PtCo-NC Catalyst Derived from the Pyrolysis of Pt-Incorporated ZIF-67 for Alcohols Fuel Electrooxidation

Bo Fang, Ligang Feng

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, Jiangsu Province, P.R.China.

Abstract:Alcohols fuel electro-oxidation is significant to the development of direct alcohols fuel cells, that are considered as a promising power source for portable electronic devices.Currently, the catalyst was restricted by the serious poisoning effect and high cost of noble metals.Developing low-cost Pt alloy with high performance and anti-CO poisoning ability was highly desired.In this work, PtCo-NC catalyst was synthesized by combining Pt nanoparticles with ZIF-67 after annealing in the tube furnace and the in situ generated N-doped carbon from ZIF-67 was functionalized to support the PtCo alloy nanoparticle.The structure and morphology were probed by X-ray diffraction, scanning electron microscope and transmission electron microscope, and the electrochemical performance was evaluated for alcohols of methanol and ethanol oxidation in the acid electrolyte.Compared with the reference sample of Pt/C, several times performance enhancement for alcohols fuel oxidation was found on PtCo-NC catalyst as well as the good catalytic stability.Specifically, the peak current density of PtCo-NC was 79.61 mA∙cm-2 for methanol oxidation, about 2.2 times higher than that of the Pt/C electrode (36.97 mA∙cm-2) and 2.5 times higher than that of the commercial Pt/C electrode (31.23 mA∙cm-2); it was 62.69 mA∙cm-2 for ethanol oxidation, about 1.65 times higher than that of Pt/C catalyst(37.99 mA∙cm-2) and commercial Pt/C electrode (37.77 mA∙cm-2).These catalytic performances were also much higher than some analogous catalysts developed for alcohols fuel oxidation.A much higher anti-CO poisoning ability was demonstrated by the CO stripping voltammetry experiment, in which the COad oxidation peak potential for PtCo-NC was 0.46 V, ca.110 mV negative shift compared with Pt/C catalyst at 0.57 V.A strong electronic effect was indicated by the peak position shifting to the lower binding energy direction by 0.3 eV on PtCo-NC compared with Pt/C reference catalyst.According to the d-band center theory, the electron-enriched state of Pt will decrease the interaction strength of poisoning intermediates adsorbed on its surface; Moreover, according to the bifunctional catalytic mechanism, the presence of Co can form the adsorbed oxygen-containing species (―OH) more easily than Pt at low potentials, and this oxygen-species were helpful in the oxidation of COad at neighboring Pt sites.The high catalytic performance for alcohols fuel oxidation could be due to the largely improved anti-CO poisoning ability and the synergistic effect between the in situ formed PtCo nanoparticles and the N-doped carbon support.

Key Words:Alcohols oxidation reaction;Direct alcohol fuel cell;Electro catalyst;Pt-base catalyst;N-doped carbon

1 Introduction

The direct alcohols fuel cells have received considerable attention as power sources for automobiles and portable electronics due to their high energy density, low pollutant emission, and mild operating temperature1-3.Developing highperformance catalysts for the alcohols oxidation reaction is still highly desired to realize the commercialization of this technology as the alcohols fuels oxidation process is very complex and sluggish4,5.Platinum is the most effective monometallic catalyst for alcohols fuel oxidation but its surface is prone to poisoning by the strongly adsorbed intermediate species leading to reduced catalytic activity and stability6.PtRu is the most efficient catalyst system for alcohols fuels oxidation while the high cost of noble metal Ru and its instability reduces the value for large-scale applications7,8.

Challenging is then given to combining Pt with a cheap 3dtransition metal such as Fe, Co and Ni with the aim to obtain more practicable catalysts.The electronic structure variation of Pt induced by the introduced metals may tune the adsorb energy of alcohols molecule and CO-like mediate to promote the performance of catalystviathe electronic effect; meanwhile, the CO-like mediate could be removed by oxidation reactionviaOH-species generated by the oxyphilic metals through the bifunction mechanism9,10.By simple alloy with Pt, catalysts like Pt-Co1,6,11-13, Pt-Ni11, Pt-Fe4, Pt-Sn14,15have been synthesized by wet chemical fabrication approaches.PtCo is very promising for alcohols fuel oxidation, several strategies have been proposed to boost its catalytic performance.For example, PtCo nanodendrites had been fabricated by a modified solvothermal method and the improved catalytic performance was found by weakening the chemical adsorption of CO on the catalyst surface6,3D Pt-Co alloy networks exhibited excellent electrocatalytic activity and stability for the methanol oxidation reaction16.

