Fe–P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction

It has been long thought that Fe–N–C structure, where Fe is bonded with an electronegative heteroatom N, plays a key role as nonprecious metal catalyst for oxygen reduction reaction (ORR) in fuel cells. However, electrocatalytic activity of Fe bonded with electropositive heteroatom P has never been considered for ORR. Herein we report the electrocatalytic activity for ORR of new Fe–P–C

Bicontinuous Spider Network Architecture of Free-Standing MnCoO<i>_X</i>@NCNF Anode for Li-Ion Battery

Herein, a smart strategy is proposed to tailor unique interwoven nanocable architecture consisting of MnCoO<i>_x</i> nanoparticles embedded in one-dimensional (1D) mesoporous N-doped carbon nanofibers (NCNFs) by using electrospinning technique. The as-prepared network mat of N-doped carbon nanofibers with embedded MnCoO<i>_x</i> nanoparticles (MnCoO<i>_x</i>@NCNFs) is tested as a current collector-free and binder-free flexible anode, which eliminates slurry preparation process during electrode fabrication in the Li-ion battery (LIB). The MnCoO<i>_x</i>@NCNFs possess versatile structural characteristics that can address simultaneously different issues such as poor conductivity, low cycling stability, volume variation, flexibility, and binder issue associate with the metal oxide. The free-standing mat electrode shows not only high initial discharge and charge capacities but also reversible discharge cycling stability of almost 80% retention up to 100 cycles and 60% retention up to 500 cycles at 1.0 A/g. Such high Li storage capacity and excellent cycling stability are attributed to the unique flexible and free-standing spider network-like architecture of the 1D MnCoO<i>_x</i>@NCNFs that provides the platform for bicontinuous electron/ion pathways for superior electrochemical performance. Along with excellent electrochemical performance, simple synthesis procedure of unique binder-free MnCoO<i>_x</i>@NCNFs can achieve cost-effective scalable mass production for practical use in a flexible mode, not merely in LIBs but also in a wide spectrum of energy storage fields

Topological Transformation of Thioether-Bridged Organosilicas into Nanostructured Functional Materials

The strong interest in nanostructured functional materials has motivated the scalable production of high quality mesoporous silicas and carbonaceous materials. Although many approaches have been explored for this goal, it is highly desired and still remains a challenge to develop a straightforward strategy for simple and cost-effective fabrication of nanostructured functional materials. Here we demonstrate a simple sol–gel preparation of bis­[3-(triethoxysilyl)­propyl]­tetrasulfide-based organosilica nanostructured materials and their topological transformations, through which porous spherical silica or carbon and hollow silica or carbon capsule are synthesized. As a representative application, the hollow carbon capsule is employed as a catalyst support for dispersion of high loading of Pt, which exhibits much higher catalytic activity toward oxygen reduction reaction than other porous carbon materials prepared in this work due to its larger surface area and mesoporous volume, particularly the unique architecture composed of a hollow macroporous core and a mesoporous shell, facilitating not only small size and good dispersion of Pt nanoparticles but also fast mass transport

Hierarchical Nanostructured Carbons with Meso–Macroporosity: Design, Characterization, and Applications

