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TECH BEAT
Dr. Neil Canter/Contributing Editor

Bouncing batteries

An extensive evaluation has been conducted to see if there is a relationship between the health of the battery and how high it bounces.

Battery performance over an extended operating time continues to be a focus of much research. Attention has mainly been placed on lithium-ion batteries because of their superior performance. Unfortunately, development of lithium-ion batteries has been hindered by concerns about their durability and flammability in use.


A previous TLT article revealed how researchers believe the instability in lithium-ion batteries is caused by the growth of metallic filaments known as dendrites as the battery ages. These dendrites grow initially at the anode into a fern-like structure that can eventually short circuit the battery.1 Researchers prepared a new ion-conducting membrane based on aramid nanofibers and poly(ethylene oxide) that stops dendrite growth and improves durability while minimizing concern about failure.

A battery that most of us are more familiar with and use almost every day is the “alkaline” AA battery, also known as the LR6 form factor zinc-manganese dioxide battery. This battery type has been in use for more than 50 years and commands annual global sales of $ 1.8 billion.

Daniel Steingart, assistant professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment at Princeton University in Princeton, N.J., says, “Electrical testing is the main way to evaluate the health of a battery, but there are other non-electrical techniques that can be used to assess the chemistry of the battery. A favored approach is to analyze both the open circuit potential of the battery and the coulomb count, which measures how much charge is in the battery.”  Steingart points out that other factors—such as temperature rise—need to be evaluated in order to evaluate the safety limits in using the battery.

Bouncing has become a popular technique to determine whether an AA battery is dead. Steingart found out about this from a colleague. He says, “We were shown a video that demonstrates there can be a correlation between the ability of a battery to bounce and its performance.”

Steingart wanted a better understanding of whether bouncing a battery is a reliable technique for determining battery health and if the degree of bouncing provides any indication about the health of the battery.

Work has now been done to more systematically evaluate AA batteries.

Coefficient of Restitution

Steingart and his colleagues have conducted an extensive evaluation of AA batteries to determine if there is a relationship between the health of the battery and how high it bounces. He says, “We evaluated a couple hundred batteries by dropping them through a 25 cm tall acrylic tube onto an epoxy benchtop and evaluating the audio to determine the number of bounces, height of the bounce and a term known as the coefficient of restitution (COR).”  COR refers to the ratio of the relative speed of an object after collision compared to before a collision. In the case of the collision between the battery and the epoxy benchtop, the COR refers to the square root of the height of one bounce to the earlier bounce.


Figure 1 shows an illustration of the testing done by Steingart’s research group. The state of charge of each battery was evaluated after each bouncing experiment. The battery was then discharged for one hour at 280 milliamps and retested. This process was repeated until the battery was completely discharged.  Data generated by the researchers shows that the COR of the batteries remains relatively stable until the state of charge drops to 80%. Then a dynamic increase in bouncing measured by COR takes place as the batteries state of charge drops to 50%. This is followed by the COR leveling off until the battery completely discharges.  The researchers hypothesized that the change in COR may be attributed to one of four effects including mass loss, reduction of the cathode, water consumption and oxidation of the zinc anode. Steingart says, “We evaluated each of these effects and concluded that the only one that is in agreement with the data is the oxidation of zinc to zinc oxide.”

Steingart continues, “As the battery discharges, zinc oxide forms through oxidation on the edges of the anode and eventually bridges develop between particles leading to the establishment of a network of springs that gives a battery the ability to bounce.”  This bridging effect was confirmed through the use of in situ energy-dispersive x-ray diffraction. Steingart also reveals that zinc oxide is used as a component to add bounce to golf balls.  The group hypothesizes that the COR levels off because the zinc oxide is more uniformly distributed through the anode and a second type of zinc oxide forms, which does not contribute to the bounce.  The important conclusion from this study is that bouncing a battery does not tell the user if it is dead but, rather, only that the battery is not fresh. Additional information about this work can be found in a recent study.2

Steingart indicates that follow-up work has focused on using sound to determine the state of charge in a battery. He says, “We have hooked up a speaker and a microphone to a battery to evaluate how the sound moves through a battery. A direct correlation is found between the state of charge in a battery and the density distribution as measured by sound.”

