Highly efficient nonprecious metal catalyst prepared with metal–organic framework in a continuous carbon nanofibrous network

Abstract

Fuel cell vehicles, the only all-electric technology with a demonstrated >300 miles per fill travel range, use Pt as the electrode catalyst. The high price of Pt creates a major cost barrier for large-scale implementation of polymer electrolyte membrane fuel cells. Nonprecious metal catalysts (NPMCs) represent attractive low-cost alternatives. However, a significantly lower turnover frequency at the individual catalytic site renders the traditional carbon-supported NPMCs inadequate in reaching the desired performance afforded by Pt. Unconventional catalyst design aiming at maximizing the active site density at much improved mass and charge transports is essential for the next-generation NPMC. We report here a method of preparing highly efficient, nanofibrous NPMC for cathodic oxygen reduction reaction by electrospinning a polymer solution containing ferrous organometallics and zeolitic imidazolate framework followed by thermal activation. The catalyst offers a carbon nanonetwork architecture made of microporous nanofibers decorated by uniformly distributed high-density active sites. In a single-cell test, the membrane electrode containing such a catalyst delivered unprecedented volumetric activities of 3.3 A·cm −3 at 0.9 V or 450 A·cm −3 extrapolated at 0.8 V, representing the highest reported value in the literature. Improved fuel cell durability was also observed.nanofibrous | nonprecious metal catalyst | metal-organic framework | fuel cell | oxygen reduction P olymer electrolyte membrane fuel cells (PEMFCs) electrochemically convert the chemical energy of hydrogen and oxygen to electricity while producing water as a byproduct. They have significantly higher power and energy densities than the competing electrochemical devices, such as Li-ion batteries and supercapacitors, and represent the only all-electric technology with a demonstrated cruising range of over 300 miles between refueling (1). Current PEMFCs use platinum as a catalyst to promote an oxygen reduction reaction (ORR) at the cathode and a hydrogen oxidation reaction at the anode. The Pt use at the cathode is typically three to four times more than that at the anode to overcome the kinetically more sluggish ORR. Because platinum is expensive and there are limited worldwide reserves, technologies that could substantially reduce or replace its use have to be realized before widespread PEMFC commercialization. Nonprecious metal catalysts (NPMCs) represent one such technology.* Among NPMCs, transition metal (TM) and N-doped carbonaceous composites (TM/N/Cs) have demonstrated promising ORR catalytic activities in both acidic and alkaline media, whereas TM-free composites (N/Cs) showed activities primarily in an alkaline medium (2-17). The initial discovery of ORR catalytic activity by N-ligated cobalt was reported half a century ago (18). However, it was not until recently that breakthrough performances were achieved (19-23). New surface property and synthesis strategies for continuously improving catalytic activity were also identified. For example, Lef...

Single‐Atom to Single‐Atom Grafting of Pt1 onto FeN4 Center: Pt1@FeNC Multifunctional Electrocatalyst with Significantly Enhanced Properties

“…[5,6] The initial performances of FeNC based PEMFCs are superior to other NPMCs and even comparable with the commercial Pt/C(20%) nowadays. [7][8][9][10][11] Nonetheless, highly active FeNC catalysts usually suffer a quick current decay in acidic PEMFCs under a harsh operating condition, e.g., a constant voltage of 0.5 V with H 2 /O 2 . [12] It has been shown that MNC with highly graphitized carbon as catalyst supports, can exhibit decent fuel cell stabilities.…”

Direct Observation of Fe-N4 Species as Active Sites for the Electrocatalytic Oxygen Reduction

“…The maximum power density values of ~0.87 and 0.94 W cm -2 of as-prepared Fe-N-C catalyst, obtained at p O2 of 1.0 and 2.0 bar, respectively, were the highest ever achieved with PGM-free ORR catalysts operating on oxygen. 7 Under H 2 -air condition, the current density recorded in the kinetic range (> 0.75 V) of cathode operation, decreased with the increased thickness of catalyst-layer, illustrating the importance of catalyst-layer thickness on mass transport properties.…”

Rational Design and Synthesis of Low-Temperature Fuel Cell Electrocatalysts

“…One is to tune pore distribution using framework substrates, and the other is to tune surface wettability using hydrophobic additives. [162]. c Tafel plot for the kinetic activity of Fe/N/CF obtained from a single fuel cell test [162].…”

“…[162]. c Tafel plot for the kinetic activity of Fe/N/CF obtained from a single fuel cell test [162]. d Illustration of hydrophobic dimethyl silicon oil (DMS) just partially penetrating into the micropores with masstransport channels open, forming a robust triple-phase interface in micropores [163].…”

“…d Illustration of hydrophobic dimethyl silicon oil (DMS) just partially penetrating into the micropores with masstransport channels open, forming a robust triple-phase interface in micropores [163]. e Polarization curves and power density curves of DMFCs with Fe/N/C-DMS cathodes [163] An ideal electrode structure as proposed by Liu et al [162] should possess micropores with high densities of active sites and reactant/product should transfer directly to and from these active sites through macropores with minimal transport resistance. Based on this, Liu et al [162] prepared an interconnected porous nano-network Fe/N/C catalyst as shown in Fig.…”