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Ni2P as an electron donor stabilizing Pt for highly efficient isopropanol fuel cell
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Ni2P as an electron donor stabilizing Pt for highly efficient isopropanol fuel cell Dan Chai a, Xiongwen Zhang a,*, Shicheng Yan b,**, Guojun Li a a
Key Laboratory of Thermal-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, 710049, China b Eco-materials and Renewable Energy Research Center (ERERC), Collaborative Innovation Center of Advanced Microstructures, College of Engineering and Applied Sciences, Nanjing University, No. 22 Hankou Road, Nanjing, Jiangsu 210093, PR China
highlights
graphical abstract
A facile and effective method to PteNi2P/RC
synthesize nanocomposite.
PteNi2P/RC is an active and stable isopropanol
electro-oxidation
catalyst. Mass activity of PteNi2P/RC is 1.90 times higher compared to Pt/C. Direct isopropanol fuel cell with Pt eNi2P/RC generates a high power density.
article info
abstract
Article history:
A key challenge for isopropanol fuel cell is to find efficient catalyst for the catalytic ability
Received 22 October 2019
improvement of isopropanol electro-oxidation reaction. Here, we anchor Pt nanoparticles
Received in revised form
on the Ni2P/resin carbon (RC) to form a PteNi2P/RC assembly. Strong interactions at Ni2P
18 December 2019
ePt interface induce the Ni2P to donate electrons to stabilize Pt, thus effectively decreasing
Accepted 31 December 2019
the adsorbed energy of isopropanol. As a result, PteNi2P/RC exhibits much higher mass
Available online 22 January 2020
activity and better stability than those of Pt/RC and Pt/C. When assembled into a direct isopropanol fuel cell, PteNi2P/RC shows a peak power density of 0.095 W cm2, which is
Keywords:
31.9% greater than that of Pt/C catalyst. Our results offer a new strategy to stabilize Pt by an
Isopropanol fuel cell
electron donor for developing affordable direct isopropanol fuel cells.
Electron donor
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
PteNi2P/RC catalyst Strong interactions
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhang), [email protected] (S. Yan). https://doi.org/10.1016/j.ijhydene.2019.12.221 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction
Experimental
Exploring novel energy conversion routes is effective strategy to overcome environmental pollution and fossil energy crisis. In recent years, the direct alcohol fuel cells (DAFCs) are widely regarded as efficient alternative energy sources, because they can directly convert chemical energy to electric power with much higher efficiency and lower green-house gases emissions than the conventional fossil fuel combustion [1e4]. Among various alcohol fuels, isopropanol can be as an alternative fuel to typical methanol and ethanol mainly due to its particular molecular structure with the feature of lower toxicity, higher boiling point, and smaller crossover current [5]. In addition, during the isopropanol oxidation process, less CO adsorption on surface of catalysts happened due to the great fully oxidized ability and large energy density [6,7]. Meanwhile, isopropanol can be obtained from biomass materials, which provides a strong guarantee for practical application. State-of-the-art DAFCs employ Pt/C as the active anode catalyst for alcohol oxidation reaction [8e10]. However, the high consumption and poor durability of Pt/C have severely hindered the commercialization of DAFCs. Recently, much attention has been paid to increase the electrocatalytic performance by using of different catalyst carriers such as carbon nanotubes, carbon nanofiber, and graphene nanosheets, or adding of earth-abundant and less expensive elements (such as Ni, Co, Fe, P, Cu or N) into Pt [11,12]. Unfortunately, the dissolution or poor stability of such electrocatalysts under working conditions resulted in a rapid decay of the catalytic performance. Therefore, the development of heterogeneous electrocatalysts with high activity and stability is attractive for DAFCs. Compared with other promoters, phosphide are more stable and the co-catalytic effect is more obvious in electrocatalysis [13]. In addition, the large-scale and low-cost preparation approach has made phosphide widely used in the commercialization process [14e18]. Inspired by these previous findings, we examined the possible promotion of Pt by Ni2P, it would be feasible to boost the electrocatalyst performance for isopropanol electrooxidation. Strong interactions at Ni2PePt interface induce the Ni2P to donate electrons to stabilize Pt, thus effectively decreasing the adsorbed energy of isopropanol. Moreover, the unique Ni2P nanoparticles supported on resin carbon (RC) can lead to a high surface area, good electric conductivity and corrosion resistance of the support, as well as the immobilization of Pt nanoparticles. And then, it can efficiently prevent the agglomeration of Pt nanoparticles, thereby, improving utilization of electrocatalysts. Therefore, the prepared PteNi2P/RC exhibits superior electrocatalytic activity and durability in comparison with Pt/C and Pt/RC. Notably, it also has a high power density when assembled into a single cell. The desirable performance in this study demonstrates the great promising for applications of PteNi2P/RC in direct isopropanol fuel cell.
Preparation of resin carbon (RC) and Ni2P/RC The resin carbon (RC) support was prepared by carbonizing the amino phosphonic acid chelating resin at 1000 C for 1 h under N2 atmosphere. For the synthesis of Ni2P loaded resin carbon (Ni2P/RC), amino phosphonic acid chelating resin was used as carbon and phosphate source. Firstly, amino phosphonic acid chelating resin (10 g) was ground into powder. Then, it was added into NiCl26H2O solution (250 mL, 0.15 mol L1) under stirring for 10 h (750 r/min). Afterwards, the suspension was centrifuged and washed by deionized water. Next, the sample was dried at 50 C for 6 h. And then, it was calcined in a tube furnace at 1000 C for 1 h under N2 atmosphere to form the Ni2P/RC composite.
Preparation of PteNi2P/RC, Pt/RC, and Pt/C To synthesize the PteNi2P co-loaded RC catalyst (PteNi2P/RC), the sodium citrate (33.5 mg) was completely dissolved into the H2PtCl6 (0.019 mol L1, 2.95 mL) containing ethylene glycol (30 mL) by magnetic stirring for 1 h. Subsequently, the pH value of the solution was adjusted to about 10 via dropping 5% KOH/ethylene glycol solution. After that, the Ni2P/RC support (100 g) was added into the solution with stirring and sonication for 0.5 h, respectively, and then heated at 180 C for 4 h in N2 atmosphere. Finally, the sample was obtained by centrifugation and washing with deionized water, and the obtained products were denoted as PteNi2P/RC (the Pt loading amount is 10 wt%). The 10 wt% Pt/RC (using RC as the support) and 10 wt% Pt/C (using Vulcan XC-72R carbon black as the support) were also synthesized in the same way.
Morphology and surface characterization The PteNi2P/RC, Pt/RC and Pt/C catalysts were tested by X-ray diffraction (XRD) on a Shimadzu Lab XRD-1600 diffractometer (Cu-Kɑ radiation) for their phase analysis. Transmission electron microscopy (TEM, FEI G2 F20) were used to determine the surface morphology and the particle size of the catalysts. Energy-dispersive X-ray spectroscopy (EDX) and elemental mapping were obtained during the scanning electron microscopy (SEM, Gemini 500) measurements. X-ray photoelectron spectroscopy (XPS) was conducted on a spectrometer (XPS, Kratos Axis Ultrabld) with the monochromatic Al Kɑ X-ray source.
