cell catalysts for electro oxidation of methanol in acidic media. Mater. Sci. Eng. 2014, 60, 012031. [Google Scholar] [CrossRef] [Green Version]
  11. Justin, P.; Charan, P.H.K.; Rao, G.R. High performance Pt–Nb2O5/C electrocatalysts for methanol electrooxidation in acidic media. Appl. Catal. B-Environ. 2010, 100, 510–515. [Google Scholar] [CrossRef]
  12. Palani, R.J.; Shanthi, K.; Velayutham, G. Enhanced methanol electro-oxidation over in-situ carbon and graphene supported one dimensional NiMoO4 nanorods. J. Power Sources 2015, 277, 350–359. [Google Scholar] [CrossRef]
  13. Tan, Q.; Du, C.; Sun, Y.; Du, L.; Yin, G.; Gao, Y. Nickel-doped ceria nanoparticles for promoting catalytic activity of Pt/C for ethanol electrooxidation. J. Power Sources 2014, 263, 310–314. [Google Scholar] [CrossRef]
  14. Liu, Y.; Liu, C.; Yu, X.; Osgood, H.; Wu, G. CeO2-modified α-MoO3 nanorods as a synergistic support for Pt nanoparticles with enhanced COads tolerance during methanol oxidation. Phys. Chem. Chem. Phys. 2017, 19, 330–339. [Google Scholar] [CrossRef] [PubMed]
  15. Scofield, M.E.; Koenigsmann, C.; Bobb-Semple, D.; Tao, J.; Tong, X.; Wang, L.; Lewis, C.S.; Vukmirovic, M.; Zhu, Y.; Adzic, R.R.; et al. Correlating the chemical composition and size of various metal oxide substrates with the catalytic activity and stability of as-deposited Pt nanoparticles for the methanol oxidation reaction. Catal. Sci. Technol. 2016, 6, 2435–2450. [Google Scholar] [CrossRef]
  16. Bai, Y.; Wu, J.; Qiu, X.; Xi, J.; Wang, J.; Li, J.; Zhu, W.; Chen, L. Electrochemical characterization of Pt-CeO2/C and Pt-CexZr1-xO2/C catalysts for ethanol electro-oxidation. Appl. Catal. B-Environ. 2007, 73, 144–149. [Google Scholar] [CrossRef]
  17. An, N.; Yuan, X.; Pan, B.; Li, Q.; Li, S.; Zhang, W. Design of a highly active Pt/Al2O3 catalyst for lowtemperature CO oxidation. RSC Adv. 2014, 4, 38250–38257. [Google Scholar] [CrossRef]
  18. Dicks, O.A.; Cottom, J.; Shluger, A.L.; Afanas’ev, V.V. The origin of negative charging in amorphous Al2O3 films: The role of native defects. Nanotechnology 2019, 30, 205201. [Google Scholar] [CrossRef]
  19. Gu, D.M.; Chu, Y.Y.; Wang, Z.B.; Jiang, Z.Z.; Yin, G.P.; Liu, Y. Methanol oxidation on Pt/CeO2–C electrocatalyst prepared by microwave-assisted ethylene glycol process. Appl. Catal. B-Environ. 2011, 102, 9–18. [Google Scholar] [CrossRef]
  20. Brandhorst, M.; Cristol, S.; Capron, M.; Dujardin, C.; Vezin, H.; Le bourdon, G.; Payen, E. Catalytic oxidation of methanol on Mo/Al2O3 catalyst: An EPR and Raman/infrared operando spectroscopies study. Catal. Today 2006, 113, 34–39. [Google Scholar] [CrossRef]
  21. Das, S.; Kundu, P.P. Pt–Ru/Al2O3–C nanocomposites as direct methanol fuel cell catalysts for electrooxidation of methanol in acidic medium. RSC Adv. 2015, 5, 93539–93546. [Google Scholar] [CrossRef]
  22. Zhang, X.; Zhao, M.; Ye, Z.; Liu, S.; Chen, Y. BaO-Modified Pd-based Catalyst for Methanol Oxidation. Adv. Mat. Res. 2013, 610–613, 577–580. [Google Scholar] [CrossRef]
  23. Chen, L.; Niu, X.; Li, Z.; Dong, Y.; Wang, D.; Yuan, F.; Zhu, Y. The effects of BaO on the catalytic activity of La1.6Ba0.4NiO4 in directdecomposition of NO. J. Mol. Catal. A-Chem. 2016, 423, 277–284. [Google Scholar] [CrossRef]
