ITO Oxygen Reduction Electrocatalysts

Dec 27, 2012 - Department of Chemical and Biomolecular Engineering, University of Connecticut, 191 Auditorium Drive, Storrs, Connecticut. 06269, Unite...
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High Stability, High Activity Pt/ITO Oxygen Reduction Electrocatalysts Ying Liu, and William Earl Mustain J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja307635r • Publication Date (Web): 27 Dec 2012 Downloaded from http://pubs.acs.org on December 30, 2012

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High Stability, High Activity Pt/ITO Oxygen Reduction Electrocatalysts Ying Liu and William E. Mustain* Department of Chemical, Materials and Biomolecular Engineering, University of Connecticut, 191 Auditorium Drive, Storrs, Connecticut 06269, United States Supporting Information Placeholder pressed 20. Oxide suppression has the potential to improve ABSTRACT: Sn-doped indium oxide (ITO) nanoparticles both the ORR activity of Pt by providing a clean surface over were conceived as a high stability non-carbon support for Pt a wide potential range and the stability of Pt since oxide spenanoparticles and the activity and stability of Pt/ITO for the cies are a key intermediate in Pt dissolution process 21,22. Howoxygen reduction reaction (ORR) was probed. Sn was emever, raw SnO2 suffers from two important limitations that ployed as the In2O3 dopant to exploit the strong interaction may preclude its use as a PEMFC cathode support. First, SnO2 between Sn and Pt that was previously reported to enhance the undergoes redox processes at potentials relevant to the ORR activity of Pt on Pt/SnO2, while concomitantly avoiding the and is at least somewhat unstable. Second, SnO2 is a wide intrinsic stability limitations of SnO2 and leveraging the high bandgap semiconductor with electrical resistivity varying from stability of bulk In2O3 at ORR relevant potentials. The mass 10 to 106 Ω·cm 23,24; this low conductivity leads to high elecactivity of Pt was extremely high on Pt/ITO, 621 ± 31 trical resistance within the catalyst layer, which is detrimental mA/mgPt, which far exceeded the 2015 DOE goal for Pt mass to fuel cell performance. One way to overcome these limitaactivity of 440 mA/mgPt The enhanced ORR activity was tions to realize a high stability support is to dope Sn into a linked to the faceting of the Pt nanparticles, which overwhelmhigh conductivity material with good stability in acid at ORR ingly consisted of Pt (111) facets. The stability of Pt/ITO was relevant potentials, such as In2O3 25. In this study, tin was also very impressive, with the electrochemically active area doped into the structure of indium oxide, and the resulting tinunchanged and the Pt half wave potential shifting only 4 mV doped indium oxide (ITO) particles with high conductivity over 1,000 potential cycles to 1.4 V vs. RHE, a very harsh were used as supports for Pt. The ORR activity and stability of condition for ORR electrocatalysts where state-of-the-art Pt/C Pt/ITO were explored to understand the enhancement of the electrocatalysts typically shows very poor stability. ORR on Pt due to the interaction between Pt and surface Sn. To prepare the ITO nanoparticles, a slurry of precursors, indium acetylacetonate (In(acac)3) and tin bis(acetylacetonate) Proton exchange membrane fuel cells (PEMFCs) have drawn dichloride (Sn(acac)2Cl2) with variable ratios (Table S1) in significant attention as a 21st century alternative to environoleylamine, was prepared and heated at 250°C for 3h in air to mentally unfriendly power sources, for example, fossil fuel produce a dark yellow suspension. The ITO nanoparticles power plants and internal combustion engines. However, to precipitated by adding ethanol in the suspension and the sumake PEMFCs economically viable, one of the main problems pernatant was removed by centrifugation. Washing with ethato be solved is to find catalysts with sufficient activity and nol was repeated and gave a white powder of ITO nanopartistability for the oxygen reduction reaction (ORR) 1,2. To adcles. The ITO powder was heated at 80°C under vacuum in the dress this problem, a number of methods have been proposed vacuum oven for 24h and then heated at 500°C for 3h in the for developing an electrocatalyst with improved performance, furnace in the air. including alloying Pt with secondary transition metals or deFigure 1a shows XRD patterns for tin-doped indium oxide positing monolayers of Pt onto other fine metal particles to nanoparticles with various Sn doping. All patterns showed a 3-9 create core-shell structures . Although Pt supported on carbixbyite In2O3 cubic structure (JCPDS 06-0416) with no secbon is still the most widely used catalyst, oxide supports as ondary phases, indicating the effective incorporation of Sn alternatives to carbon have shown improved corrosion reinto the In2O3. The lattice parameters, calculated from the sistance and reduced electrochemically active area (ECA) XRD patterns, are plotted as a function of the Sn concentration 10-17 degradation rates . In addition, oxide supports are also able in Figure 1b. At 1 at% Sn, the lattice parameter decreased to affect the electrocatalytic activity of the supported noble slightly compared to that of the undoped In2O3 due to the remetals due to metal-support interactions. These interactions placement of In3+ (0.8 Å) by Sn4+ (0.71 Å), which is smaller. manifest in several ways including: (i) modification of the At 5 at% Sn, the lattice parameter reached an inflection point electronic states or Fermi level of Pt that pushes the formation where the lattice parameter increased with the concentration of of Pt-OH groups to higher potentials 18; (ii) spillover of OH Sn. One possible explanation for the lattice expansion is ingroups onto the oxide support; and (iii) reduction of OH covcorporation of larger Sn2+ ions (1.12 Å) at In3+ sites, which 19 erage by lateral repulsion between Pt-OH and oxide surfaces . was confirmed by XPS (Figure 1c). A detailed analysis of the Ota and coworkers found that tin dioxide (SnO2) influences XPS data is provided in the Supplemental Information. the oxidation and reduction behavior of supported Pt, where the formation of Pt-oxide species at high potentials was sup-

