3. Tulemused ja arutelu
3.3 Katalüsaatormaterjalide kütuseelemendi testide tulemused
Kuna Fe-N-Gra näitas parimat O2 redutseerumise elektrokatalüütilist aktiivsust võrreldes teiste katalüsaatormaterjalidega (RDE meetodil), otsustati seda testida ka reaalses kütuseelemendis.
Joonisel 13 on näha PEM kütuseelemendi voolu- ja võimsustiheduse kõveraid, mille puhul oli
32 katoodina kasutuses Fe-N-Gra ja anoodina Pt/C. Mõõtmine viidi läbi 80 °C juures ning membraanina kasutati Nafion NRE-211. Saavutatud maksimaalne võimsustihedus (Pmax) oli 182 mW cm–2, mis on pigem madal võrreldes varem kirjanduses avaldatud tulemustega [126], kuid see võib tuleneda sellest, et antud mõõtmise puhul ei olnud elektroodide ja membraani ettevalmistus eelnevalt optimeeritud. Avatud vooluringi pinge (open circuit voltage, OCV) oli üsna kõrge (0,82 V) ja kineetilises alas oli materjal aktiivne, kuid väiksematel potentsiaalidel kasvas vool väga aeglaselt optimeerimata katalüsaatori kihi ning kõrge massiülekande takistuse tõttu. Voolutihedus 0,8 V juures oli umbes 5,9 korda kõrgem (6,5 mA cm–2) võrreldes RDE mõõtmistel saadud jk väärtusega (1,11 mA cm–2). Siiski näitavad tulemused, et Fe-N-Gra on lootustandev katalüsaatormaterjal PEM kütuseelemendi katoodina.
Joonis 13. PEM kütuseelemendi polarisatsiooni- ja võimsustiheduse kõverad. Katoodil oli 4 mg cm–2 Fe-N-Gra. Temperatuur = 80 °C, 100% RH O2/H2, rõhk 1 baari.
AEM kütuseelemendi testimisel saadud voolu- ja võimsustihedused on toodud joonisel 14.
Nagu võis oodata ka RDE tulemustest, oli aktiivsus kütuseelemendis aluselistes tingimustes palju parem kui happelistes tingimustes. Voolutihedus 0,8 V juures oli 125 mA cm–2, RDE mõõtmiste puhul oli jk väärtus 0,8 V juures 4,14 mA cm–2. Eriti muljetavaldavad olid tulemused kõrgemal potentsiaalil, kus Fe-N-Gra jõudis väärtuseni 125 mA cm–2, võrreldes Pt/C katalüsaatoriga, mille puhul on selleks väärtuseks 174 mA cm–2. Saavutati võimsustihedus Pmax
= 243 mW cm–2, mis on hea tulemus võrreldes kirjanduses leiduvate tulemustega (vt tabel 5).
Oluline on ära märkida, et võrreldes Fe-N-Gra testimisel saadud tulemustega on saavutatud
33 veelgi paremaid tulemusi teiste anioonvahetusmembraanide ja ionomeeridega. Võrreldes tulemusi mitteväärismetalli sisaldavate katalüsaatoritega, mis kasutavad sama membraani (HMT-PMBI), on Fe-N-Gra tulemus siiani parim.
Joonis 14. AEM kütuseelemendi polarisatsiooni- ja võimsustiheduse kõverad. Katoodil 2 mg cm–2 Fe-N-Gra ja anoodil PtRu/C. Temperatuur = 60 °C, 100% RH O2/H2 0,2/0,3 NLPM ja rõhk 200 kPa.
Tabel 5. AEM kütuseelemendi tulemuste võrdlus erinevate mitte-väärismetallkatalüsaatoritega
Katalüsaator j0,8 V, mA cm–2
Pmax, mW cm–2
Kogus katoodil, mg cm–2
Temperatuur, ºC Membraan Ionomeer Viide
Fe-N-Gra 125 243 2,0 60 HMT-PMBI HMT-PMBI See töö
F,N,S-rGO 0 46 1,5 85 HMT-PMBI HMT-PMBI [127]
MH-DCNT 30 250 0,34 85 HMT-PMBI HMT-PMBI [128]
Pürolüüsitud
KB/FePc 32 186 2,0 60 °C HMT-PMBI HMT-PMBI [129]
Fe-LC-900 10 50 2,0 Pole märgitud FAA Pole märgitud [130]
Fe-N-comp-0.5 25 160 2,6 60 Tokuyama A201 Tokuyama AS4 [131]
Fe-NMG 100 218 2,625 60 Tokuyama A201 Tokuyama AS4 [132]
Fe/N/C
nanotorud 75 485 2,0 60 aQAPS-S8 aQAPS-S14 [133]
MCS 250 1100 1,45 60 aQAPS-S8 aQAPS-S14 [134]
N-C-CoOx 300 1050 2,4 65 LDPE-BTMA ETFE [135]
CF-VC 60 1350 2,4 70 ETFE-BTMA ETFE [136]
34
Kokkuvõte
Käesolevas magistritöös uuriti hapniku redutseerumist raua ja lämmastikuga ning väävli ja lämmastikuga dopeeritud süsinikmaterjalidel. Raua ja lämmastikuga dopeeritud materjalide valmistamiseks kasutati süsinikuallikana grafeenoksiidi või kommertsiaalselt kättesaadavat grafeeni, lämmastikuallikana 1,10-fenantroliini ja rauaallikana raud(II)atsetaati. Väävli ja lämmastikuga dopeeritud materjalid sünteesiti kasutades süsinikuallikana kommertsiaalset grafeeni ning lämmastiku- ja väävliallikana tiosemikarbasiidi või tritiotsüanuurhapet.