This procedure works well for any battery, including a lithium-ion battery. Further information can be found in a recent reference2 or by contacting Steingart at steingart@princeton.edu.

KEY CONCEPTS

  • A battery’s health is usually determined through electrical testing, but bouncing the battery has become a popular alternative.
  • Researchers have bounced hundreds of AA batteries and found that bouncing increases as the state of charge drops between 80% and 50%.
  • Zinc oxide formation through oxidation of the zinc anode causes the increase in bouncing, but this only tells the user that the battery is not fresh.

References

  1. Canter, N. (2015), “Dendrite-suppressing battery technology,” TLT, 71 (4), pp. 14-15.
  2. Bhadra, S., Hertzberg, B., Hsieh, A., Croft, M., Gallaway, J., Tassell, B., Chamoun, M., Erdonmez, C., Zhong, Z., Sholklapper, T. and Steingart, D. (2015), “The relationship between coefficient of restitution and state of charge of zinc alkaline primary LR6 batteries,” J Mater. Chem. A, 3, pp. 9395-9400.

High power factor thermoelectric material

Researchers evaluate new thermoelectric material based on magnesium, tin and germanium.

Heat generated as a by-product of processes such as combustion in an automobile engine has been typically wasted. With the growing emphasis on efficiency, more attention is now being paid to utilize this energy.


Thermoelectric materials are well-suited to convert heat into electricity, which is a more useful energy form. They function by taking advantage of a difference in temperature in a specific application, such as the tailpipe of an automobile where the temperature of the exhaust gas from a diesel engine is between 300-400 C and ambient temperature to generate electricity. Excellent electrical conductivity along with poor thermal conductivity of the thermoelectric materials is required. The latter is important because the thermoelectric material cannot conduct heat well, otherwise it will be difficult to maintain the temperature difference in a specific application.

Finding efficient thermoelectric materials that can exhibit these features particularly at a temperature of about 300 C has been difficult. Zhifeng Ren, Anderson Chair professor of physics at the University of Houston in Houston, Texas, says, “Thermoelectric materials that have been evaluated include skutterudites and lead chalcogenides. Both types exhibit good thermoelectric figure of merit (ZT) values in the 300-500 C range and low thermal conductivities. But skutterudites exhibit low ZT values below 300 C and lead chalcogenides are toxic, exhibit poor mechanical properties and are thermally unstable above 400 C.”

In a previous TLT article, a more environmentally friendly thermoelectric material known as indium-doped tin telluride was developed by Ren and researchers in Professor Gang Chen’s group at MIT.1 Testing done on indium-doped tin telluride showed a peak ZT of 1.1 at an indium concentration of 0.25 atom percent at 600 C.  Ren points out that there has been too much emphasis placed on developing thermoelectric materials with high peak ZT values. He says, “Figure of merit characterizes how efficiently a thermoelectric material converts heat into electricity. Two other parameters that need to be taken into consideration are the power factor and the leg length.”

Power factor is a measure of how much power can be generated by the thermoelectric material. There is a direct correlation between a higher power factor and the eventual output power generated by the thermoelectric material. Leg length is a measure of the height of the thermoelectric legs. Ren says, “Leg lengths are typically about 2 mm. Reducing the distance more may seem to be a way to improve the output power, but thermal stress can act as a negative factor due to the large temperature gradient.”  In Ren’s view, thermoelectric materials are needed that combine both high ZT values and power factors. Such a material has now been developed.  

Magnesium Tin Germanium Species

Ren—in combination with colleagues at the University of Houston, Boston College and the Lawrence Berkeley National Laboratory—has developed and evaluated a new thermoelectric material based on magnesium, tin and germanium. He says, “In looking for better-performing materials, we knew that researchers had prepared an alloy of magnesium silicon with magnesium tin that shows a peak ZT value of approximately 1 at 500 C. Adjusting the stoichiometry of the mixture increased the ZT value to between 1.1 and 1.3. We decided to develop a magnesium tin compound with germanium because the latter provided to be an ideal element in providing good electrical conductivity and low thermal conductivity.”  The n-type thermoelectric material has the following composition: Mg2Sn0.75Ge0.25 doped by a minor amount of antimony to tune the carrier concentration. It is prepared though ball milling for up to 20 hours followed by hot pressing into bulk samples through a direct current-induced process at a temperature between 600-750 C for two minutes.

