Methanol Oxidation on Ru- or Ni-Modified Pd-Electrocatalysts in Alkaline Media: A Comparative Differential Electrochemical Mass Spectrometry Study

Abstract

The electrochemical oxidation of methanol on electrocatalysts is intensively investigated since direct methanol fuel cells (DMFCs) were proposed as a promising alternative as power source for mobile and back-up power applications. In DMFCs, methanol is oxidized at the anode and oxygen is reduced at the cathode to convert the chemical energy of the fuels to electric power. In recent years, the electrochemical oxidation of methanol in alkaline environment is rising in interest, because other electrocatalysts than the expensive platinum are feasible as anode and cathode catalysts in alkaline DMFCs. While multiple catalysts are feasible on the cathodic site, Palladium was proven to be a suitable substitute for Pt as anode catalyst for the electrochemical oxidation of alcohols in alkaline media [1], but needs further improvement to compete with Pt especially regarding methanol oxidation reaction. The catalytic activity of Pd catalysts was improved by alloying or mixing other metals (or metal oxides) to Pd [2,3] or by controlling the size and shape of Pd particles [4]. Besides high catalytic activity, the complete oxidation of methanol to CO2is of enormous interest, because the complete oxidation results in the highest possible faradaic efficiency with six electrons transferred per methanol molecule. In contrast, two or four electrons are transferred to the anode per methanol molecule, if methanol is not completely oxidized. Differential electrochemical mass spectrometry (DEMS) is used by different research groups to quantify a catalyst’s ability to completely oxidize methanol. While the methanol electrooxidation on Pt-based catalysts was investigated intensively via DEMS in acidic media, DEMS studies on Pd-based catalysts for methanol electrooxidation in alkaline media are rare. In former studies, we reported on PdXNi/C and PdXRu/C catalysts for methanol electrooxidation in alkaline media synthesized by wet-chemical reduction with sodium borohydride [2,3]. It was possible to show that these oxophilic metals have a positive effect on the catalytic activity towards methanol oxidation of Pd-based catalysts. Taking these studies into account, we herein investigated the influence of these metals on Pd-based electrocatalysts regarding methanol oxidation reaction via DEMS. Besides analyzing the product distribution from methanol bulk oxidation on PdXNi/C or PdXRu/C, the main interest of this work is to investigate how the addition of the metals to Pd influences the methanol oxidation mechanism and catalyst poisoning. Hence, standard electrochemical tests like cyclic voltammetry, chronoamperometry, COadsstripping or methanol adsorbate stripping were conducted in the DEMS flow cell. Figure 1 shows cyclic voltammograms and corresponding MS signals for produced CO2 (m/z = 44) and methyl formate (m/z = 60) for Pd/C, Pd5Ni/C and Pd3Ru/C during methanol electrooxidation. While both modified catalysts show higher current densities and lower onset potentials than Pd/C for the electrooxidation of methanol, the product distribution differs strongly depending on the added metal. For Ni-modified catalyst the formation of CO2 is significantly lower than for Pd/C. While the addition of Ni to Pd leads to a negative effect on the ability to completely oxidize methanol, Pd3Ru/C is able to oxidize methanol more efficiently to CO2 than Pd/C. For Pd3Ru/C, the CO2 formation is shifted to lower potentials while methyl formate is produced at higher potentials. This hints to a more selective production of CO2 at low potentials on Pd3Ru/C than on Pd/C or Pd5Ni/C. Besides DEMS experiments, measurements were carried out in dependence of methanol concentration or oxidation temperature using a 3-electrode setup with stationary electrolyte to determine the rate determining step and the activation energy of methanol electrooxidation on Pd/C and the modified catalysts. Fig. 1 – Methanol bulk oxidation on Pd/C, Pd5Ni/C and Pd3Ru/C investigated via cyclic voltammetry in a DEMS flow cell at 20 mV s-1 from 0.1 to 1.2 V (vs. RHE) with 0.1 M CH3OH + 0.5 M KOH solution as electrolyte. Corresponding mass spectrometry signals for CO2 (m/z 44) and HCOOCH3(m/z 60) are displayed to evaluate the product distribution during methanol oxidation reaction.