(December 16, 2010) — Channels in transmembrane proteins are small enough to allow ions or molecules of a certain size to pass through, while keeping out larger objects. Artificial fluidic nanochannels that mimic the capabilities of transmembrane proteins are highly prized for advanced technologies in biomedical, battery, and other technology sectors. However, it has been difficult to make individual artificial channels of this nanoscale size, until research performed by the US DOE’s Berkeley Lab.

Schematic of a 2nm nanochannel device, with two microchannels, ten nanochannels and four reservoirs. (Image courtesy of Chuanhua Duan)

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have been able to fabricate nanochannels that are only 2nm in size, using standard semiconductor manufacturing processes. Already they’ve used these nanochannels to discover that fluid mechanics for passages this small are significantly different not only from bulk-sized channels, but even from channels that are merely 10nm in size.

"We were able to study ion transport in our 2nm nanochannels by measuring the time and concentration dependence of the ionic conductance," says Arun Majumdar, director of DOE’s Advanced Research Projects Agency – Energy (ARPA-E), who led this research while still a scientist at Berkeley Lab. "We observed a much higher rate of proton and ionic mobility in our confined hydrated channels — up to a fourfold increase over that in larger nanochannels (10 to 100nm). This enhanced proton transport could explain the high throughput of protons in transmembrane channels."

Majumdar is the co-author with Chuanhua Duan, a member of Majumdar’s research group at the University of California (UC) Berkeley, of a paper on this work, which was published in the journal Nature Nanotechnlogy. The paper is titled "Anomalous ion transport in 2nm hydrophilic nanochannels."

In their paper, Majumdar and Duan describe a technique in which high-precision ion etching is combined with anodic bonding to fabricate channels of a specific size and geometry on a silicon-on-glass die. To prevent the channel from collapsing under the strong electrostatic forces of the anodic bonding process, a thick (500nm) oxide layer was deposited onto the glass substrate.

Chuanhua Duan was part of a successful Berkeley Lab effort to fabricate nanochannels that measured only two nanometers in size, using standard semiconductor manufacturing processes. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)

"This deposition step and the following bonding step guaranteed successful channel sealing without collapsing," says Duan. "We also had to choose the right temperature, voltage and time period to ensure perfect bonding. I compare the process to cooking a steak, you need to choose the right seasoning as well as the right time and temperature. The deposition of the oxide layer was the right seasoning for us."

The nanometer-sized channels in transmembrane proteins are critical to controlling the flow of ions and molecules across the external and internal walls of a biological cell, which, in turn, are critical to many of the biological processes that sustain the cell. Like their biological counterparts, fluidic nanochannels could play critical roles in the future of fuel cells and batteries.

"Enhanced ion transport improves the power density and practical energy density of fuel cells and batteries," Duan says. "Although the theoretical energy density in fuel cells and batteries is determined by the active electrochemical materials, the practical energy density is always much lower because of internal energy loss and the usage of inactive components. Enhanced ion transport could reduce internal resistance in fuel cells and batteries, which would reduce the internal energy loss and increase the practical energy density."

The findings by Duan and Majumdar indicate that ion transport could be significantly enhanced in 2nm hydrophilic nanostructures because of their geometrical confinements and high surface-charge densities. As an example, Duan cites the separator, the component placed between the between the cathode and the anode in batteries and fuel cells to prevent physical contact of the electrodes while enabling free ionic transport.

"Current separators are mostly microporous layers consisting of either a polymeric membrane or non-woven fabric mat," Duan says. "An inorganic membrane embedded with an array of 2nm hydrophilic nanochannels could be used to replace current separators and improve practical power and energy density."

Artificial fluidic nanochannels, like these 30nm channels shown under fluorescence, mimic the capabilities of transmembrane proteins and are highly prized for advanced technology applications. (Image courtesy of Majumdar group)

The 2nm nanochannels also hold promise for biological applications because they have the potential to be used to directly control and manipulate physiological solutions. Current nanofluidic devices utilize channels that are 10 to 100nm in size to separate and manipulate biomolecules. Because of problems with electrostatic interactions, these larger channels can function with artificial solutions but not with natural physiological solutions.

"For physiological solutions with typical ionic concentrations of approximately 100 millimolars, the Debye screening length is 1nm," says Duan. "Since electrical double layers from two-channel surfaces overlap in our 2nm nanochannels, all current biological applications found in larger nanochannels can be transferred to 2nm nanochannels for real physiological media."

