An Overview of the Functional Ceramic and Composite Materials for Microbiological Fuel Cells

22 Pages·2017·0.83 MB·English

J.Ceram.Sci.Technol.,08[4]433-454(2017)
DOI:10.4416/JCST2017-00041
availableonlineat:http://www.ceramic-science.com
©2017GöllerVerlag
Review
An Overview of the Functional Ceramic and
Composite Materials for Microbiological Fuel Cells
A.S.Galushko1,A.G.Ivanova2,M.S.Masalovich2,O.A.Zagrebelnyy2,
G.G.Panova1,I.Yu. Kruchinina2,3,O.A.Shilova*2,3
1AgrophysicalResearchInstitute,Grazhdanskiypr.,14,SaintPetersburg,195220,Russia
2InstituteofSilicateChemistryofRussianAcademyofSciences,
nab. Makarova,2,St. Petersburg,199034,Russia
3SaintPetersburgStateElectrotechnicalUniversity“LETI”,
ul. Prof. Popova,5,St. Petersburg,197376,Russia
receivedJune2,2017;receivedinrevisedformAugust18,2017; acceptedNovember8,2017
Abstract
Thisreviewcoversthecontemporarystateofaproblemconcerningtheselectionandusageofceramicmaterialsfor
fabricatingthebasicfunctionalcomponents(anode,cathodeandmembrane-separator)ofmicrobiologicalfuelcells
(MFCs).Electrogenesisonthesurfaceofelectrodespreparedfromceramicmaterialsisanalysed.Theelectrochemical
and catalytic processes taking place on the surface of ceramic and composite electrode materials, functions and
peculiarities of the membrane-separator structure and conductivity are evaluated in detail for the first time. The
advantagesanddrawbacksofusingceramicmaterialsforthefabricationoffunctionalcomponentsofMFCsandthe
methodsoftheirsynthesisareanalysed.
Keywords:Microbiologicalfuelcells,bioelectrogenesis,ceramicandcompositeanodematerials,ceramicandcompositecathodematerials,
ceramicandcompositeseparatingmembrane
I. Introduction mersandcompositematerialsonthebasisofpolymersand
Microbiological fuel cells are prospective bioelectro- metaloxides.Inthepresentreview,ceramicmaterialsare
chemicaldevicesabletofacilitateandimprovethequality considered in detail as the basic functional elements for
ofhumanlifeastheycanutilizewastes,wastewater,and MFCs.Ceramicmaterialsareknowntobeconventionally
differentorganicsubstratesbyconvertingthemintopow- usedforthefabricationofelectrodeandelectrolytemate-
er and pure drinking water without significant expense. rialsofsolidoxidefuelcells(SOFCs)3–6.
At present, commercially available microbiological fuel
MFCsonthebasisofcarbonmaterialshavesofarbeen
cells (MFCs) are not produced on a large scale because
advanced furthest. At the same time, interest in ceramic
a number of problems obstructing their introduction in
manufacturehavenotbeenresolved1–3. materials as functional elements for the MFCs is grow-
Theseproblemsincludelimitedpowergeneration,which ing7,8. In the paper9, ceramics are shown as an excel-
is directly caused by low-efficiency electron transport lentmaterialforadvancingMFCs.Ceramic-basedsepara-
from exogenous bacteria to the anode surface as well as torsarecomparablewithconventionalionexchangemem-
bybacteriaenergyexpenditureformaintainingtheirown branes and also considerably less expensive. Meanwhile
metabolism; high cost of separators and catalytic elec- theopportunitiesofceramicsforelectrodematerialfab-
trode materials; the necessity of prolonged maintenance
ricationhavenotbeenhighlightedenough.
of the bacteria cells in active state; the necessity of pro-
In the present review, the ceramic materials with best
tectingtheelectrodesfromseverefouling,whichreduces
prospectsforuseasanodes, cathodesormembrane-sep-
theefficiencyofelectrontransporttotheanode.Tosolve
thesedifficultproblemsrequiringaninterdisciplinaryap- aratorsareshownwithinabroadscope.Moreover,thein-
proach, intensive R&D work has been conducted in the teractingprocessesoftheceramicmaterialsandbacteriaas
worldscientificarena. wellasthevalidityandfeasibilityofthechoiceofthece-
Diversesubstancesandmaterialsaregenerallyusedasthe ramicsforthefabricationofMFCarediscussed.
functionalelementsforMFCs:carbonmaterials(carbon
Tobeginthereview,thefundamentalsofpowergenera-
black,nanotubes,graphene,etc.),electroconductivepoly-
tionbymicroorganismsisbrieflydiscussedandthestruc-
* Correspondingauthor:[email protected] tureofaMFCdescribed.
434 JournalofCeramicScienceandTechnology—A.S.Galushkoetal. Vol.8,No.4
II. Bacterial Electrogenesis, Microbial Fuel Cells and idationoforganicsubstratesbyGeobacterspp.proceeds
CompatibilitywithCeramicMaterials viathecitricacidcycleandnetATPsynthesisoccursexclu-
sively through electron transport phosphorylation12,33.
The fundamental feature of microorganisms capable of
Theaverageredoxpotentialoftheoxidationstepsinthe
performingtherespiratorytypeofmetabolismisvectori-
citricacidcycleisaround–247mV.Anotherknownpath-
alelectrontransfer.Thatmeansthatbasicallyallofthem
wayofacetateoxidationviaCOdehydrogenasehasamuch
areelectroactiveandhavethecapacitytogenerateanin-
loweraverageredoxpotential(–339mV;allvaluesaregiv-
tercellularcurrent, andunderspecialcircumstancesmay
enforpH7.034).Thispathwaymightalsobepotentially
generatethecurrentintheextracellularenvironment.The
beneficial for an MFC if electrons released from acetate
electrogenicpropertyisusedinthedevelopmentandop-
oxidationweretransferredtotheanodeviamediatorsor
erationofmicrobialfuelcells(MFCs).Thus, anMFCis
componentsofelectrontransportchainwithalowerredox
anelectrochemicaldevicethatconvertsthechemicalener-
potential than NAD(P)+ (Nicotinamide Adenine Dinu-
gyoforganiccompoundsviametabolicactionofthemi-
cleotide(Phosphate))35.Underanoxicconditions,species
croorganismstoelectricalenergy.Microorganismsdeliv-
of the genus Shewanella normally degrade lactate to ac-
erelectronsreleasedduringoxidationoforganicmatterto
etateincompletely36.Recently, ithasbeenreportedthat
ananode.Theseelectronsareutilizedforreductivechemi-
thecompletecitricacidcyclecouldbeswitchedoninthe
calorbiochemicalprocessesonacathode10,11.Anodeand
electrodepotential-dependentmodeinthecellsofseveral
cathodecompartmentsofaMFCaretypicallyseparated
strainsofShewanellaspp.underMFCconditions37,38,39.