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uct Pt/ITO reveal that a number of well-dispersed Pt nanoparticles were deposited on the ITO surface (Figure S2), which was also confirmed by XPS (Figure S3). Energy-dispersive xray spectroscopy applied directly to these particles showed the presence of ca. 20 wt.% Pt on ITO nanoparticles. Pt/ITO was further characterized by HRTEM; Figure 2f shows the tight attachment of a characteristic Pt nanoparticle to the ITO surface with the diameter of ~4 nm. In addition, the surface of the Pt nanoparticle was dominated by {111} facets, and revealed continuous lattice fringes from the ITO substrate to supported Pt particles, indicating that the Pt was grown epitaxially on the ITO surface.

Figure 1. (a) Powder XRD patterns for pure tin-doped indium oxide crystals with various Sn content. (b) Lattice parameter of tin-doped indium oxide as a function of Sn content. (c) Variation of the concentration of Sn2+ and Sn4+ ions as a function of Sn concentration. Figure S1 shows the XPS spectra of Sn 3d5/2 in tin-doped indium oxide nanparticles with various Sn concentrations. The Sn 3d5/2 peaks shifted toward lower binding energy as the Sn concentration increased. Deconvoluted peaks for Sn4+ (487.1 eV) and Sn2+ (486.1 eV) were obtained to determine the relative concentrations of Sn2+ and Sn4+ species. By combining these relative concentrations and the total Sn concentrations in these ITO nanoparticles, the Sn2+ and Sn4+ content were obtained and are shown in the Figure 1c. At 1 at.%Sn, almost all of the Sn was in the Sn4+ state. As the Sn concentration increased, the content of the Sn2+ ion increased from 0.75% to 2.73%, whereas that of Sn4+ increased from 7.57% to 12.4%. It is worthy to note that the Sn2+ ion content increased greatly relative to that of the Sn4+ ion as the Sn concentration increased. Since the In3+ is larger than Sn4+ and smaller than Sn2+, the structural disturbances due to the differences in ionic radii and electrical charges between Sn and In ions can be relaxed through substitution by Sn2+. Thus, the change of the lattice parameters of the tin-doped indium oxides when the Sn content increases, can be explained by substitutional incorporation of both Sn4+ and Sn2+. Another important consideration is that the surface concentration of Sn is more than twice than the bulk because of Sn surface segraration in the ITO stucture. The structure of the ITO nanoparticles was examined by highresolution transmission electron microscopy. The 5 at% tindoped indium oxide nanoparticles prepared without high temperature treatment had an average size of 5 nm (Figure 2a). The amorphous edges indicate that these nanoparticles were not well-crystalized, especially compared with those heated at 500°C for 3h (Figure 2b), which were fully crystalized with an average size of 18 nm. TEM images of individual nanoparticles (Figure 2, c to e) show their crystalline structure with a highly ordered continuous fringe pattern over a range of Sn concentration. Most of the exposed facets were found to be {222}, although some {220} and {022} were also identified. We also found that the heat treatment temperature had an impact on the ITO morphology, which is discussed in the Supplemental Information. Pt clusters were deposited on the ITO nanoparticles through galvanic displacement of Cu by Pt. TEM images of the prod-

