Table Of ContentC H A P T E R
1
Introduction
O U T L I N E
1.1. William Grove Inventsthe Fuel Cell 1 1.3. The Unfulfilled Promise 5
1.2. FuelCells:CommercialSuccessRemains
Elusive 4
1.1. WILLIAM GROVE INVENTS toconventionalbatteriesinitscompositionsand
THE FUEL CELL purpose, but was based on an entirely new
concept of electric generation, and indeed
Thefuelcellwasinventedin1839byWilliam became theforerunneroffuel cells.
Grove (1811e1890). Grove was a scientist and In 1839, Grove observed the production of
a lawyer in the courts of Common Law an electric current in his experimental setup.2
(Fig. 1.1). He was born in Swansea, Wales, and In the experiment, the two platinum electrodes
became a barrister at Lincoln’s Inn and were submerged halfway into a beaker of
amemberoftheRoyalInstitutionofGreatBrit- aqueous sulfuric acid, and tubes were inverted
ain.1 He later became a professor of experi- over each of the electrodes, one containing
mental philosophy at the Royal Institution in hydrogen gas and the other containing oxygen
London and then its vice-president. He subse- gas (Fig. 1.2). When the tubes were lowered,
quentlywasknightedandbecameaLordJustice the gases displaced the electrolyte, leaving
of theQueen’sBench. only a thin coating of the acid solution on the
William Grove invented two electrochemical electrode. A galvanometer deflected to indicate
cells, the first in 1839 and the second in a flow of electrons between the two electrodes,
1844. In the days before electrical distribution leading Grove to claim that his device
networks became available, batteries were the “produced a voltaic current.”3 He published
onlypracticalsourceofelectricity,andhisintent this process in Philosophical Magazine.2 Three
wastoinventarobust,practicalbattery.Thefirst years later, Grove described in his paper that
invention,the“gasvoltaicbattery,”wassimilar electric energy was generated at the three-
FuelCells
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2
1. INTRODUCTION
understand why the fuel cell produced heat
and “novel gaseous andliquid products.”4
Grovealsoreportedonotherexperimentson
fuel cells.2 He experimented with stronger
sulfuric acid, hydrogen, and air foroxygen. He
also reported on a hydrogenechlorine fuel cell
and established that different hydrocarbon
fuels, such as camphor, oils, ether, and alcohol,
produced electricity. He foresaw the fuel cell
asapossiblesourceofcommercialelectricity,es-
peciallyifhydrogenasafuelcouldbereplaced
by coal, wood, or other combustibles. He,
however, was troubled by the lack of reliability
incellperformance.Healsowasunabletobuild
a fuel cell that generated enough electricity to
serve as a practical source of electricity. Grove,
thus, never made an effort to commercialize
the invention.5
Grove, however, made a major commercial
successoutofhissecondinvention, the“Grove
cell,” which became a popular battery for the
telegraph industry. Grove’s invention in the
year 1844 consisted of a zinc anode dipped in
FIGURE 1.1 Portrait of William Robert Grove
sulfuric acid and a platinum cathode dipped
(1811e1896): Father of the fuel cell. Source: Smithsonian
in nitric acid, separated by porous earthen-
InstitutionLibraries,US,2012.
ware.6 The Grove cell provided a high current
and nearly twice the voltage of the Daniell cell
phase contact where the liquid, gas, and plat- and became the favorite battery of the early
inum met; he sought to extend this contact American telegraph (1840e1860).5
by coating the electrodes with spongy The first battery in history was invented in
platinum. 1800 by Alessandro Volta. It was of great value
for science, but was unsafe to handle because
(cid:129) The processwas exactly the reverseof
of sulfuric acid in the battery and the power of
aprocess,electrolysisddiscoveredin1800by
the cell diminished over time. Later batteries,
British scientists WilliamNicholson and
starting with John Frederic Daniell’s wet cell in
Anthony Carlisledthat splitwater into
1836, provided more reliable currents and
hydrogen andoxygen by electricity.
wereadoptedbyindustriesforuseinstationary
Accordingto Grove,combining thegasesto
devices,particularly in telegraph networks.6
produce electricityand water was “a step
further that anyhitherto recorded.”4 (cid:129) Astelegraphictraffic increased, however,it
(cid:129) Grove, however, could notexplain the basic was found that theGrove cell discharged
principles underlying thebehavioroffuel poisonous nitric dioxide gas.5 Platinum was
cells.Grove demonstrated strongevidence also very expensive. The Grovecell was
that producing anelectric current required replacedby the cheaper, safer, and better
bothhydrogen andoxygen, but did not performing gravity cell in the1860s.
