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Journal of Applied Electrochemistry

, Volume 48, Issue 6, pp 627–637 | Cite as

Electrochemical energy storage for renewable energy integration: zinc-air flow batteries

  • B. Amunátegui
  • A. Ibáñez
  • M. Sierra
  • M. Pérez
Research Article

Abstract

A 1 kW–4 kWh zinc-air flow battery has been built at Técnicas Reunidas facilities. The battery is divided in three different stacks connected in parallel, each of them comprising 20 cells connected in series and 0.25 m3 of electrolyte. The main challenges found on scaling up include the necessity of using three electrodes per cell, electrolyte leakage, ohmic resistance and gas diffusion electrode flooding. The three electrode solution is not optimal because it adds mechanical complexity and cost but it is a robust solution to evaluate the performance of the system as a baseline. During pilot plant operation, a phenomenon called shunt currents has revealed to reduce coulombic efficiency by 18%. In order to better understand this phenomenon, a mathematical model has been developed and it has been validated using real measurements taken on one of the 20-cell stacks. The major part of the challenges were solved and technical viability has been evaluated. However, efficiency, durability and the bifunctional air electrode are the challenges to overcome before this technology becomes ready for commercialization.

Graphical Abstract

Keywords

Zinc-air Flow battery Scale-up Energy storage Shunt currents Renewable energy integration 

1 Introduction

High penetration of intermittent renewable energy in electrical power networks will require increasing amounts of reserve capacity, currently provided by conventional gas fired peaking power plants [1]. Redox flow batteries could become a low cost, low emissions alternative to store electrical energy and provide the flexibility required to achieve high renewable energy penetrations [2, 3].

The potential advantages of zinc-air flow batteries are high theoretical energy density, safety, low cost and environmental friendliness [4]. The main technical obstacles to their development are low round-trip efficiency due to oxygen overpotentials and low durability due to air electrode flooding and corrosion and non-uniform zinc plating.

The half-cell and overall cell reactions are shown in equations (1), (2) and (3).
$${\text{Zn}}+4{\text{O}}{{\text{H}}^ - } \leftrightarrow {\text{Zn}}({\text{OH}})_{4}^{{2 - }}+2{{\text{e}}^ - }\quad {{\text{E}}^0}= - {\text{1}}.{\text{2 V}}$$
(1)
$$\frac{1}{2}{{\text{O}}_2}+{{\text{H}}_2}{\text{O}}+2{{\text{e}}^ - } \leftrightarrow 2{\text{O}}{{\text{H}}^ - }\quad {{\text{E}}^0}=0.{\text{4 V}}$$
(2)
$${\text{Zn}}+\frac{1}{2}{{\text{O}}_2}+{{\text{H}}_2}{\text{O}}+2{\text{O}}{{\text{H}}^ - } \leftrightarrow {\text{Zn}}({\text{OH}})_{4}^{{2 - }}$$
(3)

Operation of a zinc-air flow battery is illustrated in Fig. 1. The electrolyte in the tank is a solution of zinc oxide in potassium hydroxide. The electroactive species is the zincate ion \({\text{Zn}}({\text{OH}})_{4}^{{2 - }}\). During charge zinc is electroplated on the negative electrode substrate and oxygen is evolved at the air electrode. During discharge zinc is oxidised to zincate ions and oxygen from a stream of atmospheric air is reduced to hydroxide ions.

Fig. 1

Electrochemical reactions in the zinc-air flow battery

Unlike pure flow batteries, such as vanadium redox flow batteries (VRFB), zinc-air flow batteries are hybrid or flow-assisted batteries, because power and energy are not completely decoupled. Energy stored in the battery depends on the quantity of electrodeposited zinc and its thickness is physically limited by the gap between the air and zinc electrodes. A flowing electrolyte improves zinc deposit morphology and removes gases and heat generated inside the cell.

