Open source all-iron battery 2.0

Graphical abstract


Hardware in context
Inexpensive, safe energy storage has many applications. Renewable energy can only displace a percentage of fossil fuel energy unless it can be efficiently and cost-effectively stored [1]. Lithium-ion batteries have emerged as the dominant energy storage system for mobile applications, but they have safety [2] and cost issues [3]. For stationary applications, it may be advantageous to move to a cheaper, safer, but lower energy density chemistry. We demonstrated a small-scale all-iron battery [4]. This cell was highly rechargeable with modest but useful energy density suitable only for low-power applications. We here report an improved version of this chemistry with similar energy density and much higher power density as well as a more convenient form factor. Several all-iron batteries have been demonstrated in the literature and at large scale for commercial applications. For a recent summary, see Anarghya et al. [5]. Such batteries are often implemented as flow batteries [6,7]. Flow batteries have the advantage of decoupling the energy capacity (determined by the size of the electrolyte tank) from the power capacity (determined by the size of the flow cell). This comes at the cost of relatively complex plumbing and pumps.
The all-iron battery presented here is a conventional battery and not a flow battery. Although the chemical reactions that move and store electrons are the same (i.e., the oxidation of Fe and the reduction of Fe 3+ ), the physical design is much simpler. Rather than using a high-performance flow cell to achieve practical power levels, our approach is to modify the anode and cathode materials to achieve useful energy and power performance.

Hardware description
We describe a design for an energy storage battery with an iron-based anode and cathode. The overall strategy is shown in Fig. 1. Iron metal is oxidized to ferrous iron at the anode while ferric iron is reduced to ferrous iron at the cathode allowing electrons to flow. This system provides high rechargeability and customizability, but the power and energy density are low compared to commercial batteries. We also describe the simple synthesis of a suitable separator membrane made from cellulose acetate impregnated paper. This separator keeps the anode and cathode chemistry separate but allows ion transfer. The area of the polymer sheet determines the maximum current. For more power, the battery should be thin with a large surface area. If less power and more energy capacity is needed, the battery can be thicker with a smaller separator membrane area (i.e., a thicker acrylic plastic or multiple layers can be used).
Our primary improvement to the original open-source all-iron battery is to increase the current density. The original formulation [4] was a mix of iron chloride and potassium sulfate adjusted to pH 7.5 with sodium hydroxide. Raising the pH causes a solid to precipitate. The conductivity of the precipitated particles is low and the concentration of iron ions in solution is also low. This limited the battery's maximal electrical current. We determined that the maximal discharge and charge current could be significantly increased by adding conductive carbon. By decreasing the distance between the carbon electron conductor (originally, carbon felt) and the iron precipitate, we were able to increase power density. Adding ketjen black  [Conductive Carbon] to the anode and cathode pastes allows electrons to migrate to within a much shorter distance of the active material through a conductive matrix (see Fig. 2) We added a range of concentrations of Ketjen black [conductive carbon] to the precipitate and determined that 4% carbon by mass was sufficient to reach near-optimal performance. More than 4% carbon means that there is less room for iron and so higher concentrations are not recommended. As a secondary improvement we sought to simplify the original iron battery chemistry. In the original chemistry, the iron salt solutions contained dissolved iron, sodium, potassium, chloride and sulfate ions. This is shown at left, using the previous best membrane. The result was a stable chemistry with a large voltage drop (Fig. 3, far left). We replaced the sodium polyacrylate membrane with an improved membrane (cellulose acetate drop cast on printing paper). This improved the power density resulting in less internal resistance and a smaller voltage drop during discharge. We also systematically eliminated each component to determine which were necessary. Sulfate, chloride and potassium ions are necessary to generate a high performance, rechargeable cell. Only the removal of the sodium ions was tolerated with high performance (see Fig. 3, No Na + ). Eliminating any of the other ions resulted in cell degradation and a changing charge-discharge profile. The newest formulation includes iron chloride and potassium sulfate precipitated with potassium hydroxide. Relative to the original chemistry, the new formulation is slightly simpler. However, depending on availability of materials, other formulations are possible (e.g., the use of ammonium iron sulfate) so long as the appropriate ions are dissolved in solution prior to precipitation.  [9] Open source environmental data loggers [10] Open source weather stations [11] 3. Design files Design Files Summary: Design file name File type Open source license Location of the file Battery Design: The design files are included online at https://doi.org/10.17605/OSF.IO/YV2E6. The specified design includes a cell with external dimensions of 165 mm Â 85 mm Â 5 mm and an internal active volume of 9.6 ml. The cell is assembled from four acrylic parts and held together by a combination of solvent welding, glue and machine screws.

Bill of materials
The following is for 3 cells containing 8 ml each of the electrolyte pastes.   *Above mentioned prices are exclusive of local tax and shipping.

