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TiN–TiO2 catalyst preparation and its performance in oxygen reduction reaction
Journal of Power Sources 454 (2020) 227934
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Pt/TiN–TiO2 catalyst preparation and its performance in oxygen reduction reaction Lai Wei, Jicheng Shi *, Guowei Cheng, Lu Lu, Hongfeng Xu, Yang Li College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian, 116028, Liaoning Province, PR China
H I G H L I G H T S
� Alkali treatment induces the spherical TiN partly transform into needle-like TiO2. � TiN–TiO2 support has good electrical conductivity and corrosion resistance. � TiN–TiO2 support micropore ratio significantly increases after alkali treatment. � Alkali treatment greatly increases the specific surface area of TiN–TiO2 support. � Pt/TiN–TiO2 has a higher catalytic activity for ORR and it’s more stable than Pt/C. A R T I C L E I N F O
A B S T R A C T
Keywords: Oxygen reduction reaction Conductivity Corrosion resistance Pt/TiN–TiO2 catalyst PEMFC
Easy corrosion and Pt loss are severe problems in traditional Pt/C catalysts. Herein, a conductive and corrosionresistant TiN–TiO2 support was prepared by the alkali treatment of TiN. The micropore ratio of the support remarkably increases after alkali treatment, and the specific surface area increases from 41.0 m2 g 1 of TiN to 108 m2 g 1 of TiN–TiO2. The scanning electron microscopy image reveals the needle-like and spherical struc tures, which are attributed to TiO2 and aggregated TiN, respectively. TiO2 is the anatase crystal in the TiN–TiO2 support. The ohmic polarization of proton-exchange membrane fuel cell (PEMFC) with Pt/TiN–TiO2 in the cathode is lower than that with Pt/C in the cathode. The peak power and peak current density of PEMFC with Pt/ TiN–TiO2 catalyst increase by 36% and 28%, respectively, compared with PEMFC containing Pt/C catalyst. Overall, TiN–TiO2 catalyst has higher stability and electrochemical performance than Pt/C catalyst.
1. Introduction The commercialization of fuel-cell vehicles requires reduced cost of fuel-cell stacks, and Pt/C catalysts account for 46% of the cost of fuel-cell stacks. Thus, developing new oxygen reduction catalysts with high ef ficiency and low cost is necessary. Catalyst stability must also be improved to prolong stack life. Support is the key factor that affects the activity and stability of catalysts. Traditional carbon supports inevitably corrode at instanta neous high potential formed under dynamic conditions, causing Pt particles to grow and fall off from the surface of carbon supports. In a previous study, Yang et al. [1] found that chromium nitride (CrN) has excellent chemical and electrochemical stability in acid solutions, and the electrochemical activity and stability of Pt/CrN catalyst are better than that of Pt/C catalyst. Liu et al. [2] used Pt/TiO2/carbon as cathode catalyst in proton-exchange membrane fuel cell (PEMFC), the
electrochemical specific surface area (ECSA) of Pt/TiO2/carbon remained at 75.6% after 1000 cycles of cyclic voltammetry, whereas that of Pt/carbon was only 42.6%. Kim et al. [3] found that the activity of the catalyst prepared with carbon nanotube (CNT) as support is three times higher than that of commercial Pt/C. They believed that CNT-supported Pt could reduce the adsorption of OH species on the surface of Pt, releasing active sites and improving the catalytic activity. Mirshekari et al. [4] loaded Pt onto titanium dioxide (TiO2) with different particle sizes; the catalyst life test showed that the ECSA loss is 5.9% for 12 wt% Pt/TiO2 catalyst (30 nm). Shahgaldi et al. [5] assem bled single PEMFCs with TiO2, Vulcan carbon, carbon@TiO2 (carbon core, TiO2 shell), and TiO2@carbon (TiO2 core, carbon shell) as supports and found that the power density of Pt-carbon@TiO2 reached 410 mW cm 2, which is favorably comparable with Pt/C catalyst. The stability of the Pt-carbon@TiO2 catalyst is better than that of Pt–TiO2@carbon. Ji et al. [6] prepared a porous CNT-Pt/TiO2 catalyst with certain
* Corresponding author. E-mail address: [email protected] (J. Shi). https://doi.org/10.1016/j.jpowsour.2020.227934 Received 22 August 2019; Received in revised form 29 January 2020; Accepted 19 February 2020 Available online 24 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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advantages, such as strong interaction between Pt and TiO2 nanofibers, CNT conductivity, and corrosion-resistant TiO2. The power density of the assembled single cell is 567 mW cm 2, and that of commercial Pt/carbon is 461 mW cm 2. TiO2 and other metal oxides as catalyst supports are effective in solving the corrosion resistance of platinum catalysts, but the prepared catalysts need to have high conductivity. Li et al. [7] first demonstrated that titanium nitride (TiN) as a plasmonic booster can considerably enhance the photoelectrochemical water splitting performance of TiO2. Li et al. [8] designed a novel multi-phase N-doped TiO2/TiN/graphene composite with the properties of ionic and electronic conductivities and the high performance of lithium-ion batteries at low temperatures. Anwar et al. [9] doped TiO2 with tantalum as the platinum catalyst support, in which the conductivity was improved. Pt/Ta–TiO2 catalyst has decreased specific surface area and increased durability and oxygen reduction catalytic performance comparable with that of Pt/C catalyst. Dabir et al. [10] filled TiN into the pores of 3D graphite and found that the electron escape work decreased from 5.00 eV to 4.68 eV. Xia et al. [11] synthesized highly ordered porous N-doped TiO2 nanotube arrays by reverse oxidation of porous TiN in air atmosphere. The band gap of porous N–TiO2 is reduced from 3.04 eV for TiO2 to 2.94 eV for N–TiO2. TiN and its partial formation phase (metallic conductor) promote the charge transfer process of V(II)/V(III) pairs and improve the perfor mance of vanadium liquid flow batteries [12,13]. In summary, TiN has the characteristics of high conductivity. If TiN is used as a support, then the corrosion resistance of TiN would not be as good as that of TiO2. If the characteristics of both are combined and the specific surface area meets the standard of Pt catalyst support, then TiN–TiO2 will be the ideal support for Pt catalyst. Just as it happens, TiN has the feature that can be converted into TiO2 under alkaline condi tions. In the current paper, by choosing suitable conditions, the part of TiN is transformed into TiO2. Consequently, the support has the char acteristics of conductivity, corrosion resistance, and high specific sur face area, and the supported Pt catalyst is prepared.
