High efficiency electrical energy storage using a methane–oxygen solid oxidecell

David M. Bierschenk, James R. Wilson and Scott A. Barnett*

Received 20th September 2010, Accepted 19th November 2010

DOI: 10.1039/c0ee00457j

Reversible solid oxide cells (SOCs) are potentially useful for electrical energy storage due to their good

storage scalability, but have not been seriously considered due to concerns over round-trip efficiency.

Here we propose an SOC storage chemistry where the fuel cycles between H2O–CO2-rich and CH4–H2-

rich gases. The unique feature is the formation of CH4 during electrolysis, a less endothermic process

than the usual H2- or CO-forming reactions, enabling improved efficiency. Thermodynamic

calculations and preliminary experiments show that the CH4-rich storage chemistry is produced during

SOC operation at reduced temperature (�600 �C) and/or increased pressure (�10 atm). Balance of

plant storage system requirements are discussed briefly.

1. Introduction

Electrical energy storage has a number of well-known applica-

tions for improving the ability of the grid to efficiently respond to

demand fluctuations.1–4 The need for storage is becoming more

acute as increasing amounts of intermittent renewable electrical

sources come on line, making it increasingly difficult to match

fluctuations in both supply and demand.1,3 Perhaps the most

challenging problem is storage over relatively long (several hour)

times, requiring the ability to store energy on a large scale.

Currently available methods generally fail to meet at least one of

the key storage-technology targets including cost, efficiency,

storage capacity, and widespread availability.1–3,5 Hydroelectric

water pumping and underground compressed air storage are well

established but limited to specific naturally occurring geographic

sites. Secondary batteries and ultracapacitors currently have

limitations for storing large amounts of energy cost effectively.

Flow batteries may provide more scalable storage, although

electrolyte volume and cost scale with energy stored.6,7

Reversible fuel cells have received only limited consideration

for energy storage—although they have potential for large-scale

storage, round-trip efficiency is expected to be relatively low.8,9

Most of the work has focused on regenerative proton exchange

membrane (PEM) cells,8–11 or PEM fuel cells combined with

alkaline electrolyzers.12,13 In both cases, relatively high over-

potentials are required to achieve acceptable current densities,

leading to relatively low efficiencies. Reversible solid oxide cells

(SOCs) have not been widely explored for electrical energy

storage. On the other hand, SOCs have received considerable

attention as electrolyzers for fuel production from renewable

electricity,14–18 and of course as fuel cells for electricity produc-

tion.19

Here we discuss the fundamental limitations on round-trip

efficiency of reversible SOCs and propose a storage chemistry

that can potentially yield efficiencies competitive with the storage

technologies discussed above. The new storage chemistry, which

cycles between H2O–CO2-rich and CH4–H2-rich gases, is enabled

by SOC operation at reduced temperature (�600 �C) and/or

increased pressure (�10 atm). The CH4-forming electrolysis

reactions require less heat energy input than the usual H2- orDepartment of Materials Science and Engineering, NorthwesternUniversity, Evanston, 60208, Illinois, USA. E-mail: s-barnett@northwestern.edu

Broader context

Large-scale electrical energy storage is becoming increasingly necessary due to the continued growth of intermittent renewable

sources such as wind and solar. However, currently available methods generally fail to meet at least one of the key storage-tech-

nology targets including cost, efficiency, storage capacity, and widespread availability. Reversible solid oxide cells have many

desirable attributes for this application, but have not been widely considered due to their relatively low round-trip efficiency. Here we

show a reversible solid oxide cell storage chemistry where the fuel cycles between H2O–CO2-rich and CH4-rich gases, enabled by

operating at reduced temperature and/or increased pressure. The CH4-forming electrolysis reactions require less heat energy input

than the usual H2- or CO-forming reactions, thereby allowing a much-improved round-trip efficiency.

This journal is ª The Royal Society of Chemistry 2011 Energy Environ. Sci.

Dynamic Article LinksC<Energy &Environmental Science

Cite this: DOI: 10.1039/c0ee00457j

www.rsc.org/ees PAPER

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CO-forming reactions, allowing improved round-trip storage

efficiency.

