The proton exchange membrane fuel cell (PEMFC) is a promising power source for electric vehicles owing to its high energy efficiency and environmental compatibility. However, one critical barrier to the use of the PEMFC is the extreme reliance on precious Pt on both the anode and cathode sides of the PEMFC [1-3]. Efforts have been put into the development of non-Pt cathode catalysts to reduce the cost of the PEMFC [4-6]. There are several promising alternative materials reported for Pt-free cathode catalysts for oxygen reduction [7-10]. For the anode side, however, few non-Pt catalysts with high activity and stability for hydrogen oxidation reaction (HOR) have been reported [11]. To ultimately solve the Pt-reliance of the PEMFC technology, developing a high efficiency and low cost non-Pt anode electrocatalyst is urgently needed.
Ir has attracted considerable attention as a promising Pt-alternative anode catalyst owing to its cheaper price and stability in acidic media [12]. However, its catalytic activity is much lower than that of Pt. Controlling the morphology of these Ir-based nanocrystals has great significance on the practical application because the catalytic activity and stability are strongly correlated with the shape and size of the nanocrystals. Another approach to enhance the HOR catalytic activity of Ir-based nanocrystals is to appropriately modify the electronic structure by alloying Ir with some transition metals, such as Co and Fe. Our previous report showed that Ir-based alloy catalysts produced by the solvent evaporation plus hydrogen reduction (SE-HR) method are very active for the catalysis of HOR [13, 14]. However, a definitive determination of the alloying effect remains elusive [15].
Herein, we studied the alloying effect of Ir-based alloy nanocrystals with different metal species on the catalysis of HOR. Sub 5 nm IrFe,IrNi and IrCo alloy nanocrystal catalysts with similar alloying degree and average nanoparticle size (sub-5 nm) were obtained by in situ reduction and nucleation in a H2 atmosphere (extended SE-HR method). Our experiments showed that these foreign metal species induce a contraction in the Ir lattice. The smallest lattice parameter and shortest Ir-Ir bond among these alloys were observed for the IrFe alloy and increased for IrNi and IrCo. Electrochemical experiments demonstrated that the specific activity of 152 A/gIr at 0.1 V versus RHE was obtained on IrNi/C, which was higher than that on IrFe/C (146 A/gIr),IrCo/C (133 A/gIr) and E-TEK Pt/C (116 A/gPt). The highest HOR activity of the IrNi alloy is due to the mid-sized Ni-induced lattice contraction and the modified surface electronic structure, which led to an optimal interaction between the catalyst and the hydrogen intermediates (Ir-Had or IrOH).
In a typical synthesis of IrNi/C NPs,H2IrCl6·6H2O and NiCl2·6H2O were chosen as the Ir and Ni precursors, respectively. First, a well distributed suspension of carbon black (Vulcan XC-72R) in deionized water was formed by ultrasonicating and stirring for 30 min. Then metal precursors with initial molar ratio Ir/Ni = 1:1 were added into the suspension for a total metal loading of 20 wt%. Meanwhile, an appropriate amount of sodium citrate (TCD) was added as a protective agent. After ultrasonicating and stirring constantly overnight, an appropriate amount of NH3·H2O solution was added into the suspension to make the pH equal to 12. Then, the mixture was stirred overnight in a sealed container and evaporated to dryness at 60 °C in a water bath. The obtained precipitate was heated at 500 °C in a tube furnace under a mixture gas flow of N2 and H2 (6:1) for 2 h and then cooled to room temperature. The resultant black powder was filtered, washed with deionized water, and dried at 60 °C for 6 h. The Ir/C, IrFe/C, and IrCo/C samples were synthesized by the same method. The samples (Ir/Fe = 1/1,Ir/Ni = 1/1,Ir/Co = 1/1, annealed at 500 °C) whose pH was adjusted with NH3·H2O aqueous solution were denoted as IrFe/C,IrNi/C, and IrCo/C, respectively.
The morphology of the catalyst particles was observed by transmission electron microscopy (TEM), with a JEM 2010 EX microscope operated at 200 kV. The electronic structure and surface composition analysis of IrFe/C,IrNi/C, and IrCo/C were performed on an ESCLAB MKII (VG Co.,UK). The crystalline phase X-ray diffraction (XRD) patterns were collected on a Philips PW 3040/60 powder diffractometer using a Cu Kα source at 30 kV at a scan rate of 2°/min over the 2θ range of 10°-90°. The microstructural parameters of the samples were determined using the JADE5 software.
