Aqueous batteries as grid scale energy storage solutions

Abstract

Energy storage technologies are required to make full use of renewable energy sources, and electrochemical cells offer a great deal flexibility in the design of energy systems. For large scale electrochemical storage to be viable, the materials employed and device production methods need to be low cost, devices should be long lasting and safety during operation is of utmost importance. Energy and power densities are of lesser concern. For these reasons, battery chemistries that make use of aqueous electrolytes are favorable candidates where large quantities of energy need to be stored. Herein we describe several different aqueous based battery chemistries and identify some of the research challenges currently hindering their wider adoption. Lead acid batteries represent a mature technology that currently dominates the battery market, however there remain challenges that may prevent their future use at the large scale. Nickel–iron batteries have received a resurgence of interest of late and are known for their long cycle lives and robust nature however improvements in efficiency are needed in order to make them competitive. Other technologies that use aqueous electrolytes and have the potential to be useful in future large-scale applications are briefly introduced. Recent investigations in to the design of nickel–iron cells are reported with it being shown that electrolyte decomposition can be virtually eliminated by employing relatively large concentrations of iron sulfide in the electrode mixture, however this is at the expense of capacity and cycle life.

Full text

Aqueous batteries as grid scale energy storage solutions
Jorge Omar Gil Posada
a,
n
, Anthony J.R. Rennie
a
,Sofia Perez Villar
a
, Vitor L. Martins
a,b
,
Jordan Marinaccio
c
, Alistair Barnes
c
, Carol F. Glover
c
, David A. Worsley
c
, Peter J. Hall
a
a
Chemical and Biological Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, England, UK
b
Instituto de Química, Universidade de São Paulo –C.P. 26077, CEP 05513-970, São Paulo, SP, Brazil
c
SPECIFIC, College of Engineering Swansea University, Baglan Bay Innovation and Knowledge Centre, Port Talbot SA12 7AZ, UK
article info
Keywords:
Aqueous batteries
Lead acid batteries
NiFe
Energy storage
Metal air
abstract
Energy storage technologies are required to make full use of renewable energy sources, and electro-
chemical cells offer a great deal flexibility in the design of energy systems. For large scale electrochemical
storage to be viable, the materials employed and device production methods need to be lowcost, devices
should be long lasting and safety during operation is of utmost importance. Energy and power densities
are of lesser concern. For these reasons, battery chemistries that make use of aqueous electrolytes are
favorable candidates where large quantities of energy need to be stored. Herein we describe several
different aqueous based battery chemistries and identify some of the research challenges currently
hindering their wider adoption. Lead acid batteries represent a mature technology that currently dom-
inates the battery market, however there remain challenges that may prevent their future use at the
large scale. Nickel–iron batteries have received a resurgence of interest of late and are known for their
long cycle lives and robust nature however improvements in efficiency are needed in order to make them
competitive. Other technologies that use aqueous electrolytes and have the potential to be useful in
future large-scale applications are briefly introduced. Recent investigations in to the design of nickel–iron
cells are reported with it being shown that electrolyte decomposition can be virtually eliminated by
employing relatively large concentrations of iron sulfide in the electrode mixture, however this is at the
expense of capacity and cycle life.
&2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Literature review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Lead acid batteries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1.2. Negative electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.3. Positive electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.4. Collector grids for lead acid batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.5. Research challenges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. NiFe batteries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.2. Negative electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.3. Positive electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.4. Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.5. Research challenges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Aqueous ‘Rocking-Chair’batteries................................................................................... 4
2.3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3.2. Lithium based systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3.3. Sodium based systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
http://dx.doi.org/10.1016/j.rser.2016.02.024
1364-0321/&2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
n
Corresponding author. Tel.: þ44 1142228257; fax: þ44 1142227501.
E-mail address: j.o.gil-posada@sheffield.ac.uk (J.O.G. Posada).
Please cite this article as: Posada JOG, et al. Aqueous batteries as grid scale energy storage solutions. Renewable and Sustainable Energy
Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.02.024i
Renewable and Sustainable Energy Reviews ∎(∎∎∎∎)∎∎∎–∎∎∎