3
1.2. FUELCELLS:COMMERCIALSUCCESSREMAINSELUSIVE
FIGURE 1.2 Sketch of William
Grove’s 1839 fuel cell. Source: “1839
WilliamGroveFuelCell.jpg,”File,http://
en.wikipedia.org/wiki/File:1839_William_
Grove_Fuel_Cell.jpg, Wikipedia, no date,
accessed March 2012. Originally pub-
lished in the article authored by W. R.
Grove, “On a Gaseous Voltaic Battery,”
LXXII, Third Series, the London, Edin-
burgh And Dublin Philosophical Maga-
zineAndJournalOfScience,Richardand
John E. Taylor, London, December 1842.
(For the color version of this figure,
the reader is referred to the online
versionofthisbook.)
1.2. FUEL CELLS: COMMERCIAL in the nineteenth century, readily and easily
SUCCESS REMAINS ELUSIVE available for public consumption at that time.
Thus, the possibility of using coal in fuel cells
William Grove probably never expected that drew particular interest among researchers
his invention would keep the global science and developers. Indeed, they experimented
and technology community excited as well as with nearly every type of fuel cell chemistry
exasperatedforthenext170years.SinceGrove’s and physical design, many of which foreshad-
invention, scientists, engineers, and busi- owedthefuelcellsthatweknowoftoday.These
nessmen have dreamed that the fuel cell could researchers, however, realized that there were
deliver low cost, clean energy to power too many hurdles to overcome before fuel
factories, homes, cars, appliancesdvirtually cells could be reliable and inexpensive enough
anything that uses electric power. Despite the to be commercialized. Over time enthusiasm
best of intentions, this dream remains for the device gradually withered away. By the
unfulfilled. end of the nineteenth century, the internal
In thefirst100 years after Grove’sinvention, combustion engine was invented, and the fuel
many notable European scientists followed his cell was relegated to the status of a scientific
footsteps and attempted to develop a fuel cell curiosity.7
with commercial viability. They explored Bythemid-1900s,however,fuelcelltechnol-
various electrolytes and electrodes that could ogies started to make significant progress, and
produce an electric current reliably and inex- some fuel cells began to serve practical func-
pensively. They also experimented with tions.Thefirstfuelcellusedinpracticalapplica-
a variety of fuels, including wood, oil, coal, tion was the alkaline fuel cell (AFC) developed
and coal gas. Coal was the major source offuel by British scientist Sir Francis Bacon. Bacon’s
4
1. INTRODUCTION
AFCtechnologywaslicensedbyaUScompany, sourceformaterialhandlingvehiclesandemer-
which developed it as the source of onboard gency backup power systems. A subtype of
power and drinking water for its Apollo lunar PEMFCs, direct methanol fuel cells, is also
missions in 1962. A number of companies, under development for portable applications
both in the United States and Europe, sprang to power small electronic devices. Fuel cell
into action and began developing terrestrial researchers and developers, however, are still
applicationsofAFCs.Theyproducedanumber struggling to develop reliable, durable, and
of prototype AFC products, but were unable to inexpensive fuel cells that are competitive with
bring them to the commercial market. They conventional powergeneration technologies.
foundAFCsintolerantofcarbondioxide,which
is omnipresent on Earth, and thus concluded
AFCs to be unsuitable for terrestrial applica- 1.3. THE UNFULFILLED PROMISE
tions. Many companies abandoned AFC devel-
opmentby the 1970s. Fuelcelltechnologyisanextraordinary,revo-
In the mid-1970s, phosphoric acid fuel cells lutionary technology. Without question, the
(PAFCs) came to be recognized as better suited development of a commercially feasible fuel
forstationary,terrestrialapplications.Anumber cell technology would set off a marked change
of US companies launched R&D of PAFCs. The in the world that truly could be called a para-
US Government and electric and gas utilities digm shift.
