One of the biggest obstacles to widespread adoption of clean hydrogen fuel cell-powered cars and trucks is the high price and rarity of the platinum and platinum-family catalysts that the stacks need to make electricity. The costly metals are critical to carrying out the crucial oxygen-reduction reaction at the fuel-cell cathode, the place where the water "exhaust" forms when oxygen molecules from the air combine with protons filtering through the polymer membrane and electrons arriving via the external circuit.
Recently a team of chemists and materials scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (LBNL) and Argonne National Laboratory (ANL) developed novel three-dimensional "nanoframe" substances that have demonstrated much better catalytic activity (the tendency to facilitate reactions) for the fuel cell’s key cathodic oxidation reaction. The order-of-magnitude jump in performance it exhibited compared to the state-of-the-art catalysts—platinum nanoparticles deposited onto carbon substrates—is reportedly almost unprecedented. The activity of the catalyst also exceeds by an order of magnitude the 2017 target set for the technology by DOE planners.
The bimetallic catalysts of platinum and nickel feature hollow, high-activity, high-surface-area faces both inside and out, which makes them significantly more efficient and potentially far less expensive than today’s counterparts, according to the team’s recent paper in
The catalysts also work in water-alkali electrolyzers, which split water into oxygen and hydrogen and could be a potential source of hydrogen fuel depending on the cost of the electrical power to run them. Alkaline water electrolyzers have a pair of membrane-separated electrodes immersed in a liquid electrolyte made highly alkaline with caustic potash, or potassium hydroxide. The researchers tested the new electrocatalysts in the crucial cathodic hydrogen-evolution reaction, and activity was enhanced by almost one order of magnitude compared to platinum-carbon.
In recent years, intensive worldwide research efforts have focused on creating high-performance electrocatalysts with the minimum costly precious metal content by alloying platinum with cheaper materials and hoping to maintain activity levels. Another promising strategy for improving catalysts involves the development of caged, hollow, or porous materials that contain fewer buried and thus nonfunctional precious-metal atoms. These uncommon geometries can also provide an easier route for tailoring physical and chemical properties as needed, said Peidong Yang, a noted chemist at LBNL and the University of California at Berkeley, who led the work.
New bimetallic nanocages
“We started research on nanoparticle catalysis for both solution and gas-phase reactions about 10 years ago,” Yang began. “Initially, we focused on single elements like platinum, analyzing the size- and shape-dependent catalytic properties, but the focus in time morphed to bimetallic catalysts, such as platinum-nickel (PtNi) and platinum-copper. Then three or four years ago, something unexpected happened when two of my post-docs placed a platinum-nickel sample into a solvent: two weeks later they found the bimetallic nanoparticles had evolved into new shapes.”
“It was an accidental discovery for us,” Yang continued, “but once we saw the 3-D ‘nanoframe’ structures that had emerged were covered with catalytic sites, we knew that we had something interesting.”
The Berkeley researchers consulted the existing literature and found that a group at ANL led by chemist Vojislav Stamenkovic had already done considerable work on catalytic activity of bulk single-crystal substrates. “Based on research by Voya’s team, it was fairly obvious that new bimetallic material could be an amazing electrocatalyst. So we contacted him and began a collaboration” to test it, Yang said.
Opening a hole
It turned out that in solution, the starting material, solid crystalline PtNi3 polyhedra, transformed naturally into PtNi intermediate species, and then via interior erosion into Pt3Ni nanoframes with surfaces that provide ready access to oxygen molecules. The edges of the PtNi3 polyhedra, which are platinum-rich, are maintained in the final Pt3Ni nanoframes, the paper stated. Whereas the original polyhedron consisted of three nickel atoms for every platinum one, the nanoframes had, on average, the reverse ratio.
“Polyhedra have been the usual nanostructures used for decades for catalysis research,” Stamenkovic said in a statement. “Our research shows that other options may be available. With frames, we completely opened the structure and got rid of the buried nonfunctional bulk atoms. There are still a substantial number of active sites on the nanoframes that can be approached from any direction.”
The solvent, with its dissolved oxygen, causes a natural interior erosion to occur that yields a hollowtwelve-sided, or dodecahedron, nanoframe. Running this reaction at a higher temperature shortened 2 weeks to 12 h.
After producing the basic material, the scientists wanted to ensure its stability in the harsh electrochemical environment of the fuel-cell stack, so they created a “second skin” of platinum atoms over the nanoframe, boosting the catalyst’s durability, to stay active. Annealing, or heat-treating, the nanoframes in argon gas creates a platinum skin on the nanoframe surfaces.
“We suspect that the oxygen pulls the nickel nanoparticles out onto the nanoframes,” Yang said. “Then annealing in argon pulls the platinum out onto the surfaces.”
Both the interior and exterior catalytic surfaces of this open Pt3Ni framework structure, the paper stated, are composed of a nano-segregated platinum structure that exhibits enhanced oxygen-reduction reaction activity. The nanoframe catalysts achieved more than 36- and 22-fold enhancements in mass and specific activities, respectively, for this reaction in comparison with those of the best platinum-carbon catalysts during prolonged exposure to reaction conditions. The novel material has not been tested in a real fuel-cell stack as yet, Yang noted.
“In contrast to other synthesis procedures for hollow nanostructures that involve corrosion induced by harsh oxidizing agents or applied potential, our method proceeds spontaneously in air,” Yang said. “The open structure of our platinum/nickel nanoframes addresses some of the major design criteria for advanced nanoscale electrocatalysts, including high surface-to-volume ratio, 3-D surface molecular accessibility, and significantly reduced precious metal utilization.”
By greatly reducing the amount of platinum needed for oxygen reduction and hydrogen evolution reactions, the new class of nanocatalysts could lead to the design of next-generation catalysts with greatly reduced cost but significantly enhanced activities tuned as needed, the researchers said.
Synthesis of the nanoframes can be readily scaled up to produce high-performance electrocatalysts at the gram-scale. Importantly, the method can be generalized toward the design of other multimetallic nanoframe systems. The process can be readily applied to other alloy systems such as platinum-cobalt, platinum-copper, platinum-rhenium-nickel and platinum-lead-nickel.
“We’re pretty happy with this development,” Yang said. “It would entail a significant reduction in use of platinum. And we are quite optimistic about its commercial viability.”