Electrochemical characterization A CHI 600D (Shanghai Chenhua Instrument Factory, China) electrochemical workstation was used for all electrochemical measurements in a standard three-electrode cell. A Pt wire and a leak-free Ag/AgCl electrode (saturated with KCl) was used as the counter and reference electrodes, respectively.
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Glassy carbon electrode (0.1256 cm2) was used as the working electrode to support the electrocatalysts. To prepare the catalyst-coated working electrode, 5 mg catalyst was dispersed ultrasonically in Nafion/ethanol (1 mL, 0.25% Nafion) solution, then dropped onto the glassy carbon electrode. Cyclic voltammograms (CV), linear sweep voltammogram (LSV), and I-t tests were obtained in a 0.1 M KOH or 0.1 M KOH-0.5 M isopropanol solution. Accelerated durability test (ADT) was characterized at a scan rate of 50 mV s1 for 500 cycles. All solutions were purged with high-purity N2 for 10 min before taking the measurements. And N2 flow was maintained over the electrolyte during all experiments.
Test in single alkaline isopropanol fuel cell The assembled single cell performance was conducted on a fuel cell testing system G20 (Greenlight, Canada). The temperature of the fuel cell was set at 60 C. Anode catalyst inks were prepared by ultrasonically mixing the appropriate amount of as-prepared PteNi2P/RC and Pt/C catalysts, ethyl alcohol, Nafion® ionomer (5% solution, Alfa Aesar), and DI water. The benchmark reference Pt/C catalyst ink (20 wt% Pt loading, Johnson Matthey Company) is employed for the cathode. The catalyst ink was sprayed on the two sides of the Nafion 117 membranes (DuPont) membrane with an effective area of 2 * 2 cm2 and the catalyst coverage is 0.5 mg cm2. The Pt loading at cathode and anode are kept at 0.1 and 0.05 mg cm2, respectively. Prior to use, Nafion membrane was accomplished by successively treating the membrane in 5 wt % H2O2 solution at 80, distilled water at 80 C, 8 wt% H2SO4 solution at 80 C and then in distilled water at 80 C again, for
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30 min each step. The fuel of 0.1 M KOH-0.5 M isopropanol solution was delivered into anode with a rate of 1 mL min1. The oxygen flow was regulated at 200 mL min1 in cathode.
Results and discussion As shown in Scheme 1, the PteNi2P/RC catalyst was prepared by using amino phosphonic acid chelating resin, NiCl2, and H2PtCl6 as raw materials. The mixture of chelating resin and Ni2þ was carbonized at 1000 C for forming Ni2P/RC composite. Finally, the PteNi2P/RC assembly was obtained by reducing Pt over the Ni2P/RC. For comparison, the Pt/RC and Pt/C were respectively by reducing Pt onto RC and XC-72R carbon black. The crystal structures of PteNi2P/RC, Pt/RC, and Pt/C electrocatalysts were analyzed by XRD technique, as shown in Fig. 1a. The Pt/C and Pt/RC samples exhibits the obvious XRD peak at approximately 25 , corresponding to the carbon (002) plane [19], confirming the formation of crystalline carbon by heating the amino phosphonic acid chelating resin. For Pt/C catalyst, the XRD peaks for (111), (200), (220), and (311) facets of Pt are observed [20]. The same XRD peaks with the larger full width at half maximum (FWHM) and lower diffraction intensity would indicate a smaller particle size of Pt in the Pt/ RC catalyst. The XRD peaks in Ni2P/RC and PteNi2P/RC samples are well indexed to single-phase Ni2P (JCPDS no 65e1989) with high crystalline [21e23]. While, there are no obvious peaks can be attributed to Pt in PteNi2P/RC catalyst. Possible reasons are as follows. The strong peaks of Ni2P (111) and Ni2P (210) may cover or integrate the peaks of Pt, forming the broadened peaks at approximately 40 and 46 . This result may
Scheme 1 e Schematic illustration for the preparation of Ni2P/RC and PteNi2P/RC catalyst.
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Fig. 1 e XRD and TEM analysis. a) XRD patterns for Pt/C, Pt/RC, PteNi2P/RC, and Ni2P/RC. TEM images of b) Pt/C, d) Ni2P/RC, and f) PteNi2P/RC. High-resolution TEM (HRTEM) lattice images of c) Pt/C, e) Ni2P/RC, and g) PteNi2P/RC.
be ascribed to the low amount of Pt loading and small size as well as the much stronger diffraction peak for Ni2P. But the peak intensity of Ni2P is reduced slightly due to the surface loaded of Pt nanocrystals. PteNi2P/RC, Ni2P/RC and Pt/C were visualized by TEM. For Pt/C, the 5e10 nm Pt nanoparticles were dispersed on the carbon support (Fig. 1b). For Ni2P/RC, about 5e200 nm nanocrystals were loaded on the RC support (Fig. 1d). A 0.507 nm lattice spacing in high-resolution TEM (HRTEM) lattice image (Fig. 1e) is observed on the nanocrystal and assigned to (100) facet of Ni2P, suggesting that the nanocrystals are Ni2P. In addition, the 0.34 nm lattice spacing, typically corresponding to (002) facet of carbon, suggests the formation of crystalline carbon from the carbonization of chelating resin. After loading of Pt on Ni2P/RC, the 3e10 nm particles dispersed on the carbon sheet (Fig. 1f). The average diameter of Pt
nanoparticles (shown in Fig. 2a and Fig. 2b), which are obtained by measuring over 200 particles from different areas of the TEM grid, are 4.00 nm for Pt/C and 3.39 nm for PteNi2P/RC. Meanwhile, Pt nanoparticles are homogeneously distributed on the surface of Ni2P/RC hybrid material with less agglomeration than Pt/C. It is generally believed that the high dispersity and smaller particles size of the Pt nanoparticles in PteNi2P/RC catalyst may be due to good immobilization of the Ni2P/RC, which would lead to a large specific surface area and produce more active sites for isopropanol electro-oxidation reaction. HRTEM lattice image of PteNi2P/RC catalyst displayed the lattice fringe of 0.221 and 0.228 nm (Fig. 1g), corresponding to the (111) facet of Ni2P and the (111) facet of Pt, respectively [21], demonstrating that the Pt and Ni2P were successfully anchored onto RC [24e26]. The corresponding EDX spectrum is further executed to analyze the elemental
Fig. 2 e (a) and (b) Particle size distribution of PteNi2P/RC and Pt/C.
