Highly Selective TiN-Supported Highly Dispersed Pt Catalyst: Ultra

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Highly selective TiN-supported high dispersed Pt catalyst: ultra active towards the hydrogen oxidation and inactive towards the oxygen reduction Junming Luo, Haibo Tang, Xinlong Tian, Sanying Hou, Xiuhua Li, Li Du, and Shijun Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15159 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Highly Selective TiN-Supported High Dispersed Pt Catalyst: Ultra Active towards the Hydrogen Oxidation and Inactive towards the Oxygen Reduction Junming Luo, Haibo Tang, Xinlong Tian, Sanying Hou, Xiuhua Li, Li Du, Shijun Liao∗ The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key Laboratory of New Energy Technology of Guangdong Universities, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China Abstract The severe dissolution of the cathode catalyst, caused by an undesired oxygen reduction reaction at the anode during startup and shutdown, is a fatal challenge to practical applications of polymer electrolyte membrane fuel cells. To address this important issue, according to the distinct structure-sensitivity between the σ-type bond in H2 and the π-type bond in O2, we design a HD-Pt/TiN material by highly dispersing Pt on the TiN surface to inhibit the unwanted oxygen reduction reaction. The highly dispersed Pt/TiN catalyst exhibits excellent selectivity towards hydrogen oxidation and oxygen reduction reactions. With a Pt loading of 0.88 wt.%, our catalyst shows excellent hydrogen oxidation reaction activity, close to that of commercial 20 wt.% Pt/C catalyst, and much lower oxygen reduction reaction activity than the commercial 20 wt.% Pt/C catalyst. The lack of well-ordered Pt facets is responsible for the excellent selectivity of the HD-Pt/TiN materials toward hydrogen oxidation and oxygen reduction reactions. Our work provides a new and cost-effective solution to design selective catalysts toward hydrogen oxidation and oxygen reduction reactions, making the strategy of using oxygen-tolerant anode catalyst to improve the stability of polymer electrolyte membrane fuel cells during startup and shutdown more affordable and practical. Keywords: platinum; hydrogen oxidation reaction; oxygen reduction reaction; structure sensitivity; polymer electrolyte membrane fuel cells.

∗ Corresponding author, e-mail: [email protected], fax +86 20 87113586

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1 Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have the merits of zero emissions, high energy conversion, and ease of operation, making them one of the most promising clean energy technologies. Actually，they are regarded as ideal solution for non-polluting vehicles. Unfortunately, their practical application and commercialization on a large scale are challenged by high cost, a sluggish oxygen reduction reaction rate (ORR), insufficient stability/durability, abundant low cost fossil fuels, lack of a CO2 tax and H2 infrastructures.1-2 The main factors that reduce the stability of PEMFCs include the dissolution and aggregation of catalyst particles, carbon-support corrosion, and membrane decomposition.3 Catalyst dissolution and carbon corrosion are negligible and tolerable under normal operating conditions when the potential at the cathode side is kept at ~0.7 V. However, during startup and shutdown, part of hydrogen at the anode of PEMFCs is replaced by air.4 Since the working potential at the anode is active for the ORR, besides the hydrogen oxidation reaction (HOR), the ORR can also take place at the anode. This drives the cathode to take place undesired oxygen evolution reaction (OER) and carbon oxidation reaction, for these reactions providing the protons that are requisite source for the ORR.4 Thus, the cathode potential can reach 1.5 V during startup and shutdown.3, 5 Such a high positive potential is fatal to cathode catalysts because it causes severe dissolution and corrosion, even for noble catalysts such as Pt and inert supports such as carbon. Therefore, improving the stability of cathode catalysts during startup and shutdown is extremely important to prolong the life time of PEMFCs in practical applications. To date, two types of strategies have been proposed to address the stability issues caused by startup and shutdown. One type aims to lower the high positive potential by designing a cathode catalyst with both good ORR and OER activity to minimize the OER overpotential.4, 6 The other type aims to eliminate, from the very beginning, the high positive potential at the cathode by designing an HOR catalyst that can selectively inhibit the unwanted ORR at the anode. The latter strategy was proposed by Markovic and his coworkers who opened up the research on selective catalyst towards HOR and ORR. They modified the Pt electrode ACS Paragon Plus Environment 2