2. Reversible solid oxide cell efficiency

Fig. 1 shows schematically one possible embodiment of

a reversible SOC storage device. The SOC is operated in elec-

trolysis mode (dashed arrows) to store energy in a fuel in one

tank, and then in fuel cell mode (solid arrows) to convert that

stored fuel back to electricity, producing a gas mixture that is

stored in a second tank. In electrolysis mode, pure oxygen is

produced by the SOC. A likely scenario, as shown in Fig. 1, is to

store the pure oxygen in a third tank to be consumed in fuel cell

mode, although the atmospheric air could also be employed.

Similar schemes have been proposed for reversible PEM cells.10,11

The round-trip efficiency (h) for reversible cells has been

described previously.9,14 When parasitic and heat transfer losses

are neglected, the ideal efficiency is the quotient of energy

supplied during discharging and energy consumed during

charging (eqn (1))

h ¼ VFCQFC/VELQEL (1)

where energy is the product of the cell potential (V) and charge

(Q) supplied or consumed.14 The effects of parasitic losses and

heat losses to the environment are discussed in Section 5.2. Note

that the reversible system in Fig. 1 is closed, unlike a conven-

tional fuel cell where fuel/oxidant enters and exhaust leaves the

device. Thus, fuel utilization efficiency is not a factor in deter-

mining system efficiency. If the coulometric efficiency is 100%,

i.e. the electrolyte does not have a leakage current due to mixed

conductivity and none of the reactants are lost due to gas

leakage, QFC ¼ QEL and eqn (1) reduces to:

h ¼ VFC/VEL (2)

Therefore, in order to maximize h, VFC and VEL should be as

close as possible.

In order to determine h, the specific cell reactions and cell

characteristics must be considered. An SOC performs electrolysis

by extracting oxygen from H2O (or CO2) across an oxygen-ion-

conducting electrolyte:

H2O / H2 + (1/2)O2, DH (800 �C) ¼ 248.3 kJ mol�1, (3)

or

CO2 / CO + (1/2)O2, DH (800 �C) ¼ 282.4 kJ mol�1, (4)

nominally producing a H2 (CO) enriched fuel gas and pure O2.

These reactions are reversed in fuel cell mode. Fig. 2 shows

typical results for the potential versus current density of an SOC

operated at 800 �C in 50% H2–50% H2O or 25% H2–25% CO–

50% H2O at the fuel electrode, with air at the oxygen electrode.14

VEL must be maintained above the open-circuit potential VOC

(approximately equal to the Nernst potential E) in order to drive

a current through the cell and thereby produce H2 and/or CO via

electrolysis. For the H2–H2O fuel gas and air oxidant used in the

SOC in Fig. 2, VOC ¼ 0.97 V. Imposing a potential VEL ¼ VOC +

0.1 V yields an electrolysis current density of �0.5 A cm�2,

whereas decreasing the potential to VFC ¼ VOC � 0.1 V yields

a fuel cell current density of +0.5 A cm�2. Using these potentials

in eqn (2) yields h ¼ 0.87 V/1.07 V ¼ 81%. For the cell operating

on the CO2–H2–H2O mixture, overpotentials of �0.1 V gave

currents of �0.45 A cm�2. That is, typical SOCs satisfy a key

requirement for a reversible device: sufficiently low resistance to

allow technologically useful current densities† at low over-

potentials consistent with high efficiency. Note that the low

resistance, along with the ability to work with carbon-containing

fuels (including providing expected Nernst potentials20) and

catalyze desired reactions, are key reasons for using SOCs in

electrical storage.

While the above arguments suggest that high h should be

possible, there is another factor that usually requires a higher

VEL, and thereby limits h. That is, the thermal energy DHFig. 1 Schematic diagram of a simplified reversible SOC system. Elec-

trical energy is stored by electrolyzing a H2O–CO2-rich mixture (dashed

arrows) and electricity is produced (solid arrows) in fuel cell mode

utilizing the resulting H2–CH4-rich fuel. Pure oxygen is produced during

electrolysis and is stored for use during fuel-cell operation.

Fig. 2 Potential versus current density at 800 �C for an SOC with 50%

H2–50%H2O or 25%H2—25% CO2—50%H2O at the fuel electrode, and

air at the oxygen electrode. Data are presented for operation in both

electrolysis mode (negative current) and fuel cell mode (positive

current).14

† If the current density is too small, then the device active area required tostore/produce energy at the required power level becomes too large, suchthat the device becomes excessively large and expensive.

