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\DOI{10.5802/crchim.345}
\datereceived{2024-07-25}
\daterevised{2024-09-22}
\dateaccepted{2024-09-24}
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\dateposted{2025-02-13}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\title{Versatility of gas--solid fluorination for multiple functionalities}

\alttitle{Polyvalence de la fluoration gaz-solide pour des 
fonctionnalit\'{e}s multiples}

\author{\firstname{Nicolas} \lastname{Batisse}\CDRorcid{0000-0003-2501-8496}}
\address{Universit\'{e} Clermont Auvergne, CNRS, ICCF, 24, 
Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[N. Batisse]{Nicolas.batisse@uca.fr}

\author{\firstname{Pierre} \lastname{Bonnet}\CDRorcid{0000-0003-1055-9695}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[P. Bonnet]{Pierre.m.bonnet@uca.fr}

\author{\firstname{Daniel} \lastname{Claves}\CDRorcid{0000-0001-6357-574X}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[D. Claves]{Daniel.claves@uca.fr}

\author{\firstname{Marc} \lastname{Dubois}\CDRorcid{0000-0002-9192-0931}\IsCorresp}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[M. Dubois]{marc.dubois@uca.fr}

\author{\firstname{Malika} \lastname{El~Ghozzi}\CDRorcid{0000-0001-6839-9482}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[M. El Ghozzi]{Malika.el-ghozzi@uca.fr}

\author{\firstname{Katia} \lastname{Guerin}\CDRorcid{0000-0002-6184-2143}\IsCorresp}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[K. Guerin]{katia.araujo\_da\_silva@uca.fr}

\author{\firstname{Tomy} \lastname{Falcon}\CDRorcid{0000-0003-4053-6742}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[T. Falcon]{Tomy.falcon@uca.fr}

\author{\firstname{Laurent} \lastname{Jouffret}\CDRorcid{0000-0003-0196-7128}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[L. Jouffret]{Laurent.jouffret@uca.fr}

\author{\firstname{Kevin} \lastname{Lemoine}\CDRorcid{0000-0003-0401-6416}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, 
CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\email[K. Lemoine]{Kevin.lemoine@uca.fr}

\shortrunauthors

\keywords{\kwd{Gas--solid fluorination}
\kwd{Surface engineering}
\kwd{Materials for energy}
\kwd{Materials for the environment}
\kwd{Scale-up}}

\altkeywords{\kwd{Fluoration gaz-solide}
\kwd{Ing\'{e}nierie de surface}
\kwd{Mat\'{e}riaux pour l'\'{e}nergie}
\kwd{Mat\'{e}riaux pour l'environnement}
\kwd{Mise \`a l'\'{e}chelle}}

\begin{abstract}
Gaseous fluorine is at the heart of many gas--solid reactivity, ranging
from the most fragile solids such as biomaterials or polymers to the
most stable solids such as graphite or transition metals. To address
these diverse reactivity, precise control of the fluorine/solid
stoichiometry is required to allow selective reactivity either at the
surface or in the core of the material. This control can be applied at
both laboratory and micropilot scale. Selected examples, illustrating
applications in photocatalysis, batteries, fuel cells, corrosion
protection, and healthcare will highlight the full potential of this
unique chemical element.
\end{abstract}

\begin{altabstract}
Le fluor gazeux est un \'{e}l\'{e}ment au centre de nombreuses
r\'{e}actions gaz--solide, depuis les mat\'{e}riaux les plus fragiles
comme les biomat\'{e}riaux ou les polym\`{e}res, jusqu'aux
mat\'{e}riaux les plus stables, comme le graphite ou les m\'{e}taux de
transition. Pour r\'{e}pondre \`{a} ces diff\'{e}rentes
r\'{e}activit\'{e}s, il est n\'{e}cessaire de contr\^{o}ler finement la
st\oe{}chiom\'{e}trie fluor/solide. Cela permet de choisir la
r\'{e}activit\'{e}, en surface ou dans le c\oe{}ur de la mati\`{e}re.
Il peut s'appliquer aussi bien \`{a} l'\'{e}chelle du laboratoire
qu'\`{a} celle d'un micropilote. \`{A} travers des exemples
choisis, des applications en photocatalyse, batteries, piles et piles
\`{a} combustibles, protection de la corrosion, sant\'{e}, etc.,
illustreront toutes les potentialit\'{e}s de cet \'{e}l\'{e}ment
chimique incomparable.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\let\rmmu\upmu

\section{Introduction}\label{sec1}

{\vspace*{-2pt}}

Inorganic fluorine chemistry applied to materials requires an
interdisciplinary approach that integrates chemistry, materials
science, and engineering, fostering a collaborative environment that
pushes the boundaries of what is possible with fluorinated materials.
The potential applications of the fluorination technologies are vast.
Selected examples include photocatalysis, where fluorinated materials
improve the efficiency and stability of photocatalysts, offering new
solutions for environmental remediation and energy conversion. In fuel
cells, improved catalytic activity and corrosion resistance of
fluorinated catalysts contribute to more efficient and durable fuel
cells. Fluorination also provides robust protective coatings for metals
and alloys, extending their service life in harsh environments. In
healthcare, fluorinated biomaterials and polymers are being developed
for medical devices and drug delivery systems, exploiting the unique
properties of fluorine to improve performance and biocompatibility.
Fluorine chemistry also plays a critical role in nuclear applications,
where fluorinated materials are used to develop high-performance
coatings and components that can withstand extreme conditions. This
includes the production of fuels that are essential to the safety and
efficiency of nuclear reactors and other related technologies. 

{

\advance\baselineskip by -.2pt

The Clermont-Ferrand Institute of Chemistry (ICCF) is at the forefront
of research into fluorination processes and the development of
fluorinated materials, exploiting the unique reactivity of fluorine to
innovate in a wide range of applications. The \mbox{Institute's} thematic
focus on fluorination encompasses a variety of methodologies, ranging
from gas--solid interactions to solution-based processes, each tailored
to address the specific challenges and requirements of different
material targets. A common method is gas--solid fluorination, in which
the material is exposed to fluorine gas or a fluorine-containing gas
mixture at temperatures ranging from room temperature to
650~\textdegree C. This gas--solid interaction is very versatile and
can be used either in the bulk or on the surface only. In the latter
case, this route facilitates the incorporation of fluorine atoms onto
the material surface, providing a uniform fluorine coating and is
particularly effective for materials with high surface areas. Extending
fluorination to the bulk poses several challenges: tailoring the
fluorine content, the competition with decomposition due to the
reactivity of most of fluorine species, the sublimation of the
fluoride. The fluorine group at ICCF has developed advanced techniques
to control the fluorination process at the molecular level, ensuring
uniform and controlled fluorine incorporation. 

