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

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.Polymer 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, Lefèvre and coworkers identified the importance of micropores as the hosts of the catalytic sites formed during pyrolysis (24–26) and found that the ORR catalytic activity can be significantly enhanced by infiltrating the N-coordinated iron complex within the micropores (pore diameter <2 nm) of the carbon support (19). More recently, several new synthetic approaches were also explored to produce high catalytic active site density decorated within the micropores using rationally designed zeolitic imidazolate frameworks (ZIFs) and porous organic polymers (POPs) (14, 20–22, 27).Whereas the micropore is critically important in hosting active site for ORR, the catalyst should also contain a sufficient amount of macropores (pore size >50 nm) to ensure the effective mass transfer of both reactant (O₂) and product (H₂O) to and from the active sites with minimal resistance throughout the entire electrode layer. Furthermore, efficient charge transfer of H⁺ and e⁻ to the active sites must also be established to fully use the catalyst in different electrode depths and to reduce the cell impedance. The improvements of mass and charge transports directly result in the reduction of the associated overpotentials, therefore lowering the efficiency penalties. These multifaceted requirements for catalyst morphology render the conventional carbon support no longer adequate for the ideal NPMC design. In a conventional amorphous carbon support, the micropores reside inside of primary carbon particles with the dimensions of a few tenths of a nanometer. These carbon particles agglomerate together by van der Waals force to a large cluster forming the mesopores (pore diameter 2∼50 nm) in the space between them. The macropores are generated by the voids through further stacking of these clusters (Fig. S1A). The mesopores, while serving as the secondary passage between the gas phase to catalytic site in micropores, create additional tortuosity, thus mass-transfer resistance within each cluster. Mesopores also have much higher volume-to-surface area ratios than the micropores. Therefore, they add substantial volume to the catalyst and reduce the electrode volumetric current density. Furthermore, the oxidative corrosion during fuel cell operation could reduce the primary carbon particle size, causing the loss of electric contact between the particles and the increase of the overall cell impedance. In an ideal catalyst structure, the active sites should be densely populated inside of the micropores. The reactant/product should transfer directly to and from these active sites through macropores with minimal transport resistance. The micropores should also be connected through a continuous conductive matrix robust against the corrosion-induced conductivity loss. In such a design, the mesopores are no longer necessary.Open in a separate windowFig. S1.Comparison on the morphologies of (A) conventional carbon support and (B) nanonetwork catalyst.Herein, we present a rationally designed, interconnected porous nanonetwork catalyst (Fe/N/CF) prepared by electrospinning a polymer solution containing Tris-1,10-phenanthroline iron(II) perchlorate (TPI) and ZIFs, a subgroup of metal–organic frameworks (MOFs), followed by posttreatments (Fig. S2). The network structure facilitates the mass transfers through its macroporous voids between the interconnected nanofibers. More importantly, each nanofiber was predominantly microporous, containing uniformly and densely dispersed catalytic sites throughout the fiber (Fig. S1B). Such a catalyst delivered an unprecedented volumetric activity of 450 A⋅cm⁻³ extrapolated at 0.8 V_iR-free or measured volumetric current densities of 0.25, 3.3, and 60 A⋅cm⁻³ at 0.95, 0.9, and 0.8 V_iR-free, respectively, when tested in a single cell under the standard test condition established by the US Department of Energy (28).Open in a separate windowFig. S2.The schematic diagram of Fe/N/CF synthesis by electrospinning. The spun composite fibers contain uniformly mixed polymer, TPI salt, and ZIF-8 nanoparticles. The fibrous nanonetwork retains its morphology after pyrolysis, acid wash, and posttreatment.     

End of the bare metal stent era?

• Second‐generation drug eluting stents (2g‐DES) have lower rates of stent thrombosis (ST) than bare metal stents (BMS).
• Second‐generation DES have exceedingly low rates of very late ST, significantly improving on first‐generation drug eluting stents (1g‐DES) and similar to BMS.
• Emerging drug‐coated stent (DCS) technology appears superior to BMS in patients unable to tolerate prolonged dual antiplatelet therapy (DAPT).