byaprotonexchangemembranealthoughthatcausessev-
eraloperationalproblems12–15.Theusageofothertypes Despite intensive investigations, mechanisms of elec-
ofionexchangemembranesmayalsosustainorevenim- trontransferontotheanodearenotfullyunderstood.At
provetheelectrochemicalperformanceandoperationof present, two possibilities are considered for direct and
bioelectrochemicalsystemsofdifferenttypes16–18. indirectelectrontransferontotheanode.Directelectron
transfer (DIET) assumes direct contact of the terminal
Microbiologicalelectricitygenerationisarelativelynew
componentsoftheelectron–transportchainwiththean-
area of bioenergy production that has been developing
rapidlywithinthelastcoupleofdecades19,althoughthe ode surface40,41. That might occur via physical contact
ofthecellandanodesurfaces.AnotherDIEThypothesis
historyoftheelectricalactivityofmicroorganismsstart-
edmorethan100yearsago20.Thediscoveryofelectrical doesnotrequiresurfacecontactbuttheelectrontransfer
occursviaso-callednanowires–thetypeIVpilithatare
propertiesofplantsandanimalsledtounderstandingthat
connectedtothecellsurface.Inthiscasethedistancebe-
thedisintegrationoforganiccompoundsasthenecessary
tween cells and the anode surface might be in the range
sourceofenergybymicroorganismswasaccompaniedby
thereleaseofelectricalenergy20.Notmuchattentionwas fromseveral“micrometers”uptofewmillimeters42–48.
paid to this field until the second half of the 20th centu- Both DIET cases require the presence of Mtr (metal re-
ducing)respiratorycomplexthatperformthedeliveryof
ry.Atthattime,thebiologyofelectricalcurrentgenera-
electronsfromcytoplasmtotheexternalsideoftheout-
tionwasconfinedtostudiesofenzymesandextracellular
er membrane39,41,49. The problem of electron delivery
electrontransferviamediators–redox-activecompounds
abletotransferelectronstotheanode21–24.Thereasonfor viananowireshasnotbeencompletelyelucidatedtodate
either.Inthelastyears,manystudieswithpro-andcon-
thiswasthatnormallyintactcellsofmicroorganismswere
tra-arguments have been published on the subject50,51.
electrochemicallyinactivesincethecellsurfacewascov-
However, positive results are accumulating proving the
eredbynon-conductivestructuresandthereleaseofinter-
pili hypothesis52 for the case of Geobacter spp. and ex-
nalelectronsintotheenvironmentwasoccurringmostly
tendedperiplasmcoveredwithoutermembraneenriched
bymeansofmediatorsthatmightpenetrateintoperiplasm
with cytochromes for the Shewanella spp. case51,53,54.
orcytoplasmandinreducedstatetransferelectronsonan
anodeintotheextracellularenvironment25,26. For example, it was shown that only pili consisting of
short-chain monomers had the capacity to mediate the
The interest in MFCs has sharply increased within the
transfer of electrons via metallic-like conductivity that
last 15–20 years owing to the discovery of new species
mightbeattributedtooverlappingp-porbitalsofkeyaro-
ofmicroorganismscapableofgrowingbyintercellularor-
maticaminoacids54,55.Electrontransferviaconjugated
ganiccompoundsoxidationcoupledtotheelectrontrans-
doublebondswasconsideredtofacilitatetheprogressin
fer to external electrons acceptors, including water-in-
thedevelopmentofenzymaticfuelcells56.
solubleones27.Ithasbeenfoundthatsuchmicroorgan-
ismsareabletotransferelectronsontoextracellularsol- Indirect electrons transfer (iDIET) from microorgan-
idcompounds:iron(III)oxides, electron-conductingan- ismstotheelectrodesisperformedbymeansofelectron
odes28–30.Theadditionofelectronshuttlesmediatingthe shuttling mediators. The mediators are reversibly redox
electron transfer has not been required12,28–31. Power activecompoundsthatareaddedfromoutsideorexcreted
density of the MFCs based on the latest discoveries has bymicroorganismsthemselves25,26.Thesemediatorsare
beenincreasedbyseveralordersofmagnitude11. neededtomakeelectrontransfertotheanodepossiblein
thecaseofnon-exoelectrogenicmicroorganismsbutalso
At present, it is considered that biological electricity
couldbegeneratedbyanybiodegradablematter11,32.In inthecaseofDIETmicroorganisms57.
laboratorystudies, glucose, acetateandlactatearetradi- Natural microbial communities and pure cultures
tionallyused32.Ithasbeenassumedthatonlyinsomecases of microorganisms can generate electrical current as
thecompletedegradationoftheorganicsubstratesoccurs well32,58,59.Theelectrogenicpropertyhasbeendiscov-
viareactionsofthecitricacidcycle.Indeed,completeox- eredwithinrepresentativesofseveralclassesofProteobac-
December2017 AnOverviewoftheFunctionalCeramicandCompositeMaterialsforMicrobiologicalFuelCells 435
teria, Firmicutes, Acidobacteria and others11,32,60. At ThetheoreticalandconstructivesidesoftheMFCcon-
present69speciesofmicroorganismshavebeenreported cept are currently developing. One of the directions in
to be able to transfer electrons to the anode59. Unfor- thecreationofmoreeffectiveMFCsistheapplicationof
tunately, upuntilnowonlyrepresentativesofDeltapro- newmaterialsintheconstructionofMFCelements.Such
teobacteria and Gammaproteobacteria, namely species prospective materials include ceramic-containing candi-
of genera Geobacter and Shewanella, respectively, have dates.Thebenefitofusingceramicmaterialsisthatthey
been investigated in detail for their capacity to generate are not toxic for aerobic and anaerobic microorganisms
electricityinMFCs.Surprisingly,notallknownGeobac- andareusedinwastewatertreatmentreactorsassupport-
terspp.areequallyefficientincurrentgeneration61.Itis ing78,79orfiltering80materials.Furthermore,application
stillunclearhowmicroorganismsofvariousphylogenetic ofceramicsinthedesignofMFCelectrodesisbeneficial
groupsmaygenerateelectricalenergy, forexample, how forMFCoperation81.