The metal-organic framework (MOF) derived carbon could promote the conductivity and prevent the active metal sites from agglomerating and corrosion, meanwhile, the carbon support usually has defect or heteroatom coordination site that may provide more active sites for catalysis reaction process.Therefore, MOF derived materials have received much attention for the relevant catalytic reactions.For example, N-doped carbon nanotube frameworks derived from MOFs were found active and stable for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) due to the advantageous features in composition, structure and optimum graphitic degree and N-doping level17; Shenet al.reported Co@Pd core-shell nanoparticles embedded in N-doped carbon matrix obtained from ZIF-67 served as a highly active and extremely stable catalyst toward the nitrobenzene hydrogenation reaction18,Porous GA-CNx-ZIF-8 with an open structure and abundant defects by utilizing ZIF-8 derivatives to coat GA during hydrothermal self-assembly process, which was further used to deposit Pt NPs for methanol oxidation reaction19.

Because of facile formation of N-doped carbon support and metallic Co from ZIF-67viathermal annealing, it was employed to fabricate PtCo-NC catalyst by combining Pt nanoparticles with ZIF-67 after annealing in the tube furnace and thein situgenerated N-doped carbon from ZIF-67 was functionalized to support the PtCo alloy nanoparticle.The material was etched by nitric acid to remove the unalloyed Co metal to increase its stability and the structure and morphology were probed by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM).The PtCo-NC was studied for alcohol oxidation compared with Pt/C catalyst, it was found that the synergistic effect between thein situformed PtCo nanoparticle and the N-doped carbon support could enhance the tolerance of CO-like intermediate and promote the electrochemical performance for the alcohols fuels oxidation.

2 Experimental and computational section

2.1 Reagents

Sulfuric acid, nitric acid, methanol, ethanol, cobalt nitrate hexahydrate, 2-methyl imidazole, chloroplatinic acid hexahydrate, ethylene glycol and Polyvinylpyrrolidone (PVP,average Mw = 40000) with analytic grade were employed without further purification.

2.2 Synthesis of Pt nanoparticles

Pt nanoparticles were synthesized by modifying a previously reported method20.The 10 mL water and 30 mL ethylene glycol were adding into a flask under stirring to obtain a homogeneous solution.Then 0.28 mL aqueous solution of hydrogen hexachloroplatinate (179 mmol∙L-1) and 2 g of PVP were added into that solution to obtain a light yellow solution.It was heated to 150 °C and kept 3 h in an oil bath under stirring.After the reaction, the mixture was cooled down to room temperature, the Pt nanoparticles were precipitated by adding acetone to the solution and collected by centrifugation and finally dispersed in 16 mL ethanol for further use.

2.3 Synthesis of PtCo-NC

Pt@ZIF-67 was synthesized according to a previous report after a modification21.2.052 g of 2-methyl imidazole dissolved in 14 mL of ethanol was added into 16 mL of ethanol suspension containing Pt nanoparticles.Then the mixture was stirring at room temperature for 2 h.After that, a viscous dark brown liquid was formed after evaporating the ethanol by rotary evaporator.The viscous dark brown liquid was added to the 30 mL of ethanol solution containing 0.915 g of Co(NO3)2⋅6H2O.The reaction mixture was kept at 30 °C for 20 h in an oil bath.The precipitate was collected by centrifugation and washed several times with ethanol.At last, the product as Pt@ZIF-67 was dried in vacuum at 60 °C for 12 h.

The Pt@ZIF-67 samples were pyrolysis at 400 °C for 2 h, then at 600 °C for 2 h and finally at 900 °C for 10 min in N2atmosphere with the temperature ramp rate of 2 °C∙min-1in the pyrolysis progress.Then the material obtained from the tube furnace was immersed intoexcessive 0.1 mol∙L-1HNO3solution for 2 h and three times was repeated to remove the unstable Co particles in the frameworks.Finally, PtCo-NC was obtained by water and ethanol washing and dried in vacuum overnight.