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components of the PteNi2P/RC, as shown in Fig. S1a. It exhibits that the prepared PteNi2P/RC catalyst are composed of C, P, Pt and Ni elemental without other impurities. Fig. S1b is the element mapping analyses, which show that Pt, Ni, P and C elements are homogeneously dispersed, representing a uniform distribution of PteNi2P/RC nanocatalyst. To further insight into the chemical composition and surface electronic state of the catalysts, XPS was carried out to explore the valence of Pt, Ni, and P. XPS survey spectra (Fig. 3a) clearly indicated the presence of Pt, Ni, P, and C for PteNi2P/RC catalyst, as well as Pt and C for Pt/C catalyst. Pt 4f 7/2 XPS peaks of Pt/RC can be deconvoluted into three species (Fig. 3b): 72.89, 74.83, and 74.95 eV, which are ascribed to metallic Pt, Pt2þ [PtO and/or Pt(OH)2], and Pt4þ chemical state (PtO2 phases), respectively [27]. The oxidation state of Pt may be due to oxygen chemisorption display on the Pt surface [28]. The binding energies of the three Pt species evidently decreased after loading Pt onto Ni2P/RC (Pt: 71.65 eV, Pt2þ: 72.55 eV, Pt4þ: 74.35 eV) (Fig. 3b). The molar percent of these Pt species was calculated according to their XPS peak area (shown in Table S1). The content of Pt0 in Pt 4f XPS increased from 51.9% of Pt/C to 62.3% of PteNi2P/RC, while content of Pt2þ decreased from 37.2% of Pt/C to 20.3% of PteNi2P/RC. The content of Pt4þ slightly increased from 10.9% of Pt/C to 17.4% of PteNi2P/RC. This means that the total content of high-valence Pt decreased obviously after loading Pt onto Ni2P/RC. This fact would originate from that the interactions between Ni2P and Pt induce the electrons transfer from Ni2P to Pt due to the higher work function of Pt [8,29]. Indeed, the Pt 4f binding
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energy is about 0.3 eV smaller for PteNi2P/RC (Pt 4f7/2: 71.64 and Pt 4f5/2: 74.93 eV) than Pt/C (Pt 4f7/2: 71.83 and Pt 4f5/2: 75.16 eV) (Fig. 3b), implying that the Ni2P as an electron donor donated its electrons to Pt [16]. Ni 2p3/2 XPS spectrum of Ni2P/ RC was decomposed to three peaks: 854.1 eV for Nidþ (0
Fig. 3 e a) XPS survey spectra of Pt/C and PteNi2P/RC. b) Deconvolution of Pt 4f XPS spectra of Pt/C and PteNi2P/RC. c) Deconvolution of Ni 2p XPS spectra of PteNi2P/RC and Ni2P/RC. d) P 2p core-level spectra of PteNi2P/RC.
(1)
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Fig. 4 e The electrochemical measurements of PteNi2P/RC, Pt/RC and Pt/C. a) CV in 0.1 M N2-purged KOH. b) CV in 0.1 M KOH0.5 M isopropanol. c) I-t curves for 3600 s, inserted: the current after 3600 s. d) Loss of the isopropanol oxidation activity with cycles in 0.1 M KOH-0.5 M isopropanol.
where, QH is the amount of charge exchange during the adsorption of hydrogen atoms on the Pt surface (C); m is the Pt loading in the electrode (g cm2); 0.21 mC cm2 is the charge required to oxidize the hydrogen monolayer. As illustrated in Fig. 4a, the EASA of PteNi2P/RC is roughly 95.7 m2 g1 Pt, which is dramatically higher than Pt/RC (41.9 m2 g1 Pt) and Pt/C (27.8 m2 g1 Pt), suggesting the rich available electrochemical active sites on the surfaces of PteNi2P/RC. The electrocatalytic performances of the prepared electrocatalysts toward isopropanol oxidation were investigated in 0.1 M KOH-0.5 M isopropanol solution at a sweeping rate of 50 mV s1, and the results are displayed in Fig. 4b. The onset potential of PteNi2P/RC shifts lower than that of Pt/C and Pt/ RC, which are more favorable to the electro-oxidation of isopropanol. Such a trend clearly indicates the promotion effect of Ni2P. Moreover, for all catalysts, there are two apparent peaks for forward and reverse scans. The peak of the forward scan is due to the oxidation of chemisorbed isopropanol molecules on the catalyst surface, while the peak of the backward scan is related to the further oxidation of intermediates [35]. PteNi2P/RC exhibits the highest mass activity of 403.1 mA mg1Pt, which is 1.48 and 1.90 times higher than those of Pt/RC (272.9 mA mg1Pt), and Pt/C (212.2 mA mg1Pt), respectively. These results demonstrate a good electrocatalytic activity of isopropanol oxidation on PteNi2P/RC catalyst. I-t tests is used to evaluate the stability of the electrocatalysts in 0.1 M KOH-0.5 M (the applied potential is 0.2 V). In Fig. 4c, the current density of all catalysts decay rapidly in the initial stage, possibly due to the accumulation of strongly
adsorbed intermediates on the reaction site surface [36,37]. While, initial current for PteNi2P/RC is larger than that of Pt/ RC and Pt/C, demonstrating a greater number of active sites available per mass of surface. Consequently, the relative current slowly decreases and reach a pseudo-steady state. It is obvious that the PteNi2P/RC exhibits much higher current densities over time in comparsion with Pt/C and Pt/RC. Especially, after 3600 s, the current of the PteNi2P/RC (23.8 mA mg1Pt) is about 4.25 and 8.81 times than that of Pt/RC (5.6 mA mg1Pt) and Pt/C (2.7 mA mg1Pt), respectively (inserted in Fig. 4c). These results indicate a high tolerance to the carbonaceous species produced during isopropanol oxidation and good stability of PteNi2P/RC catalysts. To further clarify the good durability of PteNi2P/RC, ADT are performed in Fig. 4d. After 500 cycles of CV measurements, the activity loss of
Table 1 e Comparative electrocatalytic properties towards isopropanol electrooxidation. Catalyst PteNi2P/RC Pd3Cu PdeAu/PPP/GCE Pd/GCE Pd3Fe/CN Pd/CN Pd/C Pd1Sn2 PdAu PdAu41/C
Ep/V
Ip/mA mg1
Ref.
0.05 (Ag/AgCl) 0.05 (Hg/HgO) 0.29 (SCE) 0.35 (SCE) 0.08 (Ag/AgCl) 0.03 (Ag/AgCl) 0.06 (Ag/AgCl) 0.69 (RHE) 0.22 (SCE) 0.32 (SCE)
403 0.79 140 21 470 360 240 150 280 453
This Work [1] [32] [32] [35] [35] [35] [7] [37] [37]
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Fig. 5 e a) LSV in 0.1 M KOH-0.5 M isopropanol. b) Corresponded Tafel plots of PteNi2P/RC, Pt/RC and Pt/C.