  24. Zhang, C.; Yang, H.; Sun, T.; Shan, N.; Chen, J.; Xu, L.; Yan, Y. Synthesis of three-dimensionally ordered macro-/mesoporous Pt with high electrocatalytic activity by a dual-templating approach. J. Power Sources 2014, 245, 579–582. [Google Scholar] [CrossRef]
  25. Silva, C.D.; Morais, L.H.; Gonçalves, R.; Matos, R.; Souza, G.L.C.; Freitas, R.G.; Pereira, E.C. The methanol and CO electro-oxidation onto Ptpc/Co/Pt metallic multilayer nanostructured electrodes: An experimental and theoretical approach. Electrochim. Acta 2018, 280, 197–205. [Google Scholar] [CrossRef]
  26. Piela, P.; Fields, R.; Zelenay, P. Electrochemical impedance spectroscopy for direct methanol fuel cell diagnostics. J. Electrochem. Soc. 2006, 153, A1902–A1913. [Google Scholar] [CrossRef]
  27. Bergamaski, K.; Pinheiro, A.L.N.; Teixeira-Neto, E.; Nart, F.C. Nanoparticle size effects on methanol electrochemical oxidation on carbon supported platinum catalysts. J. Phys. Chem. B 2006, 110, 19271–19279. [Google Scholar] [CrossRef]
  28. Kumar, N.; Biswas, K. Cryomilling: An environment friendly approach of preparation large quantity ultra refined pure aluminium nanoparticles. J. Mater. Res. Technol. 2019, 8, 63–74. [Google Scholar] [CrossRef]
  29. Teng, H.T.; Lee, T.Y.; Chen, Y.K.; Wang, H.W.; Cao, G. Effect of Al(OH)3 on the hydrogen generation of aluminumewater system. J. Power Sources 2012, 219, 16–21. [Google Scholar] [CrossRef]
  30. Fang, R.C.; Sun, Q.Q.; Zhou, P.; Yang, W.; Wang, P.F.; Zhang, D.W. High-performance bilayer flexible resistive random access memory based on low-temperature thermal atomic layer deposition. Nanoscale Res. Lett. 2013, 8, 92–99. [Google Scholar] [CrossRef] [Green Version]
  31. Santra, C.; Rahman, S.; Bojja, S.; James, O.O.; Sen, D.; Maity, S.; Mohanty, A.K.; Mazumderd, S.; Chowdhury, B. Barium, calcium and magnesium doped mesoporous ceria supported gold nanoparticle for benzyl alcohol oxidation using molecular O2. Catal. Sci. Technol. 2013, 3, 360–370. [Google Scholar] [CrossRef]
  32. Verma, A.; Basu, S. Direct alcohol and borohydride alkaline fuel cells. In Recent Trends in Fuel Cell Science and Technology; Basu, S., Ed.; Springer: New Delhi, India, 2007; pp. 157–169. [Google Scholar]
  33. Zhao, Y.; Chen, L.; Song, Y.; Meng, Y. Preparation of high catalytic activity and low platinum loading catalyst for methanol oxidation. Int. J. Electrochem. Sci. 2018, 13, 7370–7384. [Google Scholar] [CrossRef]
  34. Taguchi, M.; Takahashi, H.; Nakajima, S. Methanol oxidation activity and chemical state of platinum oxide thin film treated by electrochemical reduction. Mater. Trans. 2013, 54, 582–587. [Google Scholar] [CrossRef] [Green Version]
  35. Perevalov, T.V.; Tereshenko, O.E.; Gritsenko, V.A.; Pustovarov, V.A.; Yelisseyev, A.P.; Park, C.; Han, J.H.; Lee, C. Oxygen deficiency defects in amorphous Al2O3. J. Appl. Phys. 2010, 108, 013501. [Google Scholar] [CrossRef]
  36. Yang, W.; Yang, X.; Jia, J.; Hou, C.; Gao, H.; Mao, Y.; Wang, C.; Lin, J.; Luo, X. Oxygen vacancies confined in ultrathin nickel oxide nanosheets for enhanced electrocatalytic methanol oxidation. Appl. Catal. B-Environ. 2019, 244, 1096–1102. [Google Scholar] [CrossRef]