Materjalide dopeerimiseks kasutati erinevaid meetodeid: märg- ja kuivmeetodit. Märgmeetodi puhul (kasutati nii Fe,N kui ka S,N dopeerimisel) dispergeeriti kõik lähteained solvendis, segu kuivatati ning pürolüüsiti 800 °C juures N2 keskkonnas 1 h jooksul. Kuivmeetodi jaoks jahvatati kõik lähteained kuulveskis 2 h kiirusel 400 p min–1 ning pürolüüsiti samades tingimustes.
Pöörleva ketaselektroodi meetodil saadud tulemused näitasid, et kõik sünteesitud materjalid olid hapniku redutseerumisel aktiivsemad kui dopeerimata materjalid. Hapniku redutseerumisreaktsiooni uuringud aluselises keskkonnas Fe-N-Gra puhul näitasid palju suuremat aktiivsust võrreldes Fe-N dopeeritud GO-ga. Nagu selgus erinevate füüsikaliste mõõtmismeetodite tulemustest, siis tuleneb erinevus Fe-N-Gra suuremast eripinnast, mikro-/mesopooride suhtest ja suuremast hulgast Fe-Nx lämmastikuvormist võrreldes Fe-N-GO materjaliga. 57Fe Mössbaueri spektroskoopia andmed näitasid, et raua ja lämmastikuga dopeeritud grafeenis oli peaaegu pool rauast aktiivses Fe-Nx vormis. Samas Fe-N-Gra katalüsaatormaterjaliga happelises keskkonnas läbi viidud hapniku redutseerumise mõõtmised ei näidanud võrreldes aluselise keskkonna tulemustega nii suurt elektrokatalüütilist aktiivsust.
S,N-dopeeritud materjalidel puhul näitasid XPS tulemused, et kõik dopeeritud materjalid sisaldasid väävlit ja lämmastikku ning suurima sisaldusega lämmastikuvormiks oli püridiinne lämmastik. Samuti on huvitav märkida, et kuivdopeerimise puhul oli üleminevate elektronide arvuks O2 molekuli kohta 4, seega võib see meetod olla ka S,N-dopeerimiseks sobivam.
Aktiivseimat materjali, Fe-N-Gra, testiti ka prooton- ja anioonvahetusmembraaniga kütuseelemendi tingimustes. Sarnaselt RDE meetodil saadud tulemustele, polnud ka prootonvahetusmembraaniga kütuseelemendi testi tulemus parim. Samas anioonvahetusmembraaniga saadud tulemused olid väga paljulubavad. Nimelt, maksimaalseks võimsustiheduseks saadi 243 mW cm–2 , mis on küllaltki kõrge võrreldes kirjanduses avaldatud tulemustega.
35
Oxygen reduction on nitrogen doped and iron or sulphur-containing graphene based materials
Roberta Sibul
Summary
In this work, oxygen reduction reaction was studied on iron and nitrogen or sulphur and nitrogen doped carbon materials. To produce iron and nitrogen doped catalyst materials, graphene oxide or commercial graphene were used as carbon sources, 1,10-phenanthroline as a nitrogen source and iron(II)acetate as iron source. To synthesise S,N doped materials, commercial graphene was used as a carbon source, thiosemicarbazide and thiocyanuric acid were employed as nitrogen and sulphur sources. For the doping procedure both wet and dry methods were used.
In case of wet method (for Fe,N and S,N doping) all the materials were dispersed in the solvent, the mixture was dried and thereafter pyrolysed at 800 °C in N2 atmosphere for 1 h. For the dry synthesis method all the starting materials were ball-milled for 2 h at 400 rpm and pyrolysed in N2 flow at 800 °C for 1 h.
The rotating disc electrode results showed that all the doped catalyst materials were more active towards the oxygen reduction than undoped materials. The oxygen reduction studies for Fe-N-Gra in alkaline conditions exhibited much higher activity compared to Fe-N-GO. As shown by different physico-chemical methods, this is due to a higher specific surface area, micro/mesoporous nature and larger amount of Fe-Nx moieties in Gra compared to Fe-N-GO. 57Fe Mössbauer spectroscopy presented that almost half of the iron in Fe-N-Gra was in an active Fe-Nx form. Fe-N-Gra in acidic conditions did not show as good electrocatalytic activity as in alkaline media. The XPS analysis of S,N-doped materials showed that all the synthesised catalyst materials contained sulphur and nitrogen and the highest amount of nitrogen form in these catalysts was pyridinic nitrogen. It must also be noted that the number of electrons transferred per O2 molecule was four in case of the materials which were doped using dry ball-milling. This means that the dry method could be more beneficial for producing sulphur and nitrogen doped catalysts.
Fe-N-Gra as the most active catalyst material was also tested in anion and proton exchange membrane fuel cells. Similarly to the results obtained by RDE, the proton exchange membrane fuel cell test did not give the best results. But the anion exchange membrane fuel cell test the performance was very promising. Namely, the peak power density (Pmax) for Fe-N-Gra was 243 mW cm–2, which is a quite good result compared to the ones published in literature.
36
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