The Institute has pioneered the development and application of a
variety of fluorinating agents, each selected based on the specific
material target and desired outcome. Key agents include elemental
fluorine (F$_{2}$), the most reactive form of fluorine, used for
high-intensity fluorination processes; hydrofluoric acid (HF), commonly
used for solution-based fluorination, particularly effective for
etching and surface treatments; and fluorine-containing compounds such
as xenon difluoride (XeF$_{2}$) and terbium tetrafluoride (TbF$_{4}$),
which provide controlled fluorination environments for specific
applications. The case of the fluorination of high surface area carbons
perfectly illustrates the difficulties and how to overcome them; a
recent paper~\cite{1} has reviewed such a case. Fluorinated
(nano)carbons have been studied at ICCF for more than 40~years. 

Research at ICCF focuses on a wide range of materials, each of which
benefits uniquely from fluorination. Some targeted applications include
electrode materials for batteries, where surface fluorination of
electrodes has been shown to improve electrochemical stability, ionic
conductivity, and energy density, significantly enhancing battery
performance. Bulk fluorination processes are used to improve the
structural integrity and electrochemical performance of cathode
materials for lithium-ion batteries. In addition, the Institute is
developing high-performance fluorinated carbons for use as advanced
lubricants, offering superior performance and durability. Surface
fluorination of polymers enhances their liquid-repellent properties and
can facilitate the diffusion of plasticizers to the surface, improving
material performance in various applications. ICCF is unique in its
ability to precisely control fluorination processes at both laboratory
and micropilot scale. This versatility allows research results to be
transferred from the laboratory to potential industrial applications,
ensuring that innovations can be effectively scaled up. 

At the beginning of the timeline, simpler methods such as direct
fluorination with F$_{2}$ predominated. As research progressed, more
sophisticated techniques emerged. For example, fluorination by
anhydrous HF gained attention for its effectiveness in controlled
environments. Catalytic fluorination introduced new efficiency by using
catalysts to facilitate the fluorination process. Plasma fluorination
gained prominence for its ability to achieve high reactivity at lower
temperatures. Other advances included fluorination by the decomposition
of solid fluorinating agents and by the decomposition of fluorinated
polymers, each offering unique advantages in specific applications.
Laser or plasma torch-assisted fluorination was another leap forward,
offering precise control of the fluorination process and allowing
targeted modification of materials. Each boxed  figure in the graph
corresponds to a breakthrough or increased research activity in the
development of these advanced methods. The steady increase in the
number of fluorination methods indicates growing interest and
improvements in the control of fluorine incorporation into various
materials, reflecting the increasing complexity and precision of
fluorination techniques over the years. This trend underlines the
importance of fluorination in material science and its expanding
applications in various fields (Scheme~\ref{sch1}).

\begin{scheme*}
\vspace*{-.2pc}
\includegraphics{sc01}
\vspace*{.4pc}
\caption{\label{sch1}Evolution of fluorination methods at ICCF upon
time.}
\vspace*{-.4pc}
\end{scheme*}

In this paper, a few examples have been chosen to illustrate the
diversity of applications and methods. The aim is not to be exhaustive.
Firstly, applications that require the presence of fluorine on the
surface will be discussed.

}

\section{Surface engineering}\label{sec2}

\subsection{Control of surface energy by fluorination}\label{sec2.1.}
In many applications, such as the production of 
high-performance
filler--matrix composites or 
\mbox{hydrophobic} surfaces, 
control of the 
interface between the various interacting components is critical.
Modification of surface chemistry plays an important role in
ensuring properties (mechanical, thermal, controlled ageing, etc.)
where parameters such as adhesion or wettability are\break
dominant.

This is particularly true in the field of composites, where the
filler--matrix interaction is a key parameter to control. Carbon
fiber--polymer composites are sensitive to moisture, which can lead to
a deterioration in mechanical performance. One solution to this problem
has been to carry out gas fluorination treatments on carbon 
fibers~\cite{2,3} (which are generally coated with a thin layer of
polymers to ensure their compatibility with the polymer matrix) to
reinforce their hydrophobic character and limit water absorption. By
modifying fluorination temperatures, it is possible either to
perfluorinate the polymer-coated layer on the surface of the carbon
fiber, or to eliminate the polymer and fluorinate the fiber surface
directly  (Figure~\ref{fig1}a). This reactivity modularity makes it
possible to adjust the reaction conditions in such a way that the
mechanical properties of the original fiber are maintained, while the
surface energy of the fiber is significantly reduced, making it more
hydrophobic, but also more easily compatible with the polymer matrix.

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig01}
{\vspace*{-2pt}}
\caption{\label{fig1}(a)~Transversal section of carbon fiber
fluorinated at 250~\textdegree C by F$_{2}$(g) correlated with the EDX
fluorine repartition from the core to the surface of the fiber; 
(b)~Flax fiber fluorination treatment effects on polar, dispersive and
total surface energy; (c)~Fluorinated electrospun PVP/fluorinated
carbon nanofibers composite; (d)~F-CNF/PS nanocomposite; (e)~Laser
microtextured F-CNF/PVDF nanocomposite; Anode images ((f) raw and (g)
microtextured) during electrolysis of fluorine in KF-2HF.}
{\vspace*{-3pt}}
\end{figure*}

The latter aspect has also been extensively studied by our group in the
case of eco-composites, using flax fibers as reinforcing fillers. One
of the main drawbacks of flax fibers in this application is their
surface chemistry, rich in oxygenated groups, which induces a polar
surface character that limits their compatibility with the matrix
polymers commonly used. By grafting fluorinated groups (either by
fluorine plasma treatment~\cite{4}  or by direct fluorination~\cite{5})
directly onto the surface of the fibers (thereby significantly reducing
the polar component of the surface energy), it has been possible to
significantly reduce the overall surface energy of natural fibers 
(Figure~\ref{fig1}b), making them more hydrophobic and compatible with
the polymer resins, while maintaining their good mechanical
properties~\cite{6}.

One of the major advantages of direct fluorination is its ability to
homogeneously treat surfaces with complex geometries. This has been
applied to electrospun nanocomposites based on polyvinylpyrrolidone
(PVP) microfibers doped with fluorinated\unskip\break 
nanocarbons~\cite{7}. By
finely modulating the
reactivity\unskip\break
of molecular fluorine with the polymer
matrix, it is possible to control the competition that exists between
the perfluorination of the polymer in the solid state and its
degradation into volatile fluorocarbons inducing the etching of the
latter. This has enabled us to create a high-performance
superhydrophobic coating with multi-texturing at the micro- and
nanoscale  (Figure~\ref{fig1}c).