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using calix[4]arene so as to cover the large ensembles of Pt sites but retain small Pt sites, and found that the calix[4]arene-modified Pt electrode not only preserved the excellent Pt-like HOR activity but also significantly inhibited the Pt-like ORR activity.5, 7 Although the calix[4]arene-modified Pt electrode showed excellent selectivity towards HOR and ORR, the utilization ratio of Pt was very low, because the vast majority of Pt atoms were buried inside the Pt electrode and most of surface Pt atoms were covered by calix[4]arene. Given the high price and scarcity of Pt, the selective catalyst Markovic proposed will be more practical if we can lower Pt loading to an affordable degree while maintaining its excellent catalytic selectivity. In order to design a more cost-effective selective catalyst, we paid our attention to the molecular structure of O2 and H2 and noticed that O2 has both σ-type and π-type bonds while H2 only has σ-type bonds due to the lack of p orbitals in H atoms. As reported in the literature, π-type bonds are more sensitive to the geometric structure of crystal surfaces than σ-type bonds.8 Thus, the distinct geometric structural sensitivity between π-type and σ-type bonds can be used for designing selective catalysts toward HOR and ORR. Since the well-ordered geometric structure of a crystal disappears when the particle size decreases a certain degree, it is possible that reducing the particle size of Pt to a certain degree will affect its ORR activity but hardly affect its HOR activity. Very recently, Yang9 and Vukmirovic10 confirmed that the ORR activity of Pt can be significantly reduced by preparing single-atom Pt catalysts. Therefore, reducing the particle size of Pt seems to be a good strategy, for it not only improves the utilization ratio Pt but also may enable down-sized Pt nanoparticles acquire selectivity towards HOR and ORR. Herein, we designed and prepared a new type of selective catalyst towards HOR and ORR by highly dispersing Pt atoms on TiN nanoparticles, the Pt atoms were so highly dispersed on TiN nanoparticles that no well-ordered Pt facets could be detected. We chose TiN as the support due to its good chemical and electrochemical stability, excellent conductivity, and ability to stabilize highly dispersed Pt.9,

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expected, our highly dispersed Pt/TiN (HD-Pt/TiN) catalyst exhibited excellent selectivity towards HOR and ACS Paragon Plus Environment 3


ORR. With a Pt loading of 0.88 wt.%, our catalyst showed excellent HOR activity, close to that of commercial 20 wt.% Pt/C catalyst, and much lower ORR activity than the commercial 20 wt.% Pt/C catalyst. 2 Experimental section 2.1 Synthesis of TiN nanoparticles TiN nanoparticles were prepared through the nitridation of TiO2 nanoparticles, which had been synthesized by a hydrothermal method. A typical preparation was as follows. First, 3 mL TiO(C4H9)4 was dissolved in 30 mL absolute ethanol solution. After 0.5 h of stirring, 30 mL ammonia solution (25 wt.%) was added to the solution dropwise. The mixture was stirred for 1 h at room temperature then transferred to a sealed Teflon container (100 mL) and reacted at 180°C for 24 h. Next, the TiO2 precipitates were filtered and washed with distilled water until a neutral filtrate was obtained. The precipitates were then dried in a vacuum oven at 60°C for 12 h. TiN nanoparticles were prepared by annealing the TiO2 nanoparticles at 850°C for 2 h under an NH3 atmosphere. The heating rate was kept at 5°C min–1 from room temperature to 830°C and adjusted to 1°C min–1 from 830 to 850°C. 2.2 Preparation of HD-Pt/TiN catalysts Prior to Pt deposition, the TiN support was acid-treated to obtain a clean surface. 400 mg TiN powders were added into 20 mL HCl solution (37 wt.%), then the mixture was agitated and heated at 65°C for 1 h under a N2 atmosphere. After the mixture was cooled to room temperature, the HCl-treated TiN powders were collected by centrifugation, washed with distilled water several times until a neutral filtrate was obtained, then dried in a vacuum oven at 50°C for 10 h. A series of TiN-supported, highly dispersed Pt materials were prepared using an incipient wetness impregnation method.9 Appropriate amounts (20, 30, 60, and 100 µL) of H2PtCl6 ethanol solution (7.53 mgPt mL–1) and 1 mL absolute ethanol were added to 50 mg HCl-treated TiN. The mixture was ultrasonicated for 0.5 h and dried in a vacuum oven at 50°C. Subsequently, the collected dried powders were reduced at 100°C ACS Paragon Plus Environment 4