Técnicas Reunidas is developing zinc-air flow battery technology for stationary energy storage applications and has aimed to demonstrate the technical viability in a 1 kW–4 kWh zinc-air flow battery pilot plant. From our knowledge, small and medium sized zinc-air flow battery cells have been reported in the literature [5, 6, 7, 8] but a pilot plant of this size has never been tested and acknowledged before.

Many attempts have been made before to develop a bifunctional air electrode using carbon and non-carbon materials [9, 10, 11, 12, 13, 14, 15]. So far all electrodes suffer from electrolyte flooding, high overpotentials (ΔV between charge and discharge > 0.6 V) and limited durability (few hundreds of hours or cycles). Técnicas Reunidas is developing a proprietary carbon-free bifunctional air electrode technology but it was not ready to be implemented in this pilot plant.

The pilot plant cells have three electrodes: a negative zinc electrode and two positive air electrodes, a dimensionally stable anode (DSA) type electrode for oxygen evolution and a gas diffusion electrode (GDE) for oxygen reduction.

This paper reports details of the pilot plant design and results of power, energy and durability tests. A shunt current model is validated against measurements in the electrolyte distribution circuits.

2 Experimental section

2.1 Pilot plant

The pilot plant is designed to deliver a peak power of 1 kW and a nominal energy of 4 kWh. It is divided into three identical stacks electrically connected in parallel. Each stack is made up of 20 cells connected in series. Each cell is 40 mm wide. Considering spacers in between cells (30 mm) the total stack length is 1460 mm. Each cell has an active area of 500 cm2.

Figure 2 explains the pilot plant design and Fig. 3 shows a picture of the pilot plant as built. The pilot plant design specifications are shown in Table 1.

Fig. 2

Pilot plant description

Fig. 3

Zinc-air flow batteries pilot plant

Table 1

Pilot plant discharge design specifications

Power

1 kW

Energy

4 kWh

Voltage

20 V

Current

50 A

Capacity

200 Ah

Electrolyte volume

750 l

Depth of discharge

20%

Oxygen required for the oxygen reduction reaction comes from atmospheric air that is circulated in parallel through the air compartment of the cells. Atmospheric air is CO2-filtered using an adsorption dryer (Parker K-MT 6). Oxygen depleted air is released to the atmosphere.

Each stack has its own electrolyte tank. The electrolyte recirculates through the negative and positive cell compartments with the aid of two pumps. Cells within the stacks are hydraulically connected in parallel. A hydraulic parallel configuration was chosen in order to reduce liquid pressure on the GDEs. Figure 4 shows the inlet manifold; the primary manifold is square shaped and it has a cross sectional area of approximately 1200 mm2 and the secondary manifold (not shown on the figure) is composed of 8 mm diameter PTFE tubes. Rotameters and valves placed between the primary and secondary manifolds were used to guarantee an equal flow distribution.

Fig. 4

Electrolyte tank and cell stack

2.2 Materials and equipment

The electrolyte was a solution of 50 g/L zinc oxide (SIGMA 205532, ReagentPlus, < 5 μm particle size, 99.9%) in 7 M KOH (Unid Co. Ltd., potassium hydroxide, pellet) in deionized water (Comercial Denpol S.L.). A proprietary additive for zinc electroplating was used. In each tank there were 250 l of electrolyte.

The zinc electrode substrate is a flat nickel sheet (Goodfellow Cambridge Ltd., 99.0% pure, annealed). The oxygen evolution electrode is a nickel mesh coated with a nickel and cobalt oxide catalyst. The oxygen reduction electrode is a porous layer of carbon, PTFE and manganese oxide catalyst on a nickel wire mesh with an expanded PTFE backing layer. The total GDE thickness is 0.5 mm. The DSA and GDE are provided by specialised manufacturers and no more technical details are available. The oxygen evolution electrode is placed between two separators to protect the other electrodes from oxidation (Agfa, Zirfon Perl UTp 500). The electrodes (200 × 300 mm) are shown in Fig. 5.