PteNi2P/RC is 56.6%, while this value of Pt/C is up to 72.0%, indicating that PteNi2P/RC has superior cycling stability for the oxidation of isopropanol. The successive CV of 500 cycles for PteNi2P/RC and Pt/C are shown in Fig. S2a and Fig. S2b. To further prove that the electrocatalytic performances of the prepared PteNi2P/RC catalyst has been greatly improved, we have compared our work with some previously reported catalysts for electro-oxidation of isopropanol, and the results are listed in Table 1. Clearly, PteNi2P/RC exhibits an advantage in the improvement of peak current (Ip) compared with other catalysts, indicating that the PteNi2P/RC can be well applied to serve as an outstanding anode catalyst for direct isopropanol fuel cells. LSV is performed at a scan rate of 1 mV s1 (shown in Fig. 5a). Notably, the oxidation current of PteNi2P/RC is larger than that of the Pt/RC and Pt/C electrocatalysts, which is well consistent with CV results of Fig. 4b. Fig. 5b present Tafel plots corresponding to Fig. 5a, for further investigating isopropanol electro-oxidation process. Visibly, all curves of the figures are composed of straight lines with two slopes. The first fitted Tafel slopes values (in the low overpotential region) are 137.7, 169.9 and 157.1 mV dec1 for PteNi2P/RC, Pt/RC and Pt/C, respectively. The lower Tafel slope of PteNi2P/RC indicates the charge-transfer kinetics on it is faster than other catalysts for the isopropanol oxidation [14]. While, the second fitted Tafel slopes values (in the high overpotential region) sharply increased to 202.7, 237.7 and 296.1 mV dec1, respectively. The second slopes are almost double the first ones, which indicates that the reaction mechanism or rate-determining steps of isopropanol electro-oxidation have changed in different potential ranges. For comprehensively understanding the process of isopropanol oxidation reaction, following reaction mechanism are proposed [38]: CH3CHOHCH3þ 2OH / CH3COCH3 þ 2H2O þ 2e
(2)
CH3COCH3þ16OH / 3CO2 þ 11H2O þ 16 e
(3)
It is observed that acetone and carbon dioxide are the only products of electro-oxidation of isopropanol [38]. The process of acetone formation by electro-oxidation does not involve strong adsorption intermediates such as carbon monoxide. Fig. S3a and Fig. S3b are the XPS spectra of Pt in PtNi2P/RC and
Pt/C after isopropanol electrooxidation, respectively. For each catalyst, after isopropanol electro-oxidation reaction, the content of metal Pt decreased, and the content of Pt in the oxidation state increased, indicating that surface oxide was generated during the reaction. However, after electrooxidation of isopropanol, the content of Pt in PtNi2P/RC is also higher than that of Pt/C (shown in Table 2), indicating that the intermediate oxide adsorbs less on PtNi2P/RC, which is more favorable for the catalytic reaction. In general, the electron donation between Pt and Ni2P affects the catalytic properties of metal nanoparticles and results in the activation of the electrode processes by dispersing metal and oxide material [17]. The electrons rich in the Pt surface can weaken
Table 2 e Binding energy and relative concentration of Pt 4f XPS spectra for PteNi2P/RC and Pt/C catalysts after isopropanol electro-oxidation reaction. Pt Species
Pt 4f7/2 Binding Energy (eV)
PteNi2P/RC catalyst Pt (0) Pt (II) Pt (IV) Pt/C catalyst Pt (0) Pt (II) Pt (IV)
Relative Concentrations (%)
71.83 72.55 75.13
40 50 10
71.76 72.55 75.08
33 59 8
Table 3 e Binding energy and relative concentration of C 1s XPS spectra for PteNi2P/RC and Pt/C catalysts after isopropanol electro-oxidation reaction. C Species PteNi2P/RC catalyst CeC CH3eC CaO CO2/CO23 Pt/C catalyst CeC CH3eC CaO CO2/CO23
Binding Energy (eV)
Relative Concentrations (%)
284.79 285.51 288.3 288.7
66.1 27.8 4.2 1.9
284.78 285.65 288.3 288.76
73.6 19.6 1.3 5.5
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Fig. 6 e Polarization curves and corresponding power density plots of the direct isopropanol fuel cells.
the adsorption ability of CO-poisoning species according to dband theory, thus the catalytic activity of alcohols oxidation was increased greatly compared with Pt/C catalyst [31]. Fig. S3c and Fig. S3d are the XPS spectra of C in PtNi2P/RC and Pt/C after electro-oxidation of isopropanol, respectively. The content of C]O group in PtNi2P/RC is higher than that of Pt/C catalyst, which indicates that PtNi2P/RC promotes the kinetics of acetone production (shown in Table 3). It is reported that acetone is weakly adsorbed on the electrode surface. However, the further electro-oxidation of acetone proceeds through strongly adsorbed intermediates, which will hinder the electro-oxidation of isopropanol [39]. In PtNi2P/RC, the content of CO2/CO23 is lower than that in Pt/C, which indicates that the further oxidation of acetone is less in PtNi2P/RC electrode, so the intermediate adsorbed on PtNi2P/RC electrode is less, which is more conducive to the electro-oxidation of isopropanol. Based on the Density Functional Theory (DFT) calculation reported in literature, electron transfer can reduce the d-band center of Pt and change the reaction pathway and mechanism of the surface sensitive reaction. Then, the adsorption energy of the poisoning intermediates on the metal surfaces can be decreased, which would contribute positively to the removal of it [29]. Meanwhile, the presence of Ni2P can activate water, producing -OHads to oxidize poisoning intermediates adsorbed at Pt sites, through the so-called bifunctional mechanism, which would result in an improvement in the electrocatalytic performance [40]. In addition, it is reported that the oxidation state of the phosphide can facilitate the production of adsorbed oxophilic species, which would provide the necessary oxygen-containing sites for the poisoning intermediates oxidation. To demonstrate the potential of the PteNi2P/RC in a direct isopropanol fuel cell, the catalysts are integrated at the anode with a homemade fuel cell. Fig. 6 shows the steady-stead polarization of the fuel cells. The open circuit voltage are 0.90 V and 0.97 V using PteNi2P/RC and Pt/C as an anode,
respectively. As the discharging current density increases, the voltages of the test cells gradually decrease. However, using PteNi2P/RC as the anode electrocatalyst, the cell voltage drops more slowly as the current density increases. Meanwhile, the maximum power densities in the test cells using PteNi2P/RC and Pt/C are 0.095 and 0.072 W cm2, respectively, which confirms the promising activity of PteNi2P/RC for direct isopropanol fuel cell [6,25]. These performance data of direct isopropanol fuel cell are comparable to those of the previously demonstrated DAFCs. During the isopropanol oxidation process, oxidation product is mainly acetone, and less COads absorption phenomenon happened, which provides significantly important advantages over the other DAFCs systems.
Conclusions In summary, PteNi2P/RC was prepared using a scalable and facile procedure. The Ni2P nanoparticles significantly promote the activity and stability of Pt in isopropanol electro-oxidation reaction, mainly due to the advantages of greater number of active sites and synergistic effects. Meanwhile, PteNi2P/RC was successfully integrated into a direct isopropanol fuel cell, showing superior performance to Pt/C electrocatalyst. Furthermore, compared to other previously-reported catalysts, the prepared PteNi2P/RC also exhibits higher oxidation current. We believe that this work is a significant step toward the development of more efficient catalysts for practical applications of direct isopropanol fuel cells.
Acknowledgement This work is funded by the National Natural Science Foundation of China (Grant No.51676161).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.221.