Electrochemical experiments were conducted in 0.1 mol/L HClO4 (or NaOH) at room temperature with a rotating disk electrode (RDE) using a Solartron electrochemistry station. Ag/AgCl (saturated KCl) and a Pt wire were used as reference and counter electrodes, respectively. All the potentials in this study are given relative to the reversible hydrogen electrode (RHE). The working electrode was prepared as follows. 2 mg 20 wt% E-TEK Pt/C (conventional Pt/C catalyst from E-TEK Co. USA) or 2 mg 20 wt% IrNi/C catalyst was dispersed in 1600 μL ethanol with 20 μL Nafion solution (0.1 wt% in isopropyl alcohol) and ultrasonicated to form a uniform catalyst ink. Subsequently, a total of 20 μL well dispersed catalyst ink was applied onto the pre-polished RDE. After drying at room temperature, a drop of 0.01 wt% Nafion solution was placed onto the surface of the catalyst layer to form a thin protective film. The apparent surface area of the glassy carbon disk was 0.19625 cm2, thus, for all electrochemical experiments on the RDE, the specific loadings of Pt and IrNi were equivalent, i.e., 25.48 μg-Pt/cm2 and 25.48 μg-IrNi/cm2. Cyclic voltammograms (CVs) for the catalysts were obtained in N2 purged 0.1 mol/L HClO4 (or NaOH) from 0.0 to 1.1 V versus RHE at 50 mV/s. When the CVs did not change, the polarization curve for the HOR was recorded in H2-saturated 0.1 mol/L HClO4 (or NaOH) by sweeping the potential from 0.0 to 0.5 V versus RHE at a scan rate of 10 mV/s and a rotation rate of 1600 r/min.
Figure 1 shows representative TEM images of IrFe/C,IrNi/C and IrCo/C. Spherical shaped alloy NPs were well dispersed on the carbon support, and only a small amount of agglomeration of nanoparticles was observed even after high temperature reduction (500 °C). The particle size distribution analysis, based on more than 200 particles, showed an average particle size of 3 nm with the size distribution between 1 and 6 nm for all the Ir alloy nanocrystals.
The alloy structure of the synthesized catalyst was confirmed by XRD analysis, shown in Fig. 2. These bimetallic catalysts have the same face centered cubic (fcc) structure as Ir, but with the diffraction peaks slightly shifted toward higher angles. The peak at 2θ of 40.61°, 41.98°, 42.07° and 42.16° corresponded to Ir(111),IrCo(111),IrNi(111) and IrFe(111), respectively. This observation together with the absence of peaks for an impure crystal phase demonstrated the distribution of alloy elements within the NPs. The lattice parameter for IrFe/C,IrNi/C, and IrCo/C were 3.722, 3.734, and 3.757 Å, respectively, indicating that the 3d transition metal atoms in the Ir lattice reduced the Ir-Ir bond length in the solid solutions. The asymmetrical reflection peaks with tails toward the 2θ values of the Ir planes indicated an Ir enriched surface alloy structure owing to Ir segregation to the alloy surface during annealing. The alloying degree for the alloy catalysts was estimated according to the line shift in the XRD pattern of the Ir-M alloys [16], and was in the range of 48.5% to 76.4% (Table 1). The average nanocrystal sizes of IrFe,IrNi, and IrCo were estimated [17] as 3.5, 2.6, and 2.9 nm, respectively, which were consistent with TEM observations.
CV curves in acidic medium are shown in Fig. 3. These displayed two peaks corresponding to hydrogen adsorption and desorption in the potential range of 0.05-0.3 V on IrFe/C,IrNi/C,IrCo/C, and Pt/C catalyst modified electrodes. The electrochemical surface areas (ECSAs) based on the Hupd adsorption-desorption region were 65.6 m2/gIr, 65.3 m2/gIr, 45.0 m2/gIr, and 58.4 m2/gPt for IrFe/C,IrNi/C,IrCo/C and Pt/C, respectively. The potential of Had desorption on IrNi/C,IrFe/C, and IrCo/C modified electrodes was negatively shifted to 0.25, 0.25, and 0.3 V, respectively, compared to that on the Pt/C modified electrode (0.4 V). This indicated that the adsorption of Had was weakened on the alloy surfaces. The catalytic activities of IrFe,IrNi, and IrCo alloys were evaluated with a rotating disk electrode (RDE) in H2-saturated 0.1 mol/L HClO4 solution. The kinetic current of HOR obtained from RDE data was normalized by the precious metal mass of the IrM (M = Fe,Ni,Co) alloy to give a mass activity (MA), which was used for the comparison of the intrinsic activity of the catalysts. The highest HOR activity, as estimated from the mass activity, was obtained with IrNi/C and it decreased in order for IrFe/C and IrCo/C in acidic medium. The specific activity on IrNi/C reached 152 A/gIr (at 0.1 V vs RHE) in acidic medium, and it was 146 A/gIr, 133 A/gIr, and 116 A/gPt on IrFe/C,IrCo/C, and E-TEK Pt/C, respectively. According to previous studies, the particle size effect on HOR activity can be ignored when the particle size is less than 5 nm [18, 19]. Therefore, the difference in HOR activity for all the catalysts was attributed to the alloying effect. On the basis of the HOR activity and structural properties, the HOR activity of the alloy was mainly affected by the second metal-induced lattice contraction that changed the electronic structure of the Ir enriched surface. However, the smallest lattice parameter and shortest Ir-Ir bond among these alloys were observed for IrFe alloy and increased for IrNi and IrCo. This means that there is an optimum Ir-Ir bond distance for HOR. Therefore, a mid-sized lattice parameter and Ir-Ir bond were why the IrNi alloy showed the best HOR activity.