The next step for the researchers will be to study the transport of ions and molecules in hydrophilic nanotubes that are even smaller than 2nm. Ion transport is expected to be even further enhanced by the smaller geometry and stronger hydration force.

"I am developing an inorganic membrane with embedded sub-2nm hydrophilic nanotube array that will be used to study ion transport in both aqueous and organic electrolytes," Duan says. "It will also be developed as a new type of separator for lithium-ion batteries."

This work was supported by DOE’s Office of Science, plus the Center for Scalable and Integrated Nanomanufacturing, and the Center of Integrated Nanomechanical Systems at UC Berkeley.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, CA. It conducts unclassified scientific research and is managed by the University of California for the DOE Office of Science. Visit http://www.lbl.gov.

More info:

Arun Majumdar: http://www.me.berkeley.edu/faculty/majumdar/
ARPA-E: http://arpa-e.energy.gov/
Center for Scalable and Integrated Nanomanufacturing (SINAM): http://www.sinam.org/
Center of Integrated Nanomechanical Systems (COINS): http://mint.physics.berkeley.edu/coins/

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July 22, 2010 – Researchers at Oregon State U. have come up with yet another unique application for nanocoatings — help produce more electricity from sewage.

Their work, published a few weeks ago in Biosensors and Bioelectronics, focuses on the anodes of microbial electrochemical cells (MEC), the core of efforts to clean biowaste and produce useful levels of electricity, realizing twin goals of wastewater treatment and renewable energy. (OSU has been working on MECs for several years, hoping to develop systems for producing electricity from hydrogen fuel cells for automobiles.)

Bacteria from biowaste (i.e., sewage) is placed in an anode chamber, where they form a biofilm, consume nutrients, and grow, and that process releases electrons. Coating graphite anodes with a gold nanolayer increased the electricity production by 20×, they found; similar palladium coatings also produced an increase (50%-150%). They think iron nanoparticle coatings could produce similar electricity increases as gold — and cost a lot less. And a similar approach could be applied to producing hydrogen gas instead of electricity, toward use in hydrogen fuel cells e.g. in cars.

From the paper abstract:

Significant positive linear regression was obtained between the current density and the particle size (average Feret’s diameter and average area), while the circularity of the particles showed negative correlation with current densities. On the contrary, no significant correlation was evident between the current density and the particle density based on area fraction and particle counts. These results demonstrated that nano-decoration can greatly enhance the performance of microbial anodes, while the chemical composition, size and shape of the nanoparticles determined the extent of the enhancement.

More work is needed to get the process working beyond a lab environment, to lower its cost (e.g. identify the lowest-cost materials to use), and improve efficiency and electrical output even more. "We still need some improvements in design of the cathode chamber, and a better understanding of the interaction between different microbial species," added Frank Chaplen, an associate professor of biological and ecological engineering, in a statement. "But the new approach is clearly producing more electricity."

Ultimately, the researchers see the technology being used to reduce the cost of wastewater treatment, or in developing nations where wastewater treatment is impractical due to a lack of adequate power supply. Sewage treatment plants could be made to be completely self-sufficient in terms of energy usage, they say.

The research is supported by the Oregon Nanoscience and Microtechnologies Institute (ONAMI) and the National Science Foundation.

by Takehiko Yaza, Seika Machinery

Executive overview
In developing new fuel cells, there are many obstacles to overcome. For example, gas diffusion must be improved, the volume of catalysts must be reduced and recondensation must be prevented. Another major difficulty is managing water in the fuel cell so that the proton exchange membrane is moisturized and water is quickly discharged from the cathode. Failure to control the water discharge properly will cause flooding at the cathode and a reduction of voltage. There will be a considerable difference depending on the materials selected and the treatment and processing of the materials. Failure to understand the properties of each individual material will make it impossible to develop the optimal fuel cell (Figure 1).

February 25, 2010 – Key factors to consider when developing new fuel cells include: controlling the gas diffusion, permeation, condensation, the reactive area, as well as moisturizing and discharging water. All of these factors depend on the particle size, fiber diameter of the materials and the through-pore structures in them at the electrodes. This article introduces tools that enable better understanding of these properties.