Figure 2. TEM images of tin-doped indium oxide nanoparticles: (a) Unheated 5 at%, inset shows an uncrystalized structure. (b) Heated at 500°C, 5 at%, inset shows a fully crystalized structure. HRTEM images of those: (c) Heated at 500°C, 1 at%. (d) Heated at 500°C, 5 at%. (e) Heated at 500°C, 10 at%. (f) Pt nanoparticle supported on ITO prepared by heating to 500°C, 5 at%. Figure S5 shows cyclic voltammograms (CVs) for Pt/ITO and Pt/C catalysts (20% wt. Pt on Vulcan XC-72) recorded at room temperature in N2-purged 0.1M HClO4 solution at a sweep rate of 50 mV/s. All potentials are discussed relative to the reversible hydrogen electrode (RHE). The CVs exhibited two distinctive potential regions associated with Hupd adsorption/desorption processes (H+ + e- = Hupd) between 0 < E < 0.35 V and the formation of a OHad layer (2H2O = OHad + H3O+ + e-) beyond 0.6 V, where Hupd and OHad refer to underpotentially deposited hydrogen and adsorbed hydroxyl species, respectively. The electrochemically active surface area (ECSA) was calculated by integrating the double layer corrected Hupd

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charge in the cathodic scan, and dividing by a value of 210 µC/cm2 for the adsorption of a hydrogen monolayer on polycrystalline Pt. The specific ECSA of Pt/ITO (83.1 m2/gPt) was found to be three times Pt/C (27.3 m2/gPt).

rection to compare the specific activity of the catalysts. The Pt/ITO catalyst exhibited a specific activity of 0.750 ± .04 mA/cm2 at 0.9 V, which was 3 times greater than Pt/C (0.235 0.01 ± mA/cm2). Normalizing to the loading amount of Pt metal, the mass activity of Pt/ITO catalyst was found to be 621 ± 31 mA/mgPt, which was 4 times greater than Pt/C (156 ± 9 mA/mgPt). A characteristic voltammogram for Pt/ITO is presented in Figure 3c. The value for Pt/C was nearly identical to the literature accepted value reported by Gasteiger et.al 26, validating our experimental setup and approach. The higher specific activity of the Pt/ITO is likely a result of the synergistic effects between surface Sn of ITO and the supported Pt nanoparticles. Considering the small changes in the ITO lattice parameter as a function of Sn content, it is unlikely that lattice strain was the root cause for electron transfer between the catalyst and support, which suggests that electronic effects play a larger role in determining where the Pt nanoparticles are deposited and their shape. Another important effect is the preferential exposure of {111} facets on the Pt nanoparticles on Pt/ITO compared to Pt nanoparticles supported on carbon blacks, which typically contain a mix of {100} and {111} facets 27. The ORR activity on low-index crystallographic facets of Pt in a nonadsorbing electrolyte such as perchloric acid is known to increase on the order of Pt(100)