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for 1 h under a mixed H2/N2 atmosphere (20 vol% H2) to obtain the final products. 2.3 Characterization of catalysts The actual Pt loadings of the Pt/TiN materials were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements. X-ray diffraction (XRD) was conducted on a TD-3500 powder diffractometer (Tongda, China) operated at 30 kV and 20 mA, using Cu-Kα radiation sources. High-angle annular dark field (HAADF) and energy dispersive spectrometer (EDS) elemental mapping analysis were acquired from a JEM-2100F microscope (JEOL, Japan) operated at 200 kV. Transmission electron microscopy (TEM) images were acquired from a JEM-2100HR microscope (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) was performed on an Axis Ultra DLD X-ray photoelectron spectrometer employing monochromated Al-Kα X-ray sources (hν = 1486.6 eV). 2.4 Evaluation of catalysts The ORR and HOR activity measurements of the catalysts in 0.1 M HClO4 solution were conducted on an electrochemical workstation (Autolab PGSTAT302N, Netherlands) at room temperature (25±1°C), using a three-electrode system with a rotating disk electrode (RDE) system (Pine Research Instrumentation, USA). The cell consisted of a Ag/AgCl (3 M NaCl solution) reference electrode, a Pt wire counter electrode, and a glassy carbon-based (GC) working electrode (0.196 cm2). All potentials were calibrated to the reversible hydrogen electrode (RHE). All electrochemical signals were recorded without iR-compensation. Without specified, all working electrodes were prepared as follows. First, a catalyst ink was prepared by ultrasonicating a mixture of 1 mL 0.25 wt% Nafion ethanol solution and 5 mg catalyst for 30 min. Then, 5 µL catalyst ink was pipetted onto the GC working electrode. Finally, the catalyst-loaded GC working electrode was dried under an infrared lamp for 10 min. The diluted JM Pt/C catalyst ink was prepared by ultrasonicating a mixture of 2.6 mL 0.25 wt% Nafion ethanol solution and 1 mg JM 20 wt.% Pt/C catalyst for 30 min, yielding a Pt loading on the GC working electrode equal to that of a 1.54 wt.% Pt/C catalyst. In rotating ring disk electrode (RRDE) tests, the Pt ring was polarized at 1.087 V vs. RHE. The electron ACS Paragon Plus Environment 5


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transfer number (n) was calculated by n = 4Idisk/(Idisk + Iring/N), and the H2O2 yield was calculated by H2O2(%) = (200Iring/N)/(Idisk + Iring/N), where Idisk and Iring were the absolute values of the disk current and ring current, respectively, and N was the collection efficiency at the ring electrode (N = 0.21). 3 Results and discussion We prepared four HD-Pt/TiN materials with various Pt contents. Measured by ICP-AES analysis, these contents were 0.29, 0.44, 0.88, and 1.46 wt.%. We therefore denote our prepared four catalysts as 29HD-Pt/TiN, 44HD-Pt/TiN, 88HD-Pt/TiN, and 146HD-Pt/TiN, respectively. TEM images of the HD-Pt/TiN materials are presented in Figure 1. It can be seen that no Pt nanoparticles were detectable in the samples with Pt contents of 0.29 and 0.44 wt.%, whereas some small nanoparticles 2~5 nm in diameter were found in the samples with Pt contents of 0.88 and 1.46 wt.%, confirming the high dispersion of Pt in 29HD-Pt/TiN and 44HD-Pt/TiN.

Figure 1. TEM images of 29HD-Pt/TiN (a-b), 44HD-Pt/TiN (c-b), 88HD-Pt/TiN (e-f), and 146HD-Pt/TiN (g-h).

Taking into consideration the resolution limitations, and aiming to explore the real dispersion of Pt on TiN surface, we used EDS elemental mapping to further characterize our catalysts. Figure 2 shows HAADF ACS Paragon Plus Environment 6

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images and EDS elemental mapping of the HD-Pt/TiN materials. As shown in Figure 2(a4) and 2(b4), the Pt species in 29HD-Pt/TiN and 44HD-Pt/TiN were highly dispersed on the TiN surface, with no detectable Pt nanoparticles. The distribution of highly dispersed Pt species was denser in the latter than in the former due to a higher Pt loading. When the Pt loading was increased to 0.88 wt.%, both small Pt nanoparticles and highly dispersed Pt species could be found, as shown in Figure 2(c4). With a Pt loading of 1.46 wt.%, the distribution of Pt nanoparticles and highly dispersed Pt species became denser, as shown in Figure 2(d4). These results confirm that highly dispersed Pt species could be found in all of the HD-Pt/TiN materials, but some small Pt nanoparticles could only be found in the HD-Pt/TiN materials with a high Pt loading. The results of HRTEM and EDS mapping clearly reveal the high dispersion of Pt in our catalysts, especially in 29HD-Pt/TiN and 44HD-Pt/TiN.