Figure 1. Current issues for developing fuel cells.

Milling

In North America, ball mills are the most common method for reducing the size of catalyst particles and micro-porous layers of slurry. In a ball mill, the pulverizing energy is generated by steel balls in a gravity-based process. Generally, the milling energy is 1G. By contrast, in a bead mill, the dispersing slurry contains carbon blacks, catalysts and beads, which are mixed together by an agitator inside the milling vessel where particle communion breaks them down into smaller particles. The high-speed of the stirring device generates 100-500G of centrifugal force, providing very high energy. The bead mill makes a large difference in the velocity between the beads and the milled particles. Occasionally, the beads and slurry rotate together and the slurry is not pulverized efficiently. To solve this problem, the AIMEX Alpha Mill (Figure 2) uses an orifice contractile flow vessel. In this configuration, even low-speed rotation generates a very large velocity difference between beads and the slurry. By applying this principle, the milling operation can be carried out very effectively using 30μm beads. Consequently, micrometer-sized carbon black and platinum catalyst particles can be transformed into particles with nm sizes.

Figure 2. a) Alpha Mill generates high ΔV with small power; b) comparison with the existing model.

When evaluating the efficiency of the slurry pulverization by particle size distribution, we recommend using Horiba Partica LA-950V2. This instrument has the closest correlation to data for NIST traceable to particle size standards and it provides the widest measuring range in the industry.

When the average diameter of the particle to be pulverized is ~0.5μm, even if the materials are crushed, agglomeration is prone to happen. To prevent this phenomenon, a dispersant can be added to the slurry, but this method does have limitations. Therefore, maintaining a high degree of dispersion while coating catalyst and micro-porous layers becomes more important and the Ultrasonic System PRISM series meets this requirement. This coating system uses ultrasonic energy and a nozzleless feed system to eliminate nozzle-blocking problems. The result of these modifications is a 1mm uniform coating thickness. In screen printers and other typical coating systems, it is difficult to provide a thin and uniform thickness without using a pressing machine.

Understanding pore structure

It is necessary to fully understand the pore structure of the coated catalyst layer and the micro porous layer, and the properties relative to water. Although the traditional method of mercury porosimetory generally is used to measure the opened pore structure and volume, the high-pressure measurement method will destroy the pore structure and cannot pick up the functional pores on the application. In contrast, the PMI Capillary Flow Porometer series can measure pore size distribution to determine gas diffusion, water permeability, and the repellent characteristics of the gas diffusion and catalyst layers, without the use of mercury and liquid nitrogen. It realizes automatic and short duration measurements based on the ASTM, bubble point and half-dry methods.

Figure 3 compares measurements made by the Mercury Porosimeter and the Capillary Flow Porometer on a polymer membrane that has only through-pores. The former shows 1-4μm of pore size distribution (peak pore size: 3μm) while the latter shows 1-1.2μm peak pore size:1.1μm difference.

Figure 3. a) Measurement pore size distribution by Mercury Porosimeter; b) Measurement pore size distribution by Capillary Flow Porometer.

Gas diffusion

Specifically, gas diffusion is the phenomenon in which high-concentration gas flows towards low concentration gas, and eventually the concentration becomes even in the differentiated space with a concentration gradient. Differential pressure results in permeation, but this cannot be called diffusion. There are several measuring instruments for the water vapor transmission rate in the market, however, these instruments can measure only membranes without pores. When fuel cells use high permeability samples, it is difficult to evenly control the pressure at the primary and secondary sides, and the measurement instruments that can measure diffusion at high accuracy are not available. The Seika Moisture Vapor Diffusion Permeameter (MVDP) is integrated using the technologies to maintain high humidity to some degree and prevent condensation by means of temperature control and pressure control. Instead of using batch methods in gas chromatography, MVDP uses real-time technologies to measure concentrations.

As shown in Figure 4, the equipment can measure water vapor diffusion of gas diffusion layers and a proton exchange membrane, as well as oxygen diffusion of moisturized gas. In addition, it can measure gas permeability of generated condensation in the inside of a sample by simulating the flooding phenomenon.

Figure 4. a) Measurement in-plane gas permeability test for carbon papers by MVDP; b) oxygen diffusion test for carbon papers by MVDP.

