Renxuan Liu

Methanol Oxidation on Single‐Phase Pt‐Ru‐Os Ternary Alloys

Methanol oxidation was studied on arc-melted Pt-Ru-Os alloys and on fuel cell catalysts prepared by the NaBH{sub 4} reduction of metal chloride salts. Both the arc-melted alloys and the high surface area catalysts have x-ray diffraction patterns indicative of single-phase face-centered cubic lattices. Hydrogen adsorption/desorption measurements on the polished alloy electrodes, in the presence of adsorbed CO (25 C), show that selected ternary alloys have significant hydrogen adsorption/desorption integrals at adsorption potentials where Pt:Ru (1:1) was fully blocked and higher integrals at all adsorption potentials studied up to 400 mV vs. the reference hydrogen electrode. In situ diffuse reflection Fourier transform infrared spectroscopy of the fuel cell anodes showed that the alloy catalysts had reduced CO coverage relative to Pt, with the ternary catalyst showing the least coverage. Steady-state voltammetry of the arc-melted alloys at 25 C confirmed that Pt-Ru-Os (65:25:10) is more active than Pt-Ru (1:1), particularly above 0.6 V. Pt-Ru-Os (65:25:10) methanol fuel cell performance curves were consistently superior to those of Pt-Ru (1:1) (e.g., typically at 90 C, 0.4 V; 340 mA/cm{sup 2} with Pt-Ru-Os vs. 260 mA/cm{sup 2} with Pt-Ru).

Carbon supported and unsupported Pt–Ru anodes for liquid feed direct methanol fuel cells

A comparative study of the use of supported and unsupported catalysts for direct methanol fuel cells has been performed. The effect of catalyst loading, fuel concentration and temperature dependence on the anode, cathode and full fuel cell performance was determined in a fuel cell equipped with a reversible hydrogen reference electrode. Although the measured specific activities of supported catalysts were as much as 3-fold greater than the unsupported catalysts, membrane electrode assemblies prepared with supported catalysts showed no improvement with loadings above 0.5 mg/cm 2 . Fuel cells utilizing 0.46 mg/cm supported catalyst electrodes performed as well as unsupported catalyst electrodes with 2 mg/cm 2 . The temperature dependence and methanol concentration dependence studies both suggest increased methanol permeation through the thinner supported catalyst layers relative to the unsupported catalyst layers.

Array membrane electrode assemblies for high throughput screening of direct methanol fuel cell anode catalysts

A fuel cell using an array membrane electrode assembly has been developed for the high throughput screening of fuel cell electrocatalysts. Standard membrane and electrode assembly methods are used. The use of modified fuel cell hardware permits the use of realistic catalyst exposure histories and steady state reaction conditions. The array fuel cell requires no supplemental electrolytes. The performance of the array fuel cell is demonstrated by the testing of one prepared in-house and three commercially available fuel cell catalysts. Within the potential range of a DMFC anode (i.e. 0.3–0.4 V), the catalyst rankings were PtRu (Johnson Matthey)>PtRu oxide (E-Tek)>PtRu (reduced by NaBH4)>Pt.

Comparison of High-Throughput Electrochemical Methods for Testing Direct Methanol Fuel Cell Anode Electrocatalysts

The screening and testing of fuel cell electrocatalysts often involves comparisons under conditions that do not closely match their use in membrane electrode assemblies. We compared the activities of several commercial and homemade Pt and PtRu catalysts for electrochemical methanol oxidation by four different techniques; disk electrode linear sweep voltammetry in aqueous methanol/ sulfuric acid solutions, optical fluorescence detection in aqueous methanol solutions containing a fluorescent acid-base indicator, steady-state voltammetry in a 25 electrode array fuel cell with a large common counter electrode, and steadystate voltammetry in a conventional direct methanol fuel cell. The fluorescence detection method, which is a high-throughput technique developed for large arrays of electrocatalysts, can distinguish active from inactive catalysts, but it does not accurately rank active catalysts. Both the disk electrode and array fuel cell methods gave a reliable ranking of the catalysts studied. The best agreement occurred between the array fuel cell and single electrode fuel cell catalyst rankings. A wide range of catalytic activities was found for PtRu catalysts of the same nominal composition that were prepared by different methods.

Deuterium isotope analysis of methanol oxidation on mixed metal anode catalysts

The widely accepted mechanism for methanol oxidation on Pt based catalyst surfaces has held that the rate determining step is activation of water, and/or oxidation of surface-bound CO to CO2. In fact on pure Pt, water activation is always rate limiting at potentials negative of 0.6 V. Anode potentials greater than 0.4 V are outside the useful potential window of direct methanol fuel cells when using Nafion 117 at 60 °C. Enhancement of the water activation kinetics on Pt has been effected by the use of oxophilic transition metal promoters including Ru, W and Sn. For decades the search for improved methanol oxidation electrocatalysts has focused on water activation. A systematic deuterium isotope study on Pt black and two active mixed metal catalysts (PtRu and PtRuOsIr) shows that for each catalyst there is a characteristic transition potential above which the primary reaction in the rate-determining step changes from water activation to CH bond activation. On the mixed metal catalysts, this crossover potential is ca. 0.35 V, which is within the direct methanol fuel cell potential window (0–0.400 V). This study confirms that on these active catalysts there is a potential above which further improvements in water activation must be concomitant with acceleration of CH bond activation. Thus the catalyst search strategy involving Pt promoter metals must also consider the kinetic importance of CH bond activation.