Figure 2. HAADF images and EDS elemental mapping of 29HD-Pt/TiN (a1-4), 44HD-Pt/TiN (b1-4), 88HD-Pt/TiN (c1-4), and 146HD-Pt/TiN (d1-4); resolution bar 80 nm.

Figure 3 shows the XRD patterns of TiN nanoparticles, HD-Pt/TiN materials, and JM 20 wt.% Pt/C catalyst. It can be seen in Figure 3a that the patterns of TiN nanoparticles and the HD-Pt/TiN materials are identical, and no peaks related to Pt can be observed in the patterns of all HD-Pt/TiN materials. The XRD patterns of the HD-Pt/TiN materials, recorded using fine scanning and shown in Figure 3b, further confirm the absence of peaks related to Pt. These results may imply that the Pt in the Pt/TiN materials was so highly ACS Paragon Plus Environment 8

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dispersed that no Pt nanoparticles or well-ordered Pt facets could be formed. In contrast, the JM 20 wt.% Pt/C catalyst has large well-ordered facets, and four clear Pt peaks occurred in its XRD pattern. In summary, no Pt peaks were observable for the HD-Pt/TiN materials, either because the highly dispersed Pt species were too small to form well-ordered facets, or because the Pt atoms in the small nanoparticles existed in a disordered rather than an ordered form.

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Figure 3. (a) XRD patterns of JM 20wt.% Pt/C, TiN, 29HD-Pt/TiN, 44HD-Pt/TiN, 88HD-Pt/TiN, and 146HD-Pt/TiN; (b) XRD patterns of 29HD-Pt/TiN, 44HD-Pt/TiN, 88HD-Pt/TiN, and 146HD-Pt/TiN recorded with fine scanning.

Figure 4 shows the XPS spectra of Pt 4f for the HD-Pt/TiN materials and for JM 20 wt.% Pt/C catalyst. Compared with the Pt/C catalyst, the binding energy peaks of Pt 4f 7/2 and Pt 4f 5/2 for our nitride-supported catalysts showed a significant positive shift, and the shifts increased as the Pt content decreased. For the sample with 0.29 wt.% Pt, the shifts were up to 1.25 eV (Pt 4f 7/2) and 1.0 eV (Pt 4f 5/2), respectively, indicating a very strong interaction between the Pt and the TiN support in our low-Pt catalyst. As shown in Figure 4a, the Pt 4f spectra of the HD-Pt/TiN materials fit into three valence states: Pt(0) (71.5 eV, metallic Pt), Pt(II) (72.6 eV, PtO), and Pt(IV) (75.2 eV, PtO2). Figure 4b shows the distributions of Pt(0), Pt(II), and Pt(IV) in the catalysts, based on the deconvolution results of our XPS data. Interestingly, the distribution varied significantly with the Pt loading. For sample 29HD-Pt/TiN, the proportion of Pt(0) ACS Paragon Plus Environment 9


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approached zero. As the Pt content increased, the proportion of Pt(0) also increased, further confirming the strong interaction between the Pt clusters and the TiN support for our low Pt content catalyst, as well as the very high Pt dispersion of our catalyst with very low Pt content. The valence state of a metal atom is sensitive to its coordination environment. Metal atoms in high coordination environments typically exhibit a zero state, which is often the case in large nanoparticles. A decrease in the size of metal species will generate an increase in the unsaturated coordination environment, therefore raising the valence state of metal atoms.13 Many researchers have reported small metal clusters and metal single-atom exhibited a much higher oxidation state than metal nanoparticles.9, 14-16 The oxidation state of Pt single-atom was found even close to that of Pt in PtO2.16 Here, we also found the same ‘size–valence state’ connection. As shown in Figure 2(a4) and 2(b4), the highly dispersed Pt species in 29HD-Pt/TiN and 44HD-Pt/TiN were too small to be defined as nanoparticles, accordingly, the valence state of Pt was dominated by Pt(II) and Pt(IV). In 88HD-Pt/TiN and 146HD-Pt/TiN, where some Pt nanoparticles appeared, the proportions of Pt(II) and Pt(IV) decreased while the proportion of Pt(0) increased. Figure 4b shows that most Pt species in the Pt/C catalyst were in the Pt(0) state, contrarily, most Pt species in the HD-Pt/TiN materials were in the Pt(II) and Pt(IV) states. We may therefore deduce that most of the Pt atoms in the HD-Pt/TiN materials were in an unsaturated coordination environment, while most Pt atoms in the commercial Pt/C catalyst were in a saturated coordination environment. The highly unsaturated coordination may have been caused by high Pt dispersion and would be expected to result in special catalytic properties.


