One-step synthesis of Fe3PtPd(OH)2[Picolinic acid]8(H2O)4 hybrid nanorods: efficient and stable electrocatalyst for oxygen reduction reaction in alkaline solution

Design and synthesis of effective electrocatalysts for oxygen reduction reaction in alkaline environments is critical to reduce energy losses in alkaline fuel cells. We have systematically evaluated new approaches for reducing the Pt content while retaining the activity of a Pt-based catalyst with hydrolytic phases containing hydroxide moieties in addition to metal ions and ligands. We report for the first time architectured organic-inorganic hybrid nanorod catalyst, which is fabricated by solvothermal reaction of K2MCl4 (M = Pd, Pt) with picolinic acid (PA) (chelating agent) in the presence of FeCl2. Excess base produces isostructural coordination M-PA complex to Fe-OH chains. A generic formula can be written as Fe3PtPd(OH)2[PA]8(H2O)4. The electrocatalytic activities of the hybrid nanorods are explored for oxygen reduction reaction (ORR) in alkaline medium. The onset potential of ORR is significantly reduced with a positive shift of about 109 mV and twice the reduction current density is observed in comparison with Pt/C with the same mass loading. We believe that this work may lead towards the development of heterodoped organic- inorganic hybrid materials with greatly enhanced activity and durability for applications in catalysis and energy conversion.

NPs) than commercially available pure-Pt catalysts 19 . On the other hand, transition-metal species, such as metal hydroxides or layered double hydroxides, have recently gained noticeable popularity in various energy systems owing to their low cost and high theoretical activity 21 . For example, Lei et al. report on the highly active and durable Ni x Co 1−x (OH) 2 catalyst for ORR 22 . Li et al. observed an excellent ORR performance on NiCoFe-LDH 23 . These results demonstrate that 3D metal in the hydroxide state is a promising catalyst for ORR. These studies give the idea that the use of noble and non-noble metals together in the hydroxide or complex form, instead of the reduced form, can effectively increase ORR activity. But usually, heterogeneous electrocatalysts suffer from extensive leaching of the active metal species during reactions and eventually lose their catalytic activity.
Herein, for the first time, we report an architectured organic-inorganic hybrid nanorod electrocatalyst, which was fabricated by the solvothermal reaction of K 2 MCl 4 (M = Pd, Pt) with picolinic acid in the presence of FeCl 2 (in which hydroxide forms easily in the basic solution). The excess base produces isostructural coordination solids in which 'complex ligands' , containing palladium or platinum, coordinate to metal hydroxide chains. A generic formula can be written as: Fe 3 M 2 (OH) 2 [PA] 8 (H 2 O) 4 , where M 2+ = Pd and Pt 24 . PA is known to be stable in solvothermal conditions and also to form stable complexes with soft metals via -NH coordination and hard metals -OH coordination 25 . This synthesis method allows to form a catalyst with a uniform distribution of metal ions at structure. In these conditions, the Pt content of the catalyst decreases while the activity of a Pt-based catalyst with hydrolytic phases containing hydroxide moieties will be retained. Also, This structure decreases metal leaching in successive runs and increases reusability of the catalyst. To study the effect of the presence of Pt and Pd on ORR activity, three types of hybrid nanorods, including Fe 3 Pt 2 [PA] 8 4 , were synthesized, and the effect of their compositions on ORR activity was studied. These hybrid nanorods displayed substantially enhanced ORR activity as compared with that of commercial Pt/C catalysts in 0.1 M KOH solution. It is critical to highlight that, to the best of our knowledge, there have been no previous reports of either supported or activated Fe(OH) 2 with Pt and Pd complexes. These hybrid nanorods are a promising new catalyst candidate for practical fuel cell applications.