2.3.4. Research challenges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Other technologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4.1. Nickel–cadmium and Ni-metal hydride batteries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4.2. Zinc air batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4.3. Iron air batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4.4. Copper–zinc batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Experimental section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3.1. Electrolyte formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3.2. Electrode formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Conflict of interests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Funding....................................... ....................................... ................................... 7
Disclaimer................................... ....................................... ..................................... 7
References.............................................................. ....................................... .......... 7
1. Introduction
Due to climate change and the depletion of fossil fuel reserves,
governments have started to re-evaluate global energy policy. There-
fore, we are experiencing an increasing demand of energy from
renewable sources such as solar and wind power [1–3] and the
majority of countries face challenges in the integration of an
increasing share of energy coming from these intermittent sources [4–
13] . Renewable sources (such as solar, wind power, etc.) are changing
the energy market and they may displace significant amounts of
energy that are currently produced by conventional means; this is, for
example, an staggering 57% of the total demand of electricity in
Denmark by 2025 [1], around 15% of the total UK energy demand by
2015 and almost 16% of China by 2020 [2]. Energy storage technolo-
gies are required to facilitate the move towards the supply of low
carbon electricity [14], and are particularly useful when exploiting
intermittent energy sources. Incorporatingenergystoragehasbeen
shown to be beneficial to various sectors of the electricity industry,
including generation, transmission and distribution, while also pro-
viding services to support balancing and manage network utilization
[15,16].
In large-scale energy storage systems operational safety is of prime
importance and characteristics such as energy (Wh l
1
)andpower
density (W l
1
), which are major drivers in the development of
devices for mobile applications, are of lesser concern. Other desirable
characteristics for large scale energy storage systems are a low
installed cost, long operating life, high energy efficiency and that they
can be easily scaled from several kWh to hundreds of MWh. Different
battery chemistries demonstrated for use at this scale include lead-
acid, lithium-ion and sodium-based batteries. Lithium-ion batteries
exhibit very high round trip efficiencies (as high as 99%), energy
densities in the range of 100–200 Wh kg
1
and can typically with-
stand 1000 cycles before fading [17].Sodium-basedbatteries
(sodium–sulfur, ZEBRA) operate at temperatures in the region of 300–
350 °C and are characterized by a round trip efficiency of 80%, energy
densities up to 150 Wh kg
1
and lifetimes in excess of 3000
cycles [17].
There have been several recent incidents where lithium-ion cells
and sodium–sulfur batteries have failed which has resulted in the
release of toxic materials; this aspect has raised serious safety con-
cerns over the application of these batteries to large-scale energy
storage [18–21]. Not only there are safety concerns with these che-
mistries, but also these technologies are associated with high costs
due to the materials used, manufacturing processes and auxi-
liary systems required for their operation. As a result of these
considerations, the inherent safety and potential low cost offered by
the aqueous-based electrochemical energy storage devices discussed
in the following sections reveals that they can contribute positively to
large-scale energy storage applications.
At present, lead-acid cells are the most recognizable aqueous-
based battery system and represent a major proportion of the global
battery market. For example, it was reported that during 2010 the use
of lead acid batteries in China reached a staggering 75% usage of all
new photovoltaic systems [22]; likewise, during 2008, lead acid
technology held 79% of the US rechargeable battery market share [23].
This paper is focused on aqueous electrolyte based electrochemical
energy storage technologies suitable for large-scale applications and
discusses some of the challenges faced in the development of viable
systems. The list of systems discussed here in is not exhaustive, but
intended to give a brief overview of the area. These technologies have
the potential to be integral components in future electricity supply
systems providing that substantial reductions in cost can be achieved,
and that safe and reliable operation can be assured.
2. Literature review
2.1. Lead acid batteries
2.1.1. Overview
The oldest example of a practical rechargeable battery was
developed by Gaston Planté using metallic lead and sulfuric acid as
electrolyte in 1859 [24].Leadacidbatteriesmayhavedifferent
arrangements (i.e. series or parallel) depending on the applications.
Starting batteries are widely used in automotive applications for
engine starting, lighting and ignition (SLI), where peaks in current are
requested intermittently. This is achieved by using thinner electrodes
and separators, resulting in lower internal resistance than that of
regular lead acid batteries [25,26]. In such applications the depth of
discharge is kept low to maintain device longevity. On the other hand,
if the desired application requires constant discharge at relatively low
rates, (as in an uninterruptible power supply (UPS) or when powering
small vehicles), the deep-cycle or ‘marine’battery is used. This device
architecture incorporates thicker electrodes to allow for a much
greaterdepthofdischargetobeutilized[27–29].
In their simplest form, lead acid batteries consist of a positive
electrode composed of lead-dioxide (PbO
2
), a negative electrode
composed of metallic lead (Pb), and a dilute solution of sulfuric acid
electrolyte. During discharge, at the cathode, lead oxide is reduced to
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Please cite this article as: Posada JOG, et al. Aqueous batteries as grid scale energy storage solutions. Renewable and Sustainable Energy
Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.02.024i

Pb
2þ
as indicated by Eq. (1).
PbO
2
þ4H
þ
þ2e

2Pb
2þ
þ2H
2
OE
0
¼1:46 V ð1Þ
Once formed, Pb
2þ
ions precipitate in the form of PbSO
4
,as
shown in Eq. (2).
Pb
2þ
þSO
2þ
4
2PbSO
4
ð2Þ
Likewise, at the anode during discharge, metallic lead is oxi-
dized to Pb
2þ
as presented in Eq. (3).
Pb2Pb
2þ
þ2e