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Figure 4. (a) XPS spectra of Pt 4f and (b) peak area proportion of Pt(0), Pt(II), and Pt(IV) in 29HD-Pt/TiN, 44HD-Pt/TiN, 88HD-Pt/TiN, 146HD-Pt/TiN, and JM 20 wt.% Pt/C.

The cyclic voltammetry (CV) curves of JM 20 wt.% Pt/C catalyst and the HD-Pt/TiN materials are shown in Figure 5a. The hydrogen underpotential (Hupd) adsorption/desorption phenomenon is a fingerprint for noble metals such as Pt and Pd. As expected, the JM 20 wt.% Pt/C catalyst showed typical Hupd adsorption/desorption peaks in the low potential range. However, these peaks could not be found in the CV curves of the HD-Pt/TiN materials; instead, those curves were very similar to that of TiN. They are almost identical except that the HD-Pt/TiN materials had larger CV areas. Hupd adsorption/desorption is known to be sensitive to crystal facets. Three types of Hupd adsorption/desorption peaks can be found in the CV curve of polycrystalline Pt, derived from three high-coordination facets: Pt(100), Pt(110), and Pt(111).17-18 The formation of typical Hupd adsorption/desorption peaks requires large ensembles of Pt atoms.17 Therefore, the absence of Hupd adsorption/desorption peaks suggests that the HD-Pt/TiN materials lacked well-ordered Pt(100), Pt(110), and Pt(111) facets, which is consistent with the XRD results. To serve as an anode catalyst for PEMFCs, the HD-Pt/TiN materials should have good HOR activity. The HOR activity of TiN, the HD-Pt/TiN materials, and JM 20 wt.% Pt/C catalyst are shown in Figure 5b. It can be seen that pure TiN showed no HOR activity, while the HD-Pt/TiN materials showed excellent HOR ACS Paragon Plus Environment 11


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activity, which increased with Pt loading. Like the Pt/C catalyst, the HD-Pt/TiN materials retained Pt-like activity and needed almost no overpotential to initiate the HOR. This is consistent with Vukmirovic’s finding that RuO2 supported atomically dispersed Pt catalyst exhibited similar HOR activity compared to Pt(111).10 Due to the low Pt loading, the limiting current density of the HD-Pt/TiN materials was lower than that of JM 20 wt.% Pt/C catalyst. Nevertheless, as shown in Figure 5c, the HD-Pt/TiN materials produced more HOR current than 20% Pt/C catalyst with the same Pt loading, the HOR current of 88HD-Pt/TiN was even 12 times larger than that of 20% Pt/C. The mass activity (calculated by the exchange current density) of 88HD-Pt/TiN was also 10 times larger than that of 20% Pt/C (Figure S1(c)). These results indicate that the Pt utilization in the 88HD-Pt/TiN is much higher than that in the 20% Pt/C catalyst for HOR process. We further compared the HOR mechanisms on 88HD-Pt/TiN and 20% Pt/C. Although it is difficult to judge whether the HOR followed the Tafel-Volmer mechanism or the Heyrovsky-Volmer mechanism due to the lack of a clear defined Tafel linear region (Figure S1(a)), the Koutecky–Levich plots presented in Figure S2(c) indicate that JM 20% Pt/C and 88HD-Pt/TiN have similar electron transfer number in the HOR. Figure 5d shows the ORR activity of TiN, the HD-Pt/TiN materials, and JM 20 wt.% Pt/C catalyst. It can be seen that the HD-Pt/TiN materials needed much larger overpotential to initiate the ORR than JM 20 wt.% Pt/C catalyst, and the overpotential increased as the Pt loading decreased. Unlike the JM 20 wt.% Pt/C catalyst, which had excellent ORR activity, the HD-Pt/TiN materials had extremely low ORR activity and could not even reach the theoretical limiting current. For the HD-Pt/TiN materials, the maximum ORR current density at 0.1 V, achieved by 146HD-Pt/TiN, is still lower than 2 mA cm-2. While for JM 20 wt.% Pt/C catalyst, the ORR current density at 0.1 V is 6.5 mA cm-2. These results confirm that the ORR can be significantly inhibited by the HD-Pt/TiN materials. Considering the possibility that ORR inhibition may have originated from the low Pt loading in the HD-Pt/TiN materials, we further tested the ORR activity of a diluted JM Pt/C catalyst (1.54 wt.% Pt loading, close to that of 146HD-Pt/TiN). It can be seen in Figure 5d that the ORR activity of the 1.54 wt.% Pt/C ACS Paragon Plus Environment 12