This magnesium tin germanium species exhibits a peak power factor of 55 uW cm-1 K-2 and a peak ZT value of 1.4. Ren says, “We believe the power factor represents a significant increase as compared to other thermoelectric materials and in combination with the good ZT value means this species has promise as a material that can generate a high level of electricity from heat.”  One interesting aspect of the magnesium tin germanium species is the presence of nanoinclusions as shown in a photographic microstructure image seen in Figure 2. Ren says, “The nanoinclusions play a role in limiting the thermal conductivity in the thermoelectric material.”

Ren will be continuing to evaluate new thermoelectric materials that display both high power factors and high ZT values. He says, “The currently prepared magnesium tin germanium species meets our initial goal of a material with a peak power factor above 40 and a ZT value greater than one. We are looking to find materials that exhibit the highest possible power factors and ZT values. Currently, we are evaluating new materials with maximum power factors above 55 and even above 100. The latter material has promise, but unfortunately has a low ZT value and high thermal conductivity.”

Further information on the magnesium tin germanium species can be found in a recent article2 or by contacting Ren at zren@uh.edu.

KEY CONCEPTS

  • Thermoelectric materials that exhibit excellent electrical conductivity and poor thermal conductivity at a temperature of about 300 C have been difficult to find.
  • A new thermoelectric material based on magnesium, tin and germanium with a minor amount of antimony has been prepared that exhibits a high peak figure of merit value and a high peak power factor.
  • This material contains nanoinclusions that play a role in limiting thermal conductivity.

References

  1. Canter, N. (2013), “Environmentally friendly thermoelectric material,” TLT, 69 (11), pp. 13-14.
  2. Liu, W., Kim, H., Chen. S., Jie. Q., Lv, B., Yao, M., Ren, Z., Opeil, C., Wilson, S., Chu, C. and Rhen, Z. (2015), “N-type thermoelectric material Mg2Sn0.75Ge0.25 for high power generation,” Proceedings of the National Academy of Sciences, 112 (11), pp. 3269-3274.

Joe: Insert Figure 2.

Caption: Figure 2. Nanoinclusions, such as the one shown in this image, limit the thermal conductivity and as a result help to boost the performance of a new thermoelectric material based on magnesium, tin, germanium and antimony. (Figure courtesy of the University of Houston.)

Hydrogen production from low-cost biofuel

An alternative strategy more efficiently produces hydrogen from biomass.

Interest in hydrogen fuel-cell cars is growing as Toyota introduced a hydrogen-powered vehicle in Japan in December 2014. The vehicle will be available in Europe and North America in the second half of 2015.


For hydrogen-powered vehicles to establish a significant presence in the automotive industry, several negative concerns will have to be overcome. One issue is that lack of sufficient infrastructure in the form of hydrogen refueling stations. In a previous TLT article, the development of a transient-flow facility to evaluate flow meters used in dispensing hydrogen as a fuel is discussed.1

A second issue is finding a cost-effective process for generating hydrogen. Dr. Percival Zhang, professor in the department of biological systems engineering at Virginia Tech in Blacksburg, Va., discusses some of the currently available options. He says, “Thermochemical production options such as electrolysis of water or the use of solar energy are expensive and suffer from low conversions.”  Currently, hydrogen is produced mainly through the use of hydrocarbon fuels. But researchers are also looking to improve conversions through the use of enzymes. In a previous TLT article, the first-ever conversion of a hydrocarbon into useful energy was achieved with an enzyme cascade.2 The researchers converted the kerosene-based jet fuel known as JP-8 at room temperature into alcohols and then into aldehydes through the use of a biofuel cell. Power densities as high as 3 milliwatts per square centimeter (mW/cm2) were found with the biofuel cell.

Zhang says, “The ideal fuel to use in producing hydrogen is biomass, which is more abundant and more evenly distributed globally than hydrocarbon fuels. Based on data obtained in 2011, over one billion tons of dry biomass could be harvested in the USA by 2050.”  The components present in biomass that can best be converted to hydrogen are the fermentable sugars present in plant cell walls. Over 90% of these sugars consist of glucose (C6) and xylose (C5).

