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Breakthrough Research on Platinum–Nickel Alloys Print

 

Two out of three of the kinetic barriers to the practical use of polymer electrolyte membrane (PEM) hydrogen fuel cells in automobiles have been breached: the impractically high amount of extra energy needed for the oxidation reduction reaction (ORR) on the catalyst and the loss of catalytic surface areas available for ORR. Using a combination of probes and calculations, a group of scientists has demonstrated that the Pt3Ni(111) alloy is ten times more active for ORR than the corresponding Pt(111) surface and ninety times more active than the current state-of-the-art Pt/C catalysts used in existing PEM fuel cells. This new variation of the platinum–nickel alloy is the most active oxygen-reducing catalyst ever reported.

For practical purposes, automobiles using PEM fuel cells are still test vehicles. Historically, there have been a number of obstacles to the use of such fuel cells in vehicles. These obstacles have centered around the kinetic limitations on the oxygen reduction reaction (ORR). First, the substantial overpotential, or extra energy needed for the ORR at practical operating current densities, reduces the thermal efficiency to well below the thermodynamic limits, typically to about 43% at 0.7 V (versus a theoretical thermal efficiency of 83% at 1.23 V). Second, the dissolution and/or loss of Pt surface area in the cathode, due to degradation by unwanted byproducts such as hydroxides, must be greatly reduced. Third, an approximately fivefold reduction in the amount of platinum in current PEM fuel cell stacks is needed to meet the cost requirements of large-scale automotive applications.

PEM Fuel Cells: A Key to Reducing CO2 Emissions

According to the U.S. Environmental Protection Agency, the number of miles driven per year per passenger vehicle in the United States is 12,000. Driving ten thousand miles on an internal combustion engine releases one ton of carbon dioxide into the atmosphere. According to the Intergovernmental Panel on Climate Change (IPCC), carbon dioxide is "the most important anthropogenic greenhouse gas." An IPCC report issued at the beginning of February 2007 states that "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." "Very likely" is the IPCC's term for "at least 90 percent."

By converting chemical energy into electrical energy without combustion, fuel cells represent perhaps the most efficient and clean technology for generating electricity. This is especially true for fuel cells designed to directly run off hydrogen, producing only water as a byproduct. The hydrogen-powered fuel cells most talked about for use in vehicles are PEM fuel cells (polymer electrolyte membrane fuel cells, also known as proton exchange membrane fuel cells) because they can deliver high power in a relatively small, lightweight device. Unlike batteries, PEM fuel cells do not require recharging, but rely on a supply of hydrogen and access to oxygen from the atmosphere. The recent research on the use of platinum–nickel alloys as catalysts for PEM fuel cell cathodes is a giant step toward finding a practical way to reduce the quantity of CO2 emissions.

TOP. Pt(111). OH– bonds tightly to platinum surface atoms, leaving less room for O2 to adsorb onto Pt active sites. Since hydroxyde blocking species do not have an active role in reduction of oxygen molecules, their presence substantially hinders the rate of cathodic reaction. BOTTOM. Pt3Ni(111). With Ni in the subsurface layers, the topmost Pt atoms (Pt-skin) have a modified electronic structure, which alters different adsorption properties of Pt. Consequently, interaction between OH– ions and Pt-skin is weaker compared to the pure Pt catalysts, and surface is less covered by blocking species, leaving more Pt sites active for adsorption of O2. The overall effect generates an increase in specific activity for cathodic reaction: 10 times more active than the Pt(111) surface and 90 times more active than state-of-the-art Pt/C catalysts currently used in fuel cells.

Previous studies led to incremental improvements in catalyst performance, but large increases have been elusive. However, thanks to researchers from Berkeley Lab, Argonne National Laboratory, the University of Liverpool, and the University of South Carolina, the first and second obstacles to an efficient PEM fuel cell have been surmounted.

A combination of in situ and ex situ surface-sensitive probes and density functional theory (DFT) calculations was used to study ORR on Pt3Ni(hkl) single-crystal surfaces, identify which surface properties govern the variations in reactivity of PtNi catalysts, and determine how surface structures, surface segregation, and intermetallic bonding affect the ORR kinetics. Techniques used include low-energy electron diffraction spectroscopy(LEEDS), Auger electron spectroscopy (AES), low-energy ion scattering (LEIS), and synchrotron-based high-resolution ultraviolet photoemission spectroscopy (UPS). UPS data were obtained using ALS Beamline 9.3.2.

 

LEEDS surface characterization of the Pt3Ni(111) single crystals in ultrahigh vacuum and electrochemical environments. The green dots in this LEEDS pattern (left) for a single crystal of Pt3Ni(111) reveal a tightly packed arrangement of surface atoms that repels platinum-grabbing hydroxide ions and boosts catalytic performance.

The researchers worked with three single crystals of Pt3Ni alloy: 100, 110, and 111. All three crystals, when compared with their pure platinum counterparts, showed improvement in oxidation reduction, but the Pt3Ni(111) alloy showed the most significant improvement.

 

Influence of surface morphology and electronic surface properties on the kinetics of ORR. Rotating ring disk electrode measurements for ORR in HClO4 (0.1 M) at 333 K with 1600 revolutions per minute on Pt3Ni(hkl) surfaces as compared to the corresponding Pt(hkl) surfaces are shown. A horizontal dashed gray line marks specific activity of polycrystalline Pt. Specific activity given as a kinetic current density ik, measured at 0.9 V vs. RHE (reversible hydrogen electrode). Values of d-band center position obtained from UPS spectra are listed for each surface morphology and compared between corresponding Pt3Ni(hkl) and Pt(hkl) surfaces.

The Pt3Ni(111) surface has an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. Under operating conditions relevant to fuel cells, its near-surface layer exhibits a highly structured compositional oscillation in the outermost and third layers, which are Pt-rich, and in the second atomic layer, which is Ni-rich. This causes a weakening of the bonds between the Pt surface atoms and the OH– molecules. The weakening increases the number of active sites available for O2 adsorption. As the kinetics of O2 reduction are determined by the number of free Pt sites available for the adsorption of O2, the intrinsic catalytic activity at the fuel-cell-relevant potentials (E > 0.8 V) has been found to be ten times more active than the corresponding Pt(111) surface. The observed catalytic activity for the ORR on Pt3Ni(111) is the highest ever observed on cathode catalysts, including the Pt3Ni(100) and Pt3Ni(110) surfaces.

The next step is to engineer nanoparticle catalysts with electronic and morphological properties that mimic the surfaces of pure single crystals of Pt3Ni(111). In this way, the amount of platinum will be reduced without a loss in cell voltage, while also maintaining the maximum power density. This will drive down the cost, and the third obstacle to an efficient, affordable hydrogen fuel cell will disappear.

 

 

Research conducted by V.R. Stamenkovic (Berkeley Lab and Argonne National Laboratory), B. Fowler and C.A. Lucas (University of Liverpool, U.K.), B.S. Mun and P.N. Ross (Berkeley Lab), G. Wang (University of South Carolina), and N.M. Markovi (Argonne National Laboratory).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); General Motors Corp.; and the U.K. Engineering and Physical Sciences Research Council. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences (BES).

Publication about this research: V.R. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovi, "Improved oxygen reduction activity on Pt3Ni(111) via increased surface site and availability," Science 315, 493 (2007).