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required for the endothermic electrolysis reactions (3) and (4)

must be supplied from some source. Otherwise, the device will

uncontrollably cool below its operating temperature and cease

functioning. Some studies have envisioned using an external

high-temperature heat source, such as a nuclear reactor, for this

purpose.17 More typically, however, the energy is supplied by the

electrical energy input zFVEL (F is Faraday’s constant and z is the

number of electrons transferred). Assuming that the electrical

input must match or exceed DH yields the ‘‘thermal neutrality’’

condition:

VEL $ VTN ¼ DH/zF, (5)

where VTN is the ‘‘thermo-neutral’’ voltage. For reaction (3),

VTN,H2¼ 1.29 V and for reaction (4) VTN,CO ¼ 1.48 V; z ¼ 2 for

both reactions. In the case of renewable fuel production, where

the SOC is used exclusively as an electrolyzer, this is not

considered to be a disadvantage since all of the electrical input is

being used to produce H2 or CO fuel, and the high potentials

produce desirably large current densities.15,16 For the case of

reversible energy storage, where achieving high round-trip effi-

ciency is important, the high VTN values are a serious limitation.

For example, using the previous value of VFC ¼ 0.87 V with VEL

$VTN yields h# 67% for reaction (3) and#58% for reaction (4).

For comparison, other storage methods such as hydroelectric

water pumping, compressed air storage, and Li-ion batteries can

yield h $ 80%.1

3. Thermodynamic predictions

The present method is a means of reaching higher h by reducing

VTN and thereby allowing lower VEL. This is achieved by

selecting the fuel gas compositions, operating temperature (T),

and pressure (P) such that the electrolysis reactions are less

endothermic. The fuel compositions are conveniently represented

on the C–H–O composition map in Fig. 3a. In order to illustrate

the concept, a specific set of gas compositions was selected,

shown as the dashed line at a constant H : C ratio of 7.7 between

pts 1 and 2 in Fig. 3a. Pt 1 (8.2% C, 63.1% H, 28.7% O) is the gas

composition in the ‘‘feedstock storage’’ tank in Fig. 1, whereas pt

2 (10% C, 77% H, 13% O) is that of the ‘‘fuel storage’’ tank.

Moving to the right between pts 1 and 2 represents addition of

oxygen (fuel cell mode), whereas moving to the left represents

removal of oxygen (electrolysis mode). Fig. 3a also shows the gas

composition limits for the reversible cycle. The H2O–CO2 tie-line

on the right shows completely oxidized fuel. The solid carbon

formation boundary curves vary with T and P, bounding the

composition space on the left. Carbon formation must be avoi-

ded because it will shift the gas H : C ratio out of the desired

range and also damage the SOC.21 The criteria for selecting

optimal storage-cycle gas compositions include factors such as

SOC performance and system considerations, and are beyond

the scope of this paper.

Fig. 3b and 4–7 summarize the predicted gas constitutions

equilibrated in an SOC operated at various conditions. Fig. 3b

shows the CH4 fraction versus oxygen content for various SOC

operating conditions. For conventional SOC electrolyzer

conditions (T ¼ 750 �C and P ¼ 1 atm), the CH4 content is low.

Reducing T to 600 �C shifts the gas constitution substantially,

with CH4 increased to 14.3% at pt 2. Fig. 4 shows all the major

gas constituents versus oxygen content for this case, showing that

CH4, H2, and CO all increase, and H2O and CO2 decrease, as

oxygen is extracted. Pressurization to P¼ 10 atm at 750 �C yields

a similar increase in the CH4 content in Fig. 3b to 15% at pt 2,

while P ¼ 10 atm and T ¼ 600 �C yield a higher CH4 content of

27.6% at pt 2.

Fig. 5 shows the gas constitution at pts 1 (a) and 2 (b) versus T

for P ¼ 1 atm. Fig. 5a shows that at pt 1 and 600 to 800 �C, thegas consists of �40% H2O, �40% H2, 11–14% CO2, <10% CO,

and a trace of CH4. At typical SOC electrolyzer conditions (T $

750 �C and P ¼ 1 atm), the equilibrium gas becomes enriched

primarily in H2 and CO on going from pt 1 to pt 2. That is, the

overall electrolysis process is essentially a combination of the

reactions (3) and (4). At lower T, there is increased CH4 and H2O

production at the expense of H2 and CO. Fig. 6 shows the gas

constitution at pts 1 (a) and 2 (b) versus T for P ¼ 10 atm.