XRD and XPS Analysis of As-Prepared and Conditioned DMFC Array Membrane Electrode Assemblies

Catalysts prepared by the Adams and Reetz method were ranked against a Johnson Matthey PtRu catalyst in an array fuel cell in the direct methanol fuel cell (DMFC) mode. The Reetz and Johnson Matthey PtRu catalysts were found to be equivalent. The catalysts were analyzed in membrane and electrode assemblies (MEA) before and after fuel cell performance evaluation. The X-ray diffraction (XRD) analysis showed no changes of the crystalline phases before and after fuel cell evaluation. However, the X-ray photoelectron spectroscopy (XPS) showed that in the electrode containing the Reetz catalyst, substantial loss of Ru at the MEA surface occurred during fuel cell operation. Loss of Ru also occurred at the MEA surface containing the Johnson Matthey catalyst while an enrichment of Ru on the electrode surface containing the Adams catalyst was observed. A lattice parameter analysis of all catalysts showed Pt enrichment in the alloy phase, with the largest enrichment in the Adams catalyst.

In situ x-ray absorption fuel cell

An in situ x-ray absorption spectroscopy cell was designed to study the structural and electronic properties of high performance polymer electrolyte fuel cell electrocatalysts. The cell was operated as a standard fuel cell and x-ray absorption data were collected simultaneously as the fuel cell operating conditions were varied. The cell was used to examine platinum–ruthenium alloy electrocatalysts incorporated in the anode of a fuel cell membrane and electrode assembly. In situ Pt LIII and Ru K edge x-ray absorption near-edge structure (XANES) data were collected at five selected current densities of the operating fuel cell. The XANES data show that under normal fuel cell operating conditions, the metallic nature of the Pt–Ru catalyst is retained. The cell design enabled an in situ fuel cell study at a synchrotron source in which no additional electrolytes were required.

Combinatorial Screening of Anode and Cathode Electrocatalysts for Direct Methanol Fuel Cells

Progress in several important electrochemical technologies, including batteries, fuel cells, sensors, and electrosynthesis, is currently materials-limited. A common feature of all electrode reactions is the imbalance (i.e., loss or generation) of ions at the electrode surface. We describe in this paper a method by which excess ions in the electrode diffusion layer can be imaged, and used to identify the best electrode materials from a combinatorial array of compositions. Although in principle this method can be applied to many electrochemical problems, we have focused on finding better electrocatalysts for direct methanol fuel cells (DMFCs). The DMFC performs two half-cell reactions: oxidation of methanol, and reduction of oxygen. Two of the most important problems in DMFCs are the poor performance of the electrocatalysts, and the crossover of methanol from the anode to the cathode side of the cell. An ideal situation would be the simultaneous development of two new catalysts: an anode that oxidizes methanol at low overpotential, and a “methanol-tolerant” cathode that reduces oxygen without oxidizing methanol. Based on previously developed rules for predicting the activity of ternary alloy catalysts (Ley, et al., J. Electrochem. Soc. 1997, 144, 1543), we began searching quaternary combinations of noble metals for the anode, and ruthenium selenide-type materials for the cathode reaction. The anode and cathode reactions generate and consume protons, respectively, creating a substantial pH gradient at the electrode surface. Changes in local pH are imaged by means of an appropriate fluorescent indicator: Ni-PTP for the anode and Eosin Y for the cathode. DMFC testing confirms the utility of the screening method, in that a Pt/Ru/Os/Ir quaternary catalyst was substantially superior to the best binary and ternary catalysts prepared under similar conditions.

A Coulometric Normalization Procedure for Comparing High Surface Area Methanol Anode Catalysts by Rotating Disk Electrode Voltammetry

A new method that allows one to normalize the current observed at electrocatalytic rotating disk electrodes (RDEs), based on coulometric analysis of hydrogen desorption waves, is presented. Current-voltage curves for methanol oxidation, using various platinum alloy compositions and application methods, were analyzed by this method. Mass-normalized I-V curves showed considerable scatter among similar alloys (Pt, Pt-Ru, and Pt-Ru-Os) applied by different methods and in different degrees of dispersion. The same curves normalized coulometrically are grouped more closely with respect to composition. Since the coulometric normalization procedure takes into account the electrochemically active surface area rather than the total mass of electrocatalyst, it provides a more precise correlation of catalytic activity with composition than does mass normalization. Combined with mass normalization, the procedure gives a useful estimate of catalyst utilization for different compositions and electrode preparations, under electrochemical conditions that are mimetic of direct methanol fuel cell operation. Comparison of normalized RDE currents at 25 and 50 C clearly shows that a ternary Pt-Ru-Os (65:25:10) composition is a more active anode catalyst than Pt-Ru (50:50) when both are prepared by borohydride reduction.