[17]
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Available online at www.sciencedirect.com
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Ni2P as an electron donor stabilizing Pt for highly efficient isopropanol fuel cell Dan Chai a, Xiongwen Zhang a,*, Shicheng Yan b,**, Guojun Li a a
Key Laboratory of Thermal-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, 710049, China b Eco-materials and Renewable Energy Research Center (ERERC), Collaborative Innovation Center of Advanced Microstructures, College of Engineering and Applied Sciences, Nanjing University, No. 22 Hankou Road, Nanjing, Jiangsu 210093, PR China
highlights
graphical abstract
A facile and effective method to PteNi2P/RC
synthesize nanocomposite.
PteNi2P/RC is an active and stable isopropanol
electro-oxidation
catalyst. Mass activity of PteNi2P/RC is 1.90 times higher compared to Pt/C. Direct isopropanol fuel cell with Pt eNi2P/RC generates a high power density.
article info
abstract
Article history:
A key challenge for isopropanol fuel cell is to find efficient catalyst for the catalytic ability
Received 22 October 2019
improvement of isopropanol electro-oxidation reaction. Here, we anchor Pt nanoparticles
Received in revised form
on the Ni2P/resin carbon (RC) to form a PteNi2P/RC assembly. Strong interactions at Ni2P
18 December 2019
ePt interface induce the Ni2P to donate electrons to stabilize Pt, thus effectively decreasing
Accepted 31 December 2019
the adsorbed energy of isopropanol. As a result, PteNi2P/RC exhibits much higher mass
Available online 22 January 2020
activity and better stability than those of Pt/RC and Pt/C. When assembled into a direct isopropanol fuel cell, PteNi2P/RC shows a peak power density of 0.095 W cm2, which is
Keywords:
31.9% greater than that of Pt/C catalyst. Our results offer a new strategy to stabilize Pt by an
Isopropanol fuel cell
electron donor for developing affordable direct isopropanol fuel cells.
Electron donor
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
PteNi2P/RC catalyst Strong interactions
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhang), [email protected] (S. Yan). https://doi.org/10.1016/j.ijhydene.2019.12.221 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction
Experimental
Exploring novel energy conversion routes is effective strategy to overcome environmental pollution and fossil energy crisis. In recent years, the direct alcohol fuel cells (DAFCs) are widely regarded as efficient alternative energy sources, because they can directly convert chemical energy to electric power with much higher efficiency and lower green-house gases emissions than the conventional fossil fuel combustion [1e4]. Among various alcohol fuels, isopropanol can be as an alternative fuel to typical methanol and ethanol mainly due to its particular molecular structure with the feature of lower toxicity, higher boiling point, and smaller crossover current [5]. In addition, during the isopropanol oxidation process, less CO adsorption on surface of catalysts happened due to the great fully oxidized ability and large energy density [6,7]. Meanwhile, isopropanol can be obtained from biomass materials, which provides a strong guarantee for practical application. State-of-the-art DAFCs employ Pt/C as the active anode catalyst for alcohol oxidation reaction [8e10]. However, the high consumption and poor durability of Pt/C have severely hindered the commercialization of DAFCs. Recently, much attention has been paid to increase the electrocatalytic performance by using of different catalyst carriers such as carbon nanotubes, carbon nanofiber, and graphene nanosheets, or adding of earth-abundant and less expensive elements (such as Ni, Co, Fe, P, Cu or N) into Pt [11,12]. Unfortunately, the dissolution or poor stability of such electrocatalysts under working conditions resulted in a rapid decay of the catalytic performance. Therefore, the development of heterogeneous electrocatalysts with high activity and stability is attractive for DAFCs. Compared with other promoters, phosphide are more stable and the co-catalytic effect is more obvious in electrocatalysis [13]. In addition, the large-scale and low-cost preparation approach has made phosphide widely used in the commercialization process [14e18]. Inspired by these previous findings, we examined the possible promotion of Pt by Ni2P, it would be feasible to boost the electrocatalyst performance for isopropanol electrooxidation. Strong interactions at Ni2PePt interface induce the Ni2P to donate electrons to stabilize Pt, thus effectively decreasing the adsorbed energy of isopropanol. Moreover, the unique Ni2P nanoparticles supported on resin carbon (RC) can lead to a high surface area, good electric conductivity and corrosion resistance of the support, as well as the immobilization of Pt nanoparticles. And then, it can efficiently prevent the agglomeration of Pt nanoparticles, thereby, improving utilization of electrocatalysts. Therefore, the prepared PteNi2P/RC exhibits superior electrocatalytic activity and durability in comparison with Pt/C and Pt/RC. Notably, it also has a high power density when assembled into a single cell. The desirable performance in this study demonstrates the great promising for applications of PteNi2P/RC in direct isopropanol fuel cell.
Preparation of resin carbon (RC) and Ni2P/RC The resin carbon (RC) support was prepared by carbonizing the amino phosphonic acid chelating resin at 1000 C for 1 h under N2 atmosphere. For the synthesis of Ni2P loaded resin carbon (Ni2P/RC), amino phosphonic acid chelating resin was used as carbon and phosphate source. Firstly, amino phosphonic acid chelating resin (10 g) was ground into powder. Then, it was added into NiCl26H2O solution (250 mL, 0.15 mol L1) under stirring for 10 h (750 r/min). Afterwards, the suspension was centrifuged and washed by deionized water. Next, the sample was dried at 50 C for 6 h. And then, it was calcined in a tube furnace at 1000 C for 1 h under N2 atmosphere to form the Ni2P/RC composite.
Preparation of PteNi2P/RC, Pt/RC, and Pt/C To synthesize the PteNi2P co-loaded RC catalyst (PteNi2P/RC), the sodium citrate (33.5 mg) was completely dissolved into the H2PtCl6 (0.019 mol L1, 2.95 mL) containing ethylene glycol (30 mL) by magnetic stirring for 1 h. Subsequently, the pH value of the solution was adjusted to about 10 via dropping 5% KOH/ethylene glycol solution. After that, the Ni2P/RC support (100 g) was added into the solution with stirring and sonication for 0.5 h, respectively, and then heated at 180 C for 4 h in N2 atmosphere. Finally, the sample was obtained by centrifugation and washing with deionized water, and the obtained products were denoted as PteNi2P/RC (the Pt loading amount is 10 wt%). The 10 wt% Pt/RC (using RC as the support) and 10 wt% Pt/C (using Vulcan XC-72R carbon black as the support) were also synthesized in the same way.
Morphology and surface characterization The PteNi2P/RC, Pt/RC and Pt/C catalysts were tested by X-ray diffraction (XRD) on a Shimadzu Lab XRD-1600 diffractometer (Cu-Kɑ radiation) for their phase analysis. Transmission electron microscopy (TEM, FEI G2 F20) were used to determine the surface morphology and the particle size of the catalysts. Energy-dispersive X-ray spectroscopy (EDX) and elemental mapping were obtained during the scanning electron microscopy (SEM, Gemini 500) measurements. X-ray photoelectron spectroscopy (XPS) was conducted on a spectrometer (XPS, Kratos Axis Ultrabld) with the monochromatic Al Kɑ X-ray source.