This approach has also been used to prepare superhydrophobic films
based on fluorinated polymers such as polyvinylidene fluoride (PVDF),
or poly(ethylene oxide) (PEO) fluorinated with F$_{2}$, as well as
commercial hydrophobic polymers such as polystyrene (PS), which have
been used as a binder and/or polymer matrix to prepare 
nanocomposites~\cite{8}.  Stable superhydrophobic properties are
obtained with fluorinated carbon nanofibers (F-CNF)/PVDF
nanocomposites microtextured by femtosecond laser ablation and
F-CNF/PS nanocomposites, with water contact angles of 
157\textdegree\ and 155\textdegree, respectively  (Figures~\ref{fig1}e
and~\ref{fig1}d).

Our group's expertise in controlling the reactivity of molecular
fluorine has also enabled us to develop a new approach for its
preparation by electrolysis in KF-2HF medium. By controlling the
microtexture of the carbon anode surface using a femtosecond laser
ablation method, it is possible to radically alter the interaction of
the electrogenerated fluorine bubbles with the carbon 
surface~\cite{9}. Whereas on conventional anodes the fluorine bubbles
tend to spread out on the surface  (Figure~\ref{fig1}f), thus limiting
the electrolysis performance, our approach allows the generation of
spherical bubbles that quickly detach from the surface 
(Figure~\ref{fig1}g), thus significantly increasing the gas production
and limiting the anode ageing (by reducing the formation of fluorinated
carbons on the surface due to a shorter contact time of the molecular
fluorine with the carbon).

\subsection{Surface fluorination of polymers and its\newline 
benefit to medical devices}\label{sec2.2}
Many medical devices (MDs) made of PVC, especially disposable ones, are
used extensively in healthcare facilities every day. Indeed, PVC offers
durability, chemical resistance, and low replacement costs. The
addition of a plasticizer, usually of the ester type, gives PVC the
necessary flexibility, durability, and reliability. Plasticization also
gives tubular-type \mbox{devices} the transparency needed to monitor the flow
of drugs or biological fluids. Plasticized PVC is therefore widely
accepted for use in various flexible medical devices (bags, 
catheters, and gloves) and especially for tubing (infusion/transfusion
lines, pump tubing, and extenders). A major drawback for medical use is
the diffusion of chemicals present in the polymer matrix towards
solution in contact with it. As they are not covalently bonded to the polymer
chains, plasticizers can therefore be partially extracted by certain
fluids, resulting in patient contamination by fluids (drugs, blood,
gas, etc.) circulating in medical tubing. Some plasticizers are
therefore potentially harmful and are recognized as endocrine
disrupters, carcinogens, mutagens, or substances toxic to reproduction or certain
organs. In the following, we will describe, by means of an example, how
the chemical surface modification with fluorine of a MD made of
plasticized PVC is an innovative solution to minimize such migration
problems by creating a barrier between the plasticizer and the fluids
in contact with~it.

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig02}
\caption{\label{fig2}(a)~Elemental fluorine mapping at the inner
surface of a fluorine-treated PVC medical tubing, as seen by EDX.  
(b,c)~SEM views (${\times}$2500) of the surface before and after
fluorination, respectively.  (d)~Roughness of the fluorinated surface
measured by AFM.  (e)~Quantification of plasticizer migration in dynamic
mode.}
\vspace*{-2pt}
\end{figure*}

Surface fluorination of a polymer is a quasi-irreversible treatment
because the grafted fluorinated chains are covalently bonded to the
surface, ensuring long-term chemical stability. Direct fluorination at
low temperatures has proven to be a very advantageous chemical method
for tuning the physicochemical properties of a wide range of 
polymers~\cite{7,10}. By controlled diffusion of fluorine, it is
possible to modify only the external surface properties of the
material, without affecting its intrinsic properties. The fluorinated
layer generally exhibits a gradient consisting of a gradually
decreasing fluorine content, a transition zone where the concentration
of fluorinated functions decreases sharply, down to the unmodified
polymer bulk. Fluorination is a heterogeneous reaction between the 
gaseous molecular F$_{2}$ and the solid
surface of the material, which requires no initiation step and can
proceed spontaneously at room temperature. It is a dry, solvent-free
process that can be applied to polymer objects of any shape, which is of
industrial interest. Several studies have shown the advantage of the
surface fluorination of various polymers for applications~\cite{11,12}.
In particular, the latter allows the creation of a barrier to
permeation. In fact, fluorination induces a reduction in the free
volume due to the H/F atomic substitution and, consequently, the
permeability of fluorinated polymers is reduced. Direct fluorination
proceeds via a radical mechanism that can induce inter/intra
cross-linking as a side reaction. Such a surface effect inhibits
potential solvation/swelling effects, thereby reducing permeability.
Finally, the surface energy of fluoropolymers increases, reducing the
chemical compatibility of low-polarity organic liquids with a
fluoropolymer layer, which in turn reduces permeability. The diffusion
of compounds through a fluorinated surface can therefore be
significantly reduced.

The fluorination of PVC tubular MDs, containing esters as plasticizers,
was carried out~\cite{13} at room temperature with pure molecular
fluorine, in a closed reactor under a fixed amount of excess gas, at 
a\unskip\break
pressure of 200 mbar and for 30 minutes. This static mode is preferred
when a controlled and homogeneous fluorination of the sample is
desired. Complementary physicochemical measurements (FTIR,  XPS, 
$\mbox{}^{19}$F NMR, SEM, AFM) were carried out at different scales, in
order to finely characterize the fluorinated layer formed. The most
probable conclusions and hypotheses drawn from the former set of
characterizations were that fluorination led to the formation of CHF,
CF$_{2}$, and CF$_{3}$ groups, with an average fluorination rate of the
fluorinated units close to 1.5 F per~C, both throughout the fluorinated
thickness (NMR) and at the outermost surface (XPS). They showed the
prevalence of the  --CF$_{2}$--CH$_{2}$-- sequence on the surface (XPS
C$_{1\mathrm{s}}$ and F$_{1\mathrm{s}}$), the persistence of the ester groups despite
fluorination, and a partial chemical modification in the vicinity of
the ester groups of the plasticizers according to  (i)~fluorination in
the $\rmalpha$  or $\rmbeta$ position (e.g., --CF$_{2}$--COO) and/or
formation of the anhydride group, both in the bulk (IR) and at the
outermost surface (XPS O$_{1\mathrm{s}}$). (ii)~Down chain\unskip\break 
fluorination (e.g.,
O${=}$C--O--CF$_{2}$--) at the outermost surface (XPS C$_{1\mathrm{s}}$ and 
O$_{1\mathrm{s}}$). SEM images revealed etching of the tube surface caused by
fluorination, despite the relatively mild conditions used. Indeed, the
strong oxidizing power of molecular fluorine is known to induce
chemical scissoring of alkyl chains through the hyperfluorination
mechanism, resulting in the elimination of volatile short-chain
fluorinated species  (CF$_{4}$, C$_{2}$F$_{6}$\ldots) and in a subsequent
material loss. Elemental mapping by EDX microanalysis carried out on
cross-section of tubings showed that the thickness of a fluorinated
layer was in the order of a few microns. The significant increase in
the surface roughness of the surface of the treated MDs was
quantified by AFM. Tensile tests showed that Young's modulus of the
tubes remained constant, the fluorinated thickness being negligible
compared to the total wall thickness. Therefore, the mechanical and
transparency properties were not altered after\unskip\break 
fluorination.