started to assist those companies by funding Fuel cells are singularly remarkable in their
their R&D and demonstrations. Japan eagerly potential for efficiently converting the energy
followed the US footsteps and tried to learn thatislockedupinchemicalbondstoelectrical
the PAFC technology. By the mid-1980s, atten- energy. This efficiency is achieved because fuel
tion focused on molten carbonate fuel cells cells convert the chemical energy contained in
(MCFCs) and by the 1990s on solid oxide fuel a fuel into electrical energy in a single step,
cells. Also in the mid-1990s, proton exchange extracting more useful energy from the same
membrane fuel cells (PEMFCs) came to be amount of fuel than any other known device.
viewed as the most exciting technology; in An internal combustion engine, for example,
fact, researchers and developers were able to requires several steps: converting chemical
build an automobile powered by hydrogen- energy contained in gasoline to thermal energy
fueled PEMFCs. via combustion, using thermal energy to
Unfortunately, however, none of these fuel expand gases within the engine’s cylinders,
cellsachievedcommercialsuccessonanymajor and then converting the high pressure gas to
scale. PAFCs entered the market in the early mechanical energy via the pistons and drive
1990s as the “first generation” fuel cell tech- train. Because conventional internal combus-
nology to be commercialized. A few hundred tionenginesrelyonsomanyenergyconversion
power plants were installed, but sales of most steps, they are inherently inefficient and lose
of them were completed with government and much energy to incomplete combustion and
public subsidies. MCFCs also hit the market as exhaust heat.
“second generation” fuel cells in the early
2000. A small number of stationary power (cid:129) Afuelcell is notonly moreefficientthan
generation systems have been sold, also with a combustion engine, but also delivershigh
subsidies. PEMFCs have been put on the efficiencyevenwhileoperatingunderpartial
commercial market recently; they are used as design loads. Moreover,its highefficiency is
a small stationary power plant and the power scalablefromvery small unitsto very large
5
1.3. THEUNFULFILLEDPROMISE
units. Acombustion engine,on the other of dollars in new revenue. In addition, oil
hand,ismostefficient underfulloperational importing nations can cut down on the use of
loads and only at certain minimum engine oil, thereby gaining greater independence from
sizes. oil-producing nations, enhancing their energy
security,economicsecurity,environmentalsecu-
Because of these remarkable attributes and rity, and the national security. The world will
capabilities, fuel cells can end the way we then find a profoundly different paradigm for
generate and consume electricity today. It can socio-economic, environmental, and geopolit-
deliver power that is inherently healthy for the ical relationships that could lead to greater
environment and highly efficient. Because of stability around theglobe.
their efficiency, fuel cells create less carbon Many governments and corporations have
dioxide when operating on carbon-based fuels recognized the promise of fuel cells and are
likemethane andnoemissions whenoperating working in tandem to develop their fuel cell
on hydrogen. In addition, fuel cells do not technology, manufacturing capabilities, and
generate any other emissions like nitrogen global technological leadership. Many govern-
oxide, which will harm the earth’s atmosphere. ments have devoted tens of billions of dollars
Because of their efficiency, their widespread in R&D on fuel cell technology in the past
use would slow the depletion of the earth’s 50 years. Likewise, many corporations have
fossil fuel resources. invested billions of dollars in R&D, some for
Fuel cells most commonly use hydrogen to overhalf a century.