  37. Gu, Y.; Liu, C.; Li, Y.; Sui, X.; Wang, K.; Wang, Z. Ce0.8Sn0.2O2–δ–C composite as a co-catalytic support for Pt catalysts toward methanol electrooxidation. J. Power Sources 2014, 265, 335–344. [Google Scholar] [CrossRef]
  38. Zhang, G.; Yang, Z.; Zhang, W.; Wang, Y. Nanosized Mo-doped CeO2 enhances the electrocatalytic property of the Pt anode catalyst in direct methanol fuel cells. J. Mater. Chem. A 2017, 5, 1481–1487. [Google Scholar] [CrossRef]
  39. Meher, S.K.; Rao, G.R. Polymer-assisted hydrothermal synthesis of highly reducible shuttle-shaped CeO2: Microstructural effect on promoting Pt/C for methanol electrooxidation. ACS Catal. 2012, 2, 2795–2809. [Google Scholar] [CrossRef]
  40. Ma, T.Y.; Zheng, Y.; Dai, S.; Jaroniec, M.; Qiao, S.Z. Mesoporous MnCo2O4 with abundant oxygen vacancy defects as high-performance oxygen reduction catalysts. J. Mater. Chem. A 2014, 2, 8676–8682. [Google Scholar] [CrossRef]
  41. Zhao, X.; Zhu, J.; Liang, L.; Liao, J.; Liu, C.; Xing, W. Enhanced activity of Pt nano-crystals supported on a novel TiO2@N-doped C nano-composite for methanol oxidation reaction. J. Mater. Chem. 2012, 22, 19718–19725. [Google Scholar] [CrossRef]
  42. Li, Z.; He, T.; Matsumura, D.; Miao, S.; Wu, A.; Liu, L.; Wu, G.; Chen, P. Atomically Dispersed Pt on the Surface of Ni Particles: Synthesis and Catalytic Function in Hydrogen Generation from Aqueous Ammonia−Borane. ACS Catal. 2017, 7, 6762–6769. [Google Scholar] [CrossRef]
  43. Thomson, S.J. Catalysis on well-defined metal surfaces and nonmetallic substrates. In Catalysis; Kemball, C., Dowden, D.A., Eds.; The Chemical Society: London, UK, 1980; Volume 3, pp. 1–38. [Google Scholar]
  44. Scibioh, M.A.; Kim, S.K.; Cho, E.A.; Lim, T.H.; Hong, S.A.; Ha, H.Y. Pt-CeO2/C anode catalyst for direct methanol fuel cells. Appl. Catal. B-Environ. 2008, 84, 773–782. [Google Scholar] [CrossRef]
  45. Shepperd, S.A.; Campbell, S.A.; Smith, J.R.; Lloyd, G.W.; Ralph, T.R.; Walsh, F.C. Electrochemical and microscopic characterization of platinum-coated perfluorosulfonic acid (Nafion 117) materials. Analyst 1998, 123, 1923–1929. [Google Scholar] [CrossRef]
Catalysts 10 00708 g001 550
Figure 1. (a) CV curves in 1 M CH3OH + 0.5 M H2SO4, (b) in 0.5 M H2SO4, (c) EIS (electrochemical impedance spectra) of 20%Pt-Ba0.5AlOx/C prepared with Ba0.5AlOx calcined at different temperatures, and (d) electrical equivalent circuit model.
Figure 1. (a) CV curves in 1 M CH3OH + 0.5 M H2SO4, (b) in 0.5 M H2SO4, (c) EIS (electrochemical impedance spectra) of 20%Pt-Ba0.5AlOx/C prepared with Ba0.5AlOx calcined at different temperatures, and (d) electrical equivalent circuit model.
Catalysts 10 00708 g001
Catalysts 10 00708 g002 550
Figure 2. (a) XRD data of Ba0.5AlOx under different calcining temperatures and (b) TEM image of uncalcined Ba0.5AlOx.
Figure 2. (a) XRD data of Ba0.5AlOx under different calcining temperatures and (b) TEM image of uncalcined Ba0.5AlOx.
Catalysts 10 00708 g002
Catalysts 10 00708 g003a 550Catalysts 10 00708 g003b 550
Figure 3. The particle size distribution of (a) uncalcined Ba0.5AlOx, and Ba0.5AlOx powders calcined at (b) 200 °C and (c) 400 °C.
Figure 3. The particle size distribution of (a) uncalcined Ba0.5AlOx, and Ba0.5AlOx powders calcined at (b) 200 °C and (c) 400 °C.