Fig. 5

Electrodes. From back to front: nickel sheet, nickel mesh and gas diffusion electrode

In order to quantify the magnitude of the shunt currents and validate the model, DC leakage current sensors from ChenYang Technologies were acquired. Four of them were 20 mm diameter and another four were 85 mm diameter to measure shunt currents on the secondary and the primary circuit respectively. Their measurement range was ± 100 mA. Shunt currents were measured for every cell at the secondary circuit and only for external cells on the primary circuit (Fig. 6).

Fig. 6

DC leakage current sensor on primary and secondary electrolyte circuits

Electrolyte temperature is controlled with a heating resistance and electrolyte level and hydrogen concentration in air are monitored by sensors placed in and above the tank. The plant is controlled with National Instruments hardware and LabVIEW software. A Bitrode LCV system is used to charge and discharge the battery.

3 Shunt current model

Some authors have deeply studied and reported the shunt-current phenomenon and its consequences [16, 17, 18, 19]. Flow cells electrically connected in series suffer from shunt currents. As the electrolytes have high conductivity, electrolyte connections offer additional current paths between all battery cells which are supplied via a common hydraulic circuit. During charge shunt currents lower the effective charging current that flows through each cell and waste a part of the energy which has been injected into the battery. During discharge the current that effectively discharges the battery is increased by shunt currents without increasing the current that is available at the battery string clamps. Therefore, some of the energy that has been stored in the battery is wasted by shunt currents [12]. Corrosion of electrodes and other battery components have been recently reported as shunt currents effects [20], but these effects fall out of the scope of this project.

Figure 7 represents the additional current paths between all battery cells supplied via the primary and the secondary hydraulic circuit.

Fig. 7

Brief explanation of the shunt currents concept

A shunt currents equivalent electric circuit from [13] was adapted for 20 cells and the result is shown in Fig. 8. Each cell and each electrolyte path is considered an ohmic resistance (a) and current flow through the cell resistances (b) having stack voltage as the driving force (c). In Fig. 10 R stands for resistance, I for current and U for voltage. P and S refer to primary and secondary hydraulic circuit and A and C for anode and cathode, respectively. N is the total amount of cells, which in our case is 20.

Fig. 8

Scheme of the shunt currents model

A total of 98 variables and equations are obtained by applying Kirchoff’s Laws to set out equations at every node and mesh. Cell voltage (U1…Un) for charge was taken as 2 and 1.6 V for discharge.

4 Results and discussion

4.1 Pilot plant power and energy tests

The pilot plant was designed for 1 kW peak power. However, due to ohmic losses, maximum peak power was found to be 0.82 kW at 65 A, equivalent to 43 mA/cm2 (see “initial” curve at Fig. 9). These ohmic losses take place in the long cell interconnectors and contactors necessary to switch between positive air electrodes. Using shorter interconnectors could solve this problem in the future.

Fig. 9

Power curves: initial (black) and after 5 days (grey)

Electrolyte leakage and GDE flooding are two of the main challenges on scaling-up. Electrolyte leakage was minimized using PTFE gaskets but GDE flooding effects can also be seen in Fig. 9. After 5 days on open circuit, the battery performs as shown on the curve labelled “5 days after” on Fig. 9 where a decrease in the power delivered by the battery can be observed (from 0.82 to 0.60 kW peak power). GDE design, uniform pressure distribution along the cell stack and pressure compensation on the air electrode are the key factors to control and prevent this situation in the future. In this sense, it is important to consider the electrolyte height on the air electrode due to the outlet manifold, not adding unnecessary height on the way back from the cells to the tank.

Regarding energy specifications, only 1.8 kWh were delivered at medium power (0.5 kW) whereas 3.0 kWh were delivered at low power (0.24 kW).

The experimental results shown on Table 2 suggest zinc-air flow battery technology is suitable for long duration energy storage applications where energy is delivered at medium to low power density.