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig03}
\vspace*{-3pt}
\caption{\label{fig3}(a)~Coefficients of friction for raw and
fluorinated graphite (FG), carbon nanofibers (CNF and F-CNF),
graphitized carbon blacks (GCB) and exfoliated graphite (Exf-FG).
(b)~Intrinsic CoF of raw and exfoliated fluorinated carbons
(fluorinated graphite FG and fluorinated nanofibers CNFs) and
(c)~changes with the cycle number for raw and exfoliated FG.}
\vspace*{-2pt}
\end{figure*}

Figure~\ref{fig2} clearly illustrates, through a single example, the
effective anti-diffusion barrier obtained from a treated MD, with
respect to a plasticizer. The migration flux was compared with that
observed from the same non-fluorinated tube. Migration was evaluated in
a dynamic model (syringe pump) to approximate conditions as close as
possible to clinical practice. A simulant (ethanol/water mixture 50\%
by volume) mimicking a lipophilic (i.e., most extractive) drug solution
was infused through the tubes. After fluorination, the amount of
plasticizer released in the simulant appeared to decrease sharply. The
results obtained demonstrate that fluorination can be an effective
method of preventing the release of plasticizers from plasticized PVC
MDs already on the market.

\subsection{Fluorinated (nano)carbons as solid\newline
lubricants}\label{sec2.3}
The covalent intercalation of fluorine atoms into graphitized materials
with an F/C molar ratio higher than 0.15 reduces the coefficient of
friction (CoF). The latter, shown in  Figure~\ref{fig3}, can be
compared and discussed here because they are obtained under strictly
with the same conditions using a ball-on-plane reciprocal tribometer
consisting of an AISI 52100 steel ball rubbing against an AISI 52100
steel plane. The experimental details (surface preparation, solid
lubricant deposition, sliding speed, a normal applied load of 10~N,
burnishing method with the possible addition of pentane droplets to
enhance tribofilm formation, mean contact pressure of 0.65~GPa
according to Hertz theory) are given in
references~\cite{14,15,16,17,18,19,20}.  The benefits of the presence of
F atoms on the intrinsic CoF (without pentane) are higher when
considering carbonaceous nanomaterials are 
considered~\cite{15,16,17,18,20}. CoF ranging between 0.065 and 0.085
are achieved for carbon nanofibers (CNFs) and graphitized carbon blacks
(GCBs), which are weaker than those of graphite (CoF of 0.18) and most
other solid lubricants. An influence of the shape factor of fluorinated
nanocarbons on the tribological properties has been
observed~\cite{15,16,18}. Spherical particles such as GCBs have a
slightly lower CoF, e.g., 0.065 and 0.07 respectively for F/C ${=}$
0.65~\cite{15,16,18}. The difference could occur during the formation
of a denser tribofilm due to the easy disaggregation of the spherical
shape for carbon blacks compared to tubular carbon
nanofibers~\cite{15}. At a similar fluorination rate, the CoF of
micrometric graphite fluoride (GF) is higher, e.g., for the case of
F/C ${=}$ 1, values of 0.08 and 0.07 were recorded for GF and fluorinated
GCBs  (Figure~\ref{fig3}a). The good tribological properties of
fluorinated nanocarbons make them very promising as precursors for the
tribo-active phase in lubrication. 

Another strategy is to use fluorinated graphitized carbons as
precursors for the preparation of multilayer graphene by exfoliation
using a thermal shock  (Figure~\ref{fig3}b). Exfoliation and
defluorination occur simultaneously resulting in compounds with low
fluorine content, F/C close to 0.05~\cite{17}. The preparation method can
be applied to various fluorinated precursors, covalent graphite
fluorides and fluorinated CNFs. 
Figures~\ref{fig3}b and~\ref{fig3}c show the data
obtained with fluorinated graphite (FG) and fluorinated graphitized
nanofibers. Despite the massive defluorination, the exfoliation did not
degrade the excellent lubricating performance due to the weakening of
the interparticle interactions within the exfoliated samples. Moreover,
the exfoliated structure may facilitate the formation of a homogeneous
and stable tribofilm, as discussed above for other fluorocarbons. Low
CoFs (0.05) were then obtained from the first friction cycle, in
contrast to the raw FG
\mbox{(Figure~\ref{fig3}c)}. This section demonstrates
the potential of fluorinated (nano)carbons as solid lubricants, taking
into account the diversity of the allotropic forms, sizes and shapes of
the carbonaceous starting materials. In addition, the C--F bond can be
tailored from ionic to covalent, both to further reduce the coefficient
and friction and to favor high quality and denser tribofilms~\cite{19}.

\subsection{Surface fluorination of electrodes used in electrochemical
devices}\label{sec2.4}
Surface fluorination of battery anodes and fuel cell electrolyzers has
emerged as a promising technique to improve their performance. The
fluorination process involves the introduction of fluorine atoms onto
the surface of electrode materials, resulting in improved
electrochemical properties. This synthesis examines different methods
of surface fluorination and their respective benefits for lithium-ion
batteries and fuel cells. One common method is gas--solid fluorination,
in which electrode material is exposed to fluorine gas or a
fluorine-containing gas mixture at elevated temperatures. This
gas--solid interaction facilitates the incorporation of fluorine atoms
onto the material surface, providing a uniform fluorine coating and is
particularly effective for materials with high surface areas. This
reaction typically takes place at room temperature or under mild
heating, allowing for precise control of the fluorine content and
distribution, making it suitable for delicate materials that cannot
withstand high temperatures. Atomic Layer Deposition (ALD) of fluorine
is another advanced technique, a vapor-phase method in which the
electrode material is sequentially exposed to fluorine precursors and
co-reactants, resulting in the atomic-scale deposition of fluorine
layers. ALD provides excellent control over the thickness and
uniformity of the fluorine layer, making it ideal for applications
requiring precise atomic-scale modifications~\cite{21,22}.