Firmicutescandeliverelectronstoanodessincetheylack
outermembranes35,59,62,63. III. Electrochemistry of Interaction between Bacteria
andCeramic(Composite)AnodeMaterials
The microbiological fate of electrons delivered to the
Theanodematerialisknowntocontributesignificantly
cathode from the anode has also been attracting consid-
tothehighefficiencyofanMFCasitprovidestheinter-
erableattentionsincethegraphiteelectrodeswerediscov-
actionbetweenthebacteriallayerandtheanodesurface,
eredtoserveaselectrondonorsforbacteria64.Chemical
whichinturndeterminestheamountofpowerproduced.
reductionofmolecularoxygenwasforalongtimetheop-
As a rule, diverse carbon allotropic modifications
tionforthecathodedepartmentofMFC.Inrecentyears,
(graphite, graphene, carbon black, nanotubes, etc.) are
theconceptofmicrobiallymediatedreductionofvarious
used as anode materials82–90. The popularity of carbon
electron acceptors (including molecular oxygen reduc-
materials can be attributed to the low cost, large surface
tion)viaMFCcathodeshasbeendeveloping14,26,65–67.
areaandhighconductivity,biologicalinertness,chemical
In general, the same principles of the DIET and iDIET
stability and catalytic activity. Nevertheless, the carbon
mechanismsoftheelectrontransfertotheanodemaybe
materialisnotuniversalasitselectricalactivitythatinflu-
appliedinthecaseofMFCcathodes26,67.However,this
encestheelectrontransportfromthebacterialcelltothe
questionisstillunderinvestigationanditisgenerallyen-
anodeislow2.
visionedthatphylogeneticdiversityofelectrotrophicmi-
Thus, atpresent, anactivesearchfortheanodemateri-
croorganismsandmechanismsinmicrobioelectrochemi-
alcorrespondingtothelistedcriteriaisbeingconducted.
calsystemscouldbeevenbroaderthaninthecaseofelec-
Theanalysisofnumerouspublicationsshowsthatthesig-
trogenesis26,59.Themicroorganismscapableofusingsol-
nificantresearchinterestisfocusedonthesynthesisand
idcompoundsaselectrondonorsenlargethegroupofelec-
researchofceramicpowders,films,coatingsandcompos-
troactivemicroorganisms.However, notallofthesemi-
itesbasedonthemaspartsofaMFC9.
croorganismsmightbeveryefficientintheMFCsincethe
Ingeneral,thetestedceramicanodematerialmaybedi-
prerequisiteforMFCoperationisasufficientlyhighredox
videdintothreemaingroups,whicharediscussedinthe
potentialoftheelectronacceptorsusedbythesemicroor-
ganismstoallowcurrentgeneration67. followingsubsections.
SinceMFCoperationstrictlydependsontheactivityof (1) CeramiccatalyticconductivematerialfortheMFC
living cells, the composition of electrolytes in the MFC anode
compartmentsshouldbesuitabletomaintainthemetabol- Theauthors91obtainedporousceramicsonthebasisof
ic health of the microorganisms68. On the other hand,
titaniumoxidebasedonatemplatesynthesis.Thenthey
higher salinity of electrolyte is considered beneficial for
covered the ceramics with the conductive ceramic film
current generation owing to the higher conductivity69.
consisting of fluorine-doped tin oxide (FTO) by means
Considering both trends, it is clear that certain species
ofchemicalvapourdeposition.Theresistancerangeofthe
ofelectroactivemicroorganismswouldoptimallyoperate ceramicmaterialreached10–15X.ThenaChlorellavul-
onlyatarestrictedrangeofsaltsconcentrationintheelec-
garisbiofilmwasgrownonthefabricatedanodeoftheex-
trolyte. Thus, low NaCl content in the anode compart-
perimentalMFC.Finally,thepowerdensityoftheexper-
ment of a MFC operated with acetate would be benefi-
imentalMFCwiththeceramicanodewas16timeshigher
cial for current generation by Geobacter-like freshwater
thanthatofthestandardMFCwithacarbonanode.The
species68. Increase of NaCl content to the marine level
substantialenhancementoftheperformanceoftheexper-
would benefit the growth of the marine acetate-oxidiz-
imentalMFCisprobablycausedbytheeffectivebiofilm
ingspeciesofDesulfuromonasspp.70.Thelattermicroor- growthontheceramicanodesurface91.
ganisms are metabolically similar to Geobacter and are The authors92 described the anode made of the
adaptedtoliveinbrackishandmarinesediments,subsur-
porous conductive ceramics M AX , where n=1-3,
n+1 n
facebrines71–74.AGeobacterstraintoleranttothehigh- M–transition metal, A-a 3d or 4th subgroup element of
er NaCl content is also known75. Other compounds of
Mendeleev periodic table, X–carbon or nitrogen. Thus,
electrolytemayalsoinfluencetheperformanceofMFCs.
the anode material Ti AlC synthesized with a template
2
Thus,phosphateandcarbonateionsstimulatepowerpro-
methodpossessesatotalporosityof60–70%,highcon-
duction76. ductivity up to ∼3⋅104 S/cm and is peculiar owing to its
At present, short- and long-term application of MFCs strength and resistance to corrosion. These are the main
maybepreferredforindustrialareasthatdonotrequire characteristics of the material demonstrating the advan-
largecurrentsandintensivereactors19,77. tagesofsuchaMFCanode.Theauthorshadtheintention
436 JournalofCeramicScienceandTechnology—A.S.Galushkoetal. Vol.8,No.4
totesttheceramicanodeinthepresenceofelectroactive authors.Theceramicpowderwasobtainedbymeansof
bacteriaandexpectedpositiveresultsinthefuture.Lately, chemical synthesis with the use of the biocatalyst Kleb-
considerableattentionhasbeenpaidtotheceramicmate- siellapneumoniae(Fig.1)99.ItcanbeseenfromFig.1that
rialsbasedontransitionmetalcarbidesandnitrides,which theMFCmaximumpoweris2.230W⋅m-3(0.250W⋅m-2),
havethereputationofthematerialswithchemicalstabili- which is significantly higher than the MFC power with
ty,rigidityandhighconductivity(Table1).Moreover,the thecarbonfeltanode–0.070W⋅m-2.
carbidespossesshighcatalyticactivity.
Table 1: The electrical conductivity of some metal car-
bides93andmetalnitrides94.
Carbides Bulkresis- Nitrides Bulkresis-
tivity,lX⋅cm tivity,lX⋅cm
TiC 52,5 TiN 27
ZrC 50 ZrN 24
HfC 45 HfN 27
VC 76 VN 65
TaC 24 NbN 60
Fig.1:Polarization(a)andvolumetricpoweroutput(b)curvesof
Cr3C2 75 CrN 640 thecubeMFCwithNi3Mo3C(catalystload3.0mg⋅cm-2)(after99).
Mo C 71
2
W C 75.5
2 It is worth mentioning that the anode ceramic mate-
WC 19.2 rial based on Ni/b-Mo2C catalyses three processes in a
testMFC:1)oxidationoffermentationproducts;2)elec-
Intheresearchpaper95,96,tungstencarbidewasreported tron transfer from the biofilm to the electrode; 3) elec-
todemonstratehighcatalyticactivitytowardsanumber trontransferfromtheelectroncarrier(2,6-di-rubs-butyl-
oforganiccompoundssuchasformicacid,formaldehyde,
p-quinoneproducedbyKlebsiellapneumoniae)tothean-
aceticaldehyde,propylaldehyde,acetyleneandethylene. ode(Fig.2)100.