E
0
¼0:13 V ð3Þ
Finally, Pb
2þ
ions are precipitated in the form of PbSO
4
as
indicated by Eq. (2).
Fig. 1 provides a schematic representation of a lead acid cell
[24,30].
Lead acid batteries are well known for their low rate of self-dis-
charge, complicated production process, low cost of raw materials,
recyclability, and good performance over a wide range of operating
temperatures. Significant developments in performance have been
achieved by the introduction of a valve-regulated system and also by
the addition of carbon material to the anode. The use of carbon in the
Pb anode improves both cell efficiency and cycle life due to reduced
PbSO
4
accumulation [31].
2.1.2. Negative electrodes
Most of the problems that are usually encountered with lead acid
batteries are strongly dependent upon the negative electrode. Sulfa-
tion, the formation of nearly insoluble crystals of lead sulfate is, by far,
the most common ageing phenomenon inherent to lead acid bat-
teries. During charging these crystals regenerate to a minimal extent,
adversely affecting cell efficiency and lifetime.
One solution to this challenge has been to incorporate a highly
conductive additive (usually of carbonaceous nature) to the negative
electro-active electrode to prevent sulfation while maintaining high
electronic conductivity [25,26,30]. Also, it has been reported that
nano-structuring the negative electrode improves electrode perfor-
mance [32]. The exchange of the lead anode by a carbon electrode has
also been explored. This assembly is similar to an asymmetric
supercapacitor and resulted in a battery with longer cycle life [33].
Another promising design is the use of a double anode, containing
afoilofmetallicPbandasecondfoil of carbonaceous material; such
designsallowthebatterytooperateathighpowerdueto
supercapacitor-like behavior for an extended period in partial state of
charge operation [33–35].
Although, the corrosion of the positive electrode has always been
regarded as a major concern in lead-acid battery technology, the
corrosion of the negative electrode has drawn an increased attention
recently. The importance of corrosion control through the optimiza-
tion of the electroactive materials has recently been reported [36–38].
2.1.3. Positive electrodes
In order to enhance the performance of the lead-acid battery,
low antimony grids are commonly used. Unfortunately, low anti-
mony grids are prone to develop a passivation film between the
grid and the electroactive material of the electrode. In addition,
the corrosion of the positive electrode is well known to play a
detrimental effect on the performance of the lead-acid battery.
Therefore, the production of new formulations based on lead
oxides [39], and different additives (conductive and non-con-
ductive) [40], play a pivotal role in controlling corrosion and pre-
venting passivation of positive electrodes for lead acid batteries
[41]. Finally, it is important to highlight the importance of collec-
tors for these electrodes (see Section 2.1.2).
2.1.4. Collector grids for lead acid batteries
In order to improve the energy density of lead-acid batteries
development has focused on reducing the redundant weight in cells
by optimizing the electrode composition and the structure of the
collector grid. Improvements in the manufacture of lighter grids have
been realized by electro-depositing layers of lead on highly conductive
and low specific gravity substrates such as copper, aluminum, carbon,
barium, indium, etc.[30,37,38,42–49].
Broadly speaking there are two main types of grid used at the
positive terminal; lead–antimony and lead–calcium based grids.
Unfortunately, lead–calcium grids are unsuitable for deep-discharge
applications. Likewise, lead–antimony grids are associated with a
reduced hydrogen overpotential, which results in considerable
amounts of hydrogen being evolved during charging. It has been
reported that elements such as strontium, cadmium, silver and the
majority of rare earth elements can be used to produce lead–antimony
or lead–calcium based alloys with enhanced performance [42,50–53].
Vitreous carbons coated with lead have also been proposed as suitable
electrode grids, however due to oxygen evolution these are not well
suited for use in positive plates [49,54].
2.1.5. Research challenges
Lead acid batteries are known for their low energy density,
about 30 Wh kg
1
, which represents only 25% of the value asso-
ciated with lithium-ion batteries. Other major challenges faced by
this chemistry are limited cycle life, toxicity, and relatively low
charge/discharge efficiency [26,42,55,56].
2.2. NiFe batteries
2.2.1. Overview
Nickel–iron batteries have been successfully developed and com-
mercialized in the early 20th century. Nickel–iron or ‘NiFe’cells are
secondary batteries that fell out of favor with the advent of cheaper
lead acid cells. There is renewed interest in these batteries due to their
environmentally friendliness, longevity, and tolerance to electrical
abuse. It is believed that this technology could provide a cost effective
solution for large-scale energy storage applications, particularly where
only a relatively low specificenergyisrequired(30–50 Wh kg
1
).
The relative abundance of the raw materials required to pro-
duce NiFe cells is another aspect favoring their use. Nickel and iron
are among the most abundant elements in the Earth's crust, and
less abundant elements included in the cell (such as bismuth) are
used in relatively small proportions, therefore NiFe cells have the
potential to be manufactured at relatively low cost [57,58].Fig. 2
provides a schematic representation of a NiFe cell.
2.2.2. Negative electrodes
The main reaction during charging of an iron electrode under
basic conditions is the reduction of ferrous ion (Fe
2þ
) to elemental
iron (Fe
0
). Similarly, the oxidation of elemental iron to ferrous ions,
occurs during discharge of the same electrode. Eq. (4) illustrates
Fig. 1. Schematic representation of a lead-acid cell.
J.O.G. Posada et al. / Renewable and Sustainable Energy Reviews ∎(∎∎∎∎)∎∎∎–∎∎∎ 3
Please cite this article as: Posada JOG, et al. Aqueous batteries as grid scale energy storage solutions. Renewable and Sustainable Energy
Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.02.024i

