Efficient and stable PtPb/Pt core/shell nanoplate catalysts for ORR in fuel cells; new way of introducing tensile strain
Scientists from the US Department of Energy’s (DOE) Brookhaven National Laboratory; California State University–Northridge; Soochow University; Peking University; and Shanghai Institute of Applied Physics have developed new catalysts for the oxygen reduction reaction (ORR) in fuel cells that can undergo 50,000 voltage cycles with a negligible decay in their catalytic activity and no apparent changes in their structure or elemental composition.
In a paper published in Science, the team reports on a class of platinum-lead/platinum (PtPb/Pt) core/shell nanoplate catalysts that exhibit large biaxial strains. (Modifying the electronic structure of catalysts can improve their performance; lattice strain (either compressive or tensile) modifies the distances between surface atoms and hence modifies catalytic activity. Earlier post.) The stable Pt (110) facets of the nanoplates have high ORR specific and mass activities that reach 7.8 milliampere (mA) per cm2 and 4.3 ampere per milligram of platinum at 0.9 volts versus the reversible hydrogen electrode (RHE), respectively.
We report on a class of highly uniform PtPb/Pt core/shell nanoplates with large biaxial tensile strain for boosting ORR. Rather than use compressive strain to optimize the oxygen adsorption energy, we show that at a very high tensile strain, the Pt (110) plane located outside the nanoplates can exhibit the superior electrocatalytic activity for ORR. By integrating the strong tensile strain of PtPb to Pt (110) facet along  direction with thin two-dimensional (2D) morphology and intermetallic phase (ensuring high chemical stability), the as-prepared nanoplate can deliver specific and mass activities for ORR that are 33.9 and 26.9 times greater than those of the commercial Pt/C catalyst.
The PtPb nanoplates show negligible activity decay and no obvious structure and composition changes after a 50,000-cycle electrochemical accelerated durability test (ADT). They are also extremely active and stable for anodic oxidation reactions, largely outperforming those based on the PtPb NPs and the commercial Pt/C in both methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR).—Bu et al.
Hydrogen enters a fuel cell at the anode, which is coated with a platinum catalyst. When the hydrogen molecules come into contact with the platinum, they split into hydrogen ions and free electrons in a reaction called hydrogen oxidation. The hydrogen ions travel through the electrolyte to the cathode; the electrons travel through an external circuit to generate electricity before arriving at the cathode. In the oxygen reduction reaction, oxygen molecules combine with the electrons that have completed their circuit and the hydrogen ions to produce water.
The rate of ORR is very slow because molecular oxygen has a very strong double bond that requires significant energy to be broken. As a result, the fuel cell suffers from a reduction in voltage (overpotential) that limits its efficiency.
To date, the most successful catalysts for boosting the activity of the oxygen reduction reaction (ORR)—a very slow reaction that significantly limits fuel cell efficiency—have been of the Pt-based core-shell structure. However, these catalysts typically have a thin and incomplete shell (owing to their difficult synthesis), which over time allows the acid from the fuel cell environment to leach into the core and react with the other metals inside, resulting in poor long-term stability and a short catalyst lifetime.
The goal is to make the ORR as fast as possible with catalysts that have the least amount of platinum and the most stable operation over time. Our PtPb/Pt catalysts show high ORR activity and stability—two parameters that are key to enabling a hydrogen economy—placing them among the most efficient and stable bimetallic catalysts reported for ORR.—co-corresponding author Dong Su, a scientist at Brookhaven Lab’s Center for Functional Nanomaterials (CFN)
In previous studies, scientists have shown that ORR activity can be optimally enhanced in core-shell catalysts by compressing the Pt atoms on one specific lattice surface plane called Pt(111). This compressive strain is induced by adding metals smaller in size than Pt, such as nickel, to the shell’s core, and has the effect of weakening the binding of oxygen to the Pt surface, where the catalytic reaction takes place.
The ideal ORR catalyst needs to help break bonds (between oxygen molecules) and form bonds (between oxygen and hydrogen), so oxygen can’t be too strongly or too weakly bound to the platinum surface. Scientists have focused their research on the compressively strained Pt(111) surfaces, in which Pt atoms are squeezed across the surface, because the oxygen binding energy is optimized. In general, scientists thought that tensile strain on the same surface plane would result in overly strong binding of oxygen and thus hinder the ORR reaction.—Dong Su
Su and his collaborators showed that introducing a large tensile strain along one direction of a different surface plane, Pt(110), could also improve ORR catalytic activity. They added Pb (which is larger than Pt) to the core of the Pt shell, causing the Pt atoms to stretch across the surface.
After the research group led by Xiaoqing Huang, corresponding author from Soochow University, synthesized the nanoplates, Su characterized their structure and elemental composition at the CFN. Using electron diffraction patterns and images from high-resolution scanning transmission electron microscopy (STEM), both of which reveal the relative positions of atoms, he confirmed the core-shell structure and the composition and sequence of the atoms. To verify that the core contained Pt and Pb and that the shell contained Pt, he measured the change in energy of the electrons after they interacted with the nanoplates—a technique called electron energy-loss spectroscopy.
With this information, the team distinguished how the nanoplates formed with the individual Pt and Pb atoms. To their surprise, the surface planes were not Pt(111) but Pt(110), and these Pt(110) planes were under biaxial strain—compressive strain in one direction and tensile strain in the other—originating from the PtPb core.
|Schematic diagram of the oxygen reduction reaction (reduction of O2 into H2O) on the Pt(110) surface of the PtPb/Pt nanoplates, with purple representing Pt atoms and orange representing Pb atoms. Source: BNL. Click to enlarge.|
Microscopy and synchrotron characterization techniques revealed that the structure and elemental composition of the nanoplates did not change following the durability testing.
Compared to commercial Pt-on-carbon (Pt/C) catalysts, the team’s PtPb/Pt nanoplates have one of the highest ORR activities to date, taking the amount of Pt used into account, and excellent durability. The team’s nanoplates also showed high electrocatalytic activity and stability in oxidation reactions of methanol and ethanol.
We believe the relatively thick and complete Pt layers play an important role in protecting the core.—Dong Su
To understand how the high ORR activity originates in the nanoplates, the scientists calculated the binding energy between oxygen atoms and Pt atoms on the surface. Their calculations confirmed the tensile strain on the Pt(110) surface was responsible for the enhanced ORR activity.
This work opens up a new way to introduce large tensile strain on the stable Pt(110) plane to achieve very high activity for oxygen reduction catalysis. We believe that our approach will inspire efforts to design new nanostructured catalysts with large tensile strain for more efficient catalysis.—co-corresponding author Shaojun Guo of Peking University
Eventually, the laboratory-level electrocatalysts will need to be tested in a larger fuel cell system, where real-world variables—such as pollutants that could impact surface reactivity—can be introduced.
In addition to the electron microscopy work performed at CFN, supported by DOE’s Office of Science, this work involved theoretical calculations by Gang Lu’s group at California State University–Northridge and synchrotron characterization by Jing-Yuan Ma at Shanghai Synchrotron Radiation Facility. This work was further supported by the National Basic Research Program of China, the National Natural Science Foundation of China, the Ministry of Science and Technology of the People’s Republic of China, Soochow University, Peking University, the Young Thousand Talents Program, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the U.S. Army Research Office.
Lingzheng Bu, Nan Zhang, Shaojun Guo, Xu Zhang, Jing Li, Jianlin Yao, Tao Wu, Gang Lu, Jing-Yuan Ma, Dong Su, Xiaoqing Huang (2016) “Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis” Science Vol. 354, Issue 6318, pp. 1410-1414 doi: 10.1126/science.aah6133