Thisindiciatesthepossibilityofusingtungstencarbideas
The search for the literature referring to metal nitrides
aMFCanodematerialtocatalysethedecompositionofthe
as a MFC anode material has not returned any results.
microbialwaste.
Nevertheless,thiscatalyticmaterialiseffectivelyapplied
The anode material Mo C obtained with a chemical
2 forelectrodemodificationinalow-temperaturepolymer
methodatthetemperatureof900°C;inargonmediumwas
electrolyte membrane fuel cell (PEMFC) and performs
showntodemonstratehighcatalyticactivity97.Themax-
highcatalyticactivity.Theevidencepointsoutitspotential
imumpowerdensityoftheMFCwithKlebsiellapneumo-
applicationasaMFCanodemodifier94.
niaeasabiocatalystreached2.39W⋅m-3,whichwassig-
Therefore,theceramicmaterialspossessingcatalyticac-
nificantlyhigherthanthespecificpoweroftheMFCwith
tivityandintrinsicconductivityareasuitableprospectfor
theanodematerialofcarbonfelt–0.61W⋅m-3.Moreover,
application as anodes in MFCs. And the more so as the
theanalysisofcyclicvoltammograms(CVAs)ofthetest
overwhelmingpartofthesematerialshasnotbeentested
electrochemicalsystemfabricatedfromtheMo Ccoated
2 yetinexperimentalMFCswithdiversespeciesofbacteria.
electrode,electrolytewiththesubstrate(glucose)andthe
biocatalyst–Klebsiellapneumoniaerevealedtwoanodic
(2) Ceramiccatalyticmaterialbeingapartofacompos-
peaks. These peaks may indicate oxidation of microbial
ite
fermentationproductslikeformiatesandlactates.Atthe
Agreatnumberofresearchpapersaredevotedtocom-
sametime,theCVAsofasimilarelectrochemicalsystem
positeanodematerialsforMFCs81,101–107.
butwithanunmodifiedelectrodedidnothavetheseox-
The composite materials may be divided in relative
idation peaks. Therefore, it is molybdenum(II) carbide
thatcatalysestheoxidationprocessofmicrobialfermen- groupsasfollows:
tation waste. Besides, it had been previously revealed98 a) the composites of transition or non-transition met-
that Mo C encouraged the direct electron transfer from al oxides with carbon (carbon black, nanotubes,
2
thebiofilmtotheelectrode.Thus,theMo Canodecoat- graphene,grapheneoxide,dopedcarbon,etc.);
2
edwithKlebsiellapneumoniaecatalysestwoprocessesof b) the composites of transition or non-transition met-
the direct oxidation of microbial fermentation products al oxides with conducting polymers (polypyrrole,
and the direct electron transfer from the bacteria to the polyaniline,polythiophene);
electrode. c) the composites of transition or non-transition met-
The electrochemical performance of an experimental al oxides with conducting polymers, carbon and its
MFC with the anode fabricated from the ceramic pow- derivatives,andtransitionmetalnanoparticles;
derNi Mo Cwasstudiedintheotherpaperbythesame d) thecompositesofconductingpolymerswithcarbon.
3 3
December2017 AnOverviewoftheFunctionalCeramicandCompositeMaterialsforMicrobiologicalFuelCells 437
the electrode, biocompatibility and the synergetic effect
becauseoftheinteractionbetweengrapheneandtin(IV)
oxide108.Alllistedaboveencouragedfastformationofa
stablebacterialfilm.
Fig.3: Current obtained from bare carbon paper electrode with
E.coli,CNT/Au/TiO -1,nanocompositesmodifiedcarbonpaper
2
electrodewithE.coli-2andwithoutE.coli-3(after107).
Fig.2:SchematicmechanismforelectricitygenerationbyK.pneu-
moniaeonanodeofMFC(after100).
Asseenfromthelist,thecompositeshouldcontainama-
terialwithintrinsicconductivity.Asarule,thisfunctionis
carriedoutbycarbonmaterials.
Fig.4:PolarizationcurvesforRGO-SnO /CC(a), RGO/CC(b)
Theanodematerialwithathree-dimensionalnanostruc- 2
and bare CC (c); power density curves for RGO-SnO /CC (d),
ture is known to have been fabricated from the carbon 2
RGO/CC (e) and bare CC (f). Linear sweep voltammetry was
nanotubes/Au/TiO2107. This composite anode demon- appliedforpolarizationtestsatascanrateof0.1mV⋅s-1(after108).
stratedhighconductivity,highporosityandspecificsur-
facearea,whichguaranteedeasyabsorptionofEscherichia ThecompositeanodemadeofSnO nanotubes/glasscar-
2
coli.Moreover,themodifiedanodehadamorehydrophilic bonincreasedthemaximumpowerdensityofanMFCup
surface in contrast to the unmodified carbon paper (the to1.421W⋅m-2inthepresenceofEscherichiacoli103.
edgeangleofwettingis90°versus110°),whichcontribut- Another nanocomposite material of Fe O /carbon
3 4
edtotheeffectiveadhesionofthebacterialfilm.Finally, nanotubes obtained by means of precipitation of
theestimatedpowerdensityofthetestingMFCreached monodisperse Fe O particles onto nanotubes with the
3 4
2.4mW⋅m-2,whichwasthreetimeshigherthanthatofthe solvothermal method may contain different mass per-
MFC with the carbon paper anode. The nanocomposite centagesofFe O varyingfrom10to80%.Finally, the
3 4
anodeperformedhighercatalyticactivityinthereaction compositeanodefabricatedfrom30%Fe O nanotubes/
3 4
ofglucoseoxidationbothinthepresenceaswellasinthe carbonpaperdemonstratedthehighestpowerdensityof
absenceofbacteriathanthecarbonpaper(Fig.3)107. anMFCinthepresenceofEscherichiacoli.Itdemonstrat-
The composite anode material based on the reduced ed 0.830 W⋅m-2101, which was approximately 40 times
graphene oxide-SnO /carbon fibre where tin(IV) oxide higherthanthepowerdensityoftheanodemadeofun-
2
hadbeenobtainedbyamicrowavesynthesissignificantly modifiedcarbonpaper.Theauthorsconsideredthehigh
influenced the Escherichia coli bacterial film formation performanceofthetestMFCtobecausedbytheimproved
rate108.ThepowerofthetestMFCwiththemodifiedan- bacteriaadhesiontotheelectrodesurfaceaswellasbythe
odereached1.624W⋅m-2(Fig.4).Thisvaluewas4.8times increaseinelectrontransferefficiencyfromthebiolayer
higher than that of the unmodified anode. The perfor- totheanode.
