57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The sluggish oxygen reduction reaction is a long-standing scientific challenge, that dramatically reduces the efficiency of fuel cells. This ACS-PRF grant aims to study a novel concept 1-D Fe core- Pt Shell nanowire electrocatalysts with improved activity, enhanced durability and reduced cost, and to develop a general solution-phase synthesis method to accurately prepare metallic nanostructures for electrochemical energy-related reactions. Success of this research will acquire fundamental understanding of controlled wet-chemistry synthesis of multi-metallic nanocatalysts at the nano and atomic scale, and deepen our insights into the structure-catalytic function relationships. This research will open a new avenue for design and preparation of efficient catalytic materials, and help our efforts to diversify the current energy supply status and reduce our dependence on foreign petroleum. Partially supported by this ACS-PRF grant, our research findings in the past year (2011-2012) are summarized below:

1) We further optimized the solution-phase synthesis method to PdFe nanowire structures. Since the metal precursors in the organic solvent have intimate contacts and closer redox potentials, better multi-metallic catalysts can be obtained. The diameter, length and morphology of these 1-D nanostructures can be controlled by tuning synthesis conditions, such as ratio of two surfactants, reduction temperature, Fe content, etc. Especially, the surface energy of metal crystallographic facets can be tuned by different surfactants, leading to controlled shapes. We found that the surfactants can be removed through organic acid washing or electrochemical treatment. We also found that lowering the injection temperature of Fe(CO)₅ restricted the reduction rate of Pd(acac)₂, thus leading to a slow and continuous nucleation process, which resulted in a broader length distribution of Pd-NWs. On the other hand, immediately increasing the temperature right after the quick injection of Fe(CO)₅ without further aging will accelerate the decomposition of Fe(CO)₅. Therefore, serious PdFe-NRs entanglements/aggregations are observed. It remains a challenge to prepare isolated PdFe-NW for subsequent deposition of Pt surface layer. Future study will focus on tuning the solvent, surfactant, ratio of metal to solvent, and reaction conditions (temperature, time, etc.) to prepare desired PdFe-NW with Pd(111) surface, and then cover Pt (111) thin layer on the NW's surface to achieve PdFe core Pt shell nanowire structures.

2) We explored this solution-phase synthesis method to preparation of non-precious metal Ag nanoparticles and tested their oxygen reduction reaction activity in fuel cells. Synthesis of small size, uniform size distribution, and high metal loading supported non-PGM catalysts remains a challenge. We explored the solution phase catalyst synthesis method to prepare non-precious metal catalyst - Ag/C. By tuning the synthesis conditions, we are able to prepare Ag nanoparticles with a small average diameter (5.4 nm) and narrow size distribution of 2-9 nm well dispersed on the support of carbon black Vulcan XC-72. Glycerol is an abundant bio-alcohol and can be serve an inexpensive, renewable fuel for fuel cells. We also studied anion exchange membrane ¨C direct glycerol fuel cells with self-prepared Ag/C (5.4 nm) and commercial Ag/C (25 nm) cathode catalysts, and smaller Ag particle size is able to significantly improve the fuel cell output power density: 80 mW/cm² for Ag/C prepared by the solution-phase method and <40 mW/cm² for the commercial Ag/C catalyst (Fig. 2c). Our results indicate that ORR occurring on Ag in high pH media is a structure-sensitive reaction, and smaller particle size could facilitate a higher activity. A manuscript entitled "Carbon Supported Ag Nanoparticles with Different Particle Size as Cathode Catalysts for Anion Exchange Membrane Direct Glycerol Fuel Cells" is under preparation.

3) We investigated this solution-phase synthesis method to preparation of Au nanoparticles for electrocatalytic reactions for fuel cells.  Gold (Au) has demonstrated unique properties in catalysis. It has been found that Au is able to stabilize Pt for oxygen reduction reaction in acid electrolyte (Stabilization of platinum oxygen-reduction electrocatalysts using gold clusters, Zhang, J.L., et al., Science, 2007, 315, 220-222). However, synthesis of carbon supported Au nanoparticles with small and uniform size remain a challenge. By optimizing the solution-phase reduction method, we successfully prepared ultra-small Au nanoparticles with size of 2-6 nm centered at 3.5 nm with high loading (30 wt%), and we also deposited Au-NP on 1-D carbon nanotubes (Au/CNT)..  It is still a challenging task to prepare 1D PtAuFe, PdAuFe nanowires through a similar route as the preparation of PtFe and PdFe nanowires using quick injection of Fe(CO)₅. Future work will explore approaches to deposition of Au-NP on PtFe/PdFe-NW to further stabilize the 1D nanowires for ORR in fuel cells.

We published 5 papers and gave10 oral presentations in ACS, AIChE meetings that acknowledged this grant:

1. Zhiyong Zhang, Le Xin, Wenzhen Li, Supported gold nanoparticles as anode catalyst for anion exchange membrane ¨C direct glycerol fuel cell (AEM-DGFC), International Journal of Hydrogen Energy, 2012, 37, 9393-9401.

2. Le Xin, Zhiyong Zhang, Zhichao Wang, Ji Qi, Wenzhen Li, Simultaneous generation of mesoxalic acid and electricity from glycerol on Au anode catalyst in anion exchange membrane fuel cells, ChemCatChem, 2012, 4, 1105-1114.

3. Zhiyong Zhang, Le Xin, Wenzhen Li, Electrocatalytic oxidation of glycerol on Pt/C in anion exchange membrane fuel cell: Cogeneration of electricity and valuable chemicals, Applied Catalysis B Environmental, 2012, 119, 40-48.

4. Zhiyong Zhang, Le Xin, Ji Qi, Zhichao Wang, Wenzhen Li, Selective electro-oxidation of glycerol to glycolate on carbon nanotube supported gold catalyst, Green Chemistry, 2012,14, 2150-2152.

5. Le Xin, Zhiyong Zhang, Ji Qi, David Chadderdon, Wenzhen Li, Electrocatalytic oxidation of ethylene glycol (EG) on supported Pt and Au catalysts: Reaction pathway investigation in three-electrode cell and fuel cell reactors, Applied Catalysis B Environmental, 2012, 125, 85-94.

Partially supported by this ACS-PRF grant, Zhiyong Zhang defended his Ph.D. dissertation of "Advanced Nanostructured Electrocatalysts for Electricity Generation and Biorenewable Alcohol Conversion' in August 2012. He has published 7 first-authored and 2 second-authored papers. He is currently a postdoc researcher in the Oak Ridge National Lab at DOE.