Yushan Yan

Efficient water oxidation using nanostructured α-nickel-hydroxide as an electrocatalyst.

Electrochemical water splitting is a clean technology that can store the intermittent renewable wind and solar energy in H2 fuels. However, large-scale H2 production is greatly hindered by the sluggish oxygen evolution reaction (OER) kinetics at the anode of a water electrolyzer. Although many OER electrocatalysts have been developed to negotiate this difficult reaction, substantial progresses in the design of cheap, robust, and efficient catalysts are still required and have been considered a huge challenge. Herein, we report the simple synthesis and use of α-Ni(OH)2 nanocrystals as a remarkably active and stable OER catalyst in alkaline media. We found the highly nanostructured α-Ni(OH)2 catalyst afforded a current density of 10 mA cm(-2) at a small overpotential of a mere 0.331 V and a small Tafel slope of ~42 mV/decade, comparing favorably with the state-of-the-art RuO2 catalyst. This α-Ni(OH)2 catalyst also presents outstanding durability under harsh OER cycling conditions, and its stability is much better than that of RuO2. Additionally, by comparing the performance of α-Ni(OH)2 with two kinds of β-Ni(OH)2, all synthesized in the same system, we experimentally demonstrate that α-Ni(OH)2 effects more efficient OER catalysis. These results suggest the possibility for the development of effective and robust OER electrocatalysts by using cheap and easily prepared α-Ni(OH)2 to replace the expensive commercial catalysts such as RuO2 or IrO2.

A Soluble and Highly Conductive Ionomer for High-Performance Hydroxide Exchange Membrane Fuel Cells

Hydrogen proton exchange membrane fuel cells (PEMFCs) have been demonstrated to have high power density and reasonable energy density. Their commercialization, however, has been hampered by the high cost and low durability of their electrocatalysts. By switching from an acidic medium to a basic one, hydroxide (OH ) exchange membrane fuel cells (HEMFCs) have the potential to solve the problems of catalyst cost and durability while achieving high power and energy density. In a basic environment, the cathode oxygen reduction over-potential can be significantly reduced, leading to high fuel cell efficiency, and catalysts in basic medium are also more durable. In addition, the facile cathode kinetics allows nonprecious metals to be used as catalysts, thus drastically reducing the cost of the fuel cell. Further, HEMFCs can offer fuel flexibility (e.g., methanol, ethanol, ethylene glycol, etc.) because of their low overpotential for hydrocarbon fuel oxidation and reduced fuel crossover. One of the most significant problems for HEMFCs is the lack of a soluble ionomer that can be used in the catalyst layer to build an efficient three-phase boundary and thus drastically improve the utilization of the catalyst particles and reduce the internal resistance. One of the most desirable properties of an ionomer for use in the catalyst layer is high solubility in low-boiling-point water-soluble solvents such as ethanol and (nor 2-)propanol, because these solvents are easy and safe to handle and remove during the electrode preparation. The ionomer should also have high hydroxide conductivity and alkaline stability. For PEMFCs, Nafion has been the ionomer of choice because it meets these requirements. But for HEMFCs, the most commonly used material for the hydroxide exchange membrane (HEM) is a quaternary ammonium hydroxide containing polymer that has poor solubility in the aforementioned simple solvents, low hydroxide conductivity, and poor alkaline stability. For example, Tokuyama Co. very recently reported two types of soluble quaternary ammonium hydroxide containing polymers (product code: A3Ver2, soluble in tetrahydrofuran or n-propanol, and AS-4, soluble in n-propanol); however, as a result of their low hydroxide conductivity, their incorporation into the catalyst layers of HEMFCs only led to a moderate improvement in performance. In another case, Park et al. prepared an ionomer solution of the trimethylamine (TMA) and N,N,N’,N’-tetramethyl-1,6-hexanediamine (TMHDA) based polysulfone– methylene quaternary ammonium hydroxide (T/TPQAOH) in dimethylacetamide (DMAc, b.p. 166 8C). Similar to the Tokuyama results, the low hydroxide conductivity of the ionomer significantly limited the improvement in fuel cell performance, and in addition, removal of the high-boilingpoint solvent is considered difficult and unsafe in the presence of finely dispersed catalysts. Owing to the lack of a soluble highly conductive solid ionomer, aqueous solutions of KOH or NaOH have been previously used in the electrodes, where the introduction of metal cations (M) offsets the key advantages of a HEMFC over traditional liquid-electrolytebased alkaline fuel cells (AFCs). Furthermore, owing to the lack of a good ionomer as the binder, non-ionic conductive PTFE and proton-conductive Nafion ionomers were used as substitutes in the electrodes, even though these materials were known to have no hydroxide conductivity. Recently, Varcoe et al. reported a TMHDA-based polyvinylbenzylcrosslinked quaternary ammonium hydroxide (TPCQAOH) electrochemical interface to enhance HEMFC performance. Because the polymer used was not soluble in ionomer form, one could question its ability to form an efficient three-phase-boundary structure in the catalyst layer, thereby limiting performance. Moreover, the hydroxide conductivity and stability of the electrochemical interface are still of concern because it is based on quaternary ammonium hydroxide groups. Quaternary phosphonium containing polymers showed excellent solubility in methanol. The strong basicity of the tertiary phosphine suggests that quaternary phosphonium hydroxides are very strong bases. Therefore in this work, we synthesized a new quaternary phosphonium based ionomer that is soluble in low-boiling-point water-soluble solvents and is highly hydroxide conductive: tris(2,4,6-trimethoxyphenyl) polysulfone-methylene quaternary phosphonium hydroxide (TPQPOH; Scheme 1). The TPQPOH ionomer exhibits excellent solubility in pure methanol, ethanol, and n-propanol and in their aqueous solutions (50 wt% in water, see Table S1 in the Supporting Information). On the other hand, the TPQPOH is insoluble in pure water, even at 80 8C, suggesting that it can be used in the [*] Dr. S. Gu, Dr. R. Cai, T. Luo, Dr. Z. Chen, M. Sun, Y. Liu, Prof. Dr. Y. S. Yan Department of Chemical and Environmental Engineering University of California—Riverside Riverside, CA 92521 (USA) Fax: (+1)951-827-5696 E-mail: yushan.yan@ucr.edu Homepage: http://www.engr.ucr.edu/faculty/chemenv/ yushanyan.html