manceimprovementoftheMFCwiththenanocomposite ThecompositeanodematerialWO /Cfelt,wheretung-
3
anode was explained based on the large surface area of sten(VI) oxide was obtained by means of hydrothermal
438 JournalofCeramicScienceandTechnology—A.S.Galushkoetal. Vol.8,No.4
synthesis,demonstratedhighpowerdensityvaluesforthe Thus,itisnecessarythataceramicpowdershouldbea
testMFCwithShewanellaputrefaciens.Moreover,there- partofacompositeintheanodeofaMFC.Thispowder
ceiveddataareveryclosetotheanodematerialcharacteris- maynotpossessintrinsicelectronconductivity,butissup-
ticsoftheanodematerialofWO -Pt/C:1.3W⋅m-2versus posedtohavehighcatalyticactivity,biocompatibilityand
3
1.5W⋅m-2109.Theactualmechanismoftheinteractionbe- largesurfacearea.
tweenbacterialcellsandtheanodesurfacemodifiedwith
tungsten(VI)oxideisnotclear.Nevertheless,thereareda- (3) The ceramic material deposited on the surface of a
taconfirmingthatWO encouragesregularfixationofthe conductingsupportasafilm
3
bacteriatotheelectrodesurfaceowingtothelargesurface Itshouldbenotedthattheceramicmaterialpossessing
area.Besides,WO isamorebiocompatiblematerialthan intrinsicconductivityaswellasceramic-basedcomposite
3
thecarbonmaterial110,111.Moreover,tungsten(VI)oxide materialcanbefabricatedintheformofabulkelectrode
interactswiththeproteinsproducedbyShewanellaputre- or a film deposited onto a conductive support. That is
faciensorsituatedonthemembranesurfaceofthebacterial whytheclassificationoftheceramicmaterialsdiscussed
cell.Thiscontributestotheintensityoftheelectrontrans- insubsections(1)and(2)ofSectionIIIisfacultative.
portfromthebiofilmtotheMFCanode103. The conventional conductive supports are listed in Ta-
The MFC with the composite anode containing a con- ble2.
ductingpolymeralsoshowshighelectricalperformance. For instance, the anode material may consist of ruthe-
The polyaniline-mWO /C felt-based anode tested with nium(IV) oxide deposited electrochemically onto a car-
3
E.coli as a biocatalyst demonstrated power density of bonsupport104.TheexperimentalMFCwithShewanel-
0.98W⋅m-2112. ladecolorationisS12reachedthemaximumpowerdensity
A composite anode also fabricated on the basis of felt of0.308W⋅m-2whileRuO2-coveredanodehadthespe-
carbonwasdiscussedinthepaper113.Cerium(IV)oxide cific capacitance of 50 C⋅cm-2. Meanwhile, the unmodi-
wasusedastheceramicpartofthecomposite.Toobtaina fiedcarbonsupporthadthespecificcapacitanceofabout
homogeneousmixture,theoxidewasmixedwithcarbon 0.1C⋅cm-2. Hence, the power density of the test MFC
ultrasonically.Finally,theMFCwiththecompositeelec- withtheunmodifiedcarbonanodeis16.7timeslessthan
trodedemonstratedhigherpowerdensityperformanceof that of the modified one (Fig.6). The authors of the pa-
2.94W⋅m-2incontrastto1.70W⋅m-2fortheunmodified perconsiderthehighperformanceofthetestingMFCto
carbon material (Fig.5). The exogeneous populations of betheconsequenceofthelargesurfaceareaoftheanode
bacteriawereusedasabiocatalyst. materialaswellasthecapabilityoftheruthenium(IV)to
beoxidizedandreducedquickly,whichcontributestothe
effectiveelectrontransferfromthebiolayertotheanode
andviceversaifnecessary.
Table2:ElectrodematerialsforMFCs114.
Anodematerial Advantages Disadvantages
Stainlesssteel Highconductiv- Lowsurfacearea,
ity,cheaper,easy biocompatibility
accessibility issues,corrosion
Graphiterod Highconductiv- Difficult to in-
ityandchemical creasesurfacearea
stability,cheaper,
easyaccessibility
Graphite Highspecificarea Clogging
fibrebrush and easy con-
struction
Carboncloth Large relative Relativelyexpen-
porosity sive
Carbonpaper Easywireconnec- Fragile
tion
Carbonfelt Largesurfacearea Highresistance
RVC(reticulated High electrical Fragile,largere-
vitreouscarbon) conductivity sistance
Pengetal.105developedtheanodecoatingonthebasisof
Fig.5:Cyclicvoltammogramsforcarbonfelt(a)andnano-CeO
2 Fe O (itsintrinsicconductivityisupto2⋅102S/cm)and
modified carbon felt (b) in PBS (phosphate buffer solution) and 3 4
activatedexoelectrogenessuspensionlackingsubstrate(after113). activated carbon. This mixture was applied to the stain-
December2017 AnOverviewoftheFunctionalCeramicandCompositeMaterialsforMicrobiologicalFuelCells 439
lesssteelwiththerollprintingmethod.TheMFCwiththe
modifiedelectrodeAcFeMwhereAc–activatedcarbon,
M–steelmesh,inthepresenceofdissimilatorymetal-re-
ducingbacteriashowedgoodspecificcapacitance,reach-
ingthemaximumof574.6C⋅m-2incontrasttotheMFC
with the unmodified anode (AcM) (Fig.7). The authors
associatetheMFChighcapacitiveperformancewiththe
facilitatedelectrontransferfromthebacteriallayertothe
anodeowingtotheoxidation-reductionpairFe O /Fe2+
3 4
workingasanelectronshuttle.
Fig.8:Cyclicvoltammetryplotsofdifferentelectrodes:1–unmod-
ified,2–polypyrrole,3–MnO /polypyrrole(after115).
2
Apossiblemechanismofelectrontransferfromthebac-
teria to the anode surface is suggested in Fig.9. Man-
ganese(IV) oxide is an electron carrier. The conducting
polymerfacilitatestheelectrontransportowingtotheox-
idation-reduction activity. All this causes a high power
densityperformanceofthetestMFC.
Fig.6:Cellpolarization(1, 2, 3, 4)andpowerdensity(5, 6, 7, 8)
curvesoffourMFCsinoculatedwithS.decolorationisS12varying
in anodes modified with different RuO surface concentration.
2
TheappliedchargedensityfortheformationofRuO contained
2
in four anodes was 0, 10, 30 and 50 C⋅cm-2, respectively. The
datapointshownrepresentstheaverageoftriplicatemeasurements
obtainedfromthreeindependentexperiments±standarddeviations
(after104).
Fig.9:Electrontransfermechanismofmodifiedanode–polypyr-
role/MnO (after115).
2
Thus,ceramicanodematerialssignificantlyimprovethe
electricperformanceofMFCsowingtotheirhighcatalytic
activity,biologicalinertness,electrochemicalstabilityand
highenergycapacitance.
