Effects of Pb in Electrocatalysis of Ethanol Oxidation Reaction on Pd/C and Pdru/C Catalysts in Alkaline Media
Authors: Chen Rongrong , Guo Junsong 
Published: February 2020

Electrocatalysis of ethanol oxidation reactions (EORs) in alkaline media has gained great research interests due to the development of alkaline anion exchange membranes.  The EOR kinetics are more facile in alkaline media than those in acid media. The most importantly, there are more choices of materials to be used as the electrocatalysts for the EOR since these materials are stable in the alkaline media, but not in the acid media. Although extensive research has been done worldwide related to electrocatalysis of EORs in both acid and alkaline media, the EOR mechanisms are still not well understood.  It still remains as a major challenge of reducing the overpotentials of EORs in low temperature fuel cells. The oxidation of ethanol in alkaline media is very complicated and involves many types of intermediates, such as C1 species (most likely COad and CHx,ad), OHad, CH3COad and other forms of acetaldehyde in alkaline media.  Single metal catalysts are often found to perform much poorer than those modified with the second metal. “Multi-functional” electrocatalyst systems have been designed and tested for EORs. In this work, we aim to study effects of Pb on electrocatalysis of EORs on Pd/C and PdRu/C catalysts in alkaline media.

Pd/C and Pd2Ru/C (the atomic ratio of Pd:Ru is 2:1) catalysts with 20 wt. % Palladium loading were prepared by an impregnation method. The morphologies of the catalysts were studied by transmission electron microscope (TEM) and x-ray diffraction (XRD) characterizations. Then the catalysts were performed electrochemical characterizations in a standard tri-electrode electrochemical cell with a graphite column serving as the counter electrode and a Hg/HgO/1.0 M OH- electrode (0.098 V vs. NHE) as the reference electrode in an argon-saturated or ethanol-contained 1 M NaOH solution without or with 1mM lead(II) acetate (Sigma-Aldrich) presented.

The EOR activities of the catalysts were also characterized with a single direct ethanol fuel cell. The membrane electrode assemblies (MEA) with an active electrode area of 4.5 cm2 were comprised of a catalyst coated membrane (CCM), a Ni foam (Hohsen Corp.) as the anode backing layer and a TGP-H-090 carbon paper (Toray) as the cathode backing layer. The anode catalyst ink was prepared by mixing the 16mg catalyst (Pd/C and Pd2Ru/C) with Nafion solution. The cathode catalyst ink was prepared by ultrasonicating 8mg MnO2 nanorod catalyst, 8 mg active carbon (BP2000, Cabot Corp. ) and the A4 ionomer (Tokuyama Co.). The catalyst/ionomer (Nafion or A4) weight ratio is kept at 80:20 in both anode and cathode. A fuel cell test system (Scribner Associates Model 850e) was used for controlling the cell temperature, cathode humidity, O2 flow rate. The temperature of the fuel cell was maintained with a tolerance of ± 0.2 °C. The anode solution flow rate was controlled ESI's MP2 micro peripump (elemental scientific Inc.)

CV curves were obtained with the Pd/C and Pd2Ru/C in Ar-saturated 1.0 M NaOH or 1.0 M ethanol and 1.0 M NaOH solution without or with 1mM Pb2+ presented.  From the CV curves obtained in the 1.0 M NaOH solutions, the Pb2+ presence in the solution was found to alter surface characteristics on both Pd/C and PdRu/C catalysts. The EOR activities were also affected accordingly.  Both CV and chronoamperometry (CA) measurements indicate that the presence of Pb2+ in the solution leads to the reduction of EOR kinetics on the Pd/C or PdRu/C catalysts, which are different from what were observed with Pt-based catalysts.  Detailed fuel cell characterizations of these various catalysts were performed.  Combining the electrochemical measurements and fuel cell test results, effects of Pb on the electrocatalysis of EOR on Pd/C and PdRu/C catalysts in alkaline media will be discussed.