The benefits of surface fluorination is considerable. Improved
electrochemical stability is achieved because the fluorine atoms on the
electrode surface form a robust protective layer that mitigates
unwanted side reactions with the electrolyte, thereby increasing the
overall stability and lifetime of the electrode materials in both
Li-ion batteries and fuel cells. For example, the introduction of
fluorine can improve the capacity retention of silicon anodes to over
80\% after 200 cycles, compared to less than 60\% for non-fluorinated
anodes~\cite{23}. Surface fluorination increases ionic conductivity by
creating more pathways for ion diffusion, which is particularly
beneficial for lithium-ion batteries where fast and efficient
lithium-ion transport is critical for high-performance 
applications~\cite{24}. Increased energy density is another benefit, as
stabilizing the electrode surface with fluorine allows for higher
operating voltages and capacities, making the batteries and fuel cells
more efficient and able to store more energy. For example,
nanostructured V$_{2}$O$_{5}$ anodes produced by the
fluorination--exfoliation process have capacities of up to 300~mAh/g, a
20\% improvement over bulk sample~\cite{25}. In addition, the
introduction of fluorine reduces the charge transfer resistance at the
electrode--electrolyte interface, improving the overall electrochemical
kinetics and leading to faster charging and discharging. This is
possible because fluorine atoms act first on structural defects that
can deeply damage the electronic and ionic conductivity (more than a
few equivalents of fluorine atoms!). This can result in a 30\%
reduction in charge transfer resistance, significantly improving
battery performance.

In the field of fuel cells, the improvement of electrocatalysts by
surface fluorination is particularly noteworthy. In fuel cells,
catalysts play a critical role in the oxygen reduction reaction (ORR)
and hydrogen oxidation reaction (HOR), both of which are essential for
efficient fuel cell operation. Surface fluorination increases the
stability and activity of these catalysts by creating a more durable
and reactive surface. For example, fluorinated platinum (Pt) catalysts
used in ORR show significantly improved performance, with increased
current densities and reduced overpotentials. This is because the
fluorine atoms modify the electronic structure of the Pt surface,
improving the adsorption of oxygen molecules and facilitating their
reduction. In practice, this can result in a 50\% increase in catalytic
activity compared to non-fluorinated Pt catalysts, with a similar
reduction in degradation rates over extended operating 
periods~\cite{1,26}. Fluorination can also be applied to non-precious
metal catalysts, such as those based on transition metals like iron or
cobalt, which are often used in PEM fuel cells to reduce costs.
Fluorinated versions of these catalysts exhibit improved ORR activity
and stability, making them more viable for commercial applications.

Some applications require the presence of fluorine atoms throughout the
volume of the materials. The next session is dedicated to such cases.

\section{Material conversion by gas--solid\newline 
fluorination}\label{sec3}

\subsection{Fluoride-based cathode for lithium-ion\newline
batteries}\label{sec3.1}
The design of electrode materials based on the principles of
solid-state chemistry allows their electrochemical properties to be
modified and modulated. Goodenough 
demonstrated the effect of different
chemical groups on the material potential values, following the
inductive effect of oxides lower than the corresponding 
phosphates~\cite{27}. For this reason, LiFePO$_{4}$ (LFP) with the
olivine crystal structure was one of the most widely used cathode
materials for LIBs and is still being studied. Similarly, fluorine
atoms resulted in higher potentials than oxygen due to the greater
ionicity of M--F bonds as compared to M--O bonds~\cite{28}.
LiFePO$_{4}$F fluorophosphate has therefore been investigated as a
possible iron-based candidate that allows high potentials to be
achieved while retaining the advantage of the phosphate
groups~\cite{29}. The aim of our approach was to understand the
reactivity of a commercial nanostructured LiFePO$_{4}$ (LiFePO$_{4}$/C:
2.5 wt\%) with different sources of fluorine in order to illustrate the
possibility of direct modifying this compound by introducing fluorine
atoms into the matrix. Firstly, we tried to modify LiFePO$_{4}$ with
hydrogen fluoride HF, both in the gas phase
$(\mathrm{HF}_{(\mathrm{g})})$ and in the
aqueous phase $(\mathrm{HF}_{(\mathrm{aq})})$. LiFePO$_{4}$ is placed in a Ni metal
container, which is then introduced into a reactor that is first purged
with
$\mathrm{N}_{2(\mathrm{g})}$  to ensure the absence of air and moisture, then a
pressure of 700~mbar 
$\mathrm{HF}_{(\mathrm{g})}$ is introduced at room temperature (RT),
and the compound is left in contact with the gas for 36 hours. The
initial choice of treatment at RT was made in order to compare it with
the least aggressive treatment of 
$\mathrm{F}_{2(\mathrm{g})}$, also at RT.
\looseness=-1

The diffractogram of this compound, shown in Figure~\ref{fig4}a,
indicates that a multiphase material is obtained. Bragg peaks can be
attributed to LiF and the LiFe$_{2}$F$_{6}$ phase, but some of which
may also correspond to the FeF$_{2}$ phase~\cite{30}. It should first
be noted that the phosphate matrix is eliminated. Furthermore, since
$\mathrm{HF}_{(\mathrm{g})}$  is a very weak oxidant, only a fraction of the iron atoms
are oxidized to oxidation state $+$III, leaving some of the iron atoms
remaining in oxidation state +II in the FeF$_{2}$ and the
mixed-valence LiFe$^{2+}$Fe$^{3+}$F$_{6}$ phases. The fluorination
could lead to the extraction of Li atoms from the structure to form
LiF. Thus, without a change in the oxidation state of the iron atoms,
the Fe$^{2+}$ ions cannot ensure electrical neutrality with the
PO$_{4}^{3-}$ ions, leading to the decomposition of the phosphate
matrix and the complete fluorination of the material with the formation
of the LiFe$_{2}$F$_{6}$ phase.

