New high-activity, low-cost nickel-based catalyst for fuel cells exhibits performance similar to Pt; hydroxide exchange membrane fuel cells
Researchers at the University of Delaware, with a colleague at the Beijing University of Chemical Technology, have developed a composite catalyst—nickel nanoparticles supported on nitrogen-doped carbon nanotubes—that exhibits hydrogen oxidation activity in alkaline electrolyte similar to platinum-group metals. An open access paper on their work is published in the journal Nature Communications.
Although nitrogen-doped carbon nanotubes are a very poor hydrogen oxidation catalyst, as a support, they increase the catalytic performance of nickel nanoparticles by a factor of 33 (mass activity) or 21 (exchange current density) relative to unsupported nickel nanoparticles, the researchers reported. Owing to its high activity and low cost, the catalyst shows significant potential for use in low-cost, high-performance fuel cells, the team suggested.
Polymer electrolyte membrane (PEM) fuel cells are based on two half-cell reactions: hydrogen oxidation reaction (HOR) at the anode and oxygen reduction reaction (ORR) at the cathode. Pt is the most active catalyst for both HOR and ORR; the high price of the metal (~$50 g−1) has hindered fuel cell commercialization. This, in turn, has compelled engineers to (1) work to reduce the platinum loading in the membrane assemblies and (2) find alternate, lower-cost catalysts that offer comparable performance to platinum.
Although the various efforts have managed to reduce the total content of platinum-group metals (PGMs) in the state-of-the-art proton exchange membrane fuel cell (PEMFC) stacks, more than 0.137 g Pt kW−1 is still needed, the University of Delaware team said.
One promising approach to reduce the cost of fuel cells is to switch the operating environment from an acidic to a basic one (that is, a hydroxide exchange membrane fuel cell, HEMFC), thus opening up the possibility of using PGM-free catalysts and other cheaper components. For the cathode of the HEMFC, some PGM-free and metal-free ORR catalysts have been developed that show comparable activity to Pt in alkaline media. However, for the anode side, only a few PGMs (for example, Pt, Ir and Pd) show adequate activity. The HOR catalyzed by Pt is very fast in acidic conditions so that a very low loading of the Pt catalyst could be used relative to the cathode side in PEMFCs. However, the HOR activities of PGMs are ~100 times slower in alkaline solutions. As a result, a much higher loading of the HOR catalyst is required (0.4 mg Pt cm−2 in a HEMFC compared with 0.03 mg Pt cm−2 in a PEMFC) to achieve similar performance. Thus, it is highly desirable to develop PGM-free anode catalysts for the HOR in alkaline electrolyte.
Unlike its reverse reaction (hydrogen evolution reaction, HER), only a few PGM-free HOR catalysts have been reported. One possibility is to use Raney Ni as the HOR catalyst in liquid alkaline fuel cells. However, it is functional only under very high alkalinity (6 M KOH) while the activity remains low. It is not catalytically active for a HEMFC, which can be mimicked as 0.1–1 M KOH. Efforts have been made to improve the HOR activity of the Ni-based catalyst in the last decade. Ni alloys, such as NiMo and NiTi, have been shown to enhance the HOR activity. Our recent work has also shown that electrochemically deposited NiCoMo on an Au substrate has a high HOR activity. Zhuang and co-workers decorated Ni particles with CrOx to weaken the Ni–O bond and stabilize the Ni catalysts. A HEMFC incorporating this PGM-free catalyst has been fabricated, and it exhibits a peak power density of 50 mW cm−2. Although the power density is still low (compared with the peak power density of more than 1,000 mW cm−2 for PEMFCs), it demonstrates the possibility to fabricate low-cost PGM-free fuel cells. However, their activities are still incomparable with PGM-based catalysts.—Zhuang et al.
In the Nature Communications study, the team synthesized Ni nanoparticles supported on N-doped carbon nanotubes (Ni/N-CNT) using a wet chemical method. The nanotubes are not only the support for the Ni nanoparticles, but also a promoter for the catalytic activity.
Using density functional theory (DFT) calculations to understand the interaction between the Ni nanoparticle and the N-CNT support, the team found that, when nitrogen dopants are present at the edge of the nanoparticle, the Ni nanoparticle is stabilized on the support and locally activated for the HOR because of modulation of the Ni d-orbitals.
Owing to its high activity and low cost, Ni/N-CNT has great potential to be used as the anode in HEMFCs, thereby finally bringing to fruition a high-performance and low-cost PGM-free HEMFC.—Zhuang et al.
This new hydroxide exchange membrane fuel cell can offer high performance at an unprecedented low cost. Our real hope is that we can put hydroxide exchange membrane fuel cells into cars and make them truly affordable—maybe $23,000 for a Toyota Mirai. Once the cars themselves are more affordable, that will drive development of the infrastructure to support the hydrogen economy.—Yushan Yan, corresponding author
The experimental work was supported by the ARPA-E program of the US Department of Energy under Award Number DE-AR0000009.
The computational work was financially supported by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001004.
Stephen Giles was supported by a fellowship from the University of Delaware Energy Institute.
The research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
Zhongbin Zhuang, Stephen A. Giles, Jie Zheng, Glen R. Jenness, Stavros Caratzoulas, Dionisios G. Vlachos & Yushan Yan (2016) “Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte” Nature Communications 7, Article number: 10141 doi: 10.1038/ncomms10141