Shuang Gu

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

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.

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.

Designed synthesis of large-pore crystalline polyimide covalent organic frameworks

Covalent organic frameworks (COFs) are an emerging class of porous crystalline polymers with a wide variety of applications. They are currently synthesized through only a few chemical reactions, limiting the access and exploitation of new structures and properties. Here we report that the imidization reaction can be used to prepare a series of polyimide (PI) COFs with pore size as large as 42 × 53 Å(2), which is among the largest reported to date, and surface area as high as 2,346 m(2) g(-1), which exceeds that of all amorphous porous PIs and is among the highest reported for two-dimensional COFs. These PI COFs are thermally stable up to 530 °C. We also assemble a large dye molecule into a COF that shows sensitive temperature-dependent luminescent properties.

Quaternary Phosphonium-Based Polymers as Hydroxide Exchange Membranes

The principles behind the fuel cell were discovered in 1839, with liquid sulphuric acid as electrolyte and platinum macrowires as catalysts. Liquid-acid electrolytes have high proton conductivities but are corrosive and inconvenient to handle. They also tend to make fuel cell stacks bulky and sensitive to their orientation. Solid-acid electrolytes based on sulfonated polymers were first developed in 1950, and quickly adopted to develop proton exchange membrane fuel cells (PEMFCs) in 1955. The real breakthrough for PEMFCs, however, came when Nafion was discovered in 1962. Nafion’s high proton conductivity and excellent chemical stability singularly established PEMFCs as the dominant fuel-cell technology, drawing enormous excitement and investments from industries and governments worldwide. Over the past 20 years remarkable progresses have been made in improving the power and energy density of PEMFCs, and reducing their use of platinum. However, it has become clear that PEMFCs require platinum as catalysts, leading to PEMFCs being perceived as uneconomical and unsustainable. It has been demonstrated that liquid alkaline fuel cells (AFCs) can achieve high performances without the need of precious metals. What has been critically missing for AFC technology is a polymer hydroxide (OH ) exchange membrane (HEM) that has a high hydroxide conductivity and stability. Here, we show for the first time that quaternary phosphonium-based polymer HEMs have excellent hydroxide conductivities and stabilities. The corresponding HEMFCs show the highest power density and lowest cell resistance reported. They also provide evidence that HEMFCs have the potential to achieve cell performances that rival those of state-of-the-art Nafion-based PEMFCs. Today’s most commonly used strongly basic anion exchange membranes are exclusively quaternary ammonium-based polymers that were discovered in 1950. The dependence of PEMFCs on platinum has recently motivated researchers to explore polymer quaternary ammonium hydroxide (QAOH) membranes for HEMFCs, in which nonprecious metals have been considered to be effective catalysts. Equally important is that HEMFCs have been shown to be resistant to CO2 poisoning. [7] However, currently available polymer QAOH membranes have been found to have low hydroxide conductivities and stabilities. They also have poor solubilities in common solvents, which prevents them from being used in the catalyst layer for constructing triple phase boundaries. Although quaternary phosphonium hydroxide (QPOH) functional groups were expected to have a basicity similar to QAOH, they have not been considered for anion/hydroxide exchange membranes in the past half a century because of the reported severe instability of QPOHs. By using a unique tertiary phosphine, we recently discovered an alkaline-stable strongly basic QPOH-based polymer: tris(2,4,6-trimethoxyphenyl)polysulfone-methylene quaternaryphosphonium-hydroxide (TPQPOH). We successfully used TPQPOH as a soluble hydroxide-conducting ionomer in the catalyst layer in HEMFCs to improve their performance; a major step forward in the development of HEMFCs. However, the TPQPOH used still has a limited conductivity, and is thus unsuitable for use as a membrane. The development of an inexpensive, highly conductive, and alkaline-stable membrane is now considered the most important goal for the full realization of the benefits of alkaline fuel cells. In this study, we prepare the first functional QPOH HEMs, with the highest OH conductivity reported: TPQPOH152 with a degree of chloromethylation (DC) of 152 %, which not only maximizes the ion conductivity but also avoids excessive water uptake (see Supporting Information, Table S1). We also develop a low-curing-temperature (30 8C) membrane preparation procedure by using the high-boiling-point solvent 1-methyl-2-pyrrolidone (NMP, bp = 204 8C) that endows TPQPOH152 HEM with desirable membrane properties : it has a uniform thickness and is smooth, flexible, and tough (e.g. , it can easily sustain a fuelcell back-pressure as high as 300 kPa without any gas leakage; Table S2). The TPQPOH152 HEM has a high alkaline stability and long-term stability; it can maintain conductivity and flexibility after an immersion treatment in 2 m KOH at 60 8C or 10 m KOH at room temperature for two days; or in 1 m KOH at 60 8C or 5 m KOH at room temperature for one month, while under similar conditions typical commercial QAOH-functionalized FAA membranes become very brittle owing to severe degradation. In addition, the polymer is inexpensive (ca. 2 US

Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes

m 2 vs. Nafion’s 1000 US

Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs

m ) and its synthesis is simple, fast, and environmentally friendly, as compared with conventional QAOHbased polymers (Table S3). A high hydroxide conductivity is one of the most important requirements of an HEM. The hydroxide conductivities of all HEMs available commercially or reported by academic or industry labs are plotted against their ion exchange capacities (IECs) in Figure 1. TPQPOH152 exhibits the highest hydroxide conductivity, 45 mS cm 1 (20 8C), among all known HEMs. Usu[a] Dr. S. Gu, Dr. R. Cai, T. Luo, K. Jensen, C. Contreras, Prof. 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 Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201000074.

Ambient pressure dry-gel conversion method for zeolite MFI synthesis using ionic liquid and microwave heating.

Tertiary sulfonium is introduced as the cationic functional group for hydroxide exchange membranes (HEMs). The methoxyl-substituted triarylsulfonium functionalized HEM (i.e., PSf-MeOTASOH) exhibits excellent thermal stability (TOD: 242 °C), acceptable hydroxide conductivity (15.4 mS cm−1 at 20 °C), and good chemical stability. Our work shows that, similar to nitrogen and phosphorus, a sulfur element with designed side groups can also be used to construct HEM cationic functional groups.