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catalyst was lower than that of the JM 20 wt.% Pt/C catalyst, but they are still much higher than that of 146HD-Pt/TiN. Figure 5e shows that the normalized ORR current of 146HD-Pt/TiN was much lower than those of the 1.54 and 20 wt.% Pt/C catalysts. This implies that the ORR inhibition of the HD-Pt/TiN materials derived not just from the low Pt loading but more from the very high dispersion of Pt.

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Figure 5. (a) CV curves of TiN, HD-Pt/TiN materials and the JM commercial Pt/C catalyst in N2-saturated 0.1 M HClO4 solution, 50 mV/s scan rate; (b) HOR activity of TiN, the HD-Pt/TiN materials and the JM ACS Paragon Plus Environment 13


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commercial Pt/C catalyst in H2-saturated 0.1 M HClO4 solution, 10 mV/s scan rate, 1600 rpm rotation speed; (c) Normalized HOR current of the HD-Pt/TiN materials and the JM commercial Pt/C catalyst at 0.05 V; (d) ORR activity of TiN, the HD-Pt/TiN materials and the JM commercial Pt/C catalyst in O2-saturated 0.1 M HClO4 solution, 10 mV/s scan rate, 1600 rpm rotation speed; (e) Normalized ORR current of 146HD-Pt/TiN, 1.54 wt.% Pt/C and 20 wt.% Pt/C at 0.8 V.

To investigate how the ORR was inhibited on the HD-Pt/TiN materials, we performed RRDE tests; the results are shown in Figure 6. As Figure 6a indicates, the disk and ring current of TiN was extremely low, while the Pt-loaded TiN had a higher disk and ring current. The disk and ring current of the HD-Pt/TiN materials increased with the Pt loading. From Figure 6b it can be seen that the electron transfer number of TiN at 0.1 V was about 3.7, whereas the values for the HD-Pt/TiN materials were higher and increased with Pt loading. This indicates that the ORR occurring on TiN and the HD-Pt/TiN materials proceeded predominantly through the four-electron transfer mechanism, and this mechanism was favored as the Pt loading increased. It should be mentioned that inhibition of the ORR on the calix[4]arene-modified Pt catalyst was achieved by forcing the ORR to proceed through the two-electron transfer mechanism.5, 7 Here, the RRDE results show that the ORR can be inhibited without blocking the four-electron transfer pathway. It is well known that the ORR occurring on commercial Pt/C catalysts proceeds through the four-electron transfer pathway. Since the ORR occurred on the HD-Pt/TiN materials was also dominated by the four-electron transfer pathway, inhibition of the ORR in the HD-Pt/TiN materials was achieved unlikely by manipulating the reaction pathway but likely by manipulating the activation barrier of the reaction steps. The product of the ORR proceeding through the two-electron transfer pathway is H2O2 that degrades the Nafion membrane. As shown in Figure 6b, the H2O2 yield of the HD-Pt/TiN materials at 0.1 V was less than 15% and declined to 5% when the Pt loading reached 1.46 wt.%. Such a low H2O2 yield is more beneficial to the Nafion membrane than the high H2O2 yield generated in the two-electron transfer pathway. ACS Paragon Plus Environment 14

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Figure 6. RRDE tests of TiN and HD-Pt/TiN materials in O2-saturated 0.1 M HClO4 solution at a rotation speed of 1600 rpm, 10 mV/s scan rate: (a) disk and ring current, (b) H2O2 yield and electron transfer number, n.