Comparison of Fig. 6a and 5a shows that pressurization of the

feedstock (pt 1) increases the H2O and CH4 contents at lower T.

Comparison of Fig. 6b and 5b shows that pressurization of the

fuel (pt 2) increases H2O and CH4 at the expense of H2 and CO.

This trend, also shown in the plot of gas constitution versus P in

Fig. 7, is consistent with Le Chatelier’s principle given that the

CH4–H2O-rich gas has fewer total moles.

The increase in CH4 content upon decreasing T or increasing P

results in a lower DH/zF for the net electrolysis reaction, such

that VTN in eqn (5) is decreased. Fig. 8 and 9 show VTN versus T

Fig. 3 (a) C–H–O ternary composition diagram section. The carbon

(graphite) forming boundaries are indicated for T ¼ 600 and 750 �C at

P¼ 1 and 10 atm. (b) CH4 content at T¼ 600 and 750 �C at P¼ 1 and 10

atm as a function of oxygen fraction.

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and P, respectively, calculated for a gas at pt 1 electrolyzed to pt

2. At 750 �C and 1 atm,VTN¼ 1.272 V.With T reduced to 600 �Cand P ¼ 1 atm, VTN is reduced to 1.073 V. For P¼ 10 atm and T

< 750 �C, VTN stays in a narrow range from 1.04–1.07 V. High

efficiency is obtained by using VEL relatively close to VFC (eqn

(2)), and hence both must be near the Nernst potential E. Thus, E

values for the gas compositions ranging from pt 1 to pt 2 have

been included in Fig. 8 and 9 as shaded areas. Fig. 8 shows that it

is possible to achieve VTN z E for T# 600 �C and 1 atm, or P ¼10 atm and T# 750 �C. In this case, when aVEL value > E is used

to produce a net electrolysis current, there is also net heat

produced (proportional to the difference VEL � VTN); as dis-

cussed further below, this excess heat will be important for off-

setting thermal losses. The reduced VTN also impacts fuel cell

mode, since the excess heat produced is proportional to the

difference VTN � VFC. That is, less heat is produced under the

proposed operating conditions than under more conventional

SOC conditions. Note that the present CH4-containing fuel

composition (pt 2) is similar to that used in internal reforming,

a well-known strategy for reducing the heat production in solid

oxide fuel cells.22

Finally, note that the proposed approach is very different than

previously described strategies where H2 + CO is first produced

from H2O + CO2 in an SOC, and then converted to CH4 or other

hydrocarbons in a separate lower-temperature catalytic

Fig. 5 Equilibrium gas constitution at pt 1 (a) and pt 2 (b) for P¼ 1 atm

from T ¼ 500 to 800 �C. The exhaust constitution measured by gas

chromatography, after flowing a gas mixture containing 79% H2, 14%

CO, and 6%CO2 (pt 2) over a 0.6 g Ni–YSZ electrode (closed symbols), is

also shown in (b).

Fig. 6 Equilibrium gas constitution at pt 1 (a) and pt 2 (b) for P ¼ 10

atm from T ¼ 500 to 800 �C.

Fig. 7 Equilibrium gas constitution at pt 2 for T¼ 750 �C from P¼ 1 to

20 atm.

Fig. 4 Equilibrium gas constitution versus oxygen content at T¼ 600 �Cand P ¼ 1 atm for an H : C ratio of 7.7.

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reactor.15 In the present case, the heat of CH4 formation is

utilized to make the overall cell reaction less endothermic; when

a separate lower-temperature reactor is used, the cell reaction

remains highly endothermic for syngas production.