Quantachrome degassing station and kept in the vacuum at 80 � C for 15 min and at 120 � C for 2 h. High-purity nitrogen gas was filled into the degassing station. (2) The sample was placed into the Kanta QuadraSorb SI instrument and tested under liquid nitrogen environment. 2.3. Electrochemical test of supports and catalysts The electrochemical workstation was Autolab PGSTAT302 N. The platinum wire, saturated calomel, and thin-film electrodes were the opposite, reference, and working electrodes, respectively. Thin-film electrode was prepared as follows. Glassy carbon electrode was polished and cleaned in ethanol by ultrasound method. The sample (5.0 mg), Nafion solution (5 wt%; 50.0 μL; DuPont Company, USA), and ethanol (2.0 mL) were mixed uniformly through ultrasonic dispersion. The resulting solution (20.0 μL) was dropped on the glassy carbon electrode with an area of 0.2826 cm2 by the micro-sampler. The electrochemical stability of TiN–TiO2 and XC-72 carbon supports was tested in the electrochemical workstation. The thin-film electrode was placed in 0.5 mol L 1 H2SO4 solution and oxidized for 10 min at a constant potential of 1.2 V. The electrochemical stability of Pt/TiN–TiO2 and Pt/C catalysts was tested through cyclic voltammetry. The scanning range and speed were 0.2 V–1.2 V (vs. SCE) and 0.05 V s 1, respectively. The electrolyte used was 0.5 mol L 1 H2SO4. The ECSA of Pt in the catalyst was calculated by integrating the hydrogen desorption peak area (Eq. (1)). � � ECSA ¼ SH Qref � ν � m ; (1) where ECSA (cm2 g 1) is the electrochemical specific surface area of Pt, SH (AV cm 2) is the hydrogen desorption peak area, ν (V s 1) is the cyclic voltammetric scanning speed, Qref (2.1 � 10 4 C cm 2) is the electron quantity required for a single-layer hydrogen atom oxidation on the smooth Pt surface, and m (g cm 2) is the Pt load density on the electrode. 2.4. PEMFC measurement with Pt/C and Pt/TiN–TiO2 catalysts in the cathode
2. Experimental
PEMFC anode was prepared by mixing 5.0 mg of 30% Pt/C catalyst, 50.0 μL of 5 wt% Nafion solution, and 2.0 mL of absolute ethanol under ultrasonic vibration. The solution was sprayed on the diffusion layer of Toray carbon paper (1.0 cm � 5.0 cm), and the load density of the 30% Pt/C was 1.0 mg cm 2. Cathode was prepared using the same procedure as the anode. Pt/C and Pt/TiN–TiO2 were the catalysts, and their load density was 1.0 mg cm 2. Membrane electrode assembly was arranged in the sequence of polyester frame, anode, Nafion 1135 film, cathode, and polyester frame. The assembly was placed in a hot press at 140 � C for 60 s, then at 10 MPa for 60 s. Polarization curves of the single cell were measured at the Sunrise fuel cell test platform (New Source Power Company, China). Air and hydrogen inlet pressure levels were set at 0.11 MPa, and the humidifying and cell operation temperatures were 70 � C.
2.1. Preparation of TiN–TiO2 support and Pt/TiN–TiO2 catalyst TiN (10.00 g; Qinhuangdao Yinuo, Co. Ltd.) was added into 40.0 mL of 0.5 mol L 1 NaOH solution (Tianjin Beilian). The solution was stirred continuously at 80 � C for 2.5 h, centrifuged, and dried. The samples were heated in a vacuum tube furnace (Shanghai Jujing, China) at 400 � C for 2 h under argon protection to obtain TiN–TiO2 support. TiN–TiO2 support (90.0 mg) was placed in a 150 mL conical bottle, added with 10.0 mL of 20 mmol/L chloroplatinic acid (Shenyang Jinke Reagent)/ethylene glycol (Tianjin Fuyu Fine Chemical) solution and 30.0 mL of ethylene glycol, and stirred for 25 min. The mixture was added with 0.1 mol L 1 NaOH solution, and its pH was adjusted to 12. The mixture was transferred into 100 mL three-neck flasks, and the re action was performed at 140 � C for 1 h. The product was washed with deionized water and dried in a vacuum oven at 80 � C (Pt content: 30% in the catalyst). The 30% Pt/C catalyst with XC-72 carbon support (Cabot Corp.) serves as the control. The preparation method is the same as the above-mentioned process.
3. Results and discussion
2.2. Characterization of supports and catalysts
3.1. Specific surface area measurement
Samples were characterized by a JSM-6360LV scanning electron microscope and JEOL TEM 2000EX transmission electron microscope (voltage 120 kV, resolution 1.43 Å). The phase of the samples was determined by Empyrean X-ray diffraction (Panalytical, Netherlands, auxiliary source copper, wavelength: 0.154056 nm, working voltage: 40 kV, current: 150 mA, scanning speed: 5� min 1, and scanning scope: 20� –90� ). The specific surface area was measured as follows. (1) Approximately 0.10 g–0.20 g of the sample was placed into a
Fig. 1(a) shows the adsorption–desorption isotherms of TiN and TiN–TiO2 samples. According to the IUPAC adsorption equilibrium isotherm classification, the N2 adsorption–desorption curves of TiN and TiN–TiO2 samples are V-shaped adsorption curves. A weak interaction exists between the support and adsorbed nitrogen molecules. The highpressure region has a lag ring, which leads to capillary condensation. The starting pressure of the hysteresis ring in TiN–TiO2 is lower than that in TiN, indicating that the former forms smaller pores than the latter 2
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Journal of Power Sources 454 (2020) 227934
Fig. 1. N2 adsorption–desorption isotherm (a), linear regression curve of BET isothermal adsorption (b), and BJH pore size distribution (c) of TiN and TiN–TiO2 samples.
after alkali treatment. At relative pressure P/P0 of 0.05–0.35, the N2 adsorption–desorption curve conforms to the Brunauer, Emmett, and Teller (BET) isothermal adsorption linear equation shown in Eq. (2). Fig. 1(b) presents the linear regression curves of the BET isothermal adsorption equation.
approximately 10 nm TiN nanoparticles on the carbon matrix and high SBET of the composite of up to 148 m2 g 1.
(2)
Fig. 2 presents the SEM and TEM images of TiN and TiN–TiO2 sup ports. The particle size of the sample was measured and analyzed by Image Pro Plus 6.0 software. TiN is a spherical particle with an average particle size of 33.5 nm, and the standard deviation is 3.9 nm (Fig. 3[a]). A part of the TiN–TiO2 support has a rod-like structure, whereas the rest is spherical. This finding indicates TiN particle agglomeration caused by the alkali treatment. Moatti et al. [16] prepared a core–shell structure of TiO2/TiN nanowires integrated with silicon and sapphire as substrates. The TiN sample was hydrolyzed in thermal alkaline solution to produce titanium trihydroxide. Low-valence titanium is unstable and easily oxidized into high-valence titanium hydroxide by oxygen in air. Titanium hydroxide was dehydrated to produce TiO2. The reaction mechanism is shown as follows. The TiN sample was partly converted into TiO2 by controlling the reaction conditions. The TiN–TiO2 sample has the high conductivity of TiN and the corrosion resistance of TiO2.