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig04}
\vspace*{-1pt}
\caption{\label{fig4}(a)~XRD diagrams of fluorinated LiFePO$_{4}$ by
$\mathrm{HF}_{(\mathrm{aq})}$ treated by N$_2$ at 250 and 
350~\textdegree C and fluorinated by 
$\mathrm{HF}_{(\mathrm{g})}$ at RT, (b)~LiFePO$_4$ fluorinated by  
$\mathrm{F}_{2(\mathrm{g})}$
at RT, 125~\textdegree C and 300~\textdegree C, LiFePO$_{4}$
fluorinated at RT treated at 125~\textdegree C under N$_{2}$.} 
\vspace*{-2pt}
\end{figure*}

In the second case, fluorination is carried out in the aqueous phase
using 
$\mathrm{HF}_{(\mathrm{aq})}$. 
LiFePO$_{4}$  powder is dispersed in a Teflon beaker
containing excess 
$\mathrm{HF}_{(\mathrm{aq})}$  and placed on a sand bath at moderate
temperature (${\sim}$60~\textdegree C). The diffractogram of 
$\mathrm{LiFePO}_{4}+\mathrm{HF}_{(\mathrm{aq})}$ 
is shown in Figure~\ref{fig4}a
and indicates that a single crystalline phase identified as
Li$_{3}$FeF$_{6}$ is \mbox{obtained~\cite{31}.} When this compound is annealed
in an inert atmosphere  (N$_{2}$) at 250~\textdegree C and
350~\textdegree C  (these compounds are designated
``$\mathrm{LiFePO}_{4}+\mathrm{HF}_{(\mathrm{aq})}-\mathrm{N}_{2}$
250~\textdegree C'' and
``$\mathrm{LiFePO}_{4}+\mathrm{HF}_{(\mathrm{aq})}-\mathrm{N}_{2}$
350~\textdegree C'' respectively), new peaks attributed to 
HTB--FeF$_{3}$, for hexagonal tungsten bronze or $\rmbeta$-FeF$_{3}$,
are observed, indicating that this phase was already present but below
amorphous from RT~\cite{32}. Similar to the fluorination by 
$\mathrm{HF}_{(\mathrm{g})}$,
the phosphate matrix is degraded at RT. On the other hand, since 
$\mathrm{HF}_{(\mathrm{aq})}$ is an oxidizing medium, all the iron 
atoms are oxidized to Fe$^{3+}$. Finally, the mechanism of molecular
fluorination,  $\mathrm{F}_{2(\mathrm{g})}$
fluorination of LiFePO$_{4}$ seems to take place in two stages: the
chemical delithiation followed by the collapse of the phosphate
structure  (Figure~\ref{fig4}b). The complete chemical disinsertion of
the Li atoms by fluorine is a \mbox{favored}
process even at room temperature
$(\Delta_{\mathrm{r}}{G}\text{\textdegree}=
-612.1~\mathrm{kJ}/\mathrm{mol})$, possible because of the robustness
of the phosphate network, which does not change during this process,
and also because of the total oxidation of Fe$^{2+}$ to  Fe$^{3+}$. In
the second step, at temperatures above  250~\textdegree C, the
fluorination of  FePO$_{4}$ cores leads to the formation of POF$_{3}$
and  PF$_{5}$ gases with $\rmalpha$-FeF$_{3}$. Above 250~\textdegree C,
$\rmalpha$-FeF$_{3}$  reacts with LiF to form Li$_{3}$FeF$_{6}$. Above
350~\textdegree C, the destruction of the phosphate network is
complete, resulting in a mixture of $\rmalpha$-FeF$_{3}$  and
Li$_{3}$FeF$_{6}$~\cite{33}. It should be noted that in this scenario,
the final FeF$_{3}$  is therefore the excess not reacted with LiF. Only
when the phase fluorinated at RT, a mixture of FePO$_{4}$ and LiF, is
brought up to 550~\textdegree C in an inert atmosphere as a second
treatment, it is possible to obtain the LiFePO$_{4}$F. It is
interesting to notice that this synthesis route produces a mixture at
RT equivalent to that obtained for
$\mathrm{LiFePO}_{4}+\mathrm{HF}_{(\mathrm{aq})}-\mathrm{N}_{2}$ at
350~\textdegree C. However, it should be noted that the two synthesis
routes differ in the allotropic variety of FeF$_{3}$ obtained. The or
$\rmbeta$-FeF$_{3}$ phase is obtained at low temperature in an aqueous
medium, whereas the $\rmalpha$-FeF$_{3}$ form is obtained at high
temperature in an anhydrous medium. The structural transition from
$\rmbeta$- to $\rmalpha$-FeF$_{3}$  is expected at 394~\textdegree
C~\cite{32}. These examples show how, from the same compound treated in
{different} fluorinated media (oxidizing/non-oxidizing,
\mbox{anhydrous/hydrated,} etc.) can yield materials of 
\mbox{different} natures and
structures, but all of which are of interest as electrode materials. In
an oxidizing medium, the Li$_{3}$FeF$_{6}$ phase is obtained at a high
or low temperature depending on the nature of the fluorinating agent,
whereas the LiFe$_{2}$F$_{6}$  phase is obtained when the medium is
non-oxidizing. If the oxidizing medium is anhydrous, the
$\rmalpha$-FeF$_{3}$ form is obtained, whereas, in a hydrated medium
the $\rmbeta$-FeF$_{3}$  form is obtained. As they all offer different
lithium diffusion properties, the choice of fluorinating agent is
therefore crucial in guiding the synthesis of different
materials~\cite{34}.\looseness=-1