2.4 Physical characterizations

The scanning electron microscopy (SEM) was performed on Zeiss Supra55 scanning electron microscope operating.Transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX) were carried out on FEI Tecnai G2 F30 S-TWIN transmission electron microscope operating at 200 kV.X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB250Xi spectrometer with an AlKα radiation source.XRD was measured on Bruker D8 advance X-ray diffractometer using CuKαradiation.

2.5 Electrochemical measurements

All the electrochemical measurements were performed on an electrochemical workstation (Biologic VSP, France) and a threeelectrode test cell was used.A glassy carbon electrode (0.07 cm-2)modified by catalyst was used as the working electrode, a graphite rod was used as the counter electrode and a calibrated saturated calomel electrode (SCE) were used as the reference electrodes.All of the potentials were relative to the SCE electrode.The working electrode was prepared as follows: 1 mg PtCo-NC, 10 μL of 5% (w, mass fraction) Nafion solution were dispersed in 190 μL of ethanol by sonication for at least 30 min to form a homogeneous ink.Then 10 μL of the ink was drop cast on the glassy carbon electrode as the working electrode.The electrolyte was saturated by N2 for 15 min before measurements.Cyclic voltammetry was carried out in 0.5 mol∙L-1H2SO4 with/without 1 mol∙L-1CH3OH and 0.5 mol∙L-1H2SO4/1 mol∙L-1CH3CH2OH solution at a potential range between -0.2 and 1 V with a scan rate of 50 mV∙s-1.Chronoamperometry at a fixed potential was done for 3600 s for methanol oxidation and ethanol oxidation.COad (ad means adsorbed) stripping voltammetry was performed as reported previously22.The amount of COadwas evaluated by integrating of the COadstripping peak, assuming 420 μC∙cm-2of coulombic charge required for the oxidation of the CO monolayer.

3 Results and discussion

The Pt nanoparticles combined into ZIF-67 (Pt@ZIF-67) was firstly fabricated and they were transferred to PtCo-NC after thermal annealing at 900 °C.Rhombic dodecahedral particles were observed for the Pt@ZIF-67 as revealed by SEM image and the particle size of Pt@ZIF-67 was asymmetrical and the size was from 45.09 to 317.81 nm (Fig.1a).While the morphology was destroyed after thermal annealing for the formation of Pt@ZIF-67 because of the framework shrinkviathe ligand decomposition (Fig.1b)17.In Fig.1c, a folded layered structure was observed by transmission electron microscopy (TEM), and the nanoparticles were found embed into the carbon structure(Fig.1d, e).Elements of Pt, Co, N, C were demonstrated in the EDX spectrum (Fig.1f).The composition of Co and Pt was further proved by inductively coupled plasma optical emission spectrometry (ICP-OES) and the atom ratio of Pt/Co was measured to be 0.8.The unstable Co was removed by acid etching and the residuum was probably from the coordinate Co in N-doped carbon and metallic Co in PtCo alloy nanoparticle.

Fig.1 (a) SEM of Pt@ZIF-67; (b) SEM, (c, d) TEM, (e) HRTEM images and (f) EDX spectrum of PtCo-NC.

Fig.2 (a) Powder X-ray diffraction patterns of PtCo-NC; XPS spectra of (b) N 1s, (c) Co 2p regions for PtCo-NC and (d) Pt 4f region for Pt/C and PtCo-NC.

The crystal structure was evaluated by Powder X-ray diffraction and typical diffraction peaks can be assigned to facecentered cubic (fcc) structure of Pt in which the peak positions were shifted to the high degree direction due to the alloy formation (Fig.2a).During the thermal annealing, partial coordinated Co2+was reduced to metallic Co and alloy of PtCo was correspondingly formed at high temperature23.The surface chemical state of PtCo-NC was probed by X-ray photoelectron spectroscopy (XPS) and the binding energy was calibrated by the value of 284.8 eV of C 1sfor a valid comparison.The high resolution of N spectrum can be deconvoluted into four peaks with the types of pyridinic N (ca.398.3 eV), the pyrrolic N (ca.399.6 eV), the graphitic N (ca.401.6 eV) and the oxidized N (ca.402.4 eV) (Fig.2b)24.Four distinct chemical states of Co are also found after deconvolution of Co 2pspectrum (Fig.2c),namely metallic Co (778.7 eV), Co3+(779.9 eV), Co2+(780.7 eV)and Co-Nx(782.5 eV)23.The Pt 4fregion showed two doublets from the spin-orbital splitting of the 4f7/2and 4f5/2and chemical state of metallic Pt and Pt2+was found after deconvolution.The peak position was shifted to lower binding energy direction by 0.3 eV compared with Pt/C reference catalyst indicating partial electron density transferred to Pt due to the larger electronegativity of Pt (Fig.2d).It should be pointed out no Pt―N bond was found during the fitting, and the role of N was doped into the carbon that functionalized as support of PtCo nanoparticles.The rich electrons will down-shift thed-band center of Pt and reduce the adsorption ability of the poisoning intermediates, thus improved performance is expected for the alcohols fuel electrooxidation25.