P = ðV½P0
P�Þ ¼ ðC
1ÞP=ðCVm P0 Þ þ 1=ðCVm Þ;
3.2. Characterization with SEM and TEM
where C is the isothermal adsorption constant, Vm (cm3 g 1) is the ni trogen monolayer-saturated adsorption capacity on the surface of the sample, and V (cm3 g 1) is the actual adsorption capacity of the sample. Vm ¼ 1=ðSlope þ InterceptÞ;
(3)
C ¼ 1=ðIntercept � Vm Þ;
(4)
SBET ¼ Vm � N � Am =22414;
(5)
where SBET is the specific surface area of the sample, N is the Avogadro constant (6.02 � 1023 mol 1), and Am is the nitrogen moleculeequivalent maximum cross-section area (16.2 � 10 20 m2). The isothermal adsorption constant (C) and specific surface area (SBET) of the samples are calculated from Eqs. (3)–(5). As shown in Table 1, the SBET values of TiN and TiN–TiO2 supports are 41.0 and 108 m2 g 1, respectively, which increased remarkably after alkali treatment. Fig. 1(c) shows the pore size distribution of TiN and TiN–TiO2
alkaline
TiN þ H2 O ���������������������������! TiðOHÞ3 þ NH3
dehydration
oxidation
TiðOHÞ3 �������������������������������!TiðOHÞ4 ��������������������������������������! TiO2
samples. Both samples are mesoporous. The most probable pore sizes of TiN and TiN–TiO2 samples are 1.8 and 2.0 nm, respectively. After alkali treatment, the number of mesopores increases, and more smaller pores are formed. As such, the SBET of TiN–TiO2 is 1.63 times higher than that of TiN. Li et al. [14] developed a flash oxidation method by calcining commercial TiN nanoparticles to TiN/TiO2 composite through modu lating calcination time and temperature. The SBET and average pore size of TiN–TiO2 are 52.6 m2 g 1 and 3.55 nm, respectively. Wang et al. [15] proposed dispersion and annealing steps to synthesize TiN/carbon composite materials and reported the uniform dispersion of
Fig. 4 presents the TEM images of Pt/C and Pt/TiN–TiO2 catalysts. The dispersion of Pt in the TiN–TiO2 support is uniform. The size of Pt particles in the Pt/C catalyst is finer than that in Pt/TiN–TiO2, and the average particle size is 4.6 nm; the standard deviation is 0.7 nm (Fig. 3 [b]). The crystallization conditions of Pt atoms are different for Vulcan XC-72 carbon and TiN–TiO2 support. The SBET of Vulcan XC-72 carbon is 254 m2 g 1, whereas that of the TiN–TiO2 support is 108 m2 g 1. Moreover, the nature of XC-72 carbon and TiN–TiO2 is different in the catalyst preparation. The fluidity and dispersion of XC-72 carbon are better than those of the TiN–TiO2 support. Mirshekari et al. [17] syn thesized Pt/TiO2, Pt/TiN, and Pt/TiC catalysts to solve the catalyst instability problems in PEMFC. The average Pt sizes are 2.3, 2.3, and 2.6 nm for Pt/TiO2, Pt/TiN, and Pt/TiC catalysts, respectively. The Pt/TiC catalyst is the most stable and has the best catalytic performance and highest oxygen reduction reaction current and ECSA among the three catalysts. The conductivity of TiN reaches 4.54 � 104 S cm 1, whereas TiO2 has semiconductor properties. A portion of TiN is converted into TiO2 under alkaline conditions, and the sample contains a certain proportion of TiN
Table 1 Specific surface area (SBET) data of TiN and TiN–TiO2 samples. Sample name
Slope
Intercept
Vm/cm3 g 1
Constant C
SBET/m2 g 1
TiN
0.10544
9.4286
171.07
41.0
TiN–TiO2
0.03972
6.20473 � 10 4 3.74477 � 10 4
24.94
107.1
108
3
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Journal of Power Sources 454 (2020) 227934
Fig. 2. SEM and TEM images of TiN (a, b) and TiN–TiO2 (c, d) supports.
Fig. 3. Particle size distribution of TiN [a]and Pt/C [b]samples.
Fig. 4. TEM images of Pt/C (a) and Pt/TiN–TiO2 (b) catalysts.
and TiO2 simultaneously. The support is conductive because of TiN and resistant to corrosion because of TiO2. These properties meet the re quirements of Pt catalyst for ideal supports. The samples of Pt/TiN–TiO2 were analyzed using energy-dispersive spectroscopy (Fig. 5). Based on the element distribution data shown in Table 2, the molar and mass ratios of TiN to TiO2 are 60.0:40.0 and 53.8:46.2, respectively.
3.3. X-ray diffraction analysis of supports and catalysts Fig. 6 shows the X-ray diffraction (XRD) spectra of TiN and TiN–TiO2 supports and Pt/C and Pt/TiN–TiO2 catalysts. As shown in Fig. 6(a), the diffraction angles are 36.69� , 42.63� , 61.87� , 74.14� , and 78.04� , which correspond to the (111), (200), (220), (311), and (222) crystal planes of TiN, respectively. The result is consistent with the standard spectrum 4
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Journal of Power Sources 454 (2020) 227934
Fig. 5. Energy-dispersive spectrum of Pt/TiN–TiO2 catalyst (a) Samples’ selected area, (b) energy spectrum, (c) nitrogen distribution, (d) titanium distribution, (e) oxygen distribution, and (f) platinum distribution.
composite membranes through Ti(OC4H9)4 in situ sol–gel reaction in Nafion perfluorosulfonic acid films. TiO2 in the composite membranes has an anatase structure and an average particle size of 4 nm. As shown in Fig. 6(c), the spectrum of the Pt/C catalyst shows the characteristic peaks of Pt at 39.54� , 45.98� , 67.06� , 80.75� , and 85.15� , which corre spond to the (111), (200), (220), (311), and (222) crystal planes, respectively. This result is consistent with the diffraction peak of the standard card PDF#87–0636. Fig. 6(d) is the XRD spectrum of the Pt/TiN–TiO2 catalyst. The characteristic peaks of Pt, TiN, and TiO2 are observed in the spectrum, indicating that Pt is successfully loaded onto the TiN–TiO2 support.
Table 2 Element distribution of Pt/TiN–TiO2 catalyst by EDS. Chemical elements
C
N
O
Ti
Pt
Mass ratio, (w/w) % Atomic ratio, %
2.81 12.20
1.00 3.70
1.52 4.92
66.50 71.74
28.17 7.47
3.4. Electrochemical stability of Pt/C and Pt/TiN–TiO2 catalysts Fig. 7(a) shows the time-dependent corrosion current of the XC-72 carbon and TiN–TiO2 supports at 1.2 V (vs. SCE) constant potential. At 1.2 V, the corrosion current of the XC-72 carbon support is larger than that of the TiN–TiO2 support. The TiN–TiO2 support is more resistant to high-potential oxidation than the XC-72 carbon support. Wang et al. [19] studied the corrosion resistance of Ti, TiN, TiO2, and N–TiO2 coatings on stainless steel (SS) substrates. The corrosion potentials are 0.431, 0.185, 0.192, 0.162, and 0.169 V (vs. SCE) for SS, Ti, TiN, TiO2, and N–TiO2, respectively. The polarization test revealed that N–TiO2 coating exhibited the best corrosion resistance. Fig. 7(b) and (c) show the cyclic voltammetry curves of Pt/C and Pt/ TiN–TiO2 catalysts at a scanning range of 0.2 V–1.2 V (vs. SCE) and with 800 cycles. The peak current densities of hydrogen reduction adsorption and oxidation desorption in the hydrogen region were compared. The peak current density of the Pt/TiN–TiO2 catalyst is higher than that of the Pt/C catalysts at 0.2 V–0 V potential range. In oxygen reduction, the peak current density of the Pt/C catalyst decreased rapidly with prolonged cyclic voltammetry test time. The electrochemical activity of the Pt/TiN–TiO2 catalyst is slightly higher than that of the Pt/C catalyst because Ti and Pt belong to the same dzone and have a similar outer electronic structure. The interaction be tween the two elements occurs easily (due to the overlap of the d-orbit occupied by the precious metals and the empty d-orbit of Ti4þ) and is conducive in fixing Pt particles and preventing their migration. This
Fig. 6. XRD spectrum of TiN (a), TiN–TiO2 (b), Pt/C (c), and Pt/TiN–TiO2 (d).