Fig.7:Theamountofnetstoragecapacity(AcM–black,AcFeM– IV. CeramicsasCathodeFunctionalMaterial
red)ateachdisruptiontimebasedonthecharge-dischargeexperi-
ment.Errorbars±SDwerebasedonaveragesmeasured(after105). Intermsoffunctioning,ceramicsusedforcathodescan
beassignedtwomainpurposes:supportingandcatalytic
Intheresearchpaper115,apasteofmanganese(IV)oxide functions.
and polypyrrole (a conducting polymer) composite ob-
(1) Ceramicsassupportingmaterial
tained with a chemical method was applied to titanium
wirecoveredwithadsorbedfeltcarbon.Finally,themaxi- Thesupportingfunctionisbasedonthemechanicalsta-
mumpowerdensityoftheMFCwiththemodifiedanode bilityofcalcinedmaterials,biologicalcompatibilitywith
inthepresenceofseabottombacteriareachedthevalueof bacteriaandhighporosity.Anumberofpublicationsisde-
0.562±5W⋅m-2,whichwas2.2timeshigherthantheMFC votedtotheuseofnaturalceramicmaterialssuchasterra-
powerdensitywiththeunmodifiedanode.Asseenfrom cotta, earthenware, Trojan clay, etc. in open-to-air cath-
Fig.8,thespecificcapacitanceofthemodifiedelectrodeis odes9–120.Suchcathodesplayadoublerole,beingsimul-
3.1timeshigherthanthatoftheunmodifiedone. taneously membranes in single-chamber MFCs. In this
440 JournalofCeramicScienceandTechnology—A.S.Galushkoetal. Vol.8,No.4
section, the natural ceramic cathode materials operating Thespectrumofcathodecatalyticmaterialsisenormous-
primarilyassupportsarenotdiscussedastheywillbecon- ly wide. Hereafter, primarily heat-treated materials pos-
sideredbelowinSectionVdevotedtomembranes. sessingcrystallinestructureandthereforequalifyingasce-
Nevertheless,itisworthmentioningthatsomeceramic ramicsarehighlighted.
powderslikeTiO canbeusedinsteadofcarbonassup-
2 (a) Metaloxidesandcompositematerialsbasedonthem
ports for cathode catalysts (Pt). The search for alterna-
ascatalysts
tives to carbon is necessitated by a number of its disad-
vantages121,122.Oneofthemissignificanthydrophilici- Manganese oxides are the most popular ORR catalysts
ty,whichresultsindeterioratedmasstransportproperties among other oxide compounds owing to their low cost,
abundance, non-toxicity, environmental safety and high
ofagasdiffusionlayerinopen-to-aircathodes.Thisisof
catalyticactivity126.Thepopularityiscausedbythefact
importanceastheaircontactingwiththecatalystshould
that manganese in oxides can adopt a variety of crystal
remainhumidified,otherwiseionicconductivityandcat-
structuresandmayexistindifferentvalencestatesof2+,
alyticactivitywillbeworsenedandMFCperformanceim-
3+ and 4+. In the review126, Stoerzinger, et al. (2015)
paired.
considered the ORR kinetics for manganese(IV) oxide
Another drawback of carbon is a tendency to corrode,
obtainedbymeansofdifferenttechniquesincludinghy-
whichleadstotheworseningofthecontactbetweenthe
drothermal methods and annealing steps. Crystal struc-
support and the catalyst particles. The particles conse-
tures like perovskites and spinels were observed. It was
quentlybecomemoremobileandformlargeraggregates
presumedthatthechargetransferprocessproceededpri-
ormigrateoutofthelayer.Incontrasttocarbon,although
marilyviaafour-electronroute.
TiO possesses lower conductivity with a band gap of
2 Lietal.(2010)127reportedonahydrothermallyobtained
3.2eVforanataseand3.0eVforrutile,itshowsmoreuni-
(180 °C, 48 h) cryptomelane-type octahedral molecu-
formdispersionofmetalparticles(Pt), improvedcorro-
lar sieve (OMS-2) structured from MnO in a paper-
sionresistance,highercatalyticstabilityandactivityow- 2
like shape which was a better conductor than a bulk
ingtoitsalloyformationwithPt121,123.
OMS-2powdercathodebecauseofitsmicroporousnet-
Venkatesan et al. (2016)121 obtained TiO -supported
2 work. Besides, catalytic performance of the composites
metalnanocatalysts(Pt/TiO )withtheliquidphasepho-
2 Co-OMS-2,Cu-OMS-2andCe-OMS-2wasalsodeter-
to deposition method followed by calcination at 450 °C
mined and examined. The research revealed that among
for3h.Thecathodematerials–Pt/TiO ,Pt-Fe/TiO and
2 2 undoped and doped cathode catalysts, Co and Cu com-
Pt/C–weretestedinasingle-chamberMFCtoshowthe
positesexhibitedthebestpowergenerationwithvoltages
powerdensitiesof110.5,136.8and77mW/m2respective-
of217mVand214mVrespectively(incomparisonwith
ly, thus, demonstratingabetterperformanceofPt-com-
Pt catalyst producing 202 mV), although the maximum
positeswithtitanium(IV)oxidesupportcomparedtothat
powerdensitiesattainedbythematerialswerestillbelow
withcarbonsupport.Besides,cyclicvoltammetryexperi-
Ptcatalystperformance.
mentsrevealedahigherelectrochemicalstabilityofTiO
2 In later research128, Li et al. conducted 600-h con-
after200cyclesthancarbon.
tinuous flow tests demonstrating that the fast reac-
Thealternativematerialsincludingsemi-conductiveox-
tion rate of OMS-2 cathodes enhanced power gener-
ides (TiOx, WOx and SnO ) or carbides124 instead of
2 ation and chemical oxygen demand (COD) removal
carbonassupportareinpracticeforpolymerelectrolyte
efficiency. Co-OMS-2 cathodes had the better per-
membranefuelcells(PEMFCs).
formance than Cu-OMS-2 at high COD concentra-
tions of 2000–4000 mg/L, with the power density of
(2) Ceramicsascatalyticmaterial
897mW/m2andCODremovalefficiencyof46%.Mean-
Catalyticfunctionisundoubtedlythemajorpropertyof
while,Cu-OMS-2cathodewasmoresuitableatlowCOD
cathodematerialsasitdeterminestheefficiencyofmolec- concentrations(1000mg/L)exhibitingthehighestpower
ularoxygenbinding.Protonsaretransferredfromthean- densityof201mW/m2.