Build instructions
Short build instructions are as follows (detailed build instructions are included as Supplemental material). The cell housing was laser-cut from acrylic plastic (using a suitable CO 2 laser cutter such as our BossLaser 80 W or acquired from a commercial service like Ponoko). The design can be adjusted to a wide range of dimensions. The CAD designs for the laser cut acrylic plastic (PMMA) are shown in Fig. 4A. Two copies of each part should be made. The internal enclosed volume determines the energy storage capacity. The surface area determines the power.
Graphite foil [Elec1] was cut to shape with tabs for making an electrical connection with alligator clips. The graphite foil was glued to a flat acrylic sheet using clear seal [Glue]. A second acrylic sheet was cut with a central hole slightly smaller than the graphite foil and glued to the first. The design leaves an open cavity for active material (i.e., either anode electrolyte paste or cathode electrolyte paste) as illustrated in Fig. 4B. A separator membrane was prepared by dissolving cellulose acetate in acetone, applying the solution to paper along with an optional small volume of Nafion, and allowing to evaporate (see Fig. S.2).
Electrolyte paste was prepared by dissolving iron chloride (ferric for cathode and ferrous for anode) in water at the appropriate proportions. To this, a measured quantity of potassium hydroxide solution was added to reach pH 7.5. To the resulting precipitate, ball-milled ketjen black [conductive Carbon] was added to reach 4%. The anode cavity was filled with finely divided steel wool [Fe Metal] and then packed with anode paste. The separator membrane was placed over the anode cavity. The cathode cavity was filled with cathode paste, then the cathode assembly was pressed over the separator membrane. The full assembly was then fastened together with machine screws at the perimeter and sealed by solvent welding the acrylic plastic with methylene chloride. Copper tape was used to reinforce the graphite foil tabs which extend beyond the edge of the acrylic. As assembled, the cell is in the charged state and is ready to generate electrical current.

Operation instructions
Once the battery is assembled it can be discharged and charged like any battery or electrochemical cell. This can generate the necessary electric current for a given electronic device. For discharge, the battery can be used to power any direct current electrical device that draws less than the maximum current for the cell (4.5 mA for the 8 ml cell). Multiple cells can be wired together in series or in parallel to generate higher voltage or current. For charging, we recommend a constant, regulated charging voltage of 1.1 V per cell in series.
As one example application, we used a single cell with a ''Joule thief" circuit to boost the voltage. This circuit converts the low voltage DC to AC and drives the AC voltage through a step-up transformer. The result is sufficient for a modest brightness LED (see Fig. 5). As an alternative, five such batteries could be wired in series. They could act as a storage reservoir for solar energy to operate a light at night. This battery may represent some advantages over lithium ion batteries including lower cost and lower environmental impact of production as well as low toxicity and high recyclability.
The iron battery has significant internal resistance. Beyond its maximal current, the internal resistance results in a significantly lower operational voltage. The lower voltage reduces the available power. The maximal energy density for the 8 ml cell was 0.25 mW per ml. A battery should be designed and constructed with this limitation in mind. Depending on power requirements, a battery of the appropriate size will be needed.
During the preparation of reagents, there are several safety considerations. Potassium hydroxide is caustic and should not come into contact with skin. Ferric chloride is corrosive and strongly acidic and should not be allowed to come into contact with skin. Additionally, ferric chloride will degrade metals it contacts. Once the components are neutralized by mixing in the proper proportions, they are comparatively inert. When the cell is no longer needed, it can be considered non-hazardous waste (subject to local regulations). The plastic, neutral pH water, iron salts, and potassium salts are all non-hazardous waste (comparable to normal food and household waste).

Validation and characterization
The 8 ml version of the iron cell was tested for its total capacity and maximal power. The cell is shown in Fig. 6A. This cell contains 4 ml each of anode and cathode paste. It has a membrane of 48 cm 2 . To determine the total capacity, the cell was discharged at a constant current of 1 mA until the potential dropped to 75 mV as shown in Fig. 6B. The Total capacity on first discharge was 80 mAh. With 0.97 g of ferric iron acting as a cathode, we would expect a maximum of 160 mAh. Thus, we are  accessing 50% of the iron in the cathode. After 1000 cycles, the capacity was re-measured and was 72 mAh, showing a loss of only 10%.
To determine the maximal power, the 8 ml cell was discharged at a range of currents and the voltage was measured. The power was calculated at each condition by multiplying the current and voltage. The result was a graph of power as a function of current, as shown in Fig. 6C. The maximal power was~2 mWatts or 0.25 mW/ml at 4.5 mA. This is significantly lower than typical commercial batteries, but nearly two orders of magnitude larger than our previous best.
Capabilities and limitations: Voltage: Although the cell shows an open cell potential of 1.1 V, the voltage may drop as low as~200 mV during high current discharge. This internal resistance reduces available power. Volumetric Capacity: 10 Ah/L. This limits the battery to stationary applications. Energy density: >3 Wh/L Power density: 250 mW/L Cycle stability: Stable for greater than 1000 cycles Price: $60 per watt-hour for 8 ml cells (as built, including laser-cut housing) Price: $0.36 per watt-hour for active materials at wholesale prices Human and animal rights