PDF#87–0633 of TiN. As shown in Fig. 6(b), the characteristic peaks of PDF#83–2243 anatase-type TiO2 appear in the diffraction spectrum of TiN–TiO2 samples. The diffraction angles are 25.33� , 48.10� , 55.14� , and 68.81� , which correspond to the (101), (200), (211), and (116) diffraction crystal planes of TiO2, respectively. After alkali hydrolysis and heat treatment at 400 � C, the sample has two crystalline phases, namely, conductive TiN and corrosion-resistant TiO2 phases, which meet the double requirements of the conductivity and corrosion resis tance of Pt catalyst supports. Tian et al. [18] prepared Nafion/TiO2 5
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Journal of Power Sources 454 (2020) 227934
Fig. 7. Potentiostatic oxidation curves (a) of XC-72 carbon, TiN–TiO2 supports and cyclic voltammetry curves of Pt/C (b), and Pt/TiN–TiO2 (c) catalysts (poten tiostatic oxidation potential, 1.2 V vs. SCE; cyclic voltammetry scanning, 0.2 V–1.2 V (vs. SCE); scanning speed, 50 mV s–1; electrolyte, 0.5 mol L–1 H2SO4). Table 3 Electrochemical active surface area (ECAS) of Pt/C and Pt/TiN–TiO2 catalyst. Sample name
Parameters
1st cycle
200th cycle
400th cycle
600th cycle
800th cycle
Pt/C
SH (AV cm 2) ECSA (m2 g 1) SH (AV cm 2) ECSA (m2 g 1)
0.36
0.21
0.16
0.11
0.07
44
25
19
12
8
0.59
0.39
0.37
0.32
0.23
73
47
45
39
28
TiN–TiO2
interaction can also change the electronic structure of Pt and improve the activity of the catalyst. As shown in Fig. 7 (b) and (c), the peak areas of hydrogen oxidation desorption are integrated according to Eq. (1). The peak area values are shown in Table 3. After 800 cycles of cyclic voltammetry, the peak area of the Pt/C catalyst in the hydrogen desorption zone decreases by 82% from 44 m2 g 1 to 8 m2 g 1, and the ECSA of Pt/TiN–TiO2 decreases by 62% from 73 m2 g 1 to 28 m2 g 1. Savych et al. [20] synthesized Nb-doped TiO2 nanofiber as electrocatalyst supports for PEMFC. A higher ECSA is retained for Pt supported on Nb-doped TiO2 nanofiber (73%) than on carbon nanofiber, where only 8% of the original ECSA is conserved after 1000 voltammetric cycles. The Pt/TiN–TiO2 catalyst is more durable than the Pt/C catalyst due to the oxidation resistance of the TiO2 phase in the support. The inter action between Ti and Pt improves the immobilization of Pt particles to prevent their migration and reunion. The first 200 cycles of Pt/TiN–TiO2 catalyst decay are serious, which may be due to the lack of tight bonding and the shedding of certain Pt particles from the support during loading. This phenomenon also exists in the initial cycle of the Pt/C catalyst. Wang et al. [21] and Subban et al. [22] prepared the Pt/Ti0.7W0.3O2 catalyst and found that such catalyst exhibits the same catalytic activity for hydrogen oxidation and oxygen reduction as commercial Pt/C. The current loss of the Pt/Ti0.7W0.3O2 catalyst as anode catalysts is only 5%, whereas that of commercial Pt/C (E-TEK) catalysts is 30% after 500 cycles of cyclic voltammetry tests.
Fig. 8. Polarization and power density curves of Pt/C and Pt/TiN–TiO2 cata lysts in PEMFC (cell temperature, 70 � C; humidifier temperature, 70 � C; hydrogen and air entrance pressure, 0.11 MPa).
segments are 111 mA cm 2 at 0.80 V and 824 mA cm 2 at 0.32 V for the Pt/C catalyst, whereas the boundary points are 79 mA cm 2 at 0.78 V and 914 mA cm 2 at 0.42 V for the Pt/TiN–TiO2 catalyst. The peak power density of the Pt/C catalyst is 288 mW cm 2 at 656 mA cm 2, whereas that of the Pt/TiN–TiO2 catalyst is 392 mW cm 2 at 840 mA cm 2. Liu et al. [23] fabricated a TiN inverse opal structure on a carbon paper as Pt support in PEMFCs, which used Pt@TiN@carbon paper composite as cathode and anode. Nafion 115 was used as the membrane. The electrode area was set as 2.0 cm � 2.0 cm, and the hydrogen and air inlet pressure and cell temperature were set as 0.1 MPa and 60 � C, respectively. The power density of the PEMFC is 71 mW cm 2 at 220 mA cm 2. The power density of commercial E-Tek electrodes as cathode and anode is 154 mW cm 2 at 510 mA cm 2. In the low current density region, the Pt/TiN–TiO2 catalyst has larger electrochemical polarization and more difficult electrochemical reaction speed than the Pt/C catalyst. This result may be due to the electrode wettability difference in the initial discharge stage. The electrode wettability of the Pt/C catalyst is faster, which is conducive to proton conduction; therefore, its electrochemical polarization is smaller. In the middle current density region, the ohmic polarization of the Pt/ TiN–TiO2 catalyst is small, which indicates excellent conductivity of the Pt/TiN–TiO2 catalyst. In comparison with the Pt/C catalyst, the Pt/ TiN–TiO2 catalyst exhibits concentration polarization at a higher current density because of the higher ECSA of the Pt/TiN–TiO2 catalyst. The performance of PEMFC with the Pt/TiN–TiO2 catalyst in the cathode is better than that of the Pt/C catalyst with Vulcan XC-72 carbon support.
3.5. PEMFC test with Pt/C and Pt/TiN–TiO2 in the cathode The prepared 30% Pt/C and 30% Pt/TiN–TiO2 catalysts were loaded onto the PEMFC cathode with a load density of 1.0 mg cm 2. Moreover, 30% Pt/C catalyst was loaded onto the PEMFC anode with a load density of 1.0 mg cm 2. The membrane used was Nafion 1135 membrane. The inlet pressure of hydrogen and air was set as 0.11 MPa, and the tem perature of gas humidification and cell operation was 70 � C. Fig. 8 shows the discharge polarization and power density curves with Pt/C and Pt/ TiN–TiO2 in the PEMFC cathode. The polarization curves can be divided into three distinct segments, namely, electrochemical, ohmic, and con centration polarization. In the curves, the boundary points of the three 6
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4. Conclusions
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(1) TiN support has a high conductivity but not a corrosion resis tance. TiO2 support has a corrosion resistance but a poor con ductivity. TiN is used as a raw material and treated with alkali to transform part of TiN into TiO2. The support has the character istics of conductivity and corrosion resistance. (2) The other function of treating TiN with alkali is to increase the mesopores, which increases the specific surface area of the sup port. The specific surface area of the support is increased from 41.0 m2 g 1 of TiN to 108 m2 g 1 of the TiN–TiO2 support. (3) The stability of Pt/TiN–TiO2 catalyst is considerably improved by the interaction between the d-orbital of Pt atom and the empty dorbital of Ti4þ in the support. (4) The electrochemical active surface area of Pt/TiN–TiO2 catalyst is higher than that of Pt/C, which may be due to the interaction between Pt and Ti4þ in favor of H oxidation desorption. (5) The good conductivity and stability of TiN–TiO2 support and the high electrochemical active surface area of the prepared catalyst remarkably improve the performance of PEMFC. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was funded by the National Key Research and Devel opment Program of China (No.2016YFB0101207), Liaoning Natural Science Foundation Guidance Plan (No.201602129), and Dalian Jiao tong University Students Innovation and Entrepreneurship Training Program (No.201810150088). References [1] M.H. Yang, Z.M. Cui, F.J. DiSalvo, Phys. Chem. Chem. Phys. 15 (2013) 7041–7044, https://doi.org/10.1039/C3CP51109J.