In alkaline medium, the alloying effect on the HOR activities of these Ir alloy catalysts was different. Figure 4(a-c) shows the CV curves for the IrFe/C,IrNi/C,IrCo/C, and Pt/C catalyst modified electrodes in a de-aerated 0.1 mol/L NaOH solution. The Hupd adsorption/desorption charge (QH) was 169.36 C/gIr, 158.32 C/gIr, 119.60 C/gIr, and 163.57 C/gPt for IrFe/C,IrNi/C,IrCo/C, and Pt/C, respectively. A negative shift for Had desorption on the alloy catalyst modified electrodes compared to that on the Pt/C modified electrode was also observed, which was consistent with that in acidic medium. However, in the catalysis of HOR in alkaline medium, the performance of the Ir-alloy catalysts was not better than that of Pt/C and PtRu/C, and only the IrNi/C catalyst showed a similar activity to the Pt/C catalyst (Fig. 4(d)). This indicated that Had desorption was not the rate determine step in alkaline medium.
A Tafel analysis was used to determine the rate determining step (RDS) for HOR on the catalyst surface. Tafel plots were obtained by correcting the HOR branch of the activity data by subtracting the diffusion overpotential and determining the kinetically limited HOR current [20]. As shown in Fig. 5, all the catalysts displayed a Tafel slope ~30 mV/dec in acid medium, indicating a molecular hydrogen dissociative adsorption RDS (Tafel). In alkaline medium, however, the catalysts demonstrated a Tafel slope of ~80 mV/dec, indicating an electron transfer RDS (Volmer/Heyrovsky). Thus, the weakening in the Ir-Had interaction caused by the electronic effect of M (M = Fe,Ni,Co) alloying was most probably responsible for the enhancement of HOR activity in acidic medium. However, the oxophilic effect of the catalytic metal surface on HOR activity which affects OHad adsorption and desorption and surface Had coverage cannot be ignored in alkaline medium [21, 22].
The stability of the catalysts was tested at room temperature using continuous cycling between 0 and 0.9 V for a total of 2000 cycles. IrNi/C catalyst exhibited better stability with no obvious deterioration observed after 2000 circles, as shown in Fig. 6(a). X-ray photoelectron spectroscopy (XPS) was used to analyze the change in the electronic structure and metallic components on the IrM (M = Fe,Co,Ni) samples, as shown in Fig. 6(b). The Ir 4f XPS signals were deconvoluted into two pairs of doublets, which were assigned to Ir metal and IrO2 states. The Ir 4f XPS peak of IrNi was shifted to a lower binding energy by 0.2 eV as compared to that of the IrCo and IrFe samples, but was shifted to a higher binding energy by 0.4 eV as compared to that of Ir metal. This shift was attributed to the change in the nearest neighbors surrounding the Ir atom. These results indicate that Ni was more effective than Fe and Co in inducing an appropriate Ir-Had interaction and promoting surface reactivity for HOR.
The alloying of Ir-based alloy nanocrystals with different second metals on the catalysis of HOR was studied in both acidic and alkaline medium. The second metal played an important role in tuning the crystal structure and surface electronic structure of the alloy catalyst. Among the IrFe,IrCo and IrNi alloy catalysts,Ni induced a mid-sized contraction in the Ir lattice and gave the best HOR activity in both acidic and alkaline medium. Electrochemical experiments demonstrated that a specific activity of 152 A/gIr at 0.1 V versus RHE was obtained on IrNi/C in acidic medium, which was higher than that on IrFe/C (146 A/gIr),IrCo/C (133 A/gIr) and E-TEK Pt/C (116 A/gPt). The highest HOR activity on IrNi alloy among these alloy catalysts was mainly due to the mid-sized Ni-induced lattice contraction and the modified surface electronic structure which led to an optimal interaction between the catalyst and hydrogen intermediates (Ir-Had or IrOH). Our results showed that the weakening of the Ir-Had interaction caused by the electronic effect of M (M = Fe,Ni,Co) alloying was responsible for the enhancement HOR activity in acidic medium. The oxophilic effect of catalytic metal surface, which affects OHad adsorption and desorption property and surface Had coverage, had a large impact on HOR activity in alkaline medium.
2016,
Vol. 37 Issue (7): 1142-1148 
PDF (1425 KB)
2016,
Vol. 37 Issue (7): 1142-1148 PDF (1425 KB)