Many fuel cell materials are treated with a waterproofing chemical on the surface to prevent condensation, or are hydrophilically processed to retain moisture. The characteristics of Teflon-coated samples change with immersion time and temperature. The Seika Liquid Intrusion Meter measures the changes in properties and helps optimize fuel cell performance (Figure 5). A liquid permeability test using ultra-low differential pressure can monitor permeated water from large through pores in order of pore size.

The Mesys USM-200 thickness gauge is effective for the management of thickness for the proton exchange membrane manufacturing process. This method emits no radiation, and does not come in contact with, or destroy the samples during measurement.

Figure 5. a) Hydrophobic sample measurement; b) Hydrophilic sample measurement.

Conclusion

Because it is difficult to visualize the inner operation of a fuel cell — it is covered by separators made of metal or carbon — it is necessary to use measurements (to get structural information) and simulations (to get process information). Using these direct approaches brings in new ideas and compensates simulation results on super computers. Furthermore, using the aforementioned instruments, we developed simulators for monitoring gas permeability in chronological order on blocking pores by condensation (flooding) at GDL, while also creating a gas visualization system for gas diffusion and the concentration at GDL. However, these are only one set of requirements that need to be addressed before high-volume manufacturing of high-efficiency, cost-effective fuel cells can be realized. Going forward, we would like to discuss additional research proposals with interested parties and propose new equipment necessary to develop optimal fuel cells.

Acknowledgment

Teflon is a registered trademark of DuPont.

Biography

Takehiko Yaza has a bachelor of arts in economics from Takasaki City U. of Economics. He is a senior sales manager at Seika Machinery Inc., 3528 Torrance Blvd., Suite 100, Torrance, CA 90503 USA; ph.: (+1) 310.540.7310; e-mail [email protected]; www.seikausa.com.

July 30, 2009: NanoDynamics has ceased operations and filed Chapter 7 bankruptcy, unable to find financing after a pair of abandoned IPOs and the current stagnant funding environment. This particular filing is a liquidation move, not a restructuring as would be in a Chapter 11 filing.

The company, which spanned three sites in NY, PA, and OH and nearly 100 employees working on nanoparticles used in fuel cells, water filtration, and construction materials, had announced nearly $2.5M in federal funding for two projects in January, but apparently that wasn’t enough. “While NanoDynamics has technology and products that the world needs as well as an intelligent and dedicated team of employees, the funding to continue simply does not exist,” CEO William Cann reportedly told employees in an email, according to the Buffalo News.

The Business First of Buffalo noted 2008 regulatory filings indicating NanoDynamics had about $8.8M in sales but $34M in losses. “Given the current financial market conditions the ability to raise capital is very, very difficult,” said Raymond Fink from Harter Seacrest & Emery LLP who is representing the company in bankruptcy proceedings, told the paper, “particularly for a company that is primarily research and development and hasn’t quite yet gotten most of its products to the market.”

June 22, 2009: Researchers from the U. of Georgia have developed a method to grow molecular polymer chains into miniature fuel cells that can conduct electrical charges, with potential use in devices including pacemakers, cochlear implants, and prosthetic limbs.

The polymer chains are grown as an ultrathin (5-50nm) film from the surface in a “grafting” approach, in which a single layer of thiophene is laid down as an initial coating, followed by built-up chains of more thiophene or benzene using a controlled polymerization technique. The structure is said to resemble toothbrush bristles. Thiophene is an insulator, “but by linking many thiophene molecules together in a controlled fashion, the polymers have conducting properties,” notes Jason Locklin, UGA chemist and lead of the paper which appeared in the June issue of the journal Chemical Communications.

The technique enables systematic control to vary the polymer architecture, which opens up application in devices such as sensors, transistors, and diodes. Fuel sources within the body are difficult to harness, he pointed out, and those that are good at chemical-electrical energy conversion (e.g. enzymes) don’t transmit the electricity well due to insulating layers. “Hopefully our molecular wires will provide a better conduit for charges to flow.”

Next step in the research is to pinpoint specific applications — e.g., interfacing the polymer brush directly with living tissue as a biochemical sensor, prosthetic limb, pacemaker, or bionic ears. Other possible applications include transistors (think organic semiconductors) or photovoltaic devices.

The work was funded by the Petroleum Research Foundation.

AFM image of a 42nm PPh film prepared via SI-KCTP. (Source: Chemical Communications)