Table 2

Pilot plant experimental test results

 

Medium

Low

Power

0.50 kW

0.24 kW

Energy

1.8 kWh

3.0 kWh

Voltage

11 V

16 V

Charge current

37.5 A

37.5 A

Discharge current

45 A

15 A

Capacity

173 Ah

189 Ah

4.2 Single stack accelerated durability test

An accelerated durability test was made using only one cell stack subjected to the following cycle: 5 h charge at 10 A (20 mA/cm2) followed by short cycles of 5 min charge at 10 A (20 mA/cm2) and 10 min discharge at 5 A (10 mA/cm2). When the discharge voltage reaches 1 V cutoff the stack is discharged at 1 V until the current drops to 0.5 A. Short cycles were necessary to test durability in a reasonable amount of time. Current and voltage profiles are represented in Fig. 10.

Fig. 10

a Voltage and b current profiles, accelerated single stack durability test

At the beginning of the test the charge voltage is approximately 46 V (2.3 V per cell) and the discharge voltage is approximately 22 V (1.1 V per cell). The voltages include the drop in the cell interconnections, approximately 1.2 V in charge and 0.6 V in discharge. This yields a voltage efficiency of 47.8%. The large polarization of the oxygen electrodes are responsible for this low voltage efficiency.

After about 610 h cell #20 short-circuited and after 670 h cell #18 also short-circuited. It was decided to stop the test after 760 h (some open circuit periods have been omitted) and 2000 cycles. The battery was discharged and the cells were opened and inspected. Cells #1 to #3 and #16 to #20 were partially covered in zinc. Zinc in the rest of the cells (#4 to #15) had been completely discharged. This build-up of zinc in the first and final cells of the stack is a result of shunt currents.

Inspection of the back-side of the GDEs after the test showed they were partially flooded. Flooding of the air electrodes was also evident during operation as shown by decreasing cell voltages and a trickling of electrolyte in the air outlet tubes towards the end of the test. Progressive GDE leakage made oxygen diffusion into the electrode more difficult reducing the efficiency over time. Hydrogen evolution was not observed directly but a mixed cell voltage indicated that oxygen reduction reaction and hydrogen evolution could be taking place at the same time.

Figure 11 shows beginning of test efficiencies and end of test efficiencies. Voltage efficiency (Ev) decreased over time due to GDE flooding and coulombic efficiency (Ec) due to zinc accumulation and short-circuits.

Fig. 11

Coulombic and voltage efficiency evolution with the number of cycles

According to our own experience, the coulombic efficiency of a single 500 cm2 zinc-air flow cell is about 95% in the same conditions. However, results show that the initial value of coulombic efficiency of the 20 cell stack was already 13% lower. This coulombic efficiency loss is caused by the shunt current phenomenon.

Regarding electrolyte degradation, Fig. 12 shows a photograph from the electrolyte after the test. The electrolyte is transparent and shows no visible degradation. Dissolved species were analyzed and 30.10 g/L of carbonates were found. Carbonates are due to exchange with CO2 in the air because the electrolyte tank was partially open to the air to allow hydrogen and oxygen generated during charge to easily escape. At this concentration carbonates are still soluble but electrolyte contact with atmospheric should be prevented in future designs.

Fig. 12

Electrolyte after accelerated single stack durability test

4.3 Shunt current simulations

Simulated results of shunt currents in the primary and secondary electrolyte circuits at 5 A are represented in Fig. 13.

Fig. 13

Shunt currents at a primary circuit and b secondary circuit

4.4 Shunt current measurements

Comparison between real data and the model at 5 A is presented in Fig. 14. Average absolute error for charge and discharge is 3 and 7 mA respectively. In the case of 2.5 A, average absolute errors were 2 and 8 mA respectively. The model has been considered to have enough accuracy and its results to be realiable in the range 2.5–10 A per cell stack (the range of currents in which the ZAESS pilot plant has been tested) .