odetothecathodethroughthemembraneandelectrons Zhang et al. (2009) reported about three modifications
reach the cathode through the external circuit to reduce ofMnO –a,bandc-structures129.b-structurewassyn-
2
themolecularoxygen.Therefore,theneedarisesforame- thesized hydrothermally at 125 °C for 24h followed by
diatorhasteningthereductionreaction.Thisroleisplayed sinteringat300°Cfor5h.Theothera-andc-structures
bycatalystsowingtothepresenceofatoms/ionscapable werealsoobtainedwiththesolution-basedhydrothermal
ofchangingtheiroxidationstateand,hence,leadingtoin- method,butwithoutsubsequentcalcination.Theresults
termediatebondformation. showedthata-MnO tendedtobeoflowcrystallization
2
Preciousplatinumhasbeenconsideredthemosteffective extent and amorphous structure, and c-MnO was mi-
2
catalystsofar.Thus, researchhasbeendirectedtowards crosphericalandtendedtobeagglomerated.Meanwhile,
findingnewcommerciallyavailable,effectivecatalyticma- b-MnO appeared in loose and claviform form, which
2
terialscapableofreplacingthemoreexpensiveplatinum. contributed to it having the largest BET surface of the
Numerouspublicationsaredevotedtothecatalyticactiv- threesamples.Hence,itwastheb-MnO thatcausedthe
2
ityofthermallyprocessedmetaloxidesandtheircompos- highestMFCvoltage,closetothatfortheMFCusingPt
ites along with metal compounds enriched with carbon ascatalyst(627±18mV).Themaximumachievedoutput
andnitrogen, inoxygenreductionreactions(ORR)tak- powerwas3773±347mW/m3,andthemaximumcurrent
ingplaceatcathodes125–183. densitywas20400mA/m3whenatube-typeMFCcon-
December2017 AnOverviewoftheFunctionalCeramicandCompositeMaterialsforMicrobiologicalFuelCells 441
struction was used. The data on the high catalytic activ- In the research papers102,140,141, Gong et al. (2014),
ity of b-MnO in ORR are in accordance with Lu et al. Ge et al. (2015) and Song et al. (2015) considered the
2
(2011)130. use of cobalt oxide impregnated into a stainless steel
Liu et al. (2010)131 reported about nanostructured mesh (SSM)140, activated carbon (AC)141 and nitro-
MnOxwiththecontrollablesizeandmorphologywhich gen-doped carbon nanotubes (CNT)102 respectively. In
couldbereadilyobtainedwiththeelectrochemicaldepo- all cases the Co O underwent high-temperature cal-
3 4
sition method. The MFC on the basis of such a cathode cination (300–500 °C for 2–3 h). The first composite
catalystcangenerateelectricitywiththemaximumpow- cathodeSSM/Co O wasdesignatedtooperateinadu-
3 4
er of 772.8 mW/m3. Zhang P. et al. (2014)132 obtained al-chamber MFC while the others equipped air single-
c-MnO2 carnation-like crystals with the potentiostatic chamberMFCstoproducetheMPDof1500mW⋅m–2141
method.Themaximumpowerdensity(MPD)oftheMFC and 469±17mW⋅m–2102 correspondingly. The authors
equippedwiththeelectrodepositedMnO2/AC(activated pointedouthighorcompetitivecatalyticperformanceof
carbon)aircathodereached1554mW⋅m-2. cobalt-oxide-based electrodes in ORR and emphasized
Chengetal.(2011)133describedarapidchemicalsynthe- their importance and economic efficiency in replacing
sisofnanocrystallinespinel-likeMnO2atroomtempera- platinum.
ture, although its operation in MFCs was just proposed Ahmedetal.(2014)142,147embeddedCoO nanoparti-
x
butnottested. cles on carbon in order to enhance the catalytic activity
Liewetal.134(2015)andY.Zhangetal.135(2011)studied ofcobaltorironphthalocyanine.ThecompositeCoO -C
x
hydrothermal synthesis of MnO on carbon nanotubes
2 was calcined at 400 °C for 1h in ambient conditions for
(CNT) and functionalized nanotubes (f-CNT). Accord-
graphiticstructuralrearrangementofcarbonandforthe
ing to Liew, the microbial fuel cell test showed that the
formation of cobalt oxide. The MPD of the MFC with
compositeMnO /f-CNTdisplayedhigherpowerdensi-
2 cobaltphthalocyaninecathodecatalystdopedwithCoO
ty (520 mW⋅m-2) compared to CNT (275 mW⋅m-2) and x
nanoparticles was 50% higher than without cobalt ox-
f-CNT(440mW⋅m-2).AccordingtoY.Zhang,thepower ideandapproximatelyequalled780mW⋅m–2comparedto
density of 210 mW⋅m-2 produced by the MFC with in 654mW⋅m–2forthecathodecoveredwithironphthalo-
situ MnO /CNT cathode was 2.3 times that produced
2 cyanine.
by the MFC using mechanically mixed MnO /CNTs
2 Ironoxides.StudiesofORRcatalystsbasedonironoxide
(93mW⋅m–2), and comparable to that of the MFC with
arerepresentedintheworks148,149,150.TheMPDperfor-
aconventionalPt/Ccathode(229mW⋅m–2).Onepossible
manceofsingle-chamberedMFCsiscertainlyhigherfor
reason for the moderate performance is likely to be the
thehigh-temperature-treatedironoxides(1000°C)149,150
amorphousstructure.
than for the oxides just obtained based on decomposi-
Chen et al. (2012)136 reported about the composite
tion of Fe(CO) (165–175 °C)148. Moreover, Ma et al.
MnO /CNT on a steel mesh with the MPD of the cor- 5
2 (2015)150 investigated silver composite on the basis of
responding MFC of 2676 mW⋅m–2 (normalized to the
Fe O /GC(graphiticcarbon)whichincreasedtheMPD
cathode surface area) or maximum power of 86 W⋅m-3 3 4
from1500to1700mW⋅m–2.
(normalizedtotheanodechambervolume).Thisadvan-
Nickel oxide. The heat-treated NiO (400 °C, 2 h) was
tageovertheMFCsequippedwithMnO /CNTobserved
2 examined in the composite with CNTs by Huang et
aboveislikelytodependonthepropertiesofPMPS(poly-
al. (2015)151 although the MPD was quite moderate
methylphenylsiloxane)usedinagasdiffusionlayer,and
(670mW⋅m–2).
steelmesh.Theneedforthesiloxanederivativewaspri-
Copper oxides. Crystalline-structured copper oxides
marilycausedbyitswater-repellentproperties.Andthe
were successfully synthesized on activated carbon by
steel mesh is a more conductive and adjustable support
thanthecarboncloth/paperappliedin134,135. means of electrochemical deposition without calcina-
In research papers137,138, the graphene and graphene tion152,153. The corresponding MPD of single-chamber
MFCswasabout1400–1500mW⋅m–2,whichcanbecon-
oxide were also found to be effective substrates for
sideredquiteremarkableandcomparablewiththeMPD
manganese oxide. The MPD for a MFC operating with
performanceofmanyothercalcinedoxides.