generate electricity, but can use various other But these efforts have not delivered on the
fuels, including gasoline, natural gas, ethanol, neededfuelcelltechnology.Despitehugeinvest-
and methanol. Hydrogen, which would ment and sacrifice, few companies have made
have the least environmental impact, can be any profit. To date, major impediments to fuel
producedfromwaterthroughrenewableenergy cell commercialization remain. Daunting tech-
readily available in any country on earth, nical challenges have frustrated every effort to
including sun, wind, water, geothermal energy, create economically feasible fuel cells. The
andbiomass. problemsarewide-ranging:fuelcellshaveinsuf-
Fuel cell technology can cause a paradigm ficient longevity, reliability, and have unaccept-
shift in quality of life on a global scale. It will ablyhighcost.Forfuelcellsthatrequirespecial
give an ordinary citizen the ability to generate fuels (such as hydrogen), the lack of a fuel
electricity at home instead of buying electric infrastructurealsolimitscommercialization.8
power from a national electrical grid system Giventhelonghistoryoffuelcelltechnology,
that is often delivered over great distances it is mystery that such an important invention
withhightransmissionlossesandiscommonly has not yet achieved commercial success on
generated by polluting coal-firedpower plants. any significant scale. This book attempts to
Individuals will be able to drive cars that oper- unravel this mystery. It explores the history of
ate on hydrogen instead of gasoline, and help fuel cell development, evaluates the technical
promote new industry and manufacturing challenges, considers the successes and failures
sectors that can dramatically reduce carbon of government policy in promoting solutions to
footprints. Thus, the fuel cell technology can the technical challenges, and explains why the
spur the development of new classes of energy fuel cell industry has not been able to achieve
demand and supply systems, transportation commercial success with the technology. This
systems, and industrial and manufacturing bookthenturnstothefuture.Itoutlinesasetof
systemsinallnations,whilegeneratingtrillions policy recommendations thatmighthelprealize
6
1. INTRODUCTION
theultimatepromiseoffuelcelltechnology,and 4. CollectingtheHistoryofFuelCells.SmithsonianInstitution,
suggests who the potential winners might be in US; http://americanhistory.si.edu/csr/fuelcells/index.
htm;2007.
theraceforaviablecommercialproduct.
5. SirWilliamGrove(1811–1896),Biographies,http://www.
corrosion-doctors.org/Biographies/GroveBio.htm,
References Kingston Technical Software, no date, accessed August
2012.
1. Grove Sir William Robert. (1811e1896), Archives in 6. HistoryoftheBattery,Wikipedia,http://en.wikipedia.
LondonandtheM25Area.US:RoyalInstitutionofGreat org/wiki/History_of_the_battery, [accessed October
BritainUK;April2001. 2008].
2. LeoJ,BlomenMJ,MugerwaMichaelN.Fuelcellsystems. 7. Fuelcellsdthefirst120years.In:HistoryofFuelCells,vol.
US;Springer;1993. 24.UK,FuelCellToday;July2007.
3. HoogersGregor,editor.Fuelcelltechnologyhandbook.UK, 8. Fuelcellhandbook.7thed.,NETL,theUSDepartmentof
CRCPressLLC;July2002. Energy,US,November2004.
C H A P T E R
2
Fuel Cells and the Challenges Ahead
O U T L I N E
2.1. What Is AFuel Cell? 8 2.6. PhosphoricAcid Fuel Cells 26
2.1.1. TheUnit Cell: A Simple But 2.6.1. Principles of Operation 26
Formidable Device 8 2.6.2. ThePresumptive “First Generation”
2.1.2. Fuel Cell Stacks: PlanarorTubular Commercial Fuel Cell 26
Designs 10 2.6.3. Inferior and Expensive 27
2.1.2.1. Planar-BipolarStacking 10
2.7. Molten CarbonateFuel Cells 28
2.1.2.2. StackswithTubularCells 11
2.7.1. Principles of Operation 28
2.1.3. Fuel Cell Systems 11
2.7.2. ThePresumptive “Second
2.2. Types Of Fuel Cells:Distinct Generation” Commercial Fuel 29
Technologies 12 2.7.3. Not DurableEnough and Still
Expensive 29
2.3. Polymer Electrolyte Membrane Fuel
Cells 13 2.8. Solid Oxide FuelCells 30
2.3.1. Principles of Operationand 2.8.1. Principles of Operationand
Characteristics 13 Characteristics 30
2.3.2. AnotherDaunting Problem: 2.8.2. An Early Favorite: High
Electrolyte Performance 17 TemperatureTubularCells 31
2.3.3. Challenges with Transport 2.8.3. BriefExplorationof High
Applications 20 TemperaturePlanarCells 31
2.8.4. TheCurrentTarget: Intermediate
2.4. Direct Methanol Fuel Cells 21
TemperaturePlanarCells,Many
2.4.1. Principles of Operationand
ProblemsRemain 32
Characteristics 21
2.8.4.1. BetterElectrolytesNeeded 32
2.4.2. Experiencing the SameProblemsas
2.8.4.2. BetterAnodeNeeded 33
PEMFCs And More 22
2.8.4.3. AlternateCathode
2.4.3. ChallengeswithPortableApplications23
Needed.Perhapsthe
2.5. Alkaline Fuel Cells 24 GreatestLong-
2.5.1. Principles of Operationand TermNeed 33
Characteristics 24 2.8.4.4. LackofInterconnect
2.5.2. An Early Success, Major Setbacks, Materials 33
ThenRedemption, But. 24 2.8.4.5. SealMaterialsNeeded 34
FuelCells
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http://dx.doi.org/10.1016/B978-0-444-56325-5.00002-8 Copyright(cid:1)2013ElsevierB.V.Allrightsreserved.