Electrochemical characterization A CHI 600D (Shanghai Chenhua Instrument Factory, China) electrochemical workstation was used for all electrochemical measurements in a standard three-electrode cell. A Pt wire and a leak-free Ag/AgCl electrode (saturated with KCl) was used as the counter and reference electrodes, respectively.
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Glassy carbon electrode (0.1256 cm2) was used as the working electrode to support the electrocatalysts. To prepare the catalyst-coated working electrode, 5 mg catalyst was dispersed ultrasonically in Nafion/ethanol (1 mL, 0.25% Nafion) solution, then dropped onto the glassy carbon electrode. Cyclic voltammograms (CV), linear sweep voltammogram (LSV), and I-t tests were obtained in a 0.1 M KOH or 0.1 M KOH-0.5 M isopropanol solution. Accelerated durability test (ADT) was characterized at a scan rate of 50 mV s1 for 500 cycles. All solutions were purged with high-purity N2 for 10 min before taking the measurements. And N2 flow was maintained over the electrolyte during all experiments.
Test in single alkaline isopropanol fuel cell The assembled single cell performance was conducted on a fuel cell testing system G20 (Greenlight, Canada). The temperature of the fuel cell was set at 60 C. Anode catalyst inks were prepared by ultrasonically mixing the appropriate amount of as-prepared PteNi2P/RC and Pt/C catalysts, ethyl alcohol, Nafion® ionomer (5% solution, Alfa Aesar), and DI water. The benchmark reference Pt/C catalyst ink (20 wt% Pt loading, Johnson Matthey Company) is employed for the cathode. The catalyst ink was sprayed on the two sides of the Nafion 117 membranes (DuPont) membrane with an effective area of 2 * 2 cm2 and the catalyst coverage is 0.5 mg cm2. The Pt loading at cathode and anode are kept at 0.1 and 0.05 mg cm2, respectively. Prior to use, Nafion membrane was accomplished by successively treating the membrane in 5 wt % H2O2 solution at 80, distilled water at 80 C, 8 wt% H2SO4 solution at 80 C and then in distilled water at 80 C again, for
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30 min each step. The fuel of 0.1 M KOH-0.5 M isopropanol solution was delivered into anode with a rate of 1 mL min1. The oxygen flow was regulated at 200 mL min1 in cathode.
Results and discussion As shown in Scheme 1, the PteNi2P/RC catalyst was prepared by using amino phosphonic acid chelating resin, NiCl2, and H2PtCl6 as raw materials. The mixture of chelating resin and Ni2þ was carbonized at 1000 C for forming Ni2P/RC composite. Finally, the PteNi2P/RC assembly was obtained by reducing Pt over the Ni2P/RC. For comparison, the Pt/RC and Pt/C were respectively by reducing Pt onto RC and XC-72R carbon black. The crystal structures of PteNi2P/RC, Pt/RC, and Pt/C electrocatalysts were analyzed by XRD technique, as shown in Fig. 1a. The Pt/C and Pt/RC samples exhibits the obvious XRD peak at approximately 25 , corresponding to the carbon (002) plane [19], confirming the formation of crystalline carbon by heating the amino phosphonic acid chelating resin. For Pt/C catalyst, the XRD peaks for (111), (200), (220), and (311) facets of Pt are observed [20]. The same XRD peaks with the larger full width at half maximum (FWHM) and lower diffraction intensity would indicate a smaller particle size of Pt in the Pt/ RC catalyst. The XRD peaks in Ni2P/RC and PteNi2P/RC samples are well indexed to single-phase Ni2P (JCPDS no 65e1989) with high crystalline [21e23]. While, there are no obvious peaks can be attributed to Pt in PteNi2P/RC catalyst. Possible reasons are as follows. The strong peaks of Ni2P (111) and Ni2P (210) may cover or integrate the peaks of Pt, forming the broadened peaks at approximately 40 and 46 . This result may
Scheme 1 e Schematic illustration for the preparation of Ni2P/RC and PteNi2P/RC catalyst.
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Fig. 1 e XRD and TEM analysis. a) XRD patterns for Pt/C, Pt/RC, PteNi2P/RC, and Ni2P/RC. TEM images of b) Pt/C, d) Ni2P/RC, and f) PteNi2P/RC. High-resolution TEM (HRTEM) lattice images of c) Pt/C, e) Ni2P/RC, and g) PteNi2P/RC.
be ascribed to the low amount of Pt loading and small size as well as the much stronger diffraction peak for Ni2P. But the peak intensity of Ni2P is reduced slightly due to the surface loaded of Pt nanocrystals. PteNi2P/RC, Ni2P/RC and Pt/C were visualized by TEM. For Pt/C, the 5e10 nm Pt nanoparticles were dispersed on the carbon support (Fig. 1b). For Ni2P/RC, about 5e200 nm nanocrystals were loaded on the RC support (Fig. 1d). A 0.507 nm lattice spacing in high-resolution TEM (HRTEM) lattice image (Fig. 1e) is observed on the nanocrystal and assigned to (100) facet of Ni2P, suggesting that the nanocrystals are Ni2P. In addition, the 0.34 nm lattice spacing, typically corresponding to (002) facet of carbon, suggests the formation of crystalline carbon from the carbonization of chelating resin. After loading of Pt on Ni2P/RC, the 3e10 nm particles dispersed on the carbon sheet (Fig. 1f). The average diameter of Pt
nanoparticles (shown in Fig. 2a and Fig. 2b), which are obtained by measuring over 200 particles from different areas of the TEM grid, are 4.00 nm for Pt/C and 3.39 nm for PteNi2P/RC. Meanwhile, Pt nanoparticles are homogeneously distributed on the surface of Ni2P/RC hybrid material with less agglomeration than Pt/C. It is generally believed that the high dispersity and smaller particles size of the Pt nanoparticles in PteNi2P/RC catalyst may be due to good immobilization of the Ni2P/RC, which would lead to a large specific surface area and produce more active sites for isopropanol electro-oxidation reaction. HRTEM lattice image of PteNi2P/RC catalyst displayed the lattice fringe of 0.221 and 0.228 nm (Fig. 1g), corresponding to the (111) facet of Ni2P and the (111) facet of Pt, respectively [21], demonstrating that the Pt and Ni2P were successfully anchored onto RC [24e26]. The corresponding EDX spectrum is further executed to analyze the elemental
Fig. 2 e (a) and (b) Particle size distribution of PteNi2P/RC and Pt/C.