MnO /GNS (graphene nanosheets) achieved by Wen et
2
al.(2012)was2083mW⋅m–2comparedtotheoneobtained Vanadiumoxidesarealsoprospectivelow-costcatalytic
byGnanaKumaretal.(2014)fornanotubulara-MnO / materialsusedforORRinMFCs.Noorietal.(2016)154
2
grapheneoxide,whichwas3359mW⋅m–2. showedtheadvantageoftheheat-treated(350°C)flower-
The intensive studies of catalytic materials based on likeV2O5overa-MnO2nanotubes.whichwasrevealed
MnO have led to attempts of a more complex com- in a higher MPD, Coulombic efficiency and wastewater
2
posite fabrication involving redox active organic con- treatmentefficiency.Ghoreishietal.(2014)155compared
ducting polymers. Yuan et al. (2015)139 came up with performances of the MFCs with the composite material
the hydrothermal synthesis of multiwalled nanotubes V2O5/PANI(polyaniline)(notcalcined)andV2O5(cal-
MnO /polypyrrole/MnO .ThecorrespondingMFCin- cined),demonstratingthesuperiorityofthecomposite.
2 2
cludingthismodifiedcathodecatalystexhibitedanMPD Zirconium oxide is thoroughly examined in156. The
of721±20mW⋅m–2. achievedMPDwas600mW⋅m–2.
Cobaltoxidesarethenextfrequentlyusedcathodecata- Apart from transition metal oxides, the non-transition
lystsaftermanganeseoxides102,140–147. metaloxidesalsoexhibitcatalyticactivity.
442 JournalofCeramicScienceandTechnology—A.S.Galushkoetal. Vol.8,No.4
Leadoxide157andtinoxide158couldalsobeemployed catalystsofbiologicalimportance.Althoughtheirappli-
as an alternative to platinum catalyst and co-catalyst re- cation in MFCs without preliminary processing is con-
spectively. sideredinanumberofresearcharticles147,162–164,anew
Hybridmetaloxides.Notably, thehighestMPDswere approachbasedonpyrolysisofthecompoundswithout
demonstrated by combined dual inorganic oxides en- substantialdegradinghasbeengaininginpopularity.Var-
hanced with AC. The urchin-like NiCo2O4/AC143 ex- iousresearchgroupshavereportedthattheheattreatment
hibited 1730 mW⋅m–2 versus MnO2/AC/CeO2 with (up to 1000 °C) of transition metal macrocycles signifi-
2403 mW⋅m–2159. The complex multi component per- cantlyimprovedtheirstabilityaselectrocatalystsforoxy-
ovskite-andspinel-likeoxidesarediscussedin145,146. genreductionandinsomeinstances,enhancedtheirover-
allcatalyticactivity165whilemaintainingtheopenframe-
(b) Metal oxide-based organic-inorganic composites as
workstructurewithsufficientporosityandlargesurface
catalysts
area166.Thisclassofmaterialsistermedmetal-nitrogen-
Theelectroactiveconductingorganicpolymersarefre-
carbon (M-N-C) catalysts or metal-organic-framework
quently employed as components to metal oxides in or-
(MOF)-derivedcatalysts.
ganic-inorganiccomposites139,144,155,160.Toalargeex-
Themostwidelyusedprecursorsofthesemetal-organic
tentthedemandfortheformeriscausedbythecombina-
framework catalysts are macrocyclic complexes of inex-
tionofgoodconductivitywithelectrocapacitivebehavior.
pensivetransitionmetals(Fe,Co)withnitrogen-richor-
Theelectrochemicalactivityofmetaloxidesis,however,
ganic molecules like porphyrins14,167–170, phthalocya-
limitedbytheirsemiconductingnature.Thus,theincor-
nines14,167–169,171,172,andnaphthalocyanines173,metal
poration of metal oxides on a redox active polymer like
complexesofethylenediamine(EDA)174andethylenedi-
polyaniline(PANI)orpolypyrrole(PPy)canimprovethe
aminetetraaceticacid175aswellasmetal-dopedpolymer-
catalyticperformanceofaceramicmaterial.
ic molecules evolved from imidazole176, 2-methylim-
(c) Carbides,nitridesandboridesascatalysts idazole177, aniline178,179, pyrrole180, melamine181,
aminoantipyrene182, mebendazole182, cyanamide161,
Despite ubiquitous studies of ceramic oxides and their
dicyandiamide183,etc.
application as cathode catalytic materials in MFCs, ni-
Fig.10 depicts a possible route of the Co-MOF struc-
trides, carbides and borides have not been researched
turetransformationuponthermalactivation.X-raypho-
sufficiently. Nevertheless, Wen et al. (2012)161 reported
toelectron spectroscopy (XPS) data show that imidazo-
aboutnitrogen-enrichedcore-shellstructuredFe/Fe C-C
3
lategroupsaregraduallyconvertedtocarbonaceousforms
nanorods derived from thermal decomposition of a ni-
whereasafractionofthenitrogenisretainedaspyridinic
trogen-containing polymer. Based on thermal gravimet-
orpyrrolic-likemoietieswiththeformerbeingthedom-
ric analysis (TGA), Fourier transform infrared spec-
inantcomponent.Thelossofhydrogenandsomecarbon
troscopy (FTIR), X-ray diffraction (XRD), energy-dis-
andnitrogenaswellasthesegregationoftheexcessCo2+
persive X-ray spectroscopy (EDS), transmission elec-
tron microscopy (TEM), the authors suggested that the ionsandtheirconversionintoCo0alterthestructureofthe
heating was accompanied by simultaneous release of a frameworkwiththeformationofmesoporeswhilemain-
largeamountofcarbonnitridegases(e.g.C N +,C N +, taining a certain fraction of the micropores. Further in-
2 2 3 2
C N +)whichfinallyevolvedintothecore-shellcompos- creaseoftemperatureto900°Cgraphitizescarbon,caus-
3 3
ite N-Fe/Fe C@C. Multiple cyclic voltammetry experi- ingfurtherlossofNforactivesitesand,therefore,reduc-
3
mentsgivethefoundationtoconvincethatthedopedN ingtheactivityofthecatalyst176.Oftheabove-mentioned
and core-Fe C in the N-Fe/Fe C@C play a key role in MOF-catalysts, the highest MPD of 2437 ± 55 mW⋅m-2
3 3
improvingthecatalyticperformanceforORR. has been registered for a single-chamber MFC with the
Fe-N-CcathodederivedfromtheFe-EDAderivative174.
(d) Heat-treatedM-N-Ccomplexesascatalysts Suchahighperformanceshowstheadvantageofthesub-
Macrocycliccoordinationcompoundssuchasmetalpor- stanceswiththedevelopedframeworkand,hence,multi-
phyrins and phthalocyanines are known to be effective pleactivesitesforanchoringoxygenmolecules.
Fig.10:ProposedstructuralconversionfromtheMOFstructuretothecatalyticactivesite(after176).