The background of PtCo-NC and Pt/C reference catalyst was probed in 0.5 mol∙L-1H2SO4 solution at 50 mV∙s-1.The characteristic profile of polyline crystal Pt was observed with the peaks of hydrogen adsorption/desorption and Pt oxidation/reduction (Fig.3a).The close profile of H ad/desorption peaks observed on PtCo-NC and Pt/C was due to the surface dissolution of Co and Pt skin layer formed on the surface.The anti-CO poisoning ability was evaluated by the CO stripping voltammetry as shown in Fig.3b, in which the COad oxidation peak potential for PtCo-NC was 0.46 V, ca.110 mV negative shift compared with Pt/C catalyst at 0.57 V.Similar case was found on the onset potential for COadoxidation, the lower CO oxidation potential on PtCo-NC indicated its improved ability toward poisoning intermediates oxidation and removal.The easier oxidation of CO could be due to its lower affinity toward Pt in the PtCo-NC that was consistent with the electronic effect found in the XPS spectrum.According to thed-band center theory, the electron-enrich state of Pt will decrease the interaction strength of poisoning intermediates adsorbed on its surface; Moreover, according to the bi-functional catalytic mechanism, the presence of Co can form adsorbed oxygencontaining species (―OH) more easily than Pt at low potentials,and this oxygen-species were helpful in oxidation of COadat neighboring Pt sites.Thus, improved ability of CO-tolerance was found on the PtCo-NC catalyst.Moreover, the electrochemical surface area (ECSA) was also calculated by CO stripping, and it was 55 and 50 m2∙g-1for PtCo-NC and Pt/C,respectively.The electrochemical surface area of PtCo-NC catalyst was probably underestimated due to the coverage of carbon layer on the Pt active sites, while the facile oxidation of COadspecies demonstrated the high-efficiency for alcohols fuel oxidation.

The methanol oxidation performance was studied by cyclic voltammetry in 0.5 mol∙L-1H2SO4/1 mol∙L-1CH3OH at the scan rate of 50 mV∙s-1.As shown in Fig.4a, the peak current density of the forward scan for PtCo-NC was 79.61 mA∙cm-2, about 2.2 times higher than that of the Pt/C electrode (36.97 mA∙cm-2) and 2.5 times higher than that of the commercial Pt/C electrode(31.23 mA∙cm-2).This performance was also much higher than some analogous PtCo catalyst and other catalysts such as Pt-Co/SWCNT (28.8 mA∙cm-2)26, Pt-Co/CNT (31.3 mA∙cm-2)27and multisegment PtNi nanorods (6.26 mA∙cm-2)28.Moreover,the onset potential for PtCo-NC was 0.23 V, about 50 mV less than that of Pt/C (0.28 V) indicating the highly improved ability for methanol fuel oxidation.The mass activity and intrinsic activity was compared in Fig.4b to evaluate the catalyst efficiency.The mass activity of 557.3 mA∙mg-1was obtained on PtCo-NC catalyst for methanol oxidation judged by the peak current, about 2.15 times as high as that of Pt/C catalyst.The specific activity of PtCo-NC, normalized to the electrochemically active surface area, is 1.01 mA cm-2, with a catalytic efficiency improvement of 1.94 times compared with Pt/C catalyst. The catalytic stability revealed by chronoamperometry (CA) at 0.6 V is shown in Fig.4c.After the 3600 s test, the final current density is 18 mA∙cm-2for PtCo-NC and 3 mA∙cm-2for the Pt/C catalyst, respectively.The decay rate by normalizing the final current density to the initial value for PtCo-NC is 54%, much lower than that of 85% for Pt/C catalyst.