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components of the PteNi2P/RC, as shown in Fig. S1a. It exhibits that the prepared PteNi2P/RC catalyst are composed of C, P, Pt and Ni elemental without other impurities. Fig. S1b is the element mapping analyses, which show that Pt, Ni, P and C elements are homogeneously dispersed, representing a uniform distribution of PteNi2P/RC nanocatalyst. To further insight into the chemical composition and surface electronic state of the catalysts, XPS was carried out to explore the valence of Pt, Ni, and P. XPS survey spectra (Fig. 3a) clearly indicated the presence of Pt, Ni, P, and C for PteNi2P/RC catalyst, as well as Pt and C for Pt/C catalyst. Pt 4f 7/2 XPS peaks of Pt/RC can be deconvoluted into three species (Fig. 3b): 72.89, 74.83, and 74.95 eV, which are ascribed to metallic Pt, Pt2þ [PtO and/or Pt(OH)2], and Pt4þ chemical state (PtO2 phases), respectively [27]. The oxidation state of Pt may be due to oxygen chemisorption display on the Pt surface [28]. The binding energies of the three Pt species evidently decreased after loading Pt onto Ni2P/RC (Pt: 71.65 eV, Pt2þ: 72.55 eV, Pt4þ: 74.35 eV) (Fig. 3b). The molar percent of these Pt species was calculated according to their XPS peak area (shown in Table S1). The content of Pt0 in Pt 4f XPS increased from 51.9% of Pt/C to 62.3% of PteNi2P/RC, while content of Pt2þ decreased from 37.2% of Pt/C to 20.3% of PteNi2P/RC. The content of Pt4þ slightly increased from 10.9% of Pt/C to 17.4% of PteNi2P/RC. This means that the total content of high-valence Pt decreased obviously after loading Pt onto Ni2P/RC. This fact would originate from that the interactions between Ni2P and Pt induce the electrons transfer from Ni2P to Pt due to the higher work function of Pt [8,29]. Indeed, the Pt 4f binding
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energy is about 0.3 eV smaller for PteNi2P/RC (Pt 4f7/2: 71.64 and Pt 4f5/2: 74.93 eV) than Pt/C (Pt 4f7/2: 71.83 and Pt 4f5/2: 75.16 eV) (Fig. 3b), implying that the Ni2P as an electron donor donated its electrons to Pt [16]. Ni 2p3/2 XPS spectrum of Ni2P/ RC was decomposed to three peaks: 854.1 eV for Nidþ (0
Fig. 3 e a) XPS survey spectra of Pt/C and PteNi2P/RC. b) Deconvolution of Pt 4f XPS spectra of Pt/C and PteNi2P/RC. c) Deconvolution of Ni 2p XPS spectra of PteNi2P/RC and Ni2P/RC. d) P 2p core-level spectra of PteNi2P/RC.
(1)
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Fig. 4 e The electrochemical measurements of PteNi2P/RC, Pt/RC and Pt/C. a) CV in 0.1 M N2-purged KOH. b) CV in 0.1 M KOH0.5 M isopropanol. c) I-t curves for 3600 s, inserted: the current after 3600 s. d) Loss of the isopropanol oxidation activity with cycles in 0.1 M KOH-0.5 M isopropanol.
where, QH is the amount of charge exchange during the adsorption of hydrogen atoms on the Pt surface (C); m is the Pt loading in the electrode (g cm2); 0.21 mC cm2 is the charge required to oxidize the hydrogen monolayer. As illustrated in Fig. 4a, the EASA of PteNi2P/RC is roughly 95.7 m2 g1 Pt, which is dramatically higher than Pt/RC (41.9 m2 g1 Pt) and Pt/C (27.8 m2 g1 Pt), suggesting the rich available electrochemical active sites on the surfaces of PteNi2P/RC. The electrocatalytic performances of the prepared electrocatalysts toward isopropanol oxidation were investigated in 0.1 M KOH-0.5 M isopropanol solution at a sweeping rate of 50 mV s1, and the results are displayed in Fig. 4b. The onset potential of PteNi2P/RC shifts lower than that of Pt/C and Pt/ RC, which are more favorable to the electro-oxidation of isopropanol. Such a trend clearly indicates the promotion effect of Ni2P. Moreover, for all catalysts, there are two apparent peaks for forward and reverse scans. The peak of the forward scan is due to the oxidation of chemisorbed isopropanol molecules on the catalyst surface, while the peak of the backward scan is related to the further oxidation of intermediates [35]. PteNi2P/RC exhibits the highest mass activity of 403.1 mA mg1Pt, which is 1.48 and 1.90 times higher than those of Pt/RC (272.9 mA mg1Pt), and Pt/C (212.2 mA mg1Pt), respectively. These results demonstrate a good electrocatalytic activity of isopropanol oxidation on PteNi2P/RC catalyst. I-t tests is used to evaluate the stability of the electrocatalysts in 0.1 M KOH-0.5 M (the applied potential is 0.2 V). In Fig. 4c, the current density of all catalysts decay rapidly in the initial stage, possibly due to the accumulation of strongly
adsorbed intermediates on the reaction site surface [36,37]. While, initial current for PteNi2P/RC is larger than that of Pt/ RC and Pt/C, demonstrating a greater number of active sites available per mass of surface. Consequently, the relative current slowly decreases and reach a pseudo-steady state. It is obvious that the PteNi2P/RC exhibits much higher current densities over time in comparsion with Pt/C and Pt/RC. Especially, after 3600 s, the current of the PteNi2P/RC (23.8 mA mg1Pt) is about 4.25 and 8.81 times than that of Pt/RC (5.6 mA mg1Pt) and Pt/C (2.7 mA mg1Pt), respectively (inserted in Fig. 4c). These results indicate a high tolerance to the carbonaceous species produced during isopropanol oxidation and good stability of PteNi2P/RC catalysts. To further clarify the good durability of PteNi2P/RC, ADT are performed in Fig. 4d. After 500 cycles of CV measurements, the activity loss of
Table 1 e Comparative electrocatalytic properties towards isopropanol electrooxidation. Catalyst PteNi2P/RC Pd3Cu PdeAu/PPP/GCE Pd/GCE Pd3Fe/CN Pd/CN Pd/C Pd1Sn2 PdAu PdAu41/C
Ep/V
Ip/mA mg1
Ref.
0.05 (Ag/AgCl) 0.05 (Hg/HgO) 0.29 (SCE) 0.35 (SCE) 0.08 (Ag/AgCl) 0.03 (Ag/AgCl) 0.06 (Ag/AgCl) 0.69 (RHE) 0.22 (SCE) 0.32 (SCE)
403 0.79 140 21 470 360 240 150 280 453
This Work [1] [32] [32] [35] [35] [35] [7] [37] [37]
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Fig. 5 e a) LSV in 0.1 M KOH-0.5 M isopropanol. b) Corresponded Tafel plots of PteNi2P/RC, Pt/RC and Pt/C.