Synthesis, morphology control, and properties of porous metal–organic coordination polymers

Abstract A “direct mixing” synthesis strategy has been demonstrated for the first time that allows fast (e.g., 0.5 h) synthesis of bulk quantity of thermally stable and highly porous metal–organic coordination polymers (MOCP) nanocrystals (30–150 nm diameter) at room temperature with high yield (∼90%). The MOCP materials constructed from Zn ( NO 3 ) 2 · 6 H 2 O and 1,4-benzenedicarboxylic acid (H 2 BDC) were characterized with scanning electron microscope, powder X-ray diffraction, thermal gravimetric analysis, FT-IR, and volumetric Ar adsorption/desorption. The direct mixing method also produced a highly porous nanometer-sized MOCP material, which is likely to be a new phase that has not been discovered by the more commonly used “diffusion” approach. “Soft” and “hard” template approaches were used to successfully manipulate the morphology of MOCP materials at nanometer scale. In addition, water molecules were shown to play an important role in the synthesis and eventual composition of MOCP materials. Exposure of MOCP materials to water resulted in dramatic drop in surface area and porosity because of possible hydrolysis of the framework. An acid hydrolysis process of MOCP materials was also revealed in which the crystals could be hydrolyzed back into metal salts and organic acid under acid treatment.

3D microporous base-functionalized covalent organic frameworks for size-selective catalysis.

The design and synthesis of 3D covalent organic frameworks (COFs) have been considered a challenge, and the demonstrated applications of 3D COFs have so far been limited to gas adsorption. Herein we describe the design and synthesis of two new 3D microporous base-functionalized COFs, termed BF-COF-1 and BF-COF-2, by the use of a tetrahedral alkyl amine, 1,3,5,7-tetraaminoadamantane (TAA), combined with 1,3,5-triformylbenzene (TFB) or triformylphloroglucinol (TFP). As catalysts, both BF-COFs showed remarkable conversion (96% for BF-COF-1 and 98% for BF-COF-2), high size selectivity, and good recyclability in base-catalyzed Knoevenagel condensation reactions. This study suggests that porous functionalized 3D COFs could be a promising new class of shape-selective catalysts.

Synthesis of monodispere Au@Co3O4 core-shell nanocrystals and their enhanced catalytic activity for oxygen evolution reaction.