7
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Pt/TiN–TiO2 catalyst preparation and its performance in oxygen reduction reaction Lai Wei, Jicheng Shi *, Guowei Cheng, Lu Lu, Hongfeng Xu, Yang Li College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian, 116028, Liaoning Province, PR China
H I G H L I G H T S
� Alkali treatment induces the spherical TiN partly transform into needle-like TiO2. � TiN–TiO2 support has good electrical conductivity and corrosion resistance. � TiN–TiO2 support micropore ratio significantly increases after alkali treatment. � Alkali treatment greatly increases the specific surface area of TiN–TiO2 support. � Pt/TiN–TiO2 has a higher catalytic activity for ORR and it’s more stable than Pt/C. A R T I C L E I N F O
A B S T R A C T
Keywords: Oxygen reduction reaction Conductivity Corrosion resistance Pt/TiN–TiO2 catalyst PEMFC
Easy corrosion and Pt loss are severe problems in traditional Pt/C catalysts. Herein, a conductive and corrosionresistant TiN–TiO2 support was prepared by the alkali treatment of TiN. The micropore ratio of the support remarkably increases after alkali treatment, and the specific surface area increases from 41.0 m2 g 1 of TiN to 108 m2 g 1 of TiN–TiO2. The scanning electron microscopy image reveals the needle-like and spherical struc tures, which are attributed to TiO2 and aggregated TiN, respectively. TiO2 is the anatase crystal in the TiN–TiO2 support. The ohmic polarization of proton-exchange membrane fuel cell (PEMFC) with Pt/TiN–TiO2 in the cathode is lower than that with Pt/C in the cathode. The peak power and peak current density of PEMFC with Pt/ TiN–TiO2 catalyst increase by 36% and 28%, respectively, compared with PEMFC containing Pt/C catalyst. Overall, TiN–TiO2 catalyst has higher stability and electrochemical performance than Pt/C catalyst.
1. Introduction The commercialization of fuel-cell vehicles requires reduced cost of fuel-cell stacks, and Pt/C catalysts account for 46% of the cost of fuel-cell stacks. Thus, developing new oxygen reduction catalysts with high ef ficiency and low cost is necessary. Catalyst stability must also be improved to prolong stack life. Support is the key factor that affects the activity and stability of catalysts. Traditional carbon supports inevitably corrode at instanta neous high potential formed under dynamic conditions, causing Pt particles to grow and fall off from the surface of carbon supports. In a previous study, Yang et al. [1] found that chromium nitride (CrN) has excellent chemical and electrochemical stability in acid solutions, and the electrochemical activity and stability of Pt/CrN catalyst are better than that of Pt/C catalyst. Liu et al. [2] used Pt/TiO2/carbon as cathode catalyst in proton-exchange membrane fuel cell (PEMFC), the
electrochemical specific surface area (ECSA) of Pt/TiO2/carbon remained at 75.6% after 1000 cycles of cyclic voltammetry, whereas that of Pt/carbon was only 42.6%. Kim et al. [3] found that the activity of the catalyst prepared with carbon nanotube (CNT) as support is three times higher than that of commercial Pt/C. They believed that CNT-supported Pt could reduce the adsorption of OH species on the surface of Pt, releasing active sites and improving the catalytic activity. Mirshekari et al. [4] loaded Pt onto titanium dioxide (TiO2) with different particle sizes; the catalyst life test showed that the ECSA loss is 5.9% for 12 wt% Pt/TiO2 catalyst (30 nm). Shahgaldi et al. [5] assem bled single PEMFCs with TiO2, Vulcan carbon, carbon@TiO2 (carbon core, TiO2 shell), and TiO2@carbon (TiO2 core, carbon shell) as supports and found that the power density of Pt-carbon@TiO2 reached 410 mW cm 2, which is favorably comparable with Pt/C catalyst. The stability of the Pt-carbon@TiO2 catalyst is better than that of Pt–TiO2@carbon. Ji et al. [6] prepared a porous CNT-Pt/TiO2 catalyst with certain
* Corresponding author. E-mail address: [email protected] (J. Shi). https://doi.org/10.1016/j.jpowsour.2020.227934 Received 22 August 2019; Received in revised form 29 January 2020; Accepted 19 February 2020 Available online 24 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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advantages, such as strong interaction between Pt and TiO2 nanofibers, CNT conductivity, and corrosion-resistant TiO2. The power density of the assembled single cell is 567 mW cm 2, and that of commercial Pt/carbon is 461 mW cm 2. TiO2 and other metal oxides as catalyst supports are effective in solving the corrosion resistance of platinum catalysts, but the prepared catalysts need to have high conductivity. Li et al. [7] first demonstrated that titanium nitride (TiN) as a plasmonic booster can considerably enhance the photoelectrochemical water splitting performance of TiO2. Li et al. [8] designed a novel multi-phase N-doped TiO2/TiN/graphene composite with the properties of ionic and electronic conductivities and the high performance of lithium-ion batteries at low temperatures. Anwar et al. [9] doped TiO2 with tantalum as the platinum catalyst support, in which the conductivity was improved. Pt/Ta–TiO2 catalyst has decreased specific surface area and increased durability and oxygen reduction catalytic performance comparable with that of Pt/C catalyst. Dabir et al. [10] filled TiN into the pores of 3D graphite and found that the electron escape work decreased from 5.00 eV to 4.68 eV. Xia et al. [11] synthesized highly ordered porous N-doped TiO2 nanotube arrays by reverse oxidation of porous TiN in air atmosphere. The band gap of porous N–TiO2 is reduced from 3.04 eV for TiO2 to 2.94 eV for N–TiO2. TiN and its partial formation phase (metallic conductor) promote the charge transfer process of V(II)/V(III) pairs and improve the perfor mance of vanadium liquid flow batteries [12,13]. In summary, TiN has the characteristics of high conductivity. If TiN is used as a support, then the corrosion resistance of TiN would not be as good as that of TiO2. If the characteristics of both are combined and the specific surface area meets the standard of Pt catalyst support, then TiN–TiO2 will be the ideal support for Pt catalyst. Just as it happens, TiN has the feature that can be converted into TiO2 under alkaline condi tions. In the current paper, by choosing suitable conditions, the part of TiN is transformed into TiO2. Consequently, the support has the char acteristics of conductivity, corrosion resistance, and high specific sur face area, and the supported Pt catalyst is prepared.