the charging and discharging (forward and backward reactions
respectively) processes of an iron electrode under strong alkaline
conditions [59,60].
FeðOHÞ
2
þ2e

2Feþ2OH

E
0
¼0:87 V ð4Þ
Unfortunately during charging, water can be decomposed
yielding hydrogen, a process that adversely influences cell effi-
ciency. This process of electrolyte decomposition (and therefore
hydrogen evolution) accounts for a drastic reduction in the overall
performance of the battery, as indicated by Eq. (5).
2H
2
Oþ2e

2H
2
þ2OH

E
0
¼0:83 V ð5Þ
Mitigation of this process in NiFe cells has been traditionally
achieved by either modification of the iron electrode formulation
or by the addition of electrolyte additives (such as potassium
sulfide), in such a way that the activation energy for electrolyte
decomposition would be increased [61,62].
2.2.3. Positive electrodes
The positive electrode in NiFe cells is based on the nickel
hydroxide/oxyhydroxide couple used in nickel–cadmium and
nickel-metal hydride cells. Two polymorphs of Ni(OH)
2
exist, they
are α-Ni(OH)
2
and β-Ni(OH)
2
; they can be transformed into γ-
NiOOH and β-NiOOH, respectively. However, due to the low sta-
bility of α-Ni(OH)
2
in alkaline media, the β-Ni(OH)
2
is usually used
as a precursor material in alkaline batteries [63–65].
2.2.4. Electrolyte
NiFe cells use strongly alkaline solutions of potassium and lithium
hydroxide and selected additives (such as potassium sulfide) to pre-
vent electrolyte decomposition. Typically, the mitigation/prevention of
hydrogen evolution during charging has been achieved by either
modification of the anode or by the addition of electrolyte additives
that increase the hydrogen overpotential. Other electrolyte additives
such as wetting agents [66], long chain thiols [67] and organic acids
[68] amongst others, have been investigated [61].
2.2.5. Research challenges
As discussed above, a major challenge facing NiFe batteries is
the evolution of hydrogen, which results in low charge/discharge
efficiencies (ca. 50–60%) [59,62,69],andlowspecificenergy(30–
50 Wh kg
1
), Another consideration with this chemistry is the toxi-
city of nickel which significantly influences manufacturing costs.
2.3. Aqueous ‘Rocking-Chair’batteries
2.3.1. Overview
The concept of intercalation electrodes, used in lithium-ion cells,
has inspired research into similar systems that replace organic
solvents with aqueous-based electrolytes. This enables the use of
much lower cost materials with increased ionic conductivity. Cells
using intercalation electrodes are also known as 'rocking-chair’bat-
teries [70], as ions are inserted into and removed from electrodes
during charge and discharge.
2.3.2. Lithium based systems
An aqueous Li-ion cell was first reported where a VO
2
anode
and LiMn
2
O
4
spinel cathode in 5 mol l
1
LiNO
3
solution exhibited
an energy density of around 75 Wh kg
1
[71]. This is significantly
higher than that seen in lead-acid and nickel-based cells, however
this system exhibited a poor cycle life [72].
Aclearlimitationofaqueouselectrolytesistheirrestrictedelec-
trochemical stability as, under standard conditions, electrolysis of H
2
O
occurs at 1.23 V and involves H
2
or O
2
gas evolution. The energy
density in aqueous-based systems has been increased by expanding
the operating potential. For example, Hou et al. reported values
around 342 Wh kg
1
at an average discharge voltage of 3.32 V when
the lithium anode was covered by a polymer and LISICON film. These
layers acted as a protective coating preventing the formation of
lithium dendrites and to separate the lithium metal from the aqueous
electrolyte [73].
To gain deeper insights into the intercalation mechanisms
occurring in such cells, similar electrode materials employed in
non-aqueous batteries have been considered. However an addi-
tional consideration in these systems is pH, as this influences H
2
and O
2
evolution potentials as well as the stability of the electro-
active material [74].
In the case of cathode materials, which are quite stable in aqueous
solutions, protons (H
þ
) or water molecules inserted into the host
structure compete with lithium-ion insertion which reduces capacity
due to obstructed transport pathways [75]. The structure of the host
material is very important as each structure behaves differently
throughout the insertion processes. As an example, structures such as
spinel Li
1x
Mn
2
O
4
and olivine Li
1x
FePO
4
cannothostH
þ
meanwhile
layered structures (Li
1x
CoO
2
,Li
1x
Ni
1/3
Mn
1/3
Co
1/3
O
2
)presenteda
large amount of H
þ
concentrationintheframeworkinacidicmedia
[76,77]. Moreover, dissolution of the electrode material in the elec-
trolyte is the other limiting factor in terms of long-term cyclability. The
addition of a protective surface coating onto the electrode has been
shown to improve cycle life [78,79].
2.3.3. Sodium based systems
Increasing demand for lithium and its relatively low natural
abundance has resulted in a search for suitable alternatives. Sodium is
the most promising candidate to replace lithium as it exhibits similar
chemical behavior and has a similar ionic radius (90 pm for Na
þ
and
116 p m f o r L i
þ
)[80].Ithasbeenshownthatsodiumcanbereversibly
inserted into the tunnel structure Na
0.44
MnO
2
delivering a capacity of
45 mAh g
1
at 0.125 C [81]. It was later reported that a large format
hybrid/asymmetric aqueous intercalation batteries using λ-MnO
2
as
cathode materials and an active carbon in Na
2
SO
4
-based electrolyte
could be operated over a wider range but with a lower energy density.
This was further developed to replace some of the activated carbon
with NaTi
2
(PO
4
)
3
, and has been shown to withstand thousands of
cycles without significant capacity loss. This technology has been
commercialized by “Aquion Energy”who offers a range of systems
from 2 kWh units for residential use to off-grid applications and grid
services.
Recently, Chen et al. have shown that the use of Li
þ
/Na
þ
mixed-
ion electrolytes results in good stability [82]. In these systems, at one
electrode Li
þ
are exchanged between the electrolyte and electrode
whereas at the other electrode Na
þ
is exchanged with the con-
centration of Na
þ
/Li
þ
remaining constant upon cycling.
Fig. 2. Schematic representation of a NiFe cell.
J.O.G. Posada et al. / Renewable and Sustainable Energy Reviews ∎(∎∎∎∎)∎∎∎–∎∎∎4
Please cite this article as: Posada JOG, et al. Aqueous batteries as grid scale energy storage solutions. Renewable and Sustainable Energy
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2.3.4. Research challenges
“Rocking chair”chemistries could emerge as a potential alter-
native in the development of safer and higher energy density
