Preparation And Research Of Pd Based Catalysts Supported On Carbon-contained Composite And Their Performance For Formic Acid Electro-oxidation

Direct formic acid fuel cell (DFAFC), which has broad application prospects, can beused as the power source for mobile and portable electric devices due to its severaladvantages, such as high energy conversion efficiency, low operating temperature, and lowenvironmental pollution. At present Pd based catalysts supported on carbon materials havebeen widely used for electrooxidation of formic acid. The metal particle size and itsdispersion on the surface of support are closely related with the structure and surfaceproperties of the carbon support. The modification of supports could improve the catalyticperformance of Pd based catalyst. Previous studies showed that the forming Pd-M alloy byaddition of another metal M can improve the catalytic performance of Pd based catalysts.In this paper, two kinds of carbon contained composite supports were prepared and twokinds of Pd based catalysts were synthesized by a modified liquid phase reduction method.The composition, structure, morphology and surface properties of supports and catalystswere characterized using a series of physical and chemical methods. The electrocatalyticperformance of catalysts were tested by electrochemical methods. The specific researchcontents and results are as follows:(1) Nature clay, i.e. montmorillonite (MMT), were used as support materials of anodeelectrocatalyst in the direct formic acid fuel cell. The nitrogen contained composite support(MMT-CNx) was prepared via carbonizing MMT which was covered by polyaniline.Pd/MMT-CNxand Pd/C catalysts were prepared as formic acid oxidation catalysts by animproved liquid reduction method. The composite, structure, surface properties andelectrical conductivity of the supports and catalysts were studied by energy dispersivespectroscopy (EDS), X-ray diffraction (XRD), fourier transform infrared spectroscopy(FT-IR), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray photoelectronspectroscopy (XPS), scanning electron microscopy (SEM), transmission electronmicroscopy (TEM) and electrochemical impedance spectroscopy (EIS). The results showed that well-dispersed Pd nanoparticles with a narrow size distribution of3.5–5.5nmcould be easily prepared on the MMT-CNx. Larger electrochemical specific area (ECSA)was obtained for Pd/MMT-CNxthan Pd/C according to CO-stripping voltammetry. Theoxdiation current density of Pd/MMT-CNxcatalyst is2.34times of Pd/C catalyst. Thestability of Pd/MMT-CNxcatalyst was improved as compared with Pd/C catalyst. Theratios of I3000/I10for Pd/C and Pd/MMT-CNxis found to be2.5%and30.6%, respectively.Larger electrochemical specific area (ECSA) and doped nitrogen in the support ofPd/MMT-CNxcatalyst may be the main reason of higher activity and better stability foroxidation of formic acid than Pd/C catalyst.(2) The effects of the different concentration of phosphotungstic acid solution, whichwas used to soak activated carbon support, on the performance of catalysts were studied.Effects of the different proportion of Palladium–cobalt (PdCo) on the electrooxidationperformance of formic acid were also investigated. The results showed that the optimalconcentration of phosphotungstic acid solution is3×10-4mol L-1and Pd2Co1catalystdemostrated the highest activity in the studied PdCo catalysts.(3) Activated carbon was modified with phosphotungstic acid (PWA) by impregnation.PdCo alloy nanoparticles were loaded on the hybrid supports (PWA–C) to preparePdCo/PWA–C catalyst by an improved liquid reduction method. Uniformly dispersedcatalyst nanoparticles were prepared and their average particle sizes are in the range of3.0–5.0nm according to TEM and XRD analyses. Electrochemical measurementsdemonstrated that the oxdiation current density of the PdCo/PWA–C catalyst is2.16timesthan home made Pd/C catalyst for formic acid oxidation. A negative shift (52mV thanPd/C) of the CO stripping current peak was observed for the PdCo/PWA–C catalyst. It wasfound that the current densities of formic acid oxidation at3600s of the Pd/C andPdCo/PWA–C catalyst electrodes are2.5%and13.2%of their current densities at10s,respectively. The amount of decomposed formic acid over the PdCo/PWA–C catalyst isonly36%of the Pd/C catalyst, which indicated the excellent inhibition performance of the decomposition of formic acid for the PdCo/PWA–C catalyst. In addition, fastercharge-transfer kinetics of the formic acid oxidation for the PdCo/PWA–C catalyst wasfound through Tafel slope measurements. The better electrocatalytic activity and stabilityof PdCo/PWA–C catalyst may be attributed to excellent PdCo particle dispersing on thePWA–C support and the synergistic effect of the PWA modification and PdCo alloying.