Quantachrome degassing station and kept in the vacuum at 80 � C for 15 min and at 120 � C for 2 h. High-purity nitrogen gas was filled into the degassing station. (2) The sample was placed into the Kanta QuadraSorb SI instrument and tested under liquid nitrogen environment. 2.3. Electrochemical test of supports and catalysts The electrochemical workstation was Autolab PGSTAT302 N. The platinum wire, saturated calomel, and thin-film electrodes were the opposite, reference, and working electrodes, respectively. Thin-film electrode was prepared as follows. Glassy carbon electrode was polished and cleaned in ethanol by ultrasound method. The sample (5.0 mg), Nafion solution (5 wt%; 50.0 μL; DuPont Company, USA), and ethanol (2.0 mL) were mixed uniformly through ultrasonic dispersion. The resulting solution (20.0 μL) was dropped on the glassy carbon electrode with an area of 0.2826 cm2 by the micro-sampler. The electrochemical stability of TiN–TiO2 and XC-72 carbon supports was tested in the electrochemical workstation. The thin-film electrode was placed in 0.5 mol L 1 H2SO4 solution and oxidized for 10 min at a constant potential of 1.2 V. The electrochemical stability of Pt/TiN–TiO2 and Pt/C catalysts was tested through cyclic voltammetry. The scanning range and speed were 0.2 V–1.2 V (vs. SCE) and 0.05 V s 1, respectively. The electrolyte used was 0.5 mol L 1 H2SO4. The ECSA of Pt in the catalyst was calculated by integrating the hydrogen desorption peak area (Eq. (1)). � � ECSA ¼ SH Qref � ν � m ; (1) where ECSA (cm2 g 1) is the electrochemical specific surface area of Pt, SH (AV cm 2) is the hydrogen desorption peak area, ν (V s 1) is the cyclic voltammetric scanning speed, Qref (2.1 � 10 4 C cm 2) is the electron quantity required for a single-layer hydrogen atom oxidation on the smooth Pt surface, and m (g cm 2) is the Pt load density on the electrode. 2.4. PEMFC measurement with Pt/C and Pt/TiN–TiO2 catalysts in the cathode
2. Experimental
PEMFC anode was prepared by mixing 5.0 mg of 30% Pt/C catalyst, 50.0 μL of 5 wt% Nafion solution, and 2.0 mL of absolute ethanol under ultrasonic vibration. The solution was sprayed on the diffusion layer of Toray carbon paper (1.0 cm � 5.0 cm), and the load density of the 30% Pt/C was 1.0 mg cm 2. Cathode was prepared using the same procedure as the anode. Pt/C and Pt/TiN–TiO2 were the catalysts, and their load density was 1.0 mg cm 2. Membrane electrode assembly was arranged in the sequence of polyester frame, anode, Nafion 1135 film, cathode, and polyester frame. The assembly was placed in a hot press at 140 � C for 60 s, then at 10 MPa for 60 s. Polarization curves of the single cell were measured at the Sunrise fuel cell test platform (New Source Power Company, China). Air and hydrogen inlet pressure levels were set at 0.11 MPa, and the humidifying and cell operation temperatures were 70 � C.
2.1. Preparation of TiN–TiO2 support and Pt/TiN–TiO2 catalyst TiN (10.00 g; Qinhuangdao Yinuo, Co. Ltd.) was added into 40.0 mL of 0.5 mol L 1 NaOH solution (Tianjin Beilian). The solution was stirred continuously at 80 � C for 2.5 h, centrifuged, and dried. The samples were heated in a vacuum tube furnace (Shanghai Jujing, China) at 400 � C for 2 h under argon protection to obtain TiN–TiO2 support. TiN–TiO2 support (90.0 mg) was placed in a 150 mL conical bottle, added with 10.0 mL of 20 mmol/L chloroplatinic acid (Shenyang Jinke Reagent)/ethylene glycol (Tianjin Fuyu Fine Chemical) solution and 30.0 mL of ethylene glycol, and stirred for 25 min. The mixture was added with 0.1 mol L 1 NaOH solution, and its pH was adjusted to 12. The mixture was transferred into 100 mL three-neck flasks, and the re action was performed at 140 � C for 1 h. The product was washed with deionized water and dried in a vacuum oven at 80 � C (Pt content: 30% in the catalyst). The 30% Pt/C catalyst with XC-72 carbon support (Cabot Corp.) serves as the control. The preparation method is the same as the above-mentioned process.
3. Results and discussion
2.2. Characterization of supports and catalysts
3.1. Specific surface area measurement
Samples were characterized by a JSM-6360LV scanning electron microscope and JEOL TEM 2000EX transmission electron microscope (voltage 120 kV, resolution 1.43 Å). The phase of the samples was determined by Empyrean X-ray diffraction (Panalytical, Netherlands, auxiliary source copper, wavelength: 0.154056 nm, working voltage: 40 kV, current: 150 mA, scanning speed: 5� min 1, and scanning scope: 20� –90� ). The specific surface area was measured as follows. (1) Approximately 0.10 g–0.20 g of the sample was placed into a
Fig. 1(a) shows the adsorption–desorption isotherms of TiN and TiN–TiO2 samples. According to the IUPAC adsorption equilibrium isotherm classification, the N2 adsorption–desorption curves of TiN and TiN–TiO2 samples are V-shaped adsorption curves. A weak interaction exists between the support and adsorbed nitrogen molecules. The highpressure region has a lag ring, which leads to capillary condensation. The starting pressure of the hysteresis ring in TiN–TiO2 is lower than that in TiN, indicating that the former forms smaller pores than the latter 2
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Fig. 1. N2 adsorption–desorption isotherm (a), linear regression curve of BET isothermal adsorption (b), and BJH pore size distribution (c) of TiN and TiN–TiO2 samples.
after alkali treatment. At relative pressure P/P0 of 0.05–0.35, the N2 adsorption–desorption curve conforms to the Brunauer, Emmett, and Teller (BET) isothermal adsorption linear equation shown in Eq. (2). Fig. 1(b) presents the linear regression curves of the BET isothermal adsorption equation.
approximately 10 nm TiN nanoparticles on the carbon matrix and high SBET of the composite of up to 148 m2 g 1.
(2)
Fig. 2 presents the SEM and TEM images of TiN and TiN–TiO2 sup ports. The particle size of the sample was measured and analyzed by Image Pro Plus 6.0 software. TiN is a spherical particle with an average particle size of 33.5 nm, and the standard deviation is 3.9 nm (Fig. 3[a]). A part of the TiN–TiO2 support has a rod-like structure, whereas the rest is spherical. This finding indicates TiN particle agglomeration caused by the alkali treatment. Moatti et al. [16] prepared a core–shell structure of TiO2/TiN nanowires integrated with silicon and sapphire as substrates. The TiN sample was hydrolyzed in thermal alkaline solution to produce titanium trihydroxide. Low-valence titanium is unstable and easily oxidized into high-valence titanium hydroxide by oxygen in air. Titanium hydroxide was dehydrated to produce TiO2. The reaction mechanism is shown as follows. The TiN sample was partly converted into TiO2 by controlling the reaction conditions. The TiN–TiO2 sample has the high conductivity of TiN and the corrosion resistance of TiO2.