8
2. FUELCELLSANDTHECHALLENGESAHEAD
2.8.4.6. StackScale-UpisDaunting35 References 36
2.8.5. AreAlternative Cell Designs
Feasible? 35
AfterMorethan170YearsSinceItsInvention, cell, in contrast, is theoretically capable of
Why Hasn’t the Fuel Cell Become A Viable producing electrical energy for as long as
Product:HowtoMakeItHappen,andWhoWill fuel and oxidant are supplied to the
GetThere First? By Noriko Behling electrodes. Fuel cell life is typically limited
by the physical degradation of the
components, usually corrosion, or by
malfunction of the components.
2.1. WHAT IS A FUEL CELL?I (cid:129) Fixed versus dynamic electrochemistry: A
battery containsaphysically “fixed” set of
Afuelcellisanenergyconversiondevicethat chemicalsthat reactwith eachotherto
electrochemically converts chemical energy producepower.A fuel cell producesenergy
heldwithintworeactants,afuelandanoxidant, based onan intricate, dynamic flow of
into electrical energy. Fuel can be any ordinary chemicalsand electrical properties,which
fuel, but hydrogen is most commonly used. can be disruptedby any numberoffactors.
Theoxidantismostcommonlyoxygen,usually
intheformofambientair.Thefuelcellproduces 2.1.1. The Unit Cell: A Simple But
power as fuel andoxidant aresupplied. Formidable Device
A fuel cell has physical components and
characteristics that are similar to those of Like the battery, the fuel cell is a simple
a typical battery. But there are two critical device,consistingofonlyfourbasiccomponents
differences: thatsupportelectrochemicalreactions.Theyare
as follows:
(cid:129) Energy depletion versus continuous
refueling: A battery will cease to produce 1. An electrically negative electrode, called the
electrical energy when the chemical anodewith or without a catalyst, to which
reactants stored within the battery are a fuel issupplied;
consumed ordischarged. (In a rechargeable 2. An electrically positive electrode, called the
battery, the reactants are regenerated by cathodewith or without acatalyst,to which
recharging from an external source.) A fuel an oxidant (oxygen) is supplied;
iThissectiondrawsfromarangeofexistingdocumentsonfuelcells,includingandnotlimitedto“FuelCells:Typesof
FuelCells,”Hydrogen,FuelCells&InfrastructureTechnologiesProgram,EnergyEfficiencyandRenewableEnergy,US
DepartmentofEnergy,http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html,contentlast
updatedMarch8,2007;“FuelCells:FuelCellBasics:ABasicOverviewofFuelcellTechnology,”SmithsonianInstitution,
http://americanhistory.si.edu/fuelcells/basics.htm,accessedFebruary2009;and“FuelCellHandbook,Seventh
Edition,”NationalEnergyTechnologyLaboratory,USDepartmentofEnergy,November2004;“FuelCellResearch,”
PrincetonAppliedResearch,http://www.princetonappliedresearch.com/products/markets/fuel_cell.cfm,AMETEK,
2004.
9
2.1. WHATISAFUELCELL?
3. Electrolyte (a substance that allows is the simplest case, has a nucleus of one
movementof electrically conductive ions) proton that is balanced by a single electron
that separates thetwo electrodes; in its surrounding electron shell. A hydrogen
4. An externalelectric circuitconnecting two cation has no electron, and therefore carries
electrodes(Fig. 2.1). a net positive charge. The cation flows to the
cathode where it combines with electrons
The electrochemical reactions take place at flowing through the external circuit. Some
the electrodes as fuel and oxidant are electrodes require electrocatalysts (commonly
supplied, producing or consuming electrons precious metal platinum) for the reaction to
and ions. Electrons are produced at the anode proceed at the desired rate.
