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Formation of Tubular Assembly by Ultrathin Ti0.8Co0.2N Nanosheets as Efficient Oxygen Reduction Electrocatalyst for Hydrogen-/Metal-air Fuel Cells Xin Long Tian, Lijuan Wang, Bin Chi, Yangyang Xu, Shahid Zaman, Kai Qi, Hongfang Liu, Shijun Liao, and Bao Yu Xia ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02710 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018
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ACS Catalysis
Formation of Tubular Assembly by Ultrathin Ti0.8Co0.2N Nanosheets as Efficient Oxygen Reduction Electrocatalyst for Hydrogen-/Metal-air Fuel Cells Xin Long Tian1, Lijuan Wang1, Bin Chi2, Yangyang Xu1, Shahid Zaman1, Kai Qi1, Hongfang Liu1, Shijun Liao2* and Bao Yu Xia1* 1
Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei
Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan 430074, PR China 2
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, PR China *Corresponding authors:
[email protected] (S. Liao);
[email protected] (B.Y. Xia) Abstract: Highly stable and efficient oxygen electrocatalyst with low-cost is of prime significance for fuel cells. Herein, we report hierarchical tubular assembly of metal nitride nanosheets via a facile hydrothermal method followed by nitridizing titanium-based dioxides. The resultant Ti0.8Co0.2N nanosheets assembly demonstrates considerable oxygen reduction activities in both acidic H2-air and alkaline Zn-air fuel cells. We intend to ascribe the high performance to the integration of modified electronic effect caused by the doping of cobalt, the high surface area and unique mesoporous structure induced by its nanosheet assemblies, and inherent structural stability of interlaced nitride nanosheets. This work offers an effective approach for rational design and scalable preparation of binary nitride nanostructures as well as stable and efficient alternative electrocatalyst that represents a key step towards low-cost catalysis and energy conversion. Keywords: Metal nitrides, Hierarchical structure, Oxygen reduction, Electrocatalyst, Fuel cells. 1
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Highly efficient oxygen reduction reaction (ORR) electrocatalysts are the prime prerequisite for various energy conversion technologies.1-7 However, the high cost of noble metals such as platinum (Pt) impedes their real-world application.8-10 Numerous efforts have been afforded in exploring non-precious catalysts to substitute effective yet expensive Pt-based electrocatalysts. Various non-precious catalysts including transition metal oxides, carbides, and nano-carbon composites efficiently twinned or superior to Pt-based electrocatalysts have been reported.11-15 Nevertheless, widely held ORR catalysts are limited to alkaline conditions only, owing to corrosion of carbon materials and severe leaching of transition metals that they suffer from poor durability and activity degradation in acidic conditions.16, 17 Transition metal nitrides (TMNs) exhibit outstanding potential as stable catalyst due to their thoroughly corrosion resistance and excellent conductivity.18-25 Nonetheless, their ORR activity in acidic electrolyte is disappointing even on the rotating disk electrode (RDE) measurements, and there are few available investigations on the membrane electrode assembly (MEA) testing with TMNs as the cathodic oxygen electrocatalysts.13, 26, 27 The incorporation of second transition metal would be effective to boost up the activity through the modified electronic structure and the fortified electron donation ability of the host metal atoms to the adsorbed oxygen molecules.28-31 However, the reported TMNs suffer from the wispy structure and morphology control due to the agglomeration, which would lead to the loss of active surface area. Furthermore, abundant boundaries and defects between the mixed TMN nanoparticles (NPs) leads to an insufficient electron transport, and thus an incomplete potential catalytic performance of TMNs. To this end, the rational design and preparation of TMNs with highly porous and exposed structure is therefore favorable for enhancing catalytic performance, as the interconnected architectures offer abundant edge/corner atoms with high surface area as well as excellent structural rigidity to prevent aggregation.32, 33 In this work, the rational and scalable synthesis of tubular TMNs nanosheet assembly is apprehended by an additive/template-free solvothermal with post-annealing approach. Several binary TMNs with similar 2
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one-dimensional tubular structures are also obtained by this facile strategy. Specially, the resultant Ti0.8Co0.2N nanosheets assembly is employed as the cathode oxygen electrocatalyst for the acidic H2-air and alkaline Zn-air fuel cells, which demonstrate considerable activity and durability. Such amazing electrochemical performance is credited to the synergistic effect rising from structure and composition. This effort on TMN-based nanostructures would provide the valuable perceptions in developing activity-efficient and cost-effective alternatives for practical energy conversion technologies. The typical preparation process of titanium nitride nanosheets assembly includes the formation of hierarchical TiO2-based architectures with the subsequent annealing treatment in ammonia environment (Figure 1a). In the first step, rod-like TiOSO4 precursor undergoes a gentle hydrolysis reaction at the beginning (Figure 1b-d). Prolong reaction time accelerates the hydrolysis reaction of TiOSO4, leading to the dissolution of