P = ðV½P0
P�Þ ¼ ðC
1ÞP=ðCVm P0 Þ þ 1=ðCVm Þ;
3.2. Characterization with SEM and TEM
where C is the isothermal adsorption constant, Vm (cm3 g 1) is the ni trogen monolayer-saturated adsorption capacity on the surface of the sample, and V (cm3 g 1) is the actual adsorption capacity of the sample. Vm ¼ 1=ðSlope þ InterceptÞ;
(3)
C ¼ 1=ðIntercept � Vm Þ;
(4)
SBET ¼ Vm � N � Am =22414;
(5)
where SBET is the specific surface area of the sample, N is the Avogadro constant (6.02 � 1023 mol 1), and Am is the nitrogen moleculeequivalent maximum cross-section area (16.2 � 10 20 m2). The isothermal adsorption constant (C) and specific surface area (SBET) of the samples are calculated from Eqs. (3)–(5). As shown in Table 1, the SBET values of TiN and TiN–TiO2 supports are 41.0 and 108 m2 g 1, respectively, which increased remarkably after alkali treatment. Fig. 1(c) shows the pore size distribution of TiN and TiN–TiO2
alkaline
TiN þ H2 O ���������������������������! TiðOHÞ3 þ NH3
dehydration
oxidation
TiðOHÞ3 �������������������������������!TiðOHÞ4 ��������������������������������������! TiO2
samples. Both samples are mesoporous. The most probable pore sizes of TiN and TiN–TiO2 samples are 1.8 and 2.0 nm, respectively. After alkali treatment, the number of mesopores increases, and more smaller pores are formed. As such, the SBET of TiN–TiO2 is 1.63 times higher than that of TiN. Li et al. [14] developed a flash oxidation method by calcining commercial TiN nanoparticles to TiN/TiO2 composite through modu lating calcination time and temperature. The SBET and average pore size of TiN–TiO2 are 52.6 m2 g 1 and 3.55 nm, respectively. Wang et al. [15] proposed dispersion and annealing steps to synthesize TiN/carbon composite materials and reported the uniform dispersion of
Fig. 4 presents the TEM images of Pt/C and Pt/TiN–TiO2 catalysts. The dispersion of Pt in the TiN–TiO2 support is uniform. The size of Pt particles in the Pt/C catalyst is finer than that in Pt/TiN–TiO2, and the average particle size is 4.6 nm; the standard deviation is 0.7 nm (Fig. 3 [b]). The crystallization conditions of Pt atoms are different for Vulcan XC-72 carbon and TiN–TiO2 support. The SBET of Vulcan XC-72 carbon is 254 m2 g 1, whereas that of the TiN–TiO2 support is 108 m2 g 1. Moreover, the nature of XC-72 carbon and TiN–TiO2 is different in the catalyst preparation. The fluidity and dispersion of XC-72 carbon are better than those of the TiN–TiO2 support. Mirshekari et al. [17] syn thesized Pt/TiO2, Pt/TiN, and Pt/TiC catalysts to solve the catalyst instability problems in PEMFC. The average Pt sizes are 2.3, 2.3, and 2.6 nm for Pt/TiO2, Pt/TiN, and Pt/TiC catalysts, respectively. The Pt/TiC catalyst is the most stable and has the best catalytic performance and highest oxygen reduction reaction current and ECSA among the three catalysts. The conductivity of TiN reaches 4.54 � 104 S cm 1, whereas TiO2 has semiconductor properties. A portion of TiN is converted into TiO2 under alkaline conditions, and the sample contains a certain proportion of TiN
Table 1 Specific surface area (SBET) data of TiN and TiN–TiO2 samples. Sample name
Slope
Intercept
Vm/cm3 g 1
Constant C
SBET/m2 g 1
TiN
0.10544
9.4286
171.07
41.0
TiN–TiO2
0.03972
6.20473 � 10 4 3.74477 � 10 4
24.94
107.1
108
3
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Fig. 2. SEM and TEM images of TiN (a, b) and TiN–TiO2 (c, d) supports.
Fig. 3. Particle size distribution of TiN [a]and Pt/C [b]samples.
Fig. 4. TEM images of Pt/C (a) and Pt/TiN–TiO2 (b) catalysts.
and TiO2 simultaneously. The support is conductive because of TiN and resistant to corrosion because of TiO2. These properties meet the re quirements of Pt catalyst for ideal supports. The samples of Pt/TiN–TiO2 were analyzed using energy-dispersive spectroscopy (Fig. 5). Based on the element distribution data shown in Table 2, the molar and mass ratios of TiN to TiO2 are 60.0:40.0 and 53.8:46.2, respectively.
3.3. X-ray diffraction analysis of supports and catalysts Fig. 6 shows the X-ray diffraction (XRD) spectra of TiN and TiN–TiO2 supports and Pt/C and Pt/TiN–TiO2 catalysts. As shown in Fig. 6(a), the diffraction angles are 36.69� , 42.63� , 61.87� , 74.14� , and 78.04� , which correspond to the (111), (200), (220), (311), and (222) crystal planes of TiN, respectively. The result is consistent with the standard spectrum 4
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Fig. 5. Energy-dispersive spectrum of Pt/TiN–TiO2 catalyst (a) Samples’ selected area, (b) energy spectrum, (c) nitrogen distribution, (d) titanium distribution, (e) oxygen distribution, and (f) platinum distribution.
composite membranes through Ti(OC4H9)4 in situ sol–gel reaction in Nafion perfluorosulfonic acid films. TiO2 in the composite membranes has an anatase structure and an average particle size of 4 nm. As shown in Fig. 6(c), the spectrum of the Pt/C catalyst shows the characteristic peaks of Pt at 39.54� , 45.98� , 67.06� , 80.75� , and 85.15� , which corre spond to the (111), (200), (220), (311), and (222) crystal planes, respectively. This result is consistent with the diffraction peak of the standard card PDF#87–0636. Fig. 6(d) is the XRD spectrum of the Pt/TiN–TiO2 catalyst. The characteristic peaks of Pt, TiN, and TiO2 are observed in the spectrum, indicating that Pt is successfully loaded onto the TiN–TiO2 support.
Table 2 Element distribution of Pt/TiN–TiO2 catalyst by EDS. Chemical elements
C
N
O
Ti
Pt
Mass ratio, (w/w) % Atomic ratio, %
2.81 12.20
1.00 3.70
1.52 4.92
66.50 71.74
28.17 7.47
3.4. Electrochemical stability of Pt/C and Pt/TiN–TiO2 catalysts Fig. 7(a) shows the time-dependent corrosion current of the XC-72 carbon and TiN–TiO2 supports at 1.2 V (vs. SCE) constant potential. At 1.2 V, the corrosion current of the XC-72 carbon support is larger than that of the TiN–TiO2 support. The TiN–TiO2 support is more resistant to high-potential oxidation than the XC-72 carbon support. Wang et al. [19] studied the corrosion resistance of Ti, TiN, TiO2, and N–TiO2 coatings on stainless steel (SS) substrates. The corrosion potentials are 0.431, 0.185, 0.192, 0.162, and 0.169 V (vs. SCE) for SS, Ti, TiN, TiO2, and N–TiO2, respectively. The polarization test revealed that N–TiO2 coating exhibited the best corrosion resistance. Fig. 7(b) and (c) show the cyclic voltammetry curves of Pt/C and Pt/ TiN–TiO2 catalysts at a scanning range of 0.2 V–1.2 V (vs. SCE) and with 800 cycles. The peak current densities of hydrogen reduction adsorption and oxidation desorption in the hydrogen region were compared. The peak current density of the Pt/TiN–TiO2 catalyst is higher than that of the Pt/C catalysts at 0.2 V–0 V potential range. In oxygen reduction, the peak current density of the Pt/C catalyst decreased rapidly with prolonged cyclic voltammetry test time. The electrochemical activity of the Pt/TiN–TiO2 catalyst is slightly higher than that of the Pt/C catalyst because Ti and Pt belong to the same dzone and have a similar outer electronic structure. The interaction be tween the two elements occurs easily (due to the overlap of the d-orbit occupied by the precious metals and the empty d-orbit of Ti4þ) and is conducive in fixing Pt particles and preventing their migration. This
Fig. 6. XRD spectrum of TiN (a), TiN–TiO2 (b), Pt/C (c), and Pt/TiN–TiO2 (d).