4. Experimental results

Kinetic processes within the SOC will not necessarily produce the

equilibrium products predicted thermodynamically. For

example, the fastest fuel-electrode reaction during electrolysis is

expected to be reduction of H2O to H2.14 The production of the

predicted equilibrium CH4-containing gas compositions thus

requires catalytic reactions in the H2-enriched gas, such as the

CH4-forming Sabatier23,24 or reverse-reforming reactions. The

fuel electrode must provide both electrochemical and catalytic

activities. Experiments were thus carried out to determine if the

Ni–YSZ electrode commonly used in SOCs provides the desired

catalytic activity. A gas mixture containing 6.2% CO2, 14.4%

CO, and 79.4% H2 (pt 2 in Fig. 3) at 1 atm was used. Compared

to Fig. 5b, this was not the equilibrium constitution, as it

contained no CH4. A typical SOC Ni–8 mol% yttria-stabilized

zirconia (YSZ) electrode (0.6 g) was exposed to this mixture at

a flow rate of 22 sccm from T ¼ 500 to 700 �C. The measured

exhaust gas constitution, shown in Fig. 5b, matched the equi-

librium prediction within the experimental error for gas chro-

matography. That is, the Ni–YSZ electrode was an effective

catalyst, which is not surprising given that Ni is a common

catalyst for the reforming and Sabatier reactions.23,24

Preliminary experimental validation was obtained using

anode-supported SOCs similar to those reported previously.25

Since pressurized testing capabilities were not available, the cell

testing was done at T¼ 600 �C and P¼ 1 atm. The cells provided

limited current at 600 �C (such cells normally operate at T$ 750�C), such that it was not possible to span the full composition

range from pt 1 to pt 2. The test was done with a gas containing

79% H2 and 21% CO2, located between pts 1 and 2, and the cell

was operated in both fuel cell and electrolysis modes. Fig. 10

shows the measured exhaust constitution versus cell current. The

gas constitution variation with oxygen content (the latter deter-

mined from the cell current and the gas flow rate) was in

reasonable agreement with the thermodynamic prediction in

Fig. 4, including an increase in CH4 content from 2 to 8% on

going from fuel cell to electrolysis mode. The H2, CO, CO2, and

H2O trends also matched the equilibrium prediction.

5. Discussion

The above results show a promising pathway to achieving high

efficiency SOC electrical energy storage. A number of factors

must be addressed to design the storage system and establish the

desired characteristics of its components. These will ultimately

determine system characteristics including efficiency, power

capacity, energy storage capacity, and lifetime. A few key

issues—SOC performance and durability, system design, and

thermal losses—are discussed briefly below; a more detailed

analysis is beyond the scope of this paper.

5.1 Cell performance

SOC performance requirements are discussed briefly here. For

purposes of illustration, we assume a target ideal efficiency h of

Fig. 10 Measured exhaust gas constitution as a function of cell current

from an SOC operated at T ¼ 600 �C and P ¼ 1 atm on a gas containing

79% H2 and 21% CO2 (between pts 1 and 2) supplied at a flow rate of 30

sccm.

Fig. 8 Thermal-neutral potentials versus T at P ¼ 1 atm and 10 atm for

a cell operating over a fuel composition range from pt 1 to pt 2 (Fig. 3).

Shown for comparison are the Nernst potential ranges for fuel compo-

sitions from pt 1 to pt 2 and oxygen at the other electrode at P ¼ 1 atm

(light grey) and 10 atm (dark grey).

Fig. 9 Thermal-neutral potentials versus P at T ¼ 750 �C for a cell

operating over a fuel composition range from pt 1 to pt 2 (Fig. 3). Shown

for comparison are the Nernst potential ranges for fuel compositions

from pt 1 to pt 2 and oxygen at the other electrode.

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80%, a value competitive with other electricity storage methods.1

Using eqn (2) and assuming a Nernst potential E ¼ 1.0 V, h z80% is obtained for VFC ¼ 0.9 V and VEL ¼ 1.1 V, i.e., when the

overpotentials in both modes are 0.1 V. In order to provide

a practically useful current density $ 0.5 A cm�2 at these rela-

tively low overpotentials, the SOCs should have an area-specific

resistance RAS # 0.2 U cm2. Note that 0.5 A cm�2 is used here

since it is typical of current solid oxide fuel cell system technol-

ogies that are at a near-commercial level of development.26,27 If

the cell resistance is higher, then either the current must be

decreased, lowering the power capacity of the device, or the

overpotentials increased, decreasing the efficiency.