Nanostructured porous carbon materials have diverse applications including sorbents, catalyst supports for fuel cells, electrode materials for capacitors, and hydrogen storage systems. When these materials have hierarchical porosity, interconnected pores of different dimensions, their potential application is increased. Hierarchical nanostructured carbons (HNCs) that contain 3D-interconnected macroporous/mesoporous and mesoporous/microporous structures have enhanced properties compared with single-sized porous carbon materials, because they have improved mass transport through the macropores/mesopores and enhanced selectivity and increased specific surface area on the level of fine pore systems through mesopores/micropores. The HNCs with macro/mesoporosity are of particular interest because chemists can tailor specific applications through controllable synthesis of HNCs with designed nanostructures.An efficient and commonly used technique for creating HNCs is “nanocasting”, a technique that first involves the creation of a sacrificial silica template with hierarchical porous nanostructure and then the impregnation of the silica template with an appropriate carbon source. This is followed by carbonization of the filled carbon precursor, and subsequent removal of the silica template. The resulting HNC is an inverse replica of its parent hierarchical nanostructured silica (HNS). Through such nanocasting, scientists can create different HNC frameworks with tailored pore structures and narrow pore size distribution. Generally, HNSs with specific structure and 3D-interconnected porosity are needed to fabricate HNCs using the nanocasting strategy. However, how can we fabricate a HNS framework with tailored structure and hierarchical porosity of meso–macropores?This Account reports on our recent work in the development of novel HNCs and their interesting applications. We have explored a series of strategies to address the challenges in synthesis of HNSs and HNCs. Through careful control of experimental parameters, we found we could readily create new HNSs and HNCs with tailored structure and hierarchical porosity. In this Account, we describe the applications of the HNCs in low-temperature fuel cells, in Li ion batteries, in quantum-dot-sensitized solar cells (QDSSCs) and as hydrogen storage materials. Fuel cell and QDSSC polarization performance data reveal that both the ordered HNC and spherical HNC with uniform macro- and mesoporosity demonstrate superior catalyst support effect and considerably enhanced photovoltaic performance due to their incredible structural characteristics. For hydrogen and lithium storage applications, primary experimental results show that spherical HNCs with uniform macroporous core/mesoporous shell and ordered HNC are highly beneficial in terms of a high hydrogen (or Li) uptake, good rate capability and excellent cycling retainability. These data suggest that the innovative HNCs with tailored nanostructure may find promising applications in the rapid and efficient storage of hydrogen (or Li)

Phosphorus-Doped Ordered Mesoporous Carbons with Different Lengths as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media

Phosphorus-doped ordered mesoporous carbons (POMCs) with different lengths were synthesized using a metal-free nanocasting method of SBA-15 mesoporous silica with different sizes as template and triphenylphosphine and phenol as phosphorus and carbon sources, respectively. The resultant POMC with a small amount of P doping is demonstrated as a metal-free electrode with excellent electrocatalytic activity for oxygen reduction reaction (ORR), coupled with much enhanced stability and alcohol tolerance compared to those of platinum via four-electron pathway in alkaline medium. Interestingly, the POMC with short channel length is found to have superior electrochemical performances compared to those with longer sizes

Nitrogen-Doped Porous Carbons from Ionic Liquids@MOF: Remarkable Adsorbents for Both Aqueous and Nonaqueous Media

Porous carbons were prepared from a metal–organic framework (MOF, named ZIF-8), with or without modification, via high-temperature pyrolysis. Porous carbons with high nitrogen content were obtained from the calcination of MOF after introducing an ionic liquid (IL) (IL@MOF) via the ship-in-bottle method. The MOF-derived carbons (MDCs) and IL@MOF-derived carbons (IMDCs) were characterized using various techniques and used for liquid-phase adsorptions in both water and hydrocarbon to understand the possible applications in purification of water and fuel, respectively. Adsorptive performances for the removal of organic contaminants, atrazine (ATZ), diuron, and diclofenac, were remarkably enhanced with the modification/conversion of MOFs to MDC and IMDC. For example, in the case of ATZ adsorption, the maximum adsorption capacity of IMDC (<i>Q</i>₀ = 208 m²/g) was much higher than that of activated carbon (AC, <i>Q</i>₀ = 60 m²/g) and MDC (<i>Q</i>₀ = 168 m²/g) and was found to be the highest among the reported results so far. The results of adsorptive denitrogenation and desulfurization of fuel were similar to that of water purification. The IMDCs are very useful in the adsorptions since these new carbons showed remarkable performances in both the aqueous and nonaqueous phases. These results are very meaningful because hydrophobic and hydrophilic adsorbents are usually required for the adsorptions in the water and fuel phases, respectively. Moreover, a plausible mechanism, H-bonding, was also suggested to explain the remarkable performance of the IMDCs in the adsorptions. Therefore, the IMDCs derived from IL@MOF might have various applications, especially in adsorptions, based on high porosity, mesoporosity, doped nitrogen, and functional groups

Different Hierarchical Nanostructured Carbons as Counter Electrodes for CdS Quantum Dot Solar Cells