It has also been suggested that conversion-type processes may
challenge, or even overtake the classical insertion-type mechanism.
Transition metal fluorides are conversion materials and have some
interesting electrochemical properties such as high theoretical
capacity at high voltages, such as FeF$_{3}$ or CuF$_{2}$, which
provide almost twice the capacity value of classical insertion cathode
materials~\cite{28}. A patent has been filed by Wang, Feng et~al., with
a solid solution of mixed metal fluorides formed by mechanochemical
reactions. The final mixture allows a high reversibility and a low
hysteresis of the battery as compared with single transition metal
fluoride~\cite{35}. In this sense, starting from a multi-metallic
template fluorination, or MMTF, could be a successful strategy to
obtain an intimate mixture of mixed metal fluorides~\cite{36}. Our
interest in using such a structure was initially to mix different
metals of interest for the electrochemical application, combined with
our expertise in solid gas fluorination. The fluorination of a
three-dimensional (3D) MMTF, namely copper-based Prussian blue analog
(CuPBA), resulted in a CuF$_{2}$/FeF$_{3}$ two-phase mixture with
interesting electrochemical properties compared to a similar
composition obtained by mechanochemistry. This result was followed by
an extension to 2D MMTF, hydrotalcite-type phases, of the general
formula
$[\mathrm{M}_{1-x}^{2+}\mathrm{M}_{x}^{3+}(\mathrm{OH})_{2}]{\cdot}
\mathrm{A}_{x/n}^{n-}{\cdot}\mathrm{mH}_{2}\mathrm{O}$ where M$^{2+}$ and
M$^{3+}$ are divalent and trivalent cations, respectively, and
interlayer anions A that 
\mbox{compensate}
for the excess of charge of the
positive layers. When substituted with Cu$^{2+}$ and Fe$^{3+}$ ions, the
final composition obtained by co-precipitation was
[(Cu$_{0.31}$Mg$_{0.38}$)$^{2+}$(Fe$_{0.31}$)$^{3+}$(OH)$_{2}$]Cl$_{0.31}{\cdot}$0.45H$_{2}$O. 
Here, the presence of Mg$^{2+}$  is mandatory for the stabilization of
the 2D structure due to the Cu$^{2+}$ Jahn--Teller effect, but its
molar ratio was not optimized for this composition with also CuO as a
secondary phase. As expected, its fluorination allowed the
stabilization of a composite of MgF$_{2}$, CuF$_{2}$  and FeF$_{3}$
fluorides making them accessible to the phenomenon of electrochemical
conversion for Cu and Fe~\cite{37}. An extension of this study was
carried out to optimize the composition, since Mg$^{2+}$ is not
involved as an active redox center, but also to obtain a pure compound
without any CuO. Several synthetic strategies were explored, trying to
measure the stability limit of the LDH structure, in terms of the
Mg$^{2+}$/Cu$^{2+}$  ratio but also with the addition of other
M$^{2+}$/M$^{3+}$ cations such as Co and Al. Firstly, it was found that
Al$^{3+}$ plays the same role as Mg$^{2+}$, whose presence in a cathode
material has already been shown to be effective as a stabilizer during 
cycling~\cite{38}. Secondly, Co could also be used instead of Mg to
maintain the LDH structure, increasing the theoretical capacity by
comparison due to its redox activity. Thus, a composition was sought
for fluorination:
[(Cu$_{1.5}$Co$_{0.5}$)$^{2+}$(Fe$_{0.75}$Al$_{0.25}$)$^{3+}$(OH)$_{6}$]Cl${\cdot}$2H$_{2}$O.
After fluorination at 200~\textdegree C, we observed an amorphous phase
with an interesting porosity, which crystallized rapidly in ambient air
to form Cu$_{3}$Fe$_{1.5}$Al$_{0.5}$F$_{12}$(H$_{2}$O)$_{12}$.  A
chemical milling with several fluorinated phases and in some cases even
new structures can be obtained at higher fluorination temperatures,
such as three different fluorinated materials at 500~\textdegree C with
Fe$_{1-x}$Al$_{x}$F$_{3}$,  CuF$_{2}$, and the addition of the
orthorhombic form of CoFeF$_{5}$ (Figure~\ref{fig5}a,~\cite{39}).
Operando Raman spectroscopy was a useful technique to study the
electrochemical activity of this mixture of metal fluorides,
particularly because of the different possible reactions with lithium,
insertion or conversion. In  Figure~\ref{fig5}b, the metal fluoride
peaks could be assigned to the 100 to  700~cm$^{-1}$ 
region~\cite{40,41,42}. During the first discharge, only the
\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig05}
\vspace*{-6pt}
\caption{\label{fig5}(a)~XRD diagrams of fluorinated LDH at 200, 350
and 500~\textdegree C, (b)~Raman spectra of the F500 electrode in the
operando cell with 1M LiTFSI in TEGDME electrolyte during the reduction
from 2.8 to 2.4~V and (c)~2.4 to 1.8~V.  (d)~Raman spectra during the
oxidation from 1.5 to 2.5~V and  (e)~from 2.5 to 3.2~V (purple being
the lowest voltage and red the highest). Figure modified from~\cite{43}.}
\end{figure*}
FeF$_{3}$-type peaks show an increase in intensities, suggesting an
insertion mechanism.  Fe$^{3+}$ is known to be involved in such a
process, whereas Co$^{2+}$ or Cu$^{2+}$  could only be reduced by a
conversion reaction. Such a conversion process into metals can occur
from 1.8~V vs.\ Li$^{+}$/Li as the three identified fluorinated species
peaks are all missing  (Figure~\ref{fig5}c). During charging, above
2.9~V vs.\ Li$^{+}$/Li, peaks representative of M--F bonds reappear
along with an electrochemical activity, indicating the conversion back
to metal fluorides at elevated potentials  (Figure~\ref{fig5}d). The
FeF$_{3}$-type and CuF$_{2}$ peaks increase progressively during the
oxidation  (Figure~\ref{fig5}e). The M$^{2+}$M$^{3+}$F$_{5}$ peak,
originally located at 670~cm$^{-1}$, does not return during oxidation.
However, a large peak located at  590~cm$^{-1}$ grows progressively
with the potential, which may indicate the formation of a new
fluorinated species that has not yet been identified. This suggests
that the MMTF provides a mixture of fluorides that can be effectively
used as a positive electrode for lithium-ion batteries~\cite{43}.
Further electrochemical tests are underway in terms of reversibility.
\looseness=-1

\vspace*{-5pt}
\subsection{Fluorine for the nuclear industry}\label{sec3.2}

\vspace*{-2pt}

The world's fleet of commercial nuclear power reactors requires uranium
fuel enriched in the  isotope $\mbox{}^{235}$U, as only
$\mbox{}^{235}$U can release energy by fission. In any uranium mineral
commonly used for its extraction, U is mainly present in two isotopic
forms, namely 0.71\%  of $\mbox{}^{235}$U (fissile) and 99.28\% of
$\mbox{}^{238}$U (fertile). Uranium with a $\mbox{}^{235}$U content
between 3 and 5\% is required for fuel in nuclear power reactors. To
achieve isotopic enrichment, all techniques, current or under development, 
use a gaseous fluorinated uranium compound, namely uranium \mbox{hexafluoride}
(UF$_{6}$). UF$_{6}$  has two important properties that make it
suitable for industrial use: it sublimes at only 56.2~\textdegree C
despite its high molecular weight and is relatively chemically
stable. However, the key to the gaseous enrichment is fluorine, which
has a single stable naturally occurring isotope ($\mbox{}^{19}$F).
The industrial gaseous diffusion process has been based
on the small mass difference between the two uranium isotopes, due only
to the presence of $\mbox{}^{235}$U  and $\mbox{}^{238}$U. However, the
diffusion process is now being advantageously replaced by
centrifugation, which offers higher efficiency and lower energy
consumption. This transition is already underway, as indicated in
several \mbox{recent} studies~\cite{44,45,46}. 

\begin{figure}
\includegraphics{fig06}
\vspace*{-2pt}
\caption{\label{fig6}Synthesis routes of UF$_{6}$ showing the
importance of fluorine chemistry (in green) by using F$_{2}$, CoF$_{3}$
or HF as fluorinating agent.}
\vspace*{-2pt}
\end{figure}

UF$_{6}$ can be synthesized by a variety of routes, shown in 
Figure~\ref{fig6}. The French industrial process starts with several
steps to convert U$_{3}$O$_{8}$ to  UO$_{2}$, through nitric 
conversion 
into UO$_{2}$(NO$_{3}$)$_{2}$, UO$_{3}$  synthesis and then reduction
to UO$_{2}$. Fluorine is involved in the last two steps, where
hydrofluoric acid (HF) converts  UO$_{2}$ to UF$_{4}$ and then fluorine
gas (F$_{2}$) fluorinates UF$_{4}$  to UF$_{6}$. The various fluoride
and oxyfluoride compounds involved in the process are studied. These
numerous compounds have to be synthesized and isolated in order to
understand their crystallographic and chemical properties~\cite{47}.