Reaching the RAS target should be feasible for one of the

proposed conditions, P z 10 atm and T z 750 �C, using

conventional solid oxide fuel cells (SOFCs). Anode-supported

SOFCs are typically designed to operate at T$ 750 �C.28–30 Theyutilize �10 mm thick yttria-stabilized zirconia (YSZ) electrolytes

with a resistance of�0.05U cm2 at 750 �C,31well below the target

cell resistance. Thus, the electrode resistances combined should

be #0.15 U cm2. Current anode-supported solid oxide fuel cells

used in stack demonstrations are close to meeting the proposed

cell resistance criterion—two different reports showed over-

potentials of �0.15 V at 0.5 A cm�2 when operated at 750 �C,26,27

corresponding to�0.3 U cm2. Furthermore, pressurization of the

gases and the use of pure oxygen will significantly improve

electrode performance.32–35 Although the dependence of elec-

trode polarization resistance (RP) on reactant (O2 or H2) pressure

is relatively weak at high pressure, e.g., RP f p�0.25,36 a pressure

increase from P ¼ 1 to 10 atm decreases RP by a factor of 0.56.

The other proposed condition is T z 600 �C and P ¼ 1–10

atm, where it is more challenging to make cells with RAS # 0.2 U

cm2. It is well known that SOFC resistance increases rapidly with

decreasing temperature T. The resistance of a 10 mm thick YSZ

electrolyte at 600 �C is �0.3 U cm2, larger than the cell target

value. Thus, alternative electrolyte materials must be used. Ceria

electrolyte cells have been studied for reduced-temperature

operation, e.g., yielding a total cell resistance of RAS z 0.2 U cm2

at 600 �C,37 but they are probably not suitable for this applica-

tion because their mixed conductivity results in leakage currents

that would reduce coulometric efficiency. Sc-stabilized zirconia

(SSZ) and La0.9Sr0.1Ga0.8Mg0.2O3–d (LSGM) are promising

candidates: 10 mm thick layers of SSZ or LSGM have resistances

< 0.1 U cm2 at 600 �C,31 consistent with the RAS target. Electrode

resistances also increase rapidly with decreasing T, such that

achieving suitably low RP is challenging even with a performance

boost from pressurization. Nonetheless, there are some prom-

ising results. An SSZ tubular cell has been reported that showed

a substantial increase in power density, to 1.48W cm�2 at 650 �C,with pressurization to P¼ 6.9 atm, althoughRAS values were still

relatively large.38 In one report, an LSGM electrolyte cell yielded

RAS z 0.15 U cm2 at 600 �C.39 Oxygen electrodes with RAS # 0.1

U cm2 at 600 �C have been reported.40,41

Good durability of the SOCs is an important requirement.

Solid oxide fuel cell durability has been studied extensively, and

cells have been successfully operated for tens of thousands of

hours.42,43 SOCs operated as electrolyzers have also been studied

recently; while good durability has been reported in some cases,

there does appear to be a stability issue for cells operated at VEL

$ 1.3 V,44–47 where oxygen electrode delamination is often

observed after sustained operation.44,48 Interestingly, the lower

VEL values discussed here should substantially reduce, and

perhaps eliminate, such degradation. Little is known about the

durability of SOCs when operated reversibly, and studies in this

area will be important.

5.2 Thermal and parasitic losses

As noted in Section 3, during electrolysis VEL should exceed VTN

by enough to provide sufficient heat to overcome thermal losses

to the surroundings. Similarly, VFC < VTN is needed to provide

excess heat during fuel cell operation. Thus, thermal losses

dictate an upper limit on the ideal efficiency (VFC/VEL) and,

therefore, are discussed briefly below. A more complete and

quantitative analysis is beyond the scope of this paper, as it

would require detailed systems analysis combined with experi-

mental measurements on a full-scale system. However, there is

a considerable knowledge base on the similar solid oxide fuel cell

technology, where methods have been developed to produce

thermally efficient systems.27 The key components of heat loss

are (1) heat lost by raising the temperature of the fuel and oxidant

gases from the storage tank temperature to the cell operating

temperature, (2) heat lost from the storage tanks, and (3) heat

lost through the thermal insulation surrounding the stack. Note

that the present discussion assumes that the reactants are stored

in gaseous form, requiring tank temperatures of �100 to 200 �Cchosen to avoid condensing liquid H2O. Recuperative heat

exchangers would be used to minimize (1). Tank heat losses are

offset by excess heat in the incoming gases due to heat exchanger

inefficiency. The heat loss from (2) and (3) will depend on

component geometry, size, insulation type, and insulation

thickness. The heat lost through the tank insulation, normalized

to the amount of energy stored, decreases with increasing tank

size. The heat lost through the stack insulation, normalized to the

storage power level, decreases with increasing stack size. Thus,

larger systems can potentially be more efficient.