CdS quantum dot sensitized solar cells based on TiO₂ photoanode and nanostructured carbon as well as Pt as counter electrodes using iodide/triiodide and polysulfide electrolytes were fabricated to improve the efficiency and reduce the cost of solar cells. Compared with conventional Pt (η = 1.05%) and CMK-3 (η = 0.67%) counter electrodes, hollow core-mesoporous shell carbon (HCMSC) counter electrode using polysulfide electrolyte exhibits much larger incident photon to current conversion efficiency (IPCE = 27%), photocurrent density (<i>J</i>_sc = 4.31 mA.cm^–2) and power conversion efficiency (η = 1.08%), which is basically due to superb structural characters of HCMSC such as large specific surface area, high mesoporous volume, and 3D interconnected well-developed hierarchical porosity network, which facilitate fast mass transfer with less resistance and enable HCMSC to have highly enhanced catalytic activity toward the reduction of electrolyte shuttle

Synthesis of Water-Dispersible Single-Layer CoAl-Carbonate Layered Double Hydroxide

Despite extensive study on single-layer layered double hydroxides (SL-LDHs) with NO₃^– counterions, SL-LDHs with CO₃^2– counterions (CO₃^2– SL-LDHs) have never been prepared before. Herein, a CoAl-CO₃^2– SL-LDH which stays stable in water and powdery state is first synthesized using ethylene glycol as a reaction medium. The SL-LDH, with thickness of ∼0.85 nm, is composed of one Co­(Al)­O₆ layer sandwiched between two CO₃^2– layers. The SL-LDH powder shows high specific surface area (∼289 m²/g) and excellent electrocatalytic oxygen evolution efficiency. This work provides the first simple way to prepare CO₃^2– SL-LDHs and will open an avenue for synthesizing other SL-LDHs

Fe-Treated Heteroatom (S/N/B/P)-Doped Graphene Electrocatalysts for Water Oxidation

Anodic water splitting is driven by hydroxide (OH^–) adsorption on the catalyst surface and consequent O₂ desorption. In this work, various heteroatoms (S/N/B/P) with different electronegativities and oxophilicities are introduced to alter the catalytic activity of reduced graphene oxide (RGO) as a catalyst for the oxygen evolution reaction (OER). It is found that, surprisingly, S-doped RGO outperforms the other RGOs doped with more electropositive or electronegative and more oxophilic heteroatoms, and this effect becomes more prominent after Fe treatment of the respective catalysts. Herein, we evaluate the OER activity of a series of Fe-treated mono-heteroatom (S/N/B/P)-doped RGO (Fe-X-G) catalysts, among which interestingly S-doped RGO catalyst treated with Fe (Fe-S-G) is found to show better OER activity than the well-known active Fe-N-C catalyst, demonstrating the best activity among all of the prepared catalysts, close to that of the state of the art IrO₂/C catalyst, along with pronounced long-term stability. Density functional theory (DFT) calculations indicate that the OER activity highly depends on the electroneutrality and oxophilicity of doped heteroatoms and doping-induced charge distribution over RGO, demonstrating that S with mediocre electronegativity and the least oxophilicity exhibits optimal free energy for the adsorption of the OER intermediate and desorption of the final OER product. Furthermore, it is found that Fe treatment greatly helps in enhancing the number of active sites through the regeneration of reduced catalytically active S sites and improving the conductivity and surface area of the S-doped RGO, which are found to be key factors to furnish the Fe-S-G catalyst with the capability to catalyze the OER with high efficiency, even though Fe is found to be absent in the final catalyst

In Situ NMR Study on the Interaction between LiBH₄–Ca(BH₄)₂ and Mesoporous Scaffolds

We discuss the use of nuclear magnetic resonance (NMR) spectroscopy to investigate the physical state of the eutectic composition of LiBH₄–Ca­(BH₄)₂ (LC) infiltrated into mesoporous scaffolds and the interface effect of various scaffolds. Eutectic melting and the melt infiltration of mixed borohydrides were observed through in situ NMR. In situ and ex situ NMR results for LC mixed with mesoporous scaffolds indicate that LiBH₄ and Ca­(BH₄)₂ exist as an amorphous mixture inside of the pores after infiltration. Surprisingly, the confinement of the eutectic LC mixture within the mesopores is initiated below the melting temperature, which indicates a certain interaction between the borohydrides and the mesoporous scaffolds. The confined borohydrides remain inside of the pores after cooling. These phenomena were not observed in microporous or nonporous materials, and this observation highlights the importance of the pore structure of the scaffolds. Such surface interactions may be associated with a faster dehydrogenation of the nanoconfined borohydrides