CO Tolerance of Pt Electrocatalysts on Composite Supports in the Hydrogen Oxidation Reaction

Modification of the support can alter the performance of electrocatalysts.  Platinum is subject to poisoning in the anode of electrochemical cells operated at low temperatures required for many polymer electrolytes.  Thus, very pure hydrogen is required which adds to the complexity and cost of these technologies.  A process can be used to purify the fuel by pumping hydrogen protons through the membrane.  This technique is also an excellent means of characterizing the tolerance of anode electrocatalysts.  The surfaces become saturated with the impurity over time and electrochemical diagnostics allow us to probe this dynamic behavior with exposure.  Over time, the activity is lost until the electrocatalyst is regenerated.  Increasing the temperature, oxygen pressure, or potential are the best ways to remove the contamination.  In this case, carbon monoxide (CO) is removed by cycling the potential.  By this mechanism, the catalyst can be subject to similar driving forces causing corrosion in the cathode of the fuel cell.

Through the design of a composite support, the goal is to provide a stable, connected platform for the platinum catalyst while also improving the tolerance to this common contaminant.  Carbon blacks are used as a support to maintain high surface area and conductivity among other important reasons.  Alternatively, graphitic multi-wall carbon nanotubes can be utilized to provide a more stable framework.  Additionally, metal oxides may be added to their surface to promote a bifunctional mechanism for the CO oxidation.  This may be a more robust method when compared to alloying catalysts with choice of other metals.  The metal oxide can facilitate formation of hydroxyl groups in proximity to the Pt to aid the reaction in a similar way.  Titania is chosen as an example for several reasons including its stability in this environment and also for the formation of a strong metal-support interaction.  Addition of a donor dopant such as niobium may be chosen for ideal defect formation.  Possible advantages and disadvantages of using similar transition metals in the electrocatalyst are discussed.

Following the preparation of supports, the platinum catalyst is deposited by a polyol reduction.  This low temperature reduction is selected to avoid the pitfalls of encapsulation that occur in strong metal-support interactions which can render catalysts inactive.  After synthesis of precursor powders, the materials were characterized before being built into membrane electrode assemblies for full working cells.  A suite of electrochemical diagnostics is applied to understand differences in the tolerance behavior that is fundamentally altered by the support structure.  CO stripping (Fig 1.), chronoamperometric (Fig. 2) and chronopotentiometric (Fig. 3 a & b) tests are among the best ways to characterize the tolerance of these materials.  Attempts to correlate the dynamics with impedance can give further insight into the deactivation.  At higher currents, an oscillating phenomena is observed.  Interesting transitions in the electrocatalysts arise that provide for stimulating discussion of its overarching principles which can guide support construction.