Finally, parasitic losses must be considered. Electrical pumps

and blowers used to distribute or compress the gases consume

a portion of the electricity generated and thereby decrease the

efficiency. By analogy with solid oxide fuel cell systems, which

are quite similar to the proposed systems and are sometimes

pressurized as proposed here,32 these losses typically represent at

most a few percent of total system power.27

5.3 Balance of plant

The simplified schematic in Fig. 1 shows one possible

embodiment of the storage system including some balance of

plant components, i.e., two ‘‘fuel’’ storage tanks and an oxygen

tank, but omitting others such as pumps/blowers and heat

exchangers. An initial estimate was made of the gas storage

tank volume required to store a given amount of energy,

assuming gas-phase storage. For a cell operating between gas

compositions at pt 1 and pt 2 at T ¼ 600 �C and P ¼ 1 atm, 12

Wh of electricity is stored per mole of gas (including oxygen).

Using conservative conditions for the storage tank of P ¼ 17

atm at T ¼ 177 �C (conditions that will keep H2O in the gas

phase), the resulting energy storage density is 5.3 kWh m�3,

including all three tanks shown in Fig. 1. This system

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configuration is only one possibility and is certainly not

optimal with regards to storage volume. For example, the

oxygen tank could be eliminated, reducing system size and cost,

by utilizing atmospheric air at the oxygen electrode. On the

other hand, pure oxygen has the advantage of improving

oxygen-electrode kinetics due to the 5� increase in oxygen

partial pressure compared to air. Higher-density storage might

be achieved by reducing the feedstock storage tank tempera-

ture, such that steam (the predominant component as shown in

Fig. 5a and 6a) is condensed to liquid water. This alteration

would best be combined with a catalytic reactor operated at

a lower temperature than the SOC, �400 to 500 �C, insertedbetween the SOC and fuel storage tank for use during elec-

trolysis. This would increase the CH4 fraction in the product,

thereby increasing the fuel storage tank energy density and also

producing sufficient heat to boil the water being supplied from

the feedstock storage tank.

The proposed SOC storage method possesses a unique

advantage, relative to other electrochemical storage devices such

as batteries, that could further reduce storage tank requirements.

That is, the storage media are rich in CH4, H2O, and CO2, all

widely available. The option of connecting the SOC to an

external natural gas network, effectively using it as extended gas

storage, would allow much-reduced tank sizes. During an

extended period of high electrical demand relative to supply,

when a relatively small fuel tank would be emptied, an external

natural gas supply could be used to continue providing power.

On the other hand, during an extended period of high electrical

supply relative to demand, when a relatively small fuel tank

would become full and the feedstock tank emptied, the H2–CH4–

rich fuel mixture could be more completely converted to CH4 in

the above-mentioned catalytic reactor and exported into the

natural gas network (note that this would also require an external

supply of H2O and CO2). That is, the proposed SOCs can

provide a means for transparently exchanging between electrical

and natural gas energy supplies. The recent proposed use of

SOCs for renewable fuel production,14,15,49,50 solid oxide elec-

trolysis of H2O–CO2 mixtures at �800 �C to first produce H2 +

CO, followed by a lower-temperature catalytic reaction to

produce CH4, is similar to the electrolysis portion of the present

storage cycle. However, the present approach, where significant

CH4 is produced within the SOC versus in a separate reactor, has

significant efficiency benefits.

6. Experimental

6.1 Equilibrium gas calculations

The equilibrium gas constitutions and coking conditions were

calculated with Thermocalc using the substance database

SSUB3. Gases were assumed to behave ideally under all condi-

tions calculated. Graphite was assumed to be the solid carbon

phase.