Zhang says, “Current methods for converting biomass to hydrogen suffer from low yields. One mole of glucose can be converted to four moles of hydrogen, but the yield is typically 30%, which means that two-thirds of the chemical energy used is wasted.”  Using the enzymatic pathway within a microorganism is the logical first choice in developing an efficient way to produce hydrogen. But the process has a number of disadvantages. Zhang says, “In whole-cell biosystems, the priority for the living organism is to duplicate itself, not to produce a substance desired by researchers. Other difficulties with this approach are the need to transport the desired substance across a cell membrane and the restrictions on varying reaction conditions to optimize yields.”

the need exists for an alternative strategy to more efficiently produce hydrogen from biomass. Such an approach has now been developed.

In Vitro Metabolic Engineering

Zhang and his colleagues, including lead author, Dr. Joe Rollin (see Figure 3), have developed a very efficient enzymatic pathway for converting glucose and xylose simultaneously to hydrogen using a technique known as in vitro metabolic engineering. Zhang says, “In-vitro metabolic engineering involves constructing an enzymatic pathway outside of a living cell to conduct complex biochemical reactions. It provides us with a good deal of flexibility to maximize the reaction rates of each step in the process without being restricted by the microorganism.”

The researchers are able to achieve this result using a relatively low-cost and widely available biomass feedstock, the husks and stalks of corn plants. Cellulose and xylan (the main component of hemicellulose) represent the two main sources of biomass sugars.

A four-module pathway was designed that can theoretically lead to yields of 12 moles of hydrogen per mole of glucose and 10 moles of hydrogen per mole of xylose. Zhang says, “Our process is able to simultaneously convert glucose and xylose, which previously have been difficult to co-react and also to separate.”

The pathway directly converts cellulose into hydrogen and converts xylose into a glucose derivative that can then be plugged into the hydrogen generation pathway. Promising results were initially found that led the researchers to develop a kinetic enzymatic model to maximize hydrogen formation. Zhang says, “We used typical chemical engineering to change reaction conditions and then validated predictions from the modeling through experimentation.”

The process is run under mild conditions that include atmospheric pressure and a relatively low temperature between 50 C-60 C. Hydrogen can be readily isolated from the aqueous process because it is a gas.

The result is that hydrogen formation is increased three-fold to 32 mmoles of hydrogen per liter hour. Further adjustment of the substrate, enzyme concentrations and the reaction conditions increases production to 54 mmoles of hydrogen per liter hour.

Zhang believes that the productivity of this process is comparable to that of industrial biogas and hydrogen production. He says, “We have now validated this enzymatic pathway on a small scale, but now we are looking to increase production of a hydrogen to a liter scale and then ultimately to a commercial scale. In addition, we are also seeking to conduct the enzymatic reaction in one reaction vessel.”

Additional information can be found in a recent article3 or by contacting Zhang at ypzhang@vt.edu. n

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.

KEY CONCEPTS

  • Biomass, the most widely available fuel for producing hydrogen globally, contains a high percentage of the fermentable sugars, glucose and xylose.
  • In vitro metabolic engineering has been used to efficiently convert these sugars into hydrogen through the development of an enzymatic pathway.
  • Adjustment of the substrate, enzyme concentrations and the reaction conditions led to an increase in hydrogen production to 54 mmoles per liter hour.

References

  1. Canter, N. (2014), “Testing meters used to dispense hydrogen fuel,” TLT, 70 (10), pp. 15-16.
  2. Canter, N. (2015), “Room temperature biofuel cell,” TLT, 71 (2), pp. 14-15.
  3. Rollin, J., Campo, J., Myung, S., Sun, F., You, C., Bakovic, A., Castro, R., Chandrayan, S., Wu, C., Adams, M., Senger, R. and Zhang. Y-H. (2015), “High-yield hydrogen production from biomass by in vitro metabolic engineering: Mixed sugars coutilization and kinetic modeling,” Proceedings National Academy of Sciences, 112 (16), pp. 4964-4969.