     

Expandable metal stents in achalasia--is there a role?

OBJECTIVE: Achalasia is treated with pneumatic dilation or myotomy, and botulinum toxin injections are occasionally used. We review our community's experience with expandable metal stents in six patients who failed medical treatment or were poor surgical candidates. METHODS: Eight stents were placed in six patients between July 1995 and November 1997. Four patients had achalasia and two pseudoachalasia. Four patients underwent successive botulinum toxin injections. One patient only agreed to periodic Maloney dilatations or a stent. Pneumatic dilation was performed in one patient and considered high risk in the rest. All were poor surgical candidates. Three different stents were used: Gianturco Rosch Z stent, Wallstent I, and Wallstent II. RESULTS: One-month mortality and morbidity were 33% and 50%, respectively. Two patients were asymptomatic on a liquid diet for > or =6 months but required repeat endoscopy for recurrent dysphagia because of food bolus impaction and proximal stent migration in each. CONCLUSIONS: Expandable metal stents in achalasia or pseudoachalasia do not provide sustained symptom relief, and their use is associated with unacceptably high morbidity and mortality. We do not recommend the use of these devices in patients who have failed medical therapy or who are poor surgical candidates.     

Do airport metal detectors interfere with implantable pacemakers or cardioverter-defibrillators?

OBJECTIVES: The aim of this study was to determine whether airport metal detector gates (AMDGs) interfere with pacemakers (PMs) or implantable cardioverter-defibrillators (ICDs). BACKGROUND: It is currently unknown whether AMDGs interfere with implanted PMs or ICDs. METHODS: A total of 348 consecutive patients (200 PM and 148 ICD recipients) have been tested for the occurrence of electromagnetic interference (EMI) within the electromagnetic field of a worldwide-used airport metal detector. RESULTS: No interference, such as pacing or sensing abnormalities, was observed in any of the 200 PM and 148 ICD patients; also no reprogramming occurred. CONCLUSIONS: In vivo testing of PM and ICD systems showed no EMI with a standard AMDG. Clinically relevant interactions with implanted PMs or ICDs seem unlikely.     

Palliation of malignant esophageal Dysphagia: would you like plastic or metal?

Advanced esophageal carcinoma has a rather dismal prognosis with dysphagia to solids and liquids as a common symptom. Self-expanding metal stents provide immediate and durable relief of dysphagia. Recently, self-expanding plastic stents have been developed for refractory benign esophageal strictures but may have applications in malignant strictures as well. In this issue of The American Journal of Gastroenterology , a multicenter, prospective, randomized, comparative trial evaluates the safety and efficacy of self-expanding metal versus plastic stents for palliation of dysphagia due to esophageal cancer. While the stents compared equivalently for symptom relief, complications were observed more commonly in the plastic stent group.     

Distally migrated esophageal self-expanding metal stents: wait and see or remove?

BACKGROUND: Extraction of a migrated esophageal stent may be extremely difficult with a substantial risk of complications including esophageal perforation and hemorrhage. METHODS: Retrospectively 242 patients were evaluated who underwent implantation of self-expanding metal stents (SEMS) and 13 (5.4%) were identified with distal stent migration. In all cases of stent dislocation into the stomach, extraction of the stent was not attempted and a new stent was inserted. RESULTS: Twelve patients had dysphagia. One patient underwent surgery because of stent impaction in the colon, 3 had unrecognized passage of the stent per rectum, and 9 had evidence of the stent into the stomach. Further severe complications were not observed in any patient and all stents remained into the stomach. CONCLUSION: Complications arising from migrated esophageal stents are uncommon. Further studies are warranted to determine which patients with migrated SEMS warrant stent retrieval.