6.2 Thermal neutral and Nernst Potential calculations

The Nernst potential at each condition was calculated using the

effective oxygen partial pressure as determined by the gas

constitution predicted with Thermocalc. Using the expression for

an oxygen concentration cell, the Nernst Potential is:

E¼ RT

4Fln

�pO2 pos electrode

pO2 neg electrode

�(6)

VTN was calculated using eqn (5) and assumes that the fuel and

feedstock gas constitutions reach equilibrium in the SOC due to

their close proximity with the fuel electrode catalyst, and do not

change substantially within the gas feed lines or storage tanks.

6.3 Ni–YSZ pellet methanation activity

Ni–YSZ pellets were fabricated with a 50–50 wt% mixture of

NiO (J.T. Baker) and YSZ (Tosoh). Tapioca starch (10 wt%) and

PVB were incorporated into the mixture via ball milling in

ethanol. The resulting slurry was dried, sieved (#120 mesh), and

uniaxially pressed into 1.9 cm pellets weighing 0.6 g. The pellets

were calcined at 1400 �C for 4 hours in air. For each test, a pellet

(diameter 1.5 cm) was placed in a quartz reactor (ID ¼ 1.6 cm)

and supported with quartz wool. Gas containing 6.2% CO2,

14.4% CO, and 79.4% H2 (pt 2 in Fig. 3) at P ¼ 1 atm was

supplied with mass flow controllers so that the total flow was 22

sccm. The inlet and outlet (dried) gas compositions were

measured with gas chromatography (Agilent 3000A micro GC)

on a dry basis and the water content was estimated using the

expected carbon to hydrogen ratio in the gas.

6.4 Button cell test

The fabrication and testing configuration of the cells have been

described in detail previously.25 The anode supported SOCs

consisted of a 600 mm thick Ni–YSZ support with an �20 mm

Rh–gAl2O3 catalyst layer (AlfaAesar: Rhodium, 1% on alumina

powder), 10 mm Ni–YSZ anode functional layer, 10 mm YSZ

electrolyte, �20 mm La0.8Sr0.2MnO3�d (LSM)–YSZ cathode

functional layer, and �20 mm LSM cathode current collector.25

The cell was sealed to an alumina tube with Ag paste (DAD-87,

Shanghai Research Institute of Synthetic Resins), the cell

diameter was 2.5 cm, and the cathode area (2.5 cm2) defined the

active area of the cell. Gas compositions were set with mass flow

controllers to a mixture containing 79% H2 and 21% CO2 at

a total 30 sccm flow rate. The mixture composition is shown as

the dashed line in Fig. 3 with 19% oxygen. The cell exhaust gas

was measured with gas chromatography on a dry basis and the

water content was estimated using the expected carbon to

hydrogen ratio in the gas. The measurements indicated some air

leakage through the Ag seal into the fuel electrode chamber—

thus, expected exhaust oxygen contents are not reported.

7. Summary and conclusions

A novel reversible solid oxide cell storage chemistry, where the

fuel cycles between H2O–CO2-rich and CH4–H2-rich gases, is

proposed. Thermodynamic calculations and preliminary experi-

ments were used to show that methane-containing fuels are

produced during electrolysis operation at reduced temperature

(�600 �C) and/or elevated pressure (�10 atm). The CH4-forming

electrolysis reactions require less heat energy input than the usual

H2- or CO-forming reactions, decreasing the thermal-neutral

voltage and thereby allowing improved round-trip storage effi-

ciency. A possible set of operating conditions has been described,

and basic guidelines described as to how gas compositions, T,

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and P impact efficiency. The proposed technology poses many

interesting challenges for the solid oxide cell community, in areas

such as SOC performance, system design, and thermal manage-

ment. Further work will be needed to better assess how storage

system characteristics—including efficiency, power capacity,

energy storage capacity, and lifetime—compare to other elec-

trochemical storage technologies.

Acknowledgements

We thank Robert Kee, Robert Braun, Scott Cronin, Gareth

Hughes, Kyle Yakal-Kremski, Ann Call, and Jacob Haag for

useful discussions, and Emma Dutton for assisting with the

Thermocalc calculations. We gratefully acknowledge financial

support from the National Science Foundation, grant number

CBET-0854223, Department of Energy Basic Energy Sciences,

Award Number DE-FG02-05ER46255, and the Institute for

Sustainability and Energy at Northwestern (ISEN).

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