batteries in comparison with Pb-acid cells, however the secondary
reactions present in all aqueous system restrict their performance
and cycling life. The main challenges associated with aqueous
‘rocking chair’systems have been identified as (1) electrolyte
decomposition evolving H
2
and O
2
(2) side reactions with water or
evolved gases (3) proton (H
þ
) co-intercalation into the host
electrode, and (4) the dissolution of electrode materials [83].
2.4. Other technologies
2.4.1. Nickel–cadmium and Ni-metal hydride batteries
Exploiting the same reaction as the positive electrode in NiFe cells,
are devices with alkaline aqueous electrolytes that use metal hydride
or cadmium based negative electrodes [64]. These chemistries may be
familiar, as they have been employed for many years in the consumer
electronics sector, and were integral to the development of electric
vehicles in the 1990s. This represents a mature battery technology
that has been identified as suitable for power quality applications and
grid support. Research efforts in this area are focused on improving
their energy density and cycle life alongside and preventing the
reactionsthatresultinself-discharge. An example of this technology is
the system developed by “GVEA”that uses almost 14,000 NiCd cells
providing backup power of 27 MW for upto 15 min. This system has
been in operation since 2003.
Recent improvements to cell architecture have focused on inc-
reasing the power density of NiMH cells and it remains a viable choice
for use in light rail vehicles. NiMH cells are characterized by energy
densities in the region of 250–330 Wh l
1
,aspecificenergyofupto
100 W h kg
1
and are limited to around 1000 charge–discharge cycles.
By comparison NiCd cells can perform roughly twice the number of
cycles but are associated with a lower energy density. Not only is the
toxicity of nickel and cadmium a major drawback of this technology,
but it has been recently identified that NiCd cells are associated with
substantially higher CO
2
and SO
2
emissions during production, when
compared with lithium based cells [84].
2.4.2. Zinc air batteries
Primary zinc-air cells are a fairly mature technology that has found
commercial applications in medical and telecommunications. As with
other metal-air cells, a major driver for development is their out-
standing theoretical energy density (1086 Wh kg
1
including oxygen).
(Another research direction that has come to prominence of late is the
development of aqueous Li-air batteries which have a theoretical
energy density of 1910 Wh kg
1
[85]).
The compatibility of zinc with an aqueous alkaline electrolyte
allows for substantially reduced manufacturing costs in comparison
with non-aqueous based cells. The development of electrically rech-
argeable zinc-air cells has been hindered by the propensity of zinc to
form dendrites upon repeated charge–discharge cycling and their
low output power [86]. A further drawback of aqueous alkaline
electrolytes is that carbon dioxide can be absorbed by the solution
thus producing insoluble, electrode blocking compounds that
decrease electrolyte conductivity and impede cell performance. As a
consequence (and similar to other metal-air systems), the process of
air purification needs to be considered alongside a new engineering
cell design.
Improvements in performance require the identification of
suitable robust catalysts and electrolyte additives. Zinc-air cells
have been proposed as a suitable alternative to lithium-ion for use
in electric vehicles and were successfully demonstrated by “Elec-
tric Fuel”in 2004. Currently, “Eos Energy Storage”are developing a
grid scale zinc-air system using a hybrid zinc electrode and a near
neutral pH aqueous electrolyte.
2.4.3. Iron air batteries
An alternative cell chemistry that has received attention of late
is the iron-air cell that also operates in an aqueous alkaline elec-
trolyte. Iron-air cells do not exhibit the same stripping/redeposi-
tion problem as seen in zinc-air cells but they have a lower the-
oretical energy density of 764 Wh kg
1
compared to Zn-air bat-
teries but higher than Pb-acid or NiFe batteries. Also the electrical
rechargeable cells exhibit relatively low energy efficiencies (ca.
35%) [87]. As with zinc-air cells the development of more efficient
oxygen electrodes is required.
2.4.4. Copper–zinc batteries
Another noteworthy technology utilizing aqueous electrolytes
is the development of a rechargeable copper–zinc battery by
“Cumulus Energy Storage”. This technology is based on processes
used in metal refining, this project aims create safe, low cost
battery systems with capacities in the range from between 1 MWh
and 100 MWh.
2.5. Summary
For large scale electrochemical storage to be viable, the materials
used need to be low cost, devices should be long lasting and opera-
tional safety is of utmost importance. Energy and power densities are
of lesser concern. For these reasons, battery chemistries that make use
ofaqueouselectrolytesarefavorablecandidateswherelargequan-
tities of energy need to be stored. Table 1 lists selected figures of merit
for various aqueous battery technologies to allow for easy comparison.
It is clear that certain chemistries display desirable characteristics but
are hindered by poor performance in other areas.
Large scale energy storage does not demand high efficiency, nor
does it require very high energy densities; the capital and operating
costs of the system are more crucial design parameters. Moreover,
non-aqueous batteries require the implementation of sophisticated
safetysystemstopreventhazardoussituations(e.g.thermalrunaway
leading to fire). In addition, the cost and relative abundance of the
reactants and raw materials required to build non-aqueous batteries
remain a concern when such systems are proposed for use on the
large scale. Table 2 summarizes some of the advantages and
Table 1
Figures of merit of selected aqueous batteries [17,27,31,88–91].
Technology Cost (€kWh
1
) Energy density (Wh kg
1
) Coulombic efficiency (%) Life (no. of cycles) Self-discharge (% month
1
) Memory effect
Pb acid 25–40 30–50 50–70 300–500 30 No
NiFe 50–60 30–50 55–65 2000 þ20 No
Ni–Cd 70–80 50 65–70 1500 28 Yes
NiMH 275–550 50–80 65 500–800 30 Yes
Li-ion (LiMn
2
O
4
/VO
2
)500–700 75 60 500–3000 10Small
Na-ion (λ-MnO
2
/C) 300–400 50–60 70 –– –
Zn-air 5–10 350–500 50 200–600 20 No
Fe-air 5–10 60 –80 45 300 20 No
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Please cite this article as: Posada JOG, et al. Aqueous batteries as grid scale energy storage solutions. Renewable and Sustainable Energy
Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.02.024i