Results
Characterization of hybrid nanorods. FT-IR spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), EDX, and transmission electron microscopy (TEM) were applied for characterization. FTIR spectra for the as-prepared nanohybrides are shown in Fig. 1a. The FTIR spectra of nanohybrides were roughly attributed to what follows: 3412 cm −1 to the O-H and N-H stretching vibration of the Fe hydroxide layer and PA amine groups; 3056 cm −1 due to the alkyl C-H stretching of PA; 1687 cm −1 due to the carboxylic acid C=O stretching of PA and the OH stretching band of H 2 O; 1506-1591 cm −1 due to the aromatic C=C bending; 1327 cm −1 due to the ν -COO-(asymmetric); and 1276 cm −1 due to the ν -COO-(symmetric); 1154 cm −1 due to the -C-N stretching; and 500-900 cm −1 to the Fe-O, Fe-O-Fe, and O-Fe-O lattice vibrations. These observations confirmed the formation of Pt/Pd-PA complex and Fe-hydroxide at all three hybrid nanorods. Figure 1b illustrates the XRD pattern of hydrothermally synthesized hybrid nanorods. From Fig. 1b, it can be seen that the observed reflections clearly indicate the formation of a single phase compound without any impurity traces. XRD results revealed that all of three formed hybrid nanorods have a monoclinic crystal structure with a C2/c space group (JCPDS card no. 00-049-2426). The structure of monoclinic nanorod hybrids is composed of two different structures including metal hydroxide (C2/c space group and JCPDS card no. 00-030-0147) and M-PA complex with monoclinic structure (P21/c space group and JCPDS card no. 00-044-1812), which are held The surface composition of the Fe 3 PtPd(OH) 2 [PA] 8 (H 2 O) 4 was analyzed by X-ray photoelectron spectroscopy (XPS, Figure S1). The survey spectrum ( Figure S1a) shows carbon, oxygen, nitrogen, iron, palladium and platinum species. The high-resolution XPS spectra of C 1 s ( Figure S1b Morphological properties of all three hybrid nanorods were analyzed by FE-SEM and TEM, and the images are shown in Fig. 2. From FE-SEM and TEM images, it is apparent to notice the formation of the regular shaped hybrid nanorods with ~100 nm thickness and 2-3 µm length for all three hybrid nanorods (Fig. 2a,b). The elemental analysis of the as-prepared hybrid nanorods was obtained using EDX and ICP-AES. According to Fig. 2c 4 , which is strong evidence for the formation of the hybrid nanorods. SEM-EDX mapping analysis for the hybrid nanorods ( Figure S2) proved the uniform distribution of Pt, Pd, Fe and C, N, O (Picolinic acid) inside structure. The uniform structure lead to enhancement in both catalytic activity and durability toward the oxygen reduction reaction.
Activity and performance of hybrid nanorods for the ORR. We first compared the electrochemical behavior of the hybrid nanorods with each other and with Pt/C for ORR using cyclic voltammograms (CVs) cell in oxygen-saturated 0.1 M KOH (Fig. 3a). The detailed information can be found in Table 1. As can be seen in Fig. 3a, distinct peaks corresponding to ORRs can be observed for all the hybrid nanorods. In Fig. 3a 4 exhibits a one-step process for ORR. As Fig. 3b shows, the onset potentials measured for all the hybrid nanorods are positively moved from that of Pt/C. The observed onset potentials order for the hybrid nanorods and Pt/C was . It is clear that the ORR onset potential measured for all the hybrid nanorods are positively moved from that of Pt/C. These results indicate that ORR activity of the Pd based hybrid nanorods is better than that of the Pt based hybrid nanorod, which is in line with previous reports 30,31  Stability studies. The durability of the catalysts and the long-term stability of the electrocatalytic activity for ORR are of prominent concern in fuel cells. The stabilities of the Fe 3 PtPd[PA] 8 (OH) 2 (H 2 O) 4 and Pt/C electrodes towards oxygen reduction are shown in Fig. 5. The dotted lines are the cycling difference from the 1st cycle to the 4000th cycle. Pt/C catalysts display a rapid decay of the signal (up to 20%) current depression after the 4000th cycle, indicating a poor stability. In contrast, the response of the Fe 3 PtPd[PA] 8 (OH) 2 (H 2 O) 4 electrode retains acceptable stability throughout the entire experiment, with only 5% current diminutions after the 4000th cycle. These results demonstrate the higher durability of Fe 3 PtPd[PA] 8 (OH) 2 (H 2 O) 4 compared with Pt/C. The promoted electrochemical stability may be due to the stronger interaction force between Pt/Pd-PA and F-OH groups than the force between Pt and C.
Scan rate effect. Figure 6 shows the representative CV curves of the Fe 3 PtPd[PA] 8 (OH) 2 (H 2 O) 4 electrode in a 0.1 M KOH aqueous electrolyte at various scan rates ranging from 5 to 100 mV s −1 . Clearly, an ORR peak within 0.1 to −0.4 V is visible in all the CV curves. Furthermore, a linear relation between the peak current at different scan rates and the square root of the scan rate is observed, confirming that the redox reaction is a diffusion-controlled process.

Discussions
In summary, we have fabricated an advanced organic-inorganic hybrid nanorod catalyst with the generic formula of Fe 3 PtPd[PA] 8 (OH) 2 (H 2 O) 4 , which boasts a greater electrocatalytic ORR activity than that of current commercial Pt/C catalysts. Specifically, the enhancement in the specific activity can be attributed to a uniform distribution of Pt and Pd complexes at Fe-OH structure, ligand effect, and strain effect arising from the lattice mismatch between Pd, Pt, and Fe. The new types of heterodoped structures may provide opportunities for further development of catalysts with high activities and a long lifetime for practical ORR applications in alkaline solutions.

Materials
Synthesis of hybrid nanorods. Hybrid nanorods with a molar ratio of Pd:Pt from 1:0 to 0:1 were prepared by solvothermal treatment. A similar synthesis method was previously reported 24    autoclave and heated at 150 °C for 15 h. After being cooled to room temperature, the obtained precipitate was filtered and washed with a large amount of water until the pH value of the waste water reached 7, and then it was dried at 70 °C for 6 h.
Oxygen reduction reaction procedure. Electrochemical measurements for evaluation of ORR catalytic activity of the hybrid nanorods were performed using a computer-controlled potentiostat (CHI 760 C, CH Instrument, USA) with a typical three-electrode system. A nickel foam electrode (0.3 mm diameter) was used as the working electrode, a Pt foil as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. All the experiments were conducted at room temperature (25 °C). For working electrode preparation, 1.5 mg of the catalyst was dispersed in a mixture of 0.5 mL ethanol and 20 μL of 5% Nafion under ultrasonication for 20 min. Next, 10 μL of the dispersion was uniformly dropped onto the nickel foam electrode and dried at room temperature and under ambient conditions. The commercial Pt/C (20 wt% Pt on Vulcan XC-72) electrode was prepared by the same procedure.