Fig.3 (a) Cyclic voltammograms of PtCo-NC and Pt/C catalysts in 0.5 mol∙L-1 H2SO4 solution at the scan rate of 50 mV∙s-1.(b) CO stripping voltammograms of PtCo-NC and Pt/C catalysts in 0.5 mol∙L-1 H2SO4 solution at the scan rate of 20 mV∙s-1.

Fig.4 (a) Cyclic voltammograms of PtCo-NC, Pt/C and commercial Pt/C for methanol oxidaiton at 50 mV∙s-1; (b) Specific activity and mass activity of PtCo-NC and Pt/C for methanol oxidation.(c) Chronoamperometric curves of PtCo-NC and Pt/C for methanol electrooxidaiotn at 0.6 V.

Fig.5 (a) Cyclic voltammograms of PtCo-NC, Pt/C and commercial Pt/C for ethanol electrooxidaiton at 50 mV∙s-1; (b) Specific activity and mass activity of PtCo-NC and Pt/C for ethanol oxidation, (c) Chronoamperometric curves for PtCo-NC and Pt/C at 0.65 V for ethanol oxidation.

A similar case was observed on the catalytic performance for ethanol oxidation.From the CV curves (Fig.5a), the onset potential of PtCo-NC catalyst was 160 mV, about 130 mV less than that of Pt/C catalyst; the forward peak current density was 62.69 mA∙cm-2on the PtCo-NC electrode, about 1.65 times higher than that of Pt/C catalyst (37.99 mA∙cm-2) and commercial Pt/C electrode (37.77 mA∙cm-2).This performance was also much higher than some similar PtCo catalysts and other catalysts like Co@Pt (6.12 mA∙cm-2)29, Pt-Co/SWCNT (40.9 mA∙cm-2)26, PtNi/C (4.48 mA∙cm-2)30.

As shown in Fig.5b, the mass activity of PtCo-NC was calculated to be 438.8 mA∙mg-1higher than that of Pt/C catalyst and some other catalysts reported elsewhere.For example,Pt/ZSM-5 (374 mA∙mg-1)31, Pt-SiO2 (155.42 mA∙mg-1)32, Pt-CeO2/C (225 mA∙mg-1)33.The maximum specific activity of PtCo-NC was 0.8 mA∙cm-2, about 1.51 times higher than that of the Pt/C catalyst.Moreover, much better catalytic stability was demonstrated in the CA curves (Fig.5c).Specifically, the final current density was 11 and 6 mA∙cm-2for PtCo-NC and Pt/C,respectively.The performance decay rate of 50% for PtCo-NC was lower than that of Pt/C (60%).

From the above results, it can be found that the as-prepared PtCo-NC catalyst exhibited outstanding catalytic performance for alcohols fuel oxidation.The performance can be attributed to the well-recognized bi-functional catalytic mechanism and the electronic effect.By forming PtCo alloy, the adsorption of ―OH can happen at relatively low potentials than Pt alone, the facile formation of oxygen-containing species will be beneficial to the oxidation of CO-like intermediates adsorbed on the Pt sites as per the bi-functional catalytic mechanism.Partial transfer of electron density to Pt occurred indicating the down-shift thedband center of Pt as demonstrated in the XPS results, and the electron-enrich state of Pt will decrease the interaction of poisoning intermediates adsorbed on the Pt surface.Mover, the N-doped carbon was beneficial to the anchoring and stabilizing of PtCo nanoparticlesviathe metal-support interaction.Therefore, PtCo-NC demonstrated an excellent catalytic performance for alcohols fuel oxidation.

4 Conclusions

In summary, PtCo-NC catalyst deriving from the pyrolysis of Pt-incorporated ZIF-67 has been found efficient for alcohols fuel oxidation compared with a reference Pt/C catalyst.The alloy of PtCo nanoparticles over thein situformed N-doped carbon support was observed and a strong electronic effect was indicated in this catalyst system revealed by the XPS characterization.PtCo-NC catalyst also exhibited a much higher anti-CO poisoning ability compared with the Pt/C catalysts.The performance for alcohols fuel oxidation on PtCo-NC catalyst was largely improved compared with Pt/C catalyst as indicated by the mass activity and specific activity, and outstanding catalytic stability was also found the electrochemical oxidation process.