and are consumed at the cathode. Electrons In a fuel cell, ion creation is continuous and
move from anode to cathode via an external enabled by the electrolyte, which allows ions
circuitda process that results in the electric to move between the two electrodes. The elec-
current that the fuel cell produces. Within trolyte also serves as a physical barrier to
the fuel cell, ions flow between anode and prevent the fuel and oxidant gas streams from
cathode. Ions are individual atoms that have directlymixing.Theprocessofconsumingelec-
an uneven number of protons and electrons, trons and ions at the cathode also creates reac-
and thus possess a net positive charge (ions tion waste products and heat, which must be
with a positive charge are called cations) or transported away.
negative charge (ions with a negative charge The working of a fuel cell is nothing if not
are called anions). A hydrogen atom, which complex; its various chemical and electrical
FIGURE2.1 Schematicillustrationofasinglecell.Source:J.H.Hirschenhofer,D.B.Stauffer,R.R.Engleman,andM.G.Klett,
Reprintedinseveraleditionsof“FuelCellHandbook”producedbyEG>echnicalServices,fortheU.S.DepartmentofEnergy,
OfficeofFossilEnergy,NationalEnergyTechnologyLaboratory,differentdates.The7thedition,whichmaybethelast,waspublished
inNovember2004.(Forthecolorversionofthisfigure,thereaderisreferredtotheonlineversionofthisbook.)
10
2. FUELCELLSANDTHECHALLENGESAHEAD
components and processes are intricate, deli- reductions in the thickness of the
catelybalanced,anddynamic.Themostcritical electrolyte and the development of
part of a fuel cell is the so-called three-phase improved electrode and electrolyte
boundary or interface, which is the area where materials which broaden the temperature
the electrodes and the electrolyte meet and the range over which the cells can be
electrochemical reactions take place. For the operated.1 These improvements, however,
interface area to be active, it must be exposed are incremental in nature and do not
to the reactant (fuel or oxidant), be in electrical constitute a technical breakthrough.
contact with the electrode, also be in ionic
contact with the electrolyte, and contain suffi-
cient amountsof electrocatalyst for thereaction 2.1.2. Fuel Cell Stacks: Planar
to occur.1 or Tubular Designs
These requirements lead to complex
A single fuel cell typically generates a small
performance and cost trade-offs. The density
amountofelectricity,butcellscanbeconnected
or thickness of the electrolyte and the nature
inseriestoproducegreaterpoweroutput.Such
of the interface play a critical role in the elec-
acollectionoffuelcellsinseriesiscalledastack.
trochemical performance of both liquid and
There are two major types of stack technology:
solid electrolyte fuel cells. In fuel cells that
planar-bipolarstacking andtubular stacking.
use liquid electrolytes, the electrolyte could
wet the electrodes excessively and flood
2.1.2.1. Planar-Bipolar Stacking
them, lowering their ability to transport reac-
tants to the electrolyte and causing degrada- For flat plate or planar fuel cells, individual
tion of cell performance over its life.1 In cells are stacked in layers and are electrically
solid electrolyte fuel cells, the challenge is connected with interconnects, which are
how to engineer the interface so that the elec- called planar bipolar plates. A bipolar plate
trodes and the electrolyte remain electrically connects the entire surface of one cathode
and ionically well connected and are effi- (electrically positive) and the entire surface
ciently exposed to the reactant gases. Also, of one anode (electrically negative) of the
in fuel cells that operate at low temperatures, next celldhence the name bipolardand
rare and precious materialsdcommonly plat- provides a series electrical connections
inumdare needed at the interface to provide between adjacent cells. A bipolar plate also
sufficient electrocatalytic activity, making the has other functions. The bipolar plate contains
fuel cells extraordinarily expensive. In high channels or grooves that allow for the
temperature fuel cells, the electrocatalytic uniform distribution of fuel gas over the
activity of the bulk electrode material is often surface of the anode and the distribution of
sufficient to support the reaction, but high oxygen or air over the surface of the cathode.
operating temperatures can cause other prob- The bipolar plate also often can be hollow in
lems, including longevity issues. which case it carries a cooling fluid through
the stack with a design that keeps all the reac-
(cid:129) Over the past 20 years, the performance of tant gases and cooling fluids apart. The
at least some fuel cells has been bipolar plate keeps the reactant gases con-
substantially improved based on tained within each cell, so the edges of the
improvements in the three-phase boundary cell are designed to allow space to create
interface. The improvements included a “gas-tight” seal around each cell.