As confirmed in the XRD, CV, and XPS results, the HD-Pt/TiN materials had no well-ordered Pt facets, and most of the Pt atoms in the HD-Pt/TiN materials were in an unsaturated coordination environment. In contrast, the commercial Pt/C catalyst had well-ordered Pt facets, and most of the Pt atoms were in a saturated coordination environment. Due to the lack of a p orbital in H atoms, H2 has only a single σ-type bond. In contrast, O2 has both a σ-type bond and a π-type bond because of the s and p orbitals in O atoms. The key difference between the activation of σ-type and π-type bonds is their sensitivity to the local coordination (geometric arrangement) of the metal surface. The activation of σ-type bonds usually proceeds atop of a metal atom, so these bonds therefore are not very sensitive to the local coordination of the metal surface.8 This is why the HD-Pt/TiN materials and the commercial Pt/C catalyst showed similar HOR activity, despite their structural differences. Conversely, the activation of π-type bonds depends strongly on the local coordination of the metal surface. In high coordination surface, π-type bonds will interact not only with the metal surface d orbitals but also with antisymmetric surface metal s and p group orbital combinations; hence, the activation and dissociation of π-type bonds occur more efficiently on the ensemble of surface atoms.8, 19 This explains why the HD-Pt/TiN materials, which did not have large well-ordered Pt ACS Paragon Plus Environment 15


facets, showed much lower ORR activity than the commercial Pt/C catalyst that has large well-ordered Pt facets.

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Having demonstrating that the HD-Pt/TiN materials had good HOR activity in a H2 atmosphere and significant ORR inhibition in an O2 atmosphere, we further tested their HOR and ORR activity in a mixed H2/O2 atmosphere. Figure 7 shows the i-t curves of the JM 20 wt.% Pt/C catalyst and the HD-Pt/TiN materials in a mixed H2/O2 atmosphere. The potential was kept at 0.187 V where both the HOR and the ORR are very active. In the chronoamperometry tests, a H2 gas of 50 sccm was inputted during the whole time, while an O2 gas of 100 sccm was inputted at the time of 300 s. Since the HOR and the ORR produce positive and negative currents, respectively, the recorded current is the net current of those two reactions. As shown in Figure 7, after O2 was inputted, the positive current of the Pt/C catalyst changed to a larger negative current, indicating that the ORR occurring at the anode could barely be inhibited. In contrast, the positive current of 88HD-Pt/TiN and 146HD-Pt/TiN changed to a much smaller negative current after O2 was inputted. For 29HD-Pt/TiN and 44HD-Pt/TiN, the net current was almost zero after O2 was inputted. These results confirm that the HD-Pt/TiN materials had much better oxygen-tolerant ability than the ACS Paragon Plus Environment 16

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commercial Pt/C catalyst in mixed H2/O2 atmosphere.

4 Conclusions In summary, according to the distinct structure sensitivity between the σ-type bonds in H2 and the π-type bonds in O2, we successfully designed and prepared a selective catalyst towards HOR and ORR by highly dispersing Pt atoms on TiN nanoparticles. TEM, EDS mapping, XRD and CV results confirmed that Pt was so highly dispersed that no well-ordered Pt facets were observable. The catalyst showed high selectivity towards the ORR and HOR, it exhibited comparable HOR activity but much lower ORR activity than JM 20 wt.% Pt/C catalyst, making it a potential oxygen-tolerant HOR catalyst at the anode of PEMFCs to settle the stability issues caused by startup and shutdown. Our work provides a new and cost-effective solution to design selective catalysts toward HOR and ORR. The structure-sensitivity strategy we presented here may also apply to other fields for designing selective catalysts toward σ-type bonds reactions and π-type bonds reaction. Associated content Supporting information: Koutecky–Levich plots, Tafel plots, linear polarization curves, exchange current density and mass activity of JM 20% Pt/C and 88HD-Pt/TiN in H2-saturated 0.1 M HClO4 solution. Author Information Corresponding author: [email protected] Notes The authors declare no competing financial interest. Acknowledgments This work was supported by the State's Key Project of Research and Development Plan of China (Project No 2016YFB0101201), the National Natural Science Foundation of China (NSFC Project Nos. 21276098, ACS Paragon Plus Environment 17


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21476088, 51302091, U1301245), Natural Science Foundation of Guangdong Province (Project Nos. 2014A010105041, 2015A030312007), Guangdong Provincial Department of Science and Technology (Project No. 2015B010106012), and Educational Commission of Guangdong Province (Project No. 2013CXZDA003). References 1.

