Solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used to anal-
yse numerous graphene-sheet based materials in an attempt to study their effects
on the performance of polymer electrolyte membrane fuel cell (PEM-FC) mate-
rials. It has been noted in the literature that fuel cells which incorporated these
materials (e.g. functionalized graphene, doped carbon nanotubes (CNTs), etc.)
displayed increased performance over a wider range of environmental conditions,
chiefly temperature and relative humidity. The inter-material interactions behind
this phenomenon are poorly described at best. Due to its extreme site speci ficity
and sensitivity to minute differences in nuclear electromagnetic environments, ss-
NMR is an ideal tool for investigating the complicated interactions at work in these
systems. While the electronically conductive, amorphous, non-stoichiometric, and
low spin-density nature of these materials presented challenges to the collection
of NMR spectra, the results presented here display the remarkable utility of this
method in the study of analogues and derivatives of graphene.
Graphene Oxide (GO), a derivative of graphene, has intrinsic proton conduc-
tivity which is similar to Na fon, the most popular proton exchange membrane
material currently used in fuel cells. Research into acid-functionalized graphene
oxides and determining the role of acidic groups in increasing proton conductivity
will help to improve polymer electrolyte membrane performance in fuel cell sys-
tems. Multinuclear solid-state NMR (ssNMR) spectroscopy was used to analyse
the structure and dynamics of GO and a number of sulfonic acid derivatives of
GO, both novel and previously reported. 13C spectra showed the disappearance
of surface-based oxygen groups upon GO functionalization, and can be used to
identify linker group carbon sites in previously synthesized and novel functional-
ized GO samples with high speci city. Dehydration of these samples allows the
collection of 1H spectra with resolved acidic proton and water peaks. The effect of dehydration on the proton spectrum is partially reversible through rehydration.
Deuteration of the acidic groups in high temperature and acidic conditions was
virtually unsuccessful, indicating that only the surface and not the intercalated
functional groups play a role in the enhanced proton conductivity of ionomer /
functionalized GO composites. Increased surface area and increased delamination
of functionalized GO is suggested to be important to improved PEM-FC perfor-
mance. This synthesis and method of analysis proves the utility of ssNMR in the
study of structure and dynamics in industrially relevant amorphous carbon ma-
terials, despite the obvious di culties caused by naturally broad signals and low
sensitivity. Graphene and carbon nanotubes (CNTs) have been investigated closely in re-
cent years due to their apparent positive effect on the electrochemical performance
of new fuel cell and battery systems as catalyst stabilizers, supports, or as metal-
free catalysts. This is particularly true for doped graphene and CNTs, where
only a small amount of doping with nitrogen and/or phosphorus can have a re-
markable effect on materials performance. A direct link between structure and
function in these materials is, as of yet, unclear. Doped graphene and CNTs
were synthesized using varied chemical vapour deposition (CVD)-based methods,
and ssNMR was used to unambiguously identify dopant atom sites, revealing that
these particular synthesis methods result in highly homogeneous populations of
installed phosphorus and nitrogen atoms. We present the first experimental 15N
spectrum for graphitic nitrogen in N-doped graphene. 15N-labeled nitrogen doped
graphene synthesized as reported here produces mainly graphitic nitrogen sites
located on the edges of sheets and around defect sites. 1H-1H and 1H-15N corre-
lations were also used to probe dopant nitrogen sites in labelled and unlabelled
N-doped graphene. A nearly homogeneous population of phosphorus in P-doped
graphene is found, with an overwhelming majority of graphitic phosphorus and
a small amount of phosphate oligomer. Similar findings are noted for the phos-
phorus sites in phosphorus and nitrogen co-doped CNTs with a minor change in
chemical shift, as would be expected from two chemically similar phosphorus sites in carbon allotropes (CNTs versus graphene sheets) with signifi cantly different electronic structures.
Ionomeric sulfonated polyether ether ketone (SPEEK) membranes were doped
with functionalized graphenes, and the proton conductivities of these composite
membranes was measured at fuel cell operational temperatures and percent relative
humidities (%RH). The differences in proton conductivity between pure SPEEK
membranes and composites with different dopants and doping levels at varied
conditions were investigated through high-fi eld 1H ssNMR. It was found that high-
speed MAS was able to dehydrate membranes under water-saturated conditions,
and so lower %RH conditions were better able to produce reliable ssNMR results.
The addition of graphitic dopants appeared to have an overall detrimental effect
on the bulk proton conductivity of membranes, while concurrently these doped
membranes had a broadened operational temperature window.
In an attempt to explore the positive influence of nitrogen doping on the effec-
tive lifetime of carbon-supported platinum catalysts used in automotive hydrogen
fuel cell systems, solid-state NMR was employed to probe the difference (if any)
between doped catalyst supports made from different carbon and nitrogen sources.
1H spectroscopy showed a variety of sites present in the doped samples; some likely
from residual starting material but others from novel sites within the doped cat-
alyst supports. Double-quantum and 2D 1H experiments were used to examine
the structure of these catalysts, while 13C CPMG experiments (see Chapter 2)
revealed subtle differences in the nuclear relaxation rates of these materials, poten-
tially related to their electronic conductivity. The results of the ssNMR analysis
were insuffcient to provide an unambiguous picture of the dopant sites within
these carbon black samples; this was due in equal parts to the lack of isotopically
labelled dopants, the effects of electronic induction and ring current shifts on data
acquisition and analysis, and the broad array of different 13C chemical shift en-
vironments present in the carbon black itself. While the data is still interesting
spectroscopically, suggestions are made at the end of this chapter to expand upon
the lessons learned through this study to produce more useful results from similar
samples in the future. / Thesis / Doctor of Philosophy (PhD) / Solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used to anal-
yse numerous graphene-sheet based materials in an attempt to study their effects
on the performance of polymer electrolyte membrane fuel cell (PEM-FC) materials.
It has been noted in the literature that fuel cells which incorporated these materials
(e.g. functionalized graphene / graphite, doped carbon nanotubes (CNTs), etc.)
displayed increased performance over a wider range of environmental conditions,
chiefly temperature and relative humidity. The inter-material interactions behind
this phenomenon are poorly described at best. Due to its extreme site specifi city
and sensitivity to minute differences in nuclear electromagnetic environments, ss-
NMR is an ideal tool for investigating the complicated interactions at work in these
systems. While the electronically conductive, amorphous, non-stoichiometric, and
low spin-density nature of these materials presented challenges to the collection
of NMR spectra, the results presented here display the remarkable utility of this
method in the study of analogues and derivatives of graphene.
Covalently functionalized graphene / graphite was synthesized, and the struc-
tures of several derivatives were recorded with remarkable resolution, such that
functional group carbons were resolvable. The proton dynamics of this material
were remarkably slow, and so improvements in composite PEM ion conductiv-
ity were proposed to be caused by surface interactions between dopant and poly-
mer. The proton dynamics of ionomer graphene composites were also investigated
through ssNMR. A number of graphene and CNT samples doped with phosphorus
and 15N-labelled nitrogen were also analysed, and the synthesis methods employed
were found to produce chemically homogeneous dopant sites with few by-products.
Absent isotopic labelling, nitrogen dopant sites in carbon black samples were found
to affect the relaxation properties of protons within nitrogen doped carbon black.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/23048 |
| Date | January 2018 |
| Creators | MacIntosh, Adam Robert |
| Contributors | Goward, Gillian, Chemistry and Chemical Biology |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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