disadvantages of the aqueous batteries presented in the previous
sections.
3. Experimental section
3.1. Case study
Section 2.2 presented several reasons favouring the use of NiFe
batteries, but also discussed some of the challenges associated with
this chemistry. A major challenge preventing NiFe batteries from
wider adoption is their low coulombic efficiency, which mainly occurs
due to electrolyte decomposition during charging. Consequently, we
have investigated several aspects of the behavior of iron based elec-
trodes in such cells, and have developed NiFe batteries exhibiting
coulombic efficiencies reaching 95%, whereby electrolyte decomposi-
tion has been virtually prevented [62,69,92–94].
3.2. Experimental details
Iron based electrodes were prepared procedures as described
elsewhere [69,92,93].Briefly, electrodes were produced by mixing
varyingamountsofFe,FeS,Cu,BiandBi
2
S
3
with PTFE. Strips of nickel
foam (10 mm 40 mm 1.8 mm) were coated with the electrode
materials and then hot-pressed at 150 °Cand10kgcm
2
during
3 min, in such manner that 0.2–0.25 g of iron powder were loaded on
an area of approximately 1 cm
2
. Once produced, electrodes were
tested in different electrolyte systems so coulombic efficiency was
increased. As with the development of electrolyte systems, experi-
mental design was used to facilitate the improvement of electrode
formulations. Data extraction was automated by using an in-house
developed C/Cþþ program that interrogates all files produced by the
battery cycler. Data analysis was accomplished by utilizing Python and
the R statistical software.
ThebasicelectrolyteusedinNiFebatterydevelopmentisan
aqueous solution of potassium hydroxide, typically at a molarity of
5.1 mol l
1
.Additives investigated in electrolyte formulations include,
K
2
S, LiOH, Mucic acid, CuSO
4
, and selected thiols. Deionized water was
produced by using an Elix 10-Milli-Q Plus water purification system
(Millipore, Eschborn, Germany).
Iron-based electrodes were tested in a three-electrode cell with
potentials measured against a mercury/mercury oxide (Hg/HgO)
reference electrode (E
0
MMO
¼þ0.098 V vs. NHE). Nickel electrodes,
obtained from a commercial nickel iron battery, were employed as
counter electrodes. Electrodes were cycled from 0.9 to 1.4 V vs. Hg/
HgO at a rate of C/5, which is a standard procedure for testing iron
electrodes under galvanostatic conditions, by using an Arbin SCTS
battery cycler. Galvanostatic charge discharge experiments were
performed at room temperature until steady state was reached.
Formation and stabilization of the electrodes was typically found
to be complete by the 30th cycle of charge and discharge [62,93].
3.3. Results and discussion
3.3.1. Electrolyte formulation
Once assembled, NiFe cells were cycled as explained in Section 3.2.
Experimental results indicate that before a NiFe battery attains a
steady capacity, iron based electrodes require to reach a stable con-
figuration before the steady state was reached. Fig. 3 illustrates that
electrodes require such conditioning period however the electrolyte. It
can be clearly seen that in the early stages (before the 10th cycle),
coulombic efficiency is always very poor; however, this issues dis-
appear with the cycle number and in general, after the 30th cycle,
batteries have not only increased their coulombic efficiency, but have
reached steady state.
In our previous paper [69],wehavereportedtheuseoflithium
hydroxide seems to have a marginal incidence on cell performance (or
at least for short term testing –less than 100 cycles of charge and
discharge). Fig. 3 confirms this observation. It is noteworthy that the
efficiency of electrolytes A and B (KOH and KOHþLiOH respectively)
do not exhibit meaningful differences between them. It has long being
recognized the use of lithium hydroxide as an electrolyte would
benefit the long run operation of the iron electrode (increased elec-
trode stability). However, our experimental tests were not long
enough to either confirm or deny the veracity of this claim. Longer
testing would reveal the usefulness of LiOH as an electrolyte additive
for NiFe cells. Potassium sulfide is seen to have a positive effect on
battery performance. The efficiency of formulations C, D and E are
markedly higher than cells using electrolytes that did not contain
potassium sulfide, and the greatest efficiency is observed at a K
2
S
molarity of 0.2 mol l
1
increasing the molarity of K
2
Sto0.3moll
1
results in a significantreductioninefficiency, indicating the presence
of an optimal molarity.
Table 2
Advantages and disadvantages of selected aqueous batteries [17,27,31,88–91].
Technology Advantages Disadvantages
Pb acid Abundant raw materials –low cost Low energy density, limited cycle life, toxicity
NiFe Long cycle life and abundant raw materials Low energy density, low efficiency, self-discharge
Ni–Cd Moderate energy density and moderate coulombic efficiency Memory effects (relatively short life cycle), toxicity
NiMH Moderate energy density Memory effects (relatively short life cycle), low efficiency
Li-ion (LiMn
2
O
4
/VO
2
) High energy density, high round-trip efficiency, relatively long cycle life High cost, safety issues, low Li abundance
Na-ion (λ-MnO
2
/C) High energy density, high round-trip efficiency, relatively long cycle life. Little information –earlier development stage
Zn-air High energy density, low cost, environmental friendliness, abundant raw materials,
easy to scale up.
Short cycle life, low efficiency, self-discharge
Fe-air Low cost, environmental friendliness, abundant raw materials, easy to scale up Low coulombic efficiency, low energy density, self-
discharge
Fig. 3. Electrolyte systems for NiFe cells. Electrode formulation 80% Feþ15%
FeSþ5% PTFE. Electrolyte system formulation: (A) 5.1 M KOH, (B) 5.1 M KOHþ0.1 M
LiOH, (C) 5.1 M KOHþ0.1 M K
2
S, (D) 5.1 M KOHþ0.3 M K
2
S, and (E) 5.1 M
KOH þ0.2 M K
2
S.
J.O.G. Posada et al. / Renewable and Sustainable Energy Reviews ∎(∎∎∎∎)∎∎∎–∎∎∎6
Please cite this article as: Posada JOG, et al. Aqueous batteries as grid scale energy storage solutions. Renewable and Sustainable Energy
Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.02.024i