Catalysts 10 00708 g003aCatalysts 10 00708 g003b
Catalysts 10 00708 g004 550
Figure 4. (a) O 1s (b) Ba 3d, and (c) Al 2p XPS of Ba0.5AlOx under different calcining temperatures.
Figure 4. (a) O 1s (b) Ba 3d, and (c) Al 2p XPS of Ba0.5AlOx under different calcining temperatures.
Catalysts 10 00708 g004
Catalysts 10 00708 g005a 550Catalysts 10 00708 g005b 550
Figure 5. (a) CV curves in 1 M CH3OH + 0.5 M H2SO4 and (b) in 0.5 M H2SO4. Nyquist plots of (c) 20%Pt-Al2O3/C catalysts and (d) various catalysts.
Figure 5. (a) CV curves in 1 M CH3OH + 0.5 M H2SO4 and (b) in 0.5 M H2SO4. Nyquist plots of (c) 20%Pt-Al2O3/C catalysts and (d) various catalysts.
Catalysts 10 00708 g005aCatalysts 10 00708 g005b
Catalysts 10 00708 g006 550
Figure 6. XRD of various BaAlOx powders prepared at different ratios of Ba and Al contacts.
Figure 6. XRD of various BaAlOx powders prepared at different ratios of Ba and Al contacts.
Catalysts 10 00708 g006
Catalysts 10 00708 g007a 550Catalysts 10 00708 g007b 550
Figure 7. SEM mapping image of (a) 20%Pt-Ba0.5AlOx/C catalyst, (b) Ba, (c) Al, (d) O, (e) C, and (f) Pt element.
Figure 7. SEM mapping image of (a) 20%Pt-Ba0.5AlOx/C catalyst, (b) Ba, (c) Al, (d) O, (e) C, and (f) Pt element.
Catalysts 10 00708 g007aCatalysts 10 00708 g007b
Catalysts 10 00708 g008 550
Figure 8. (a) TEM image of 20%Pt-Ba0.5AlOx/C and (b) Pt 4f XPS of two catalysts.
Figure 8. (a) TEM image of 20%Pt-Ba0.5AlOx/C and (b) Pt 4f XPS of two catalysts.
Catalysts 10 00708 g008
Catalysts 10 00708 g009 550
Figure 9. The stability test of 20%Pt/C and 20%Pt-Ba0.5AOx/C catalysts.
Figure 9. The stability test of 20%Pt/C and 20%Pt-Ba0.5AOx/C catalysts.
Catalysts 10 00708 g009
Table
Table 1. Maximum current density, potential, and electrochemically active surface area (ECSA) of different catalysts in methanol electrooxidation reaction (MOR).
Table 1. Maximum current density, potential, and electrochemically active surface area (ECSA) of different catalysts in methanol electrooxidation reaction (MOR).
CatalystsMaximum Current Density (mA/cm2)/Potential (V)ECSA (m2/g)
Forward Peak (If)Backward Peak (Ib)
20%Pt/C1.47/0.650.4/0.4733.02
20%Pt-Al2O3/C0.58/0.640.27/0.514.78
20%Pt-Ba0.5Al0.5Ox/C1.79/0.670.42/0.4826.46
20%Pt-Ba0.5AlOx/C4.07/0.662.4/0.4649.83
20%Pt-Ba0.33AlOx/C3.07/0.661.42/0.4642.02
20%Pt-BaAl0.5Ox/C2.71/0.661.32/0.4633.08
20%Pt-BaAl0.33Ox/C1.87/0.670.77/0.4828.58

Share and Cite

MDPI and ACS Style

Chiang, T.H.; Hou, W.-Y.; Hsu, J.-W.; Chen, Y.-S. Pt-Amorphous Barium Aluminum Oxide/Carbon Catalysts for an Enhanced Methanol Electrooxidation Reaction. Catalysts 2020, 10, 708. https://doi.org/10.3390/catal10060708

AMA Style

Chiang TH, Hou W-Y, Hsu J-W, Chen Y-S. Pt-Amorphous Barium Aluminum Oxide/Carbon Catalysts for an Enhanced Methanol Electrooxidation Reaction. Catalysts. 2020; 10(6):708. https://doi.org/10.3390/catal10060708

Chicago/Turabian Style

Chiang, Tzu Hsuan, Wan-Yu Hou, Jia-Wei Hsu, and Yu-Si Chen. 2020. "Pt-Amorphous Barium Aluminum Oxide/Carbon Catalysts for an Enhanced Methanol Electrooxidation Reaction" Catalysts 10, no. 6: 708. https://doi.org/10.3390/catal10060708

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop