Low Temperature Fuel Cell Catalysis

Backgrounds:

Figure 1: Tekion Has Developed Portable DFAFC That Can Power Cellphone

Direct formic acid fuel cells (DFAFCs) have been recognized as an attractive alternative portable power source which can close this power gap, because they possess a combination of high power and high energy densities (See Figure 1). One of the factors limiting the development and use of DFAFCs is that its existing anode electrocatalyst, palladium (Pd), partially promotes the undesired reaction pathway, leading to low power density and poor long-term stability of the DFAFC. Thus, it is desired to modify the catalytic properties of Pd metal, so that it does not promote this poisoning pathway. One can achieve such modification of Pd metal by combining it with various transition metals to form a bimetallic catalyst. If non-expensive transition-metals are used as supports, it reduces a total cost of fuel cells by decreasing the required amount of Pd. The development of suitable bimetallic catalysts for formic acid oxidation requires a good understanding of how these added transition-metals influence the electronic structure and chemical property of Pd toward formic acid oxidation and its reaction pathways. However, adequate understanding is not presently available.

Research Objectives:

Our long-term goal is to advance our understanding on the bimetallic catalysts for the electrochemical oxidation of simple organic molecules and, based on this improved understanding, to develop inexpensive and high performance catalysts for alternative energy applications. As a first step to achieve our long-term goal, the scientific objectives of this application are: (1) to determine how the surface electronic structure and chemical property of Pd metal are modified by supporting it on various 3d transition-metals, (2) to correlate these modifications to the electrocatalytic activity and stability of Pd for formic acid oxidation, and (3) to use this correlation for developing a highly active and stable bimetallic catalyst for formic acid oxidation.

Research Aims:

We plan to accomplish the objectives of this research by pursuing the following three specific aims:

1. Synthesize the bimetallic particles in a core/shell structure by coating 3d transition-metal cores (Ti, V, Mn, Fe, Co, Ni, Zn) with the Pd shell via a two-step polyol method: Two-step polyol method is used to provide well protected transition-metal cores by the multilayered palladium shell, and their sizes (both the core and shell) are easily controlled by adjusting its process parameters (time, temperature, concentration and pH of precursor solutions).

Figure 2: The Electrochemical Cell

2. Determine electrocatalytic trends between the surface electronic structure, chemical property, and catalytic performance of supported Pd toward formic acid oxidation by experimentally measuring their core-level bonding energy and position of d-band center in XPS and UPS data, wavenumber and peak intensity of adsorbed carboxy species in infrared spectra (see Figure 2), and exchange current density and deactivation rate in electrokinetic data: The various 3d transition-metals used for the core leads to different electronic structures of Pd shell by creating the lattice mismatch (so called strain effect) and facilitating a flow of valence electrons between the transition-metal cores and Pd shell. We believe that these strain and charge transfer effects modify metal-metal bond strength of Pd shell. As this metallic bond strength increases, the core-level bonding energy of Pd increases and position of its d-band center downshift with respect to the Fermi level. Furthermore, the Pd shell with high metallic bond strength leads to a weak interaction with carbonxy intermediate species, which affects the surface electrokinetic behavior of formic acid oxidation.

3. Based on these established trends, identify the 3d transition-metals that lead to the highest catalytic performance of supported Pd toward the formic acid oxidation, and test them using a direct formic acid fuel cell (DFAFC) for their activity and long term stability under various practical fuel cell operating conditions: In an operating fuel cell, there are complicating factors to consider, such as mass transport effects and resistive losses. These factors can influence the overall activity and stability of the newly synthesized catalysts. Thus, their performances are tested using a test DFAFC.