DOI: 10.1002/adma.201400336 substrate, which makes the Co 3 O 4 more easily oxidized. [ 6b ] Our electrochemical study shows that our Au@Co 3 O 4 NCs have an OER activity 7 times as high as a mixture of Au and Co 3 O 4 NCs or Co 3 O 4 NCs alone, and 55 times as high as Au NCs, most likely due to a strong synergistic effect between the core and the shell, and this effect does not exist between the physically mixed NCs. Some Au and Co 3 O 4 hybrid NCs have been reported, [ 10 ] however, none of them have well-defi ned core–shell structures and uniform sizes. High-quality Au@Co 3 O 4 core–shell NCs are still desired. In our experiment, a three-step approach ( Figure 1 a) was adopted to synthesize Au@Co 3 O 4 NCs, comprising synthesis of the Au NC, growth of the Co shell, and oxidation of Co to Co 3 O 4 . First, Au NCs were prepared by reducing HAuCl 4 with tertbutylamine borane (TBAB) in the presence of oleylamine (OAm) as the ligand, following the procedure described in a previous study by Peng et al. [ 11 ] Second, Co shells were grown on the Au NC cores to prepare Au@Co NCs by using Co(acac) 2 , where acac is acetylacetonate, as the cobalt source and TBAB as the reducing agent. OAm and oleic acid (OA) were introduced to control the shape and uniformity. Third, the Au@Co NCs were loaded on carbon and then the Co shells were oxidized to Co 3 O 4 by calcination in air. The experimental details are described in the Supporting Information. Figure 1 b shows the transmission electron microscopy (TEM) image of the as-obtained Au NCs. They have a narrow size distribution with a diameter of 3.6 ± 0.5 nm. Five-fold symmetry is found in the high-resolution TEM (HRTEM) image (Figure S1, Supporting Information), which is in agreement with the literature, indicating the multiple-twinned structure of the Au NCs. [ 11 ] Figure 1 c shows a TEM image of Au@Co core–shell NCs that were synthesized by growing Co shells on the pre-synthesized Au cores with 0.5 mmol OA. A dark core corresponding to Au can clearly be seen located at the center of the NC, and a uniform lighter Co shell caps around it. The Au@Co NCs are nearly monodisperse with an overall diameter of 8.1 ± 0.7 nm. The thickness of the Co shell is ca. 2 nm. The Au@Co NCs are highly uniform so that they can assemble into an ordered structure (Figure 1 d). The energy dispersive spectrometry (EDS) spectra (Figure S2a, Supporting Information) show the signals of Au and Co (atomic ratio 1:4), which confi rms the hybrid Au@Co composition. The Co shell seems to be amorphous because no clear lattice fringe can be seen in the HRTEM image (Figure 1 e). This may be due to the lattice mismatch between Au (face-centered cubic, fcc) and Co (hexagonal close packed, hcp). The multiple twinned nature of the Au core may also infl uence the crystallinity of the Co shell. It is noted that Co NCs cannot be synthesized under the same condition The hydrogen economy can provide an effi cient energy system that is free from environmental issues related to the combustion of coal, oil, and natural gas. [ 1 ] However, such a system requires a clean and sustainable source of hydrogen, which can be provided by splitting of water either electrochemically or photoelectrochemically. [ 2 ] One of the key problems in splitting water is the kinetically sluggish anode reaction, i.e., oxygen evolution reaction (OER, 4OH − → 2H 2 O + 4e − + O 2 in base). An overpotential of several hundred millivolts is often required to achieve a current density of 10 A g catalyst −1 . [ 3 ] Recent studies have shown that spinel-type Co 3 O 4 has relatively good OER activities. [ 2e , 3c , 4 ] Hybrid materials have been proposed to further promote the OER activity of Co 3 O 4 , such as doping Co 3 O 4 with other metals to make substituted cobaltites [ 5 ] or growing Co 3 O 4 on a special substrate. [ 6 ]

Self-crosslinking for dimensionally stable and solvent-resistant quaternary phosphonium based hydroxide exchange membranes

A simple self-crosslinking strategy, without the needs of a separate crosslinker or a catalyst, is reported here. The crosslinking drastically lowers the water swelling ratio (e.g., 5-10 folds reduction) and provides excellent solvent-resistance. The self-crosslinked membrane (DCL: 5.3%) shows the highest IEC-normalized hydroxide conductivity among all crosslinked HEMs reported.

3D Porous Crystalline Polyimide Covalent Organic Frameworks for Drug Delivery

Three-dimensional porous crystalline polyimide covalent organic frameworks (termed PI-COFs) have been synthesized. These PI-COFs feature non- or interpenetrated structures that can be obtained by choosing tetrahedral building units of different sizes. Both PI-COFs show high thermal stability (>450 °C) and surface area (up to 2403 m(2) g(-1)). They also show high loading and good release control for drug delivery applications.

Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy

The hydrogen oxidation/evolution reactions are two of the most fundamental reactions in distributed renewable electrochemical energy conversion and storage systems. The identification of the reaction descriptor is therefore of critical importance for the rational catalyst design and development. Here we report the correlation between hydrogen oxidation/evolution activity and experimentally measured hydrogen binding energy for polycrystalline platinum examined in several buffer solutions in a wide range of electrolyte pH from 0 to 13. The hydrogen oxidation/evolution activity obtained using the rotating disk electrode method is found to decrease with the pH, while the hydrogen binding energy, obtained from cyclic voltammograms, linearly increases with the pH. Correlating the hydrogen oxidation/evolution activity to the hydrogen binding energy renders a monotonic decreasing hydrogen oxidation/evolution activity with the hydrogen binding energy, strongly supporting the hypothesis that hydrogen binding energy is the sole reaction descriptor for the hydrogen oxidation/evolution activity on monometallic platinum.

Zeolite Thin Films: From Computer Chips to Space Stations

Zeolites are a class of crystalline oxides that have uniform and molecular-sized pores (3-12 A in diameter). Although natural zeolites were first discovered in 1756, significant commercial development did not begin until the 1950s when synthetic zeolites with high purity and controlled chemical composition became available. Since then, major commercial applications of zeolites have been limited to catalysis, adsorption, and ion exchange, all using zeolites in powder form. Although researchers have widely investigated zeolite thin films within the last 15 years, most of these studies were motivated by the potential application of these materials as separation membranes and membrane reactors. In the last decade, we have recognized and demonstrated that zeolite thin films can have new, diverse, and economically significant applications that others had not previously considered. In this Account, we highlight our work on the development of zeolite thin films as low-dielectric constant (low-k) insulators for future generation computer chips, environmentally benign corrosion-resistant coatings for aerospace alloys, and hydrophilic and microbiocidal coatings for gravity-independent water separation in space stations. Although these three applications might not seem directly related, they all rely on the ability to fine-tune important macroscopic properties of zeolites by changing their ratio of silicon to aluminum. For example, pure-silica zeolites (PSZs, Si/Al = infinity) are hydrophobic, acid stable, and have no ion exchange capacity, while low-silica zeolites (LSZs, Si/Al < 2) are hydrophilic, acid soluble, and have a high ion exchange capacity. These new thin films also take advantage of some unique properties of zeolites that have not been exploited before, such as a higher elastic modulus, hardness, and heat conductivity than those of amorphous porous silicas, and microbiocidal capabilities derived from their ion exchange capacities. Finally, we briefly discuss our more recent work on polycrystalline zeolite thin films as promising biocompatible coatings and environmentally benign wear-resistant and antifouling coatings. When zeolites are incorporated into polymer thin films in the form of nanocrystals, we also show that the resultant composite membranes can significantly improve the performance of reverse osmosis membranes for sea water desalination and proton exchange membrane fuel cells. These diverse applications of zeolites have the potential to initiate new industries while revolutionizing existing ones with a potential economic impact that could extend into the hundreds of billions of dollars. We have licensed several of these inventions to companies with millions of dollars invested in their commercial development. We expect that other related technologies will be licensed in the near future.

Stretchable Wire-Shaped Asymmetric Supercapacitors Based on Pristine and MnO2 Coated Carbon Nanotube Fibers

While the emerging wire-shaped supercapacitors (WSS) have been demonstrated as promising energy storage devices to be implemented in smart textiles, challenges in achieving the combination of both high mechanical stretchability and excellent electrochemical performance still exist. Here, an asymmetric configuration is applied to the WSS, extending the potential window from 0.8 to 1.5 V, achieving tripled energy density and doubled power density compared to its asymmetric counterpart while accomplishing stretchability of up to 100% through the prestrainning-then-buckling approach. The stretchable asymmetric WSS constituted of MnO2/CNT hybrid fiber positive electrode, aerogel CNT fiber negative electrode and KOH-PVA electrolyte possesses a high specific capacitance of around 157.53 μF cm(-1) at 50 mV s(-1) and a high energy density varying from 17.26 to 46.59 nWh cm(-1) with the corresponding power density changing from 7.63 to 61.55 μW cm(-1). Remarkably, a cyclic tensile strain of up to 100% exerts negligible effects on the electrochemical performance of the stretchable asymmetric WSS. Moreover, after 10,000 galvanostatic charge-discharge cycles, the specific capacitance retains over 99%, demonstrating a long cyclic stability.