inner core and re-deposition of titanium-based products on the outer surface.34 Derived from this interactive process, initial superficial nanoclusters/flakes emerge around the precursor and subsequent grow into nanosheets (Figure 1e-f). Finally, the hierarchical tubular structures of nanosheet assemblies are formed and persisted even after a long reaction time (Figure 1g and Figure S1). Thus, the TiOSO4 rod precursors not only are employed as the Ti source but also serve as the self-templates to grow nanosheets and form the final hierarchical tubulars. The formation of tubular nanosheet assembly may involve the outward diffusion induced by the interfacial ion-exchange reaction around the templates.35 Subsequently, titanium nitrides are obtained by nitriding titanium oxides at presence of ammonia. The synthetic procedures of Ti0.8Co0.2N assemblies is same to that of TiN except the addition of cobalt chloride during the solvothermal process. Scanning electron microscope (SEM) observations show Ti0.8Co0.2N assemblies are ~10 µm in length and ~1 µm in diameter (Figure 2a-b), and the SEM images of TiN NPs were depicted in Figure S2. The tubular’s wall with a thickness of ~300 nm is constructed by the randomly interlaced nanosheets with the 3
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thickness of ~8 nm (Figure 2c). Compared with the morphology of TiO2 precursor (Figure 1) and tubular TiN assembly (Figure S3), the architectures of Ti0.8Co0.2N assemblies are perfectly preserved after annealing, suggesting that the nitriding process is easy going. The hollow structure is confirmed by transmission electron microscopy (TEM) image, as the lighter inner chamber is clearly observed between the darker walls of the tubulars (Figure 2d). The transparent nanosheets composed of tiny crystals with a lattice fringe of 0.215 nm (Figure 1f), corresponding to the (200) plane of face-centered cubic (fcc) TiN phase. The elemental mapping analysis by scanning transmission electron microscopy (STEM) shows the homogeneous dispersion of Co, N and Ti elements in the Ti0.8Co0.2N assembly, suggesting the successful and uniform doping of Co species in TiN nanostructures (Figure 2g). The Ti/Co atomic ratio of Ti0.8Co0.2N NTs released by the energy-dispersive X-ray spectroscopy (EDS) analysis is 80.6:19.4 (Figure S4), coincided with inductively coupled plasma atomic emission spectrometry (ICP-OES) results and its initial atomic ratio in the reactant (4:1). This facile approach can be effectively applied to prepare other transitional metal doped titanium nitrides, such as Fe, Mo and Ni (Figure S5). Nitrogen sorption results indicate that Ti0.8Co0.2N assemblies demonstrate a porous structure with a prominent specific surface area of 155 m2 g-1 (Figure 3a). The similar porous structure and specific surface area of both Ti0.8Co0.2N and TiN (148 m2 g-1, Figure S6) assembly suggest that the incorporation of second metal makes minute difference in physical morphology and structure. XRD patterns show the similar TiN phase (JCPDS No. 38-1420) for both TiN and Ti0.8Co0.2N samples (Figure 3b), approving the successful conversion of TiO2 to TiN.36 However, the diffraction peak of Ti0.8Co0.2N is slightly shifted toward higher diffraction angles due to the decreased lattice caused by the replacement of Ti atoms by the smaller Co atoms.37 Such Co doping into TiN could result in the modified electronic structure, as evidenced by the different binding energies in X-ray photoelectron spectroscopy (XPS) patterns. The Ti 2p spectrum of Ti0.8Co0.2N assemblies show a substantial negative shift as compared with TiN assemblies (Figure 3c). There 4
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is a uni-directional negative shift with increasing the doping content of Co species (Figure S7a), suggesting an increase in the d-band electrons of Ti atoms. Moreover, the peaks of Co 2p spectrum demonstrate the formation of Co-N species (782.6 eV, Figure 3d),38, 39 and the binding energy increase with the increase of Co contents (Figure S7b). The unique structure of Ti0.8Co0.2N assembly endows a new prospect for oxygen electrochemical reaction. Thus, the ORR activity and durability of Ti0.8Co0.2N NTs are thoroughly investigated in acidic electrolyte primarily. There is no apparent redox peak for titanium nitrides in N2-saturated 0.1 M HClO4 solution (Figure 4a). A well-defined oxygen reduction peak is observed for TiN (0.52 V) and Ti0.8Co0.2N (0.79 V) assembly in O2-saturated 0.1 M HClO4 electrolyte, signifying the considerable ORR activity. Linear sweep voltammetry (LSV) curves show a much better activity of TiN assemblies compared with TiN NPs (Figure 4b and Figure S7), demonstrating that the unique tubular structure of nanosheets assemblies can boost the ORR activity (Figure 4b). After introducing Co element into TiN assembly, the ORR activity of resultant nitride is dramatically enhanced, and the Ti0.8Co0.2N assemblies is the best among all the samples (Figure 4c). The onset and half-wave potentials of Ti0.8Co0.2N assemblies are up to 0.96 V and 0.79 V, respectively, with a limiting current density of 5.65 mA cm-2. It is obvious that the gap for onset and half-wave potentials of Ti0.8Co0.2N assemblies are tapered to 0.1 V with respect to Pt/C having same catalyst loadings, certifying the best activity for Ti0.8Co0.2N assemblies among the TMN-based electrocatalysts reported in acidic conditions (Table S1).13, 20, 40-43 Furthermore, the rotating ring-disk electrode (RRDE) results reveal that Ti0.8Co0.2N exhibits the dominant 4-electron pathway (3.83) and a low peroxide yield (