PteNi2P/RC is 56.6%, while this value of Pt/C is up to 72.0%, indicating that PteNi2P/RC has superior cycling stability for the oxidation of isopropanol. The successive CV of 500 cycles for PteNi2P/RC and Pt/C are shown in Fig. S2a and Fig. S2b. To further prove that the electrocatalytic performances of the prepared PteNi2P/RC catalyst has been greatly improved, we have compared our work with some previously reported catalysts for electro-oxidation of isopropanol, and the results are listed in Table 1. Clearly, PteNi2P/RC exhibits an advantage in the improvement of peak current (Ip) compared with other catalysts, indicating that the PteNi2P/RC can be well applied to serve as an outstanding anode catalyst for direct isopropanol fuel cells. LSV is performed at a scan rate of 1 mV s1 (shown in Fig. 5a). Notably, the oxidation current of PteNi2P/RC is larger than that of the Pt/RC and Pt/C electrocatalysts, which is well consistent with CV results of Fig. 4b. Fig. 5b present Tafel plots corresponding to Fig. 5a, for further investigating isopropanol electro-oxidation process. Visibly, all curves of the figures are composed of straight lines with two slopes. The first fitted Tafel slopes values (in the low overpotential region) are 137.7, 169.9 and 157.1 mV dec1 for PteNi2P/RC, Pt/RC and Pt/C, respectively. The lower Tafel slope of PteNi2P/RC indicates the charge-transfer kinetics on it is faster than other catalysts for the isopropanol oxidation [14]. While, the second fitted Tafel slopes values (in the high overpotential region) sharply increased to 202.7, 237.7 and 296.1 mV dec1, respectively. The second slopes are almost double the first ones, which indicates that the reaction mechanism or rate-determining steps of isopropanol electro-oxidation have changed in different potential ranges. For comprehensively understanding the process of isopropanol oxidation reaction, following reaction mechanism are proposed [38]: CH3CHOHCH3þ 2OH / CH3COCH3 þ 2H2O þ 2e
(2)
CH3COCH3þ16OH / 3CO2 þ 11H2O þ 16 e
(3)
It is observed that acetone and carbon dioxide are the only products of electro-oxidation of isopropanol [38]. The process of acetone formation by electro-oxidation does not involve strong adsorption intermediates such as carbon monoxide. Fig. S3a and Fig. S3b are the XPS spectra of Pt in PtNi2P/RC and
Pt/C after isopropanol electrooxidation, respectively. For each catalyst, after isopropanol electro-oxidation reaction, the content of metal Pt decreased, and the content of Pt in the oxidation state increased, indicating that surface oxide was generated during the reaction. However, after electrooxidation of isopropanol, the content of Pt in PtNi2P/RC is also higher than that of Pt/C (shown in Table 2), indicating that the intermediate oxide adsorbs less on PtNi2P/RC, which is more favorable for the catalytic reaction. In general, the electron donation between Pt and Ni2P affects the catalytic properties of metal nanoparticles and results in the activation of the electrode processes by dispersing metal and oxide material [17]. The electrons rich in the Pt surface can weaken
Table 2 e Binding energy and relative concentration of Pt 4f XPS spectra for PteNi2P/RC and Pt/C catalysts after isopropanol electro-oxidation reaction. Pt Species
Pt 4f7/2 Binding Energy (eV)
PteNi2P/RC catalyst Pt (0) Pt (II) Pt (IV) Pt/C catalyst Pt (0) Pt (II) Pt (IV)
Relative Concentrations (%)
71.83 72.55 75.13
40 50 10
71.76 72.55 75.08
33 59 8
Table 3 e Binding energy and relative concentration of C 1s XPS spectra for PteNi2P/RC and Pt/C catalysts after isopropanol electro-oxidation reaction. C Species PteNi2P/RC catalyst CeC CH3eC CaO CO2/CO23 Pt/C catalyst CeC CH3eC CaO CO2/CO23
Binding Energy (eV)
Relative Concentrations (%)
284.79 285.51 288.3 288.7
66.1 27.8 4.2 1.9
284.78 285.65 288.3 288.76
73.6 19.6 1.3 5.5
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Fig. 6 e Polarization curves and corresponding power density plots of the direct isopropanol fuel cells.
the adsorption ability of CO-poisoning species according to dband theory, thus the catalytic activity of alcohols oxidation was increased greatly compared with Pt/C catalyst [31]. Fig. S3c and Fig. S3d are the XPS spectra of C in PtNi2P/RC and Pt/C after electro-oxidation of isopropanol, respectively. The content of C]O group in PtNi2P/RC is higher than that of Pt/C catalyst, which indicates that PtNi2P/RC promotes the kinetics of acetone production (shown in Table 3). It is reported that acetone is weakly adsorbed on the electrode surface. However, the further electro-oxidation of acetone proceeds through strongly adsorbed intermediates, which will hinder the electro-oxidation of isopropanol [39]. In PtNi2P/RC, the content of CO2/CO23 is lower than that in Pt/C, which indicates that the further oxidation of acetone is less in PtNi2P/RC electrode, so the intermediate adsorbed on PtNi2P/RC electrode is less, which is more conducive to the electro-oxidation of isopropanol. Based on the Density Functional Theory (DFT) calculation reported in literature, electron transfer can reduce the d-band center of Pt and change the reaction pathway and mechanism of the surface sensitive reaction. Then, the adsorption energy of the poisoning intermediates on the metal surfaces can be decreased, which would contribute positively to the removal of it [29]. Meanwhile, the presence of Ni2P can activate water, producing -OHads to oxidize poisoning intermediates adsorbed at Pt sites, through the so-called bifunctional mechanism, which would result in an improvement in the electrocatalytic performance [40]. In addition, it is reported that the oxidation state of the phosphide can facilitate the production of adsorbed oxophilic species, which would provide the necessary oxygen-containing sites for the poisoning intermediates oxidation. To demonstrate the potential of the PteNi2P/RC in a direct isopropanol fuel cell, the catalysts are integrated at the anode with a homemade fuel cell. Fig. 6 shows the steady-stead polarization of the fuel cells. The open circuit voltage are 0.90 V and 0.97 V using PteNi2P/RC and Pt/C as an anode,
respectively. As the discharging current density increases, the voltages of the test cells gradually decrease. However, using PteNi2P/RC as the anode electrocatalyst, the cell voltage drops more slowly as the current density increases. Meanwhile, the maximum power densities in the test cells using PteNi2P/RC and Pt/C are 0.095 and 0.072 W cm2, respectively, which confirms the promising activity of PteNi2P/RC for direct isopropanol fuel cell [6,25]. These performance data of direct isopropanol fuel cell are comparable to those of the previously demonstrated DAFCs. During the isopropanol oxidation process, oxidation product is mainly acetone, and less COads absorption phenomenon happened, which provides significantly important advantages over the other DAFCs systems.
Conclusions In summary, PteNi2P/RC was prepared using a scalable and facile procedure. The Ni2P nanoparticles significantly promote the activity and stability of Pt in isopropanol electro-oxidation reaction, mainly due to the advantages of greater number of active sites and synergistic effects. Meanwhile, PteNi2P/RC was successfully integrated into a direct isopropanol fuel cell, showing superior performance to Pt/C electrocatalyst. Furthermore, compared to other previously-reported catalysts, the prepared PteNi2P/RC also exhibits higher oxidation current. We believe that this work is a significant step toward the development of more efficient catalysts for practical applications of direct isopropanol fuel cells.
Acknowledgement This work is funded by the National Natural Science Foundation of China (Grant No.51676161).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.221.
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