Although platinum (Pt) has been long known as the most efficient ORR catalyst, its high cost and scarcity have hampered the large-scale commercialization of fuel cell and metal–air battery technologies. In particular, commercialization of the fuel cell technology has been further limited by the poor operation durability, fuel crossover effect, and carbon monoxide (CO) poisoning intrinsically associated with Pt catalysts. Consequently, non-precious carbon or metal oxide catalysts have been explored as alternative electrocatalysts for ORR.

… However, the low conductivity of most carbon-based structures and poor interfacial engineering of heterostructures still greatly impede the transport of electrons and electrolyte ions during the electrochemical processes, limiting their overall oxygen reduction performance. 1D graphitic structures, which provide the necessary charge conductivity and favored 3D conductive networks when assembled as fuel cell electrodes, have been considered as a promising solution to the above challenges.

… Herein, we describe a two-step electrospinning–annealing method to produce porous and electrically conductive 1D N-doped graphitic carbon fibrous networks embedded with catalytic metal (M, i.e., Co, Ni, Fe) or metal oxide (MO_x) nano- particles.

—Tang et al.

The research was led by David Kisailus, the Winston Chung Endowed Professor in Energy Innovation in UCR’s Marlan and Rosemary Bourns College of Engineering. The researchers formed 1D nanostructures by electrospinning polyacrylonitrile (PAN) fibers containing transition metal (Co, Ni, and Fe) salts and annealing in a reducing atmosphere to yield metal nanoparticles/nanoclusters that catalyze graphitization of the surrounding polymer matrix at greatly reduced temperatures (≈800 °C).

Subsequent annealing to oxidize the metal nanoparticles creates an interconnected graphite–metal oxide framework with large pore channels, considerable number of active sites, and high specific surface area.

Synthesis strategy of metal NPs@graphitic carbon fibers by sequential steps: a) Preparation of electrospun solution of metal salts/polymer/ solvent; b) electrospinning to obtain metal ions@polymer fibers; c) controlled carbonization and graphitization to obtain metal NPs@graphitic carbon fibers by annealing in inert atmospheres. Tang et al. Click to enlarge.

Kisailus and his team, collaborating with scientists at Stanford University, determined that the new materials performed as well as the industry standard platinum-carbon systems, but at a fraction of the cost.

The key to the high performance of the materials we created is the combination of the chemistry and fiber processing conditions. The remarkable electrochemical properties were primarily attributed to the synergistic effects obtained from the engineering of the metal oxide with exposed active sites and the 3D hierarchical porous graphitic structure.

—David Kisailus

Kisailus said an added benefit of the catalytic nanocomposite was that its graphitic fiber nature provided additional strength and durability, which would enable it to serve as both a fuel cell catalyst and potentially as a structural component.

An important challenge in making high-performance vehicles is reducing weight, both from the body of the vehicle as well as extra weight from the battery or fuel cell, without affecting safety or performance. The material we created may enable automakers to turn structural components, such as the hood or the chassis, into functional elements that help power cars.

—David Kisailus

Resources

• H. Tang, W. Chen, J. Wang, T. Dugger, L. Cruz, D. Kisailus (2018) “Electrocatalytic N-Doped Graphitic Nanofiber - Metal/Metal Oxide Nanoparticle Composites” Small doi: 10.1002/smll.201703459