(5i) Slurry Electrode for An All-Iron Flow Battery for Low Cost Large-Scale Energy Storage

Slurry
Electrode for an All-Iron Flow Battery for Low Cost Large-Scale Energy Storage

Tyler
J Petek, Jesse S Wainright, and Robert F Savinell

Case
Western Reserve University

Department
of Chemical Engineering

10900
Euclid Avenue, Cleveland, OH 44106 USA

The
energy production market must change its output to match real time demand; making
production very inefficient and restricting the grid's ability to rely upon
intermittent renewable sources, i.e. solar and wind. Both of these issues can
be alleviated by grid scale energy storage that could store energy when it is
available and distribute energy as needed.

In
typical batteries, all of the active materials are stored inside the battery.
This directly couples the storage capacity and power so there is very little
economic advantage to scaling, making standard batteries cost prohibitive for
grid level energy storage. In redox flow batteries, the electrochemically
active species are dissolved in an electrolyte and stored externally. During
operation, the chemicals are pumped through the active cell where the reactions
take place. Now, the energy storage capacity is dictated by the amount of
chemicals stored and the power is dictated by the size of the cell. This allows
redox flow batteries to have a significant advantage of scaling and drives down
the cost significantly.

The
all-iron flow battery is a potentially low cost energy storage device that
utilizes domestically abundant materials (iron is the only active element),
cheap aqueous electrolytes, and inexpensive separators. During charging,
ferrous iron (Fe²⁺) is oxidized at the positive electrode to ferric
iron (Fe³⁺) while ferrous iron is reduced to form iron metal at the
negative electrode. These reactions are reversed during discharge. A schematic
of the all-iron battery is shown in figure 1. This all-iron flow battery has a
nominal cell potential of 1.2 Volts. [1] With conventional electrodes the
energy storage capacity and power rating are coupled because the iron is plated
in the cell. In order to decouple the energy and power ratings of the battery
and regain the economic advantages of a flow battery without losing the
advantages of the all-iron chemistry, a new electrode design needs to be
developed. This project is investigating the use of slurry electrodes in order
to decouple the all-iron battery.

A
slurry electrode is made by flowing
electrically conductive particles in an electrolyte containing the dissolved
iron species.  Iron is plated onto the particles in the negative side while
charging.  The particles can then carry the iron metal out of the cell to be
stored in external reservoirs. This decouples the energy storage capacity and
power rating, regaining the economic advantages of scaling inherent to true
flow batteries. Some known applications of slurry electrodes include waste
water treatment [2] and in zinc-air batteries. [3] A schematic of a slurry
electrode is shown in figure 2.

Slurry
electrodes have been tested utilizing a variety of particles including natural
carbon micro-flakes, multiwall carbon nanotubes, carbon nanofibers, nickel micro-fibers,
and copper micro-particles. All of the slurries tested thus far have exhibited
shear thinning behavior as seen in figure 3. Based upon the viscosity
characteristics, the flow of a slurry electrode was simulated in a 1000 cm²
flow field in SolidWORKS. The parasitic pumping costs for a 5 kW flow battery is
estimated to be less than 1 W at flow rates up to 23 times the stoichiometric flow
rate based upon these simulations. The stoichiometric flow rate is the minimum flow
rate required to sustain the applied current assuming 100% conversion of the
reactant. For these calculations, it was assumed that the 5 kW battery was made
up of 28 cells, each 1000 cm², and operated at 200 mA/cm².

Ideally,
to control the current distribution and to ensure iron deposition occurs on the
slurry particles and not the current collector, the electronic conductivity of
the slurry electrodes needs to be greater than the ionic conductivity of the electrolyte.
The conductivity of the base electrolyte at room temperature is about 120
mS/cm. When pressed into a dry pellet, the conductivity of most of the
particles investigated is 10,000 ? 25,000 mS/cm. However, the electronic
conductivity of the slurries while flowing has been found to be significantly
lower; about 1 ? 40 mS/cm. Figure 4 shows the electronic conductivity of a
variety of slurry electrodes. The three orders of magnitude decrease in
conductivity is thought to be due to particle-to-particle resistances. The
conductivity in figure 4 is shown against the weight percent of particles in
the slurry normalized to the maximum weight percent. The maximum weight percent
is when the particles are no longer wetted when mixed into the electrolyte and
was found experimentally.

While
the slurry electrodes investigated thus far have been shown to have
appropriately low viscosities and pumping costs, their electronic
conductivities are not high enough for flow battery electrode applications. Further
work is being done in novel electrode designs and slurry compositions, as well
as current collector designs, in an attempt to control the deposition of iron
while keeping the overpotentials low.

Acknowledgements

This
is an ARPA-E funded project, contract # DE-AR0000352. The authors would also
like to acknowledge Nicholas Sinclair, Jason Pickering, Matthew Madler, Joseph
Plazek, Mallory Miller, and Mirko Antloga.

References

[1]
LW Hruska and RF Savinell, J. Electrochem. Soc., 128, 18-25
(1981).

[2]
B Kastening, T Boinowitz, M Heins, J. Appl. Electrochem., 27,
147-152 (1997).

[3]
PC Foller, J. Appl. Electrochem., 16, 527-543 (1986).

[4]
CRC Handbook of Chemistry and Physics, 60^th ed. CRC Press: Boca
Raton, FL. 1980: D-237.

[5]
YS Muzychka, J Edge, J. Fluid. Eng., 130, 111201-1 (2008)

Figure
1: Schematic of the all-iron flow
battery. This schematic shows a slurry electrode on the negative side and a
felt electrode on the positive side.

Figure
2: A schematic of a slurry electrode.

Figure
3: This figure shows the normalized
viscosity plotted against shear rate for 20µm natural flake carbon particles at
varying weight percent. The viscosity is normalized by the base electrolyte,

2M FeCl₂, which is about 4 cP at room temperature. [4] The vertical
line at 300 s^-1 indicates the shear rate associated with 5X
stoichiometric flow rates in the 1000 cm² cells that were simulated.
[5]

Figure
4: Electronic conductivity of varying
carbon slurries. Slurry concentrations are normalized to the maximum concentrations
for each type of particle. At the maximum weight percent, the MWCNTs exhibited
an electronic conductivity of 85 mS/cm.