From an industrial point of view, whichever chemical pathway is chosen,
it is a challenge for companies to comply with international standards
of nuclear fuel purity. Contaminants come from uranium ore, chemical
products used in the conversion process, and/or fission products from
spent uranium. All elements are fluorinated during the synthesis of
UF$_{6}$ and depending on the element, its volatility and solubility
in UF$_{6}$ vary~\cite{48}. Magnesium fluoride is a chemical filter to
remove of technetium~\cite{49}, molybdenum~\cite{50}, or neptunium
fluoride~\cite{51}  from UF$_{6}$, and has been studied to remove the
selected volatile impurity: vanadium oxyfluoride (VOF$_{3}$)~\cite{52}.
In order to tailor the physicochemical properties of the chemical
filter, in particular the hydroxyl group content and the specific
surface area, a fluorination \mbox{post-treatment} was carried out on
MgF$_{2}$. 

During transport and industrial processing, UF$_{6}$ comes into contact
with various types of surfaces, either in the gas or in the liquid
phase, and undergoes heating and cooling cycles. The corrosion of
metals by UF$_{6}$  is studied to understand the 
\mbox{ageing} of the metallic
surfaces of the fluorination reactors, cooling systems (for the
liquefaction of UF$_{6}$) and storage  vessels~\cite{53,54}, and a
dedicated experimental setup called CORFU (CORrosion in uranium
hexaFlUoride) has been designed and built.

\section{Conclusion}\label{sec4}
We have developed several application examples where control of the
heterogeneous gas--solid \mbox{reaction} is critical to form the desired phase,
to confine fluorination to the extreme surface, to avoid massive
decomposition, or to tailor the nature of the C--F bond. With perfect
control of inorganic fluorine chemistry, we can address further
applications in two major societal areas: materials for energy and the
environment, and surface engineering. The range of applications is
vast. Examples include the development of composites with natural or
carbon fibers made non-polar on their surface by fluorination to
lighten structural materials, or new reflectors for very slow neutrons
with fluorinated nanodiamonds and graphites.

The group's historical themes, such as fluorinated
(nano)carbons as cathodes for primary lithium batteries, should not be
neglected, as they are promising candidates for high added-value
applications such as space and medicine. Inorganic fluorine chemistry
via gas--solid reactions offers exceptional opportunities for the
synthesis of new materials, such as fluorinated diamanes prepared by
exfoliation of graphite fluoride with (C$_{2}$F)$_{n}$ structure type
as precursors. The presence of fluorine atoms allows for bandgap
engineering of various materials such as graphene, but only if the
fluorination is perfectly \mbox{tailored}.

Many challenges remain for gas--solid fluorination, such as
incorporating fluorine atoms into highly reactive materials such as
boron nitrides or graphitic carbon nitrides. In materials science, a
promising approach is to co-dope fluorine with one or two other
heteroelements such as oxygen (as seen in fluorinated graphene oxide),
sulfur, boron, or phosphorus, to provide additional parameters for
application properties. In many cases, gas--solid fluorination can be
complemented by plasma fluorination. The complementarity of these
processes needs to be developed to meet all the challenges.

Finally, let us explore some of the many avenues. There
are numerous examples in the literature of 
fluorinated photo/electro-catalytic materials for the H$_{2}$
production, CO$_{2}$ conversion or pollutants degradation. Some of
these materials are the result of modifying a non-fluorinated active
matrix by incorporating fluorine atoms. The added value of fluorine in
these materials can be diverse: band gap adjustment or better charge
separation in photocatalysts, creation of defects or active sites,
structural or textural modification of the material, etc. In most
cases, these fluorinated materials are obtained by solid-state or
hydrothermal synthesis, and in the latter case, generally, only small
amounts of fluorine can be incorporated. Few examples of gas--solid
fluorination have been reported in the literature. As shown in this
paper, the amount of fluorine incorporated, and thus the properties
obtained, can be modulated by the choice of fluorination method and
fluorinating agent. Uranium hexafluoride is difficult to handle due to
its chemical and radiological hazards, but its availability at ICCF,
rare in the world, allows it to be used as a fluorinating agent and to
study its combination with different materials and gases. In these
cases, the objectives of reducing impurities in the UF$_{6}$
used for enrichment and exploiting large quantities of depleted uranium
are perfectly\unskip\break valid.

The reactivity of fluorine with different inorganic matrices shows
promising results with the solid/gas reactions of polyanionic and
cationic compounds, offering new properties for interesting fluorinated
candidates as positive electrodes for secondary batteries. Extensions
will be made with tuned cationic compositional templates, but also with
other polyanionic compounds, aiming at both a full understanding of the
fluorination mechanism and the fluorinated materials with the best
electrochemical properties. At a more exploratory level, it has been
shown that high-pressure synthesis (${>}$1~GPa) stabilizes structures
with different atomic distributions and possible enhanced mobility
compared to the equivalent form obtained at ambient pressure, with
recent examples being the trigonal forms of LiFe$_{2}$F$_{6}$ and
Li$_{2}$MoF$_{6}$. For the latter, Mo metal and LiF were mixed with
CuF$_{2}$ as a solid fluorine source, the decomposition of which
resulted in this fluorinated compound with Mo$^{4+}$. This fluorination
with solid fluorine sources and under drastic conditions opens a
different way to discover new properties for classical fluorinated
compounds, as well as the 
\mbox{stabilization} of completely new inorganic
materials that cannot be obtained at classical pressures.

\section*{Declaration of interests} 
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgements}
The authors would like to thank all their academic and industrial
partners and the ICCF technical team.\looseness=1
\back{}
\printbibliography
\refinput{crchim20240625-reference.tex}

\end{document}