Fig. 14

Real data and model comparison at 5 A for a charge and b discharge

Average absolute error for charge and discharge increased to 12 and 8 mA respectively at very low current density (1 A = 2 mA/cm2). This deviation may probably be related with some electrochemical process but it needs to be further investigated.

4.5 Coulombic efficiency losses

As stated before, during charge, shunt currents decrease the effective charging current that flows through each cell whereas during discharge, the current that effectively discharges the battery is increased. Figure 15 illustrates the current flowing through each cell at 10 A charge and 5 A discharge according to the model. Current vs cell number describes a parabola that has its minimum (in the case of charge) and its maximum (in the case of discharge) in the central cells (cell #10 and #11).

Fig. 15

Intensity per cell at a 10 A charge and b 5 A discharge

Current values (10 and 5 A) have been selected to compare modelled and experimental efficiency losses. According to Sect. 4.2, experimental efficiency losses due to shunt currents corresponds to 13%.

Efficiency losses can be calculated using data from Fig. 15 and applying Eqs. 4 and 5. IZ10 corresponds to the current passing through the central cell of the stack.
$$\frac{{\left( {10{\text{ A}} - 9.01{\text{ A}}} \right)}}{{10{\text{ A}}}}=\frac{{I~{\text{nominal}} - Iz10}}{{I~{\text{nominal}}}}=9.9\%$$
(4)
Equation 4 coulombic efficiency losses, 10 A charge.
$$\frac{{ - \left( {5{\text{ A}} - 5.42{\text{ A}}} \right)}}{{5{\text{ A}}}}=\frac{{ - (I{\text{ nominal}} - Iz10)}}{{I{\text{ nominal}}}}=8.4\%$$
(5)
Equation 5 coulombic efficiency losses, 5 A discharge.

Overall, modelled efficiency losses are 18.3%, which is not far from the experimentally calculated value of 13%.

5 Conclusions

  • Operation of a 1 kW, 4 kWh zinc-air flow battery pilot plant with three stacks working in parallel has been demonstrated.

  • The plant has operated safely in all circumstances. There has been no leakage of electrolyte outside the system boundaries and no fire incidents, even during short-circuit events.

  • The maximum power and energy delivered by the pilot plant have been 0.82 kW and 3.04 kWh compared to the objectives of 1 kW and 4 kWh. The maximum round-trip energy efficiency obtained has been 40%. The maximum number of cycles achieved is 2000 cycles.

  • Degradation of the system is due to zinc build-up inside the cells, short-circuiting and air electrode flooding. Zinc build-up and short-circuiting is a direct consequence of shunt currents. The electrolyte does not degrade except for a small amount of carbonate formation due to exchange with CO2 in the air because the electrolyte tank was partially open to the air to allow hydrogen and oxygen generated during charge to easily escape.

  • The coulombic efficiency has been lower than expected due shunt current losses. These losses can be reduced by improving the stack design.

  • Voltage efficiency is already limited by oxygen electrochemistry and it has been lower than expected due to ohmic losses in the cell interconnections.

  • Whereas engineering aspects can be overcame with moderate efforts, oxygen electrochemistry and bifunctional air electrode—substrate and catalyst—are the main obstacles to the development of this technology.

Notes

Acknowledgements

The LIFE ZAESS project is partially funded by the European LIFE Programme, Grant Agreement LIFE13 ENV/ES/001159.

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Copyright information

© Springer Science+Business Media B.V., part of Springer Nature 2017

Authors and Affiliations

  • B. Amunátegui
    • 1
  • A. Ibáñez
    • 1
  • M. Sierra
    • 1
  • M. Pérez
    • 1
  1. 1.División de Desarrollo de Tecnologías Propias (Centro Tecnológico José Lladó)Técnicas ReunidasSan Fernando de HenaresSpain

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