Carbon/Air Thermal Battery: New Materials and Performance
Authors: Cooper John F. 
Published: February 2020

Elemental carbon may be used to generate electrical power through the reaction C + O2 = CO2  in galvanic cells. There is particular interest in carbon conversion in primary cells because of potentially high coulombic capacity (8.9 kAh/kg-C) and efficiency above 60%- ΔH° in molten carbonate electrolyte at 725 °C (1,2). We report two key technical developments: [1] Novel composites based on calcium aluminate were used to make low-cost battery and laboratory components resistant to molten carbonate corrosion. [2] Efficiency vs. power for cells fueled by renewable charcoal and charcoal/fossil-carbon composites were determined by simultaneousmeasurement of anode off-gas composition and anode potential.

Concrete bound by calcium aluminate (CA) cement has long been used in crucibles to transport molten aluminum from smelter to casting plant. These materials resist corrosion by sodium- and potassium carbonates that result from oxidation of trace alkali metals that are co-reduced with aluminum. We modified the structure and composition of industrial materials proven to resist corrosion and with minimal dimensional change at 1250 °C to allow production of battery components by low-cost casting techniques (3). This entailed some sacrifice of strength. We confirmed corrosion resistance of our modified structure using the tri-eutectic (Li, Na, K)2CO3 at 800 °C by prolonged exposure (175 h) and rapid temperature cycling. Our composite accepts common additives (e.g., corn starch) to increase porosity. Permeability and connectivity may also be adjusted; overall strength may be increased with the use of reinforcing materials. (3) We report Darcy's-law constants, porosity, conductivities, microstructure and other properties important to component design. The raw material costs for crucibles for carbon batteries is below $2.00/kg. Such low cost offsets the disadvantage of low power density (50-150 mW/cm2-cross-section) typically found for carbon/air cylindrical cells.

Total energy efficiency of a C/O2(air) cell is the product of coulombic efficiency (effective carbon valence (n) divided by 4) and cell voltage (4). Assuming carbonate ion transference number is unity, effective valence is a simple function of mole fraction, xCO2 = CO2/(CO2 +CO), given by the equation, n = 2 (1+x)/(2-x) (5). Many researchers measure off-gas composition and cell voltage to project efficiencies greater than 60%, but rarely are such determinations made simultaneously, as previously reviewed (6). In our approach, a sample of off-gas is collected in a flow-through syringe and transferred into an inverted graduated cylinder initially filled with 1 M NaOH. Anode potential is continuously monitored. Mole fraction of CO2 is inferred from the reduction of volume (corrected for CO solubility) associated with selective hydrolysis of the CO2 to form sodium carbonate. The extraction is linear with time for excess base and CO2, the reaction being controlled by the first (slow) step in hydrolysis: CO2 + OH- = HCO3
- (7) A similar process is used in the industrial separation of CO from CO2. Base-catalyzed hydrolysis of CO is known, but is far too slow to interfere at ambient temperature.

We examine applications in off-grid communities for powering cell phones and household electronics, LED lighting and small appliances (e.g., hand tools or thermoelectric refrigerators), using modular units of approximately 50 W each. Discharge is continuous; unused energy is stored as phase-change ice packs or in large appliance batteries. Potential is stepped from parallel-connected cells to an output of 5.1 VDC. No separate storage batteries are required. Applications are also envisioned for remote or off-grid sensors (e.g., SCADA, LIDAR), where the power source is available continuously regardless of climate, latitude or time of year.

References

J. F. Cooper, J. R. Selman, K. Witt and C. Jacobson, "Refuelable Carbon/Air Battery for Off-Grid Power," Tech Connect Innovations and Expo; June 2015, Washington DC; (Recipient of National Innovation Award).

K. Hemmes, J. F. Cooper, J.R. Selman, I. J. Hydrogen Energy
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C. Wohrmeyer, C. Alt, N. Kreuels, C. Parr, M. Vialle, "Calcium aluminate aggregates for use in refractory castables", (Paper TP-GB-RE-LAF-016, 35th American Ceramic Soc. Symposium, St. Louis, MO, March 1999). 

J. F. Cooper and J. R. Selman, I. J. Hydrogen Energy, 37 19319 (2012).

J. F. Cooper and J. R. Selman, ECS Trans. 19(14) 15 (2009).

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A. F. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (John Wiley & Sons, NY (1966)).