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Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493-497. 2.

Debe, M. K., Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51.

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Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K. I.; Iwashita, N., Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem Rev 2007, 107, 3904-3951. 4.

Shao, M., Electrocatalysis in Fuel Cells: A Non-and Low-Platinum Approach; Springer: London, 2013.

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M., Selective Catalysts for the Hydrogen Oxidation and Oxygen Reduction Reactions by Patterning of Platinum with Calix[4]arene Molecules. Nat Mater 2010, 9, 998-1003. 6.

Atanasoski, R., In Durable Catalysts for Fuel Cell Protection during Transient Conditions, Kickoff Meeting for New DOE

Fuel Cell Projects, Washington DC, 2009. 7.

Genorio, B.; Subbaraman, R.; Strmcnik, D.; Tripkovic, D.; Stamenkovic, V. R.; Markovic, N. M., Tailoring the Selectivity

and Stability of Chemically Modified Platinum Nanocatalysts To Design Highly Durable Anodes for PEM Fuel Cells. Angew Chem Int Edit 2011, 50, 5468-5472. 8.

van Santen, R. A.; Neurock, M.; Shetty, S. G., Reactivity Theory of Transition-Metal Surfaces: A Bronsted-Evans-Polanyi

Linear Activation Energy-Free-Energy Analysis. Chem Rev 2010, 110, 2005-2048. 9.

Yang, S.; Kim, J.; Tak, Y. J.; Soon, A.; Lee, H., Single-Atom Catalyst of Platinum Supported on Titanium Nitride for


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Selective Electrochemical Reactions. Angew Chem Int Edit 2016, 55, 2058-2062. 10. Vukmirovic, M. B.; Teeluck, K. M.; Liu, P.; Adzic, R. R., Single Platinum Atoms Electrocatalysts: Oxygen Reduction and Hydrogen Oxidation Reactions. Croat Chem Acta 2017, 90, 225-230. 11. Yang, M. H.; Cui, Z. M.; DiSalvo, F. J., Mesoporous Titanium Nitride Supported Pt Nanoparticles as High Performance Catalysts for Methanol Electrooxidation. Phys Chem Chem Phys 2013, 15, 1088-1092. 12. Zhang, R. Q.; Lee, T. H.; Yu, B. D.; Stampfl, C.; Soon, A., The Role of Titanium Nitride Supports for Single-Atom Platinum-Based Catalysts in Fuel Cell Technology. Phys Chem Chem Phys 2012, 14, 16552-16557. 13. Yang, X. F.; Wang, A. Q.; Qiao, B. T.; Li, J.; Liu, J. Y.; Zhang, T., Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Accounts Chem Res 2013, 46, 1740-1748. 14. Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T., Single-Atom Catalysis of CO Oxidation using Pt-1/FeOx. Nat Chem 2011, 3, 634-641. 15. Lin, J.; Wang, A. Q.; Qiao, B. T.; Liu, X. Y.; Yang, X. F.; Wang, X. D.; Liang, J. X.; Li, J. X.; Liu, J. Y.; Zhang, T., Remarkable Performance of Ir-1/FeOx Single-Atom Catalyst in Water Gas Shift Reaction. J Am Chem Soc 2013, 135, 15314-15317. 16. Wei, H. S.; Liu, X. Y.; Wang, A. Q.; Zhang, L. L.; Qiao, B. T.; Yang, X. F.; Huang, Y. Q.; Miao, S.; Liu, J. Y.; Zhang, T., FeOx-Supported Platinum Single-Atom and Pseudo-Single-Atom Catalysts for Chemoselective Hydrogenation of Functionalized Nitroarenes. Nat Commun 2014, 5, 1-8. 17. Markovic, N. M.; Grgur, B. N.; Ross, P. N., Temperature-Dependent Hydrogen Electrochemistry on Platinum Low-Index Single-Crystal Surfaces in Acid Solutions. J Phys Chem B 1997, 101, 5405-5413. 18. Solla-Gullon, J.; Vidal-Iglesias, F. J.; Rodriguez, P.; Herrero, E.; Feliu, J. M.; Clavilier, J.; Aldaz, A., In Situ Surface Characterization of Preferentially Oriented Platinum Nanoparticles by using Electrochemical Structure Sensitive Adsorption Reactions. J Phys Chem B 2004, 108, 13573-13575. 19. Van Santen, R. A.; Neurock, M., Molecular Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 2006.


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