PDF#87–0633 of TiN. As shown in Fig. 6(b), the characteristic peaks of PDF#83–2243 anatase-type TiO2 appear in the diffraction spectrum of TiN–TiO2 samples. The diffraction angles are 25.33� , 48.10� , 55.14� , and 68.81� , which correspond to the (101), (200), (211), and (116) diffraction crystal planes of TiO2, respectively. After alkali hydrolysis and heat treatment at 400 � C, the sample has two crystalline phases, namely, conductive TiN and corrosion-resistant TiO2 phases, which meet the double requirements of the conductivity and corrosion resis tance of Pt catalyst supports. Tian et al. [18] prepared Nafion/TiO2 5
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Journal of Power Sources 454 (2020) 227934
Fig. 7. Potentiostatic oxidation curves (a) of XC-72 carbon, TiN–TiO2 supports and cyclic voltammetry curves of Pt/C (b), and Pt/TiN–TiO2 (c) catalysts (poten tiostatic oxidation potential, 1.2 V vs. SCE; cyclic voltammetry scanning, 0.2 V–1.2 V (vs. SCE); scanning speed, 50 mV s–1; electrolyte, 0.5 mol L–1 H2SO4). Table 3 Electrochemical active surface area (ECAS) of Pt/C and Pt/TiN–TiO2 catalyst. Sample name
Parameters
1st cycle
200th cycle
400th cycle
600th cycle
800th cycle
Pt/C
SH (AV cm 2) ECSA (m2 g 1) SH (AV cm 2) ECSA (m2 g 1)
0.36
0.21
0.16
0.11
0.07
44
25
19
12
8
0.59
0.39
0.37
0.32
0.23
73
47
45
39
28
TiN–TiO2
interaction can also change the electronic structure of Pt and improve the activity of the catalyst. As shown in Fig. 7 (b) and (c), the peak areas of hydrogen oxidation desorption are integrated according to Eq. (1). The peak area values are shown in Table 3. After 800 cycles of cyclic voltammetry, the peak area of the Pt/C catalyst in the hydrogen desorption zone decreases by 82% from 44 m2 g 1 to 8 m2 g 1, and the ECSA of Pt/TiN–TiO2 decreases by 62% from 73 m2 g 1 to 28 m2 g 1. Savych et al. [20] synthesized Nb-doped TiO2 nanofiber as electrocatalyst supports for PEMFC. A higher ECSA is retained for Pt supported on Nb-doped TiO2 nanofiber (73%) than on carbon nanofiber, where only 8% of the original ECSA is conserved after 1000 voltammetric cycles. The Pt/TiN–TiO2 catalyst is more durable than the Pt/C catalyst due to the oxidation resistance of the TiO2 phase in the support. The inter action between Ti and Pt improves the immobilization of Pt particles to prevent their migration and reunion. The first 200 cycles of Pt/TiN–TiO2 catalyst decay are serious, which may be due to the lack of tight bonding and the shedding of certain Pt particles from the support during loading. This phenomenon also exists in the initial cycle of the Pt/C catalyst. Wang et al. [21] and Subban et al. [22] prepared the Pt/Ti0.7W0.3O2 catalyst and found that such catalyst exhibits the same catalytic activity for hydrogen oxidation and oxygen reduction as commercial Pt/C. The current loss of the Pt/Ti0.7W0.3O2 catalyst as anode catalysts is only 5%, whereas that of commercial Pt/C (E-TEK) catalysts is 30% after 500 cycles of cyclic voltammetry tests.
Fig. 8. Polarization and power density curves of Pt/C and Pt/TiN–TiO2 cata lysts in PEMFC (cell temperature, 70 � C; humidifier temperature, 70 � C; hydrogen and air entrance pressure, 0.11 MPa).
segments are 111 mA cm 2 at 0.80 V and 824 mA cm 2 at 0.32 V for the Pt/C catalyst, whereas the boundary points are 79 mA cm 2 at 0.78 V and 914 mA cm 2 at 0.42 V for the Pt/TiN–TiO2 catalyst. The peak power density of the Pt/C catalyst is 288 mW cm 2 at 656 mA cm 2, whereas that of the Pt/TiN–TiO2 catalyst is 392 mW cm 2 at 840 mA cm 2. Liu et al. [23] fabricated a TiN inverse opal structure on a carbon paper as Pt support in PEMFCs, which used Pt@TiN@carbon paper composite as cathode and anode. Nafion 115 was used as the membrane. The electrode area was set as 2.0 cm � 2.0 cm, and the hydrogen and air inlet pressure and cell temperature were set as 0.1 MPa and 60 � C, respectively. The power density of the PEMFC is 71 mW cm 2 at 220 mA cm 2. The power density of commercial E-Tek electrodes as cathode and anode is 154 mW cm 2 at 510 mA cm 2. In the low current density region, the Pt/TiN–TiO2 catalyst has larger electrochemical polarization and more difficult electrochemical reaction speed than the Pt/C catalyst. This result may be due to the electrode wettability difference in the initial discharge stage. The electrode wettability of the Pt/C catalyst is faster, which is conducive to proton conduction; therefore, its electrochemical polarization is smaller. In the middle current density region, the ohmic polarization of the Pt/ TiN–TiO2 catalyst is small, which indicates excellent conductivity of the Pt/TiN–TiO2 catalyst. In comparison with the Pt/C catalyst, the Pt/ TiN–TiO2 catalyst exhibits concentration polarization at a higher current density because of the higher ECSA of the Pt/TiN–TiO2 catalyst. The performance of PEMFC with the Pt/TiN–TiO2 catalyst in the cathode is better than that of the Pt/C catalyst with Vulcan XC-72 carbon support.
3.5. PEMFC test with Pt/C and Pt/TiN–TiO2 in the cathode The prepared 30% Pt/C and 30% Pt/TiN–TiO2 catalysts were loaded onto the PEMFC cathode with a load density of 1.0 mg cm 2. Moreover, 30% Pt/C catalyst was loaded onto the PEMFC anode with a load density of 1.0 mg cm 2. The membrane used was Nafion 1135 membrane. The inlet pressure of hydrogen and air was set as 0.11 MPa, and the tem perature of gas humidification and cell operation was 70 � C. Fig. 8 shows the discharge polarization and power density curves with Pt/C and Pt/ TiN–TiO2 in the PEMFC cathode. The polarization curves can be divided into three distinct segments, namely, electrochemical, ohmic, and con centration polarization. In the curves, the boundary points of the three 6
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Journal of Power Sources 454 (2020) 227934
4. Conclusions
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(1) TiN support has a high conductivity but not a corrosion resis tance. TiO2 support has a corrosion resistance but a poor con ductivity. TiN is used as a raw material and treated with alkali to transform part of TiN into TiO2. The support has the character istics of conductivity and corrosion resistance. (2) The other function of treating TiN with alkali is to increase the mesopores, which increases the specific surface area of the sup port. The specific surface area of the support is increased from 41.0 m2 g 1 of TiN to 108 m2 g 1 of the TiN–TiO2 support. (3) The stability of Pt/TiN–TiO2 catalyst is considerably improved by the interaction between the d-orbital of Pt atom and the empty dorbital of Ti4þ in the support. (4) The electrochemical active surface area of Pt/TiN–TiO2 catalyst is higher than that of Pt/C, which may be due to the interaction between Pt and Ti4þ in favor of H oxidation desorption. (5) The good conductivity and stability of TiN–TiO2 support and the high electrochemical active surface area of the prepared catalyst remarkably improve the performance of PEMFC. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was funded by the National Key Research and Devel opment Program of China (No.2016YFB0101207), Liaoning Natural Science Foundation Guidance Plan (No.201602129), and Dalian Jiao tong University Students Innovation and Entrepreneurship Training Program (No.201810150088). References [1] M.H. Yang, Z.M. Cui, F.J. DiSalvo, Phys. Chem. Chem. Phys. 15 (2013) 7041–7044, https://doi.org/10.1039/C3CP51109J.
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