3.3.2. Electrode formulation
Traditionally, iron electrodes have been manufactured by utilizing
different additives, an in particular, the use of iron sulfide in con-
centrations not exceeding 15% has been reported to be beneficial to
the performance of the battery [92], and in fact battery performance
tend to decrease with the iron sulfide content in the region from
between fifteen and forty percent of iron sulfide. Although, most NiFe
papers tend to focus on electrode formulations below 20% FeS, we
investigated the entire composition space of electrodes ranging from
pure iron electrodes (0% FeS) to pure iron sulfide electrodes (100% FeS)
on a binder free basis.
Fig. 4 illustrates the variation of capacity and coulombic efficiency
with changing iron sulfide content. At concentrations of greater than
50% FeS, coulombic efficiency rises with FeS content; surprisingly
coulombic efficiencies of 90–95% were reached when the iron sulfide
content exceeded 80%, indicating that electrolyte decomposition has
been prevented. However, Fig. 4 also shows that at high concentra-
tions of iron sulfide, the capacity of the battery is drastically reduced
in comparison with that achieved at low FeS concentrations.
Fig. 4 highlights the existence of compromise in battery design; at
low concentrations of FeS coulombic efficiencies are low and capa-
cities large conversely, at high FeS concentrations, coulombic effi-
ciency tends to be high but capacity low. An additional problem with
batteries utilizing large amounts of iron sulfide is their reduced life
cycle. It was observed that cells utilizing large amounts of FeS faded
after only 100–150 cycles. It remains a challenge to maintain the
improved coulombic efficiency of NiFe batteries that utilize large
fractions of FeS while improving capacity and cycle life.
4. Conclusions
By pursuing the development of cost effective iron sulfide based
NiFe cells, we have identified two main electrode formulation
regions. At low concentrations iron sulfide (below 30%), cells exhibit
low coulombic efficiencies (20%) and relatively large capacities
(190 mAh g
1
). Conversely, at large concentrations of iron sulfide,
cells exhibit very large coulombic efficiencies (90%) and very low
capacities (60 mAh g
1
).
The experimental approach used in this project, has facilitated and
accelerated the development of secondary NiFe batteries. From our
experimental findings, we can conclude there is a link between
electrode performance (coulombic efficiency) and:

electrode composition, in the form of iron sulfide content,

electrolyte composition (presence of potassium sulfide), and

life cycle.
A strong correlation between cell performance and electrolyte
system was found. Potassium sulfate was identified as a key additive
to improve cell performance. Lithium hydroxide on the other way was
found to have limited effect in improving cell performance; however,
no long run testing was done, and it is not wise to rule out the
importance of this additive that is present in nearly all NiFe cell for-
mulations, so extended testing of the battery is recommended as a
future work.
For large-scale applications, the safety and low installed costs
of aqueous-based batteries make them desirable propositions if
some of their limitations can be overcome. Due to the existing
manufacturing capacity, lead acid cells are likely to remain a viable
option for many applications, however as this is a mature tech-
nology; only incremental advances in performance are likely. More
substantial improvements in performance with respect to effi-
ciency, longevity and cost are likely to be seen in other aqueous-
based chemistries, and these technologies have the potential to be
integral components in future electricity supply systems.
Finally, the ideal aqueous battery would be one that had the
longevity of a NiFe cell combined with the specificenergydensityofa
metal air battery and the environmental friendliness of a ‘rocking
chair’battery. Developmentally daunting, but a worthwhile project
requiring Manhattan Project-scale investments.
Conflict of interests
The authors declare that there is no conflict of interest regarding
the publication of this paper.
Funding
The authors would like to acknowledge the U.K. Engineering and
Physical Sciences Research Council for supporting this work (EP/
K002252/1 Energy Storage for Low Carbon Grids –Horizon Scanning).
VLM thanks FAPESP (2014/14690-1) for fellowship support.
Disclaimer
The information and views set out in this article are those of the
authors and do not necessarily reflect the official opinion of elsevier or
renewable and sustainable energy reviews.
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