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\DOI{10.5802/crchim.371}
\datereceived{2024-08-30}
\daterevised{2024-10-30}
\dateaccepted{2024-12-09}
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\dateposted{2025-03-21}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Impact of Co\textsuperscript{2+} substitution by Fe\textsuperscript{2+}
on the thermal behavior of a hydrated fluoride, precursor of a mixed
iron-based oxyfluoride with reduced cobalt content as efficient OER
electrocatalyst}

\alttitle{Impact de la substitution de Co\textsuperscript{2+} par
Fe\textsuperscript{2+} sur le comportement  thermique d'un fluorure
hydrat\'{e}, pr\'{e}curseur d'un oxyfluorure mixte \`{a} base de fer
\`{a} teneur r\'{e}duite en cobalt comme \'{e}lectrocatalyseur OER
performant}

\shortrunauthors

\author{\firstname{Alexandre} \lastname{Terry}\CDRorcid{0009-0001-6456-7423}}
\address{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}
\address{Institute of Inorganic Chemistry, University of Bonn,
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany}

\author{\firstname{Guillaume} \lastname{Duval}\CDRorcid{0009-0005-1044-2237}}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\author{\firstname{Samuel} \lastname{Mathiot}}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\author{\firstname{Jean-Marc} \lastname{Gren\`{e}che}\CDRorcid{0000-0001-7309-8633}}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\author{\firstname{Ralf}\nobreakauthor\lastname{Weisbarth}}
\addressSameAs{2}{Institute of Inorganic Chemistry, University of Bonn,
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany}

\author{\firstname{Edouard} \lastname{Boivin}\CDRorcid{0000-0002-5468-3594}}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\author{\firstname{Vincent} \lastname{Maisonneuve}\CDRorcid{0000-0003-0570-953X}}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\author{\firstname{Annie}\nobreakauthor\lastname{H\'{e}mon-Ribaud}\CDRorcid{0000-0003-4845-5971}}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\author{\firstname{Nikolay} \lastname{Kornienko}\CDRorcid{0000-0001-7193-2428}}
\addressSameAs{2}{Institute of Inorganic Chemistry, University of Bonn,
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany}

\author{\firstname{Amandine} \lastname{Guiet}\CDRorcid{0000-0001-7590-1119}\IsCorresp}
\email[A. Guiet]{amandine.guiet@univ-lemans.fr}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\author{\firstname{J\'{e}r\^{o}me}\nobreakauthor\lastname{Lhoste}\CDRorcid{0000-0002-4570-6459}\IsCorresp}
\email[J. Lhoste]{jerome.lhoste@univ-lemans.fr}
\addressSameAs{1}{Institut des Mol\'{e}cules et Mat\'{e}riaux du Mans, UMR 6283
CNRS, Le Mans Universit\'{e}, Avenue Olivier Messiaen, 72085 Le Mans
Cedex 9, France}

\keywords{\kwd{Oxyfluoride}
\kwd{Solid solution}
\kwd{Coprecipitation}
\kwd{Thermal treatment}
\kwd{Electrocatalysis}
\kwd{OER}}

\altkeywords{\kwd{Oxyfluorure}
\kwd{Solution solide}
\kwd{Copr\'{e}cipitation}
\kwd{Traitement thermique}
\kwd{\'{E}lectrocatalyse}
\kwd{OER}}

\thanks{French National Research Agency (ANR OPIFCAT,
ANR-20-CE08-0026), Fondation Banque Populaire d'Entreprise de l'Ouest,
NSERC (Discovery Grant RGPIN-2019-05927),
International Emerging Action IEA, FRQNT Samuel de Champlain program
(award 293526)}

\begin{abstract}
In a sustainable development objective, it is necessary to consider
both the performance and the preciousness of metals for the development
of new catalysts. In this way, this work aims to
reduce the amount of cobalt in the mixed Co--Fe oxyfluoride
Co\textsubscript{0.5}Fe\textsubscript{0.5}O\textsubscript{0.5}F\textsubscript{1.5}, obtained from the thermal
degradation of the hydrated fluoride precursor
CoFeF\textsubscript{5}(H\textsubscript{2}O)\textsubscript{7}, while maintaining its electrocatalytic
performance as an electrocatalyst for the Oxygen Evolution Reaction
(OER) in alkaline media. To achieve this, the proposed strategy
consists in substituting the Co\textsuperscript{2+} ions by Fe\textsuperscript{2+} ions in CoFeF\textsubscript{5}(H\textsubscript{2}O)\textsubscript{7}, chosen for its earth abundance, leading to
the solid solution
(Co\textsubscript{1--\textit{x}}Fe\textsubscript{\textit{x}})\textsuperscript{2+}Fe\textsuperscript{3+}F\textsubscript{5}(H\textsubscript{2}O)\textsubscript{7} obtained by
co-precipitation. This series of hydrated precursors was then subjected
to a thermal treatment in ambient air in order to obtain the
corresponding solid solution of the oxyfluorides \tralicstex{Co\textsubscript{(1--\textit{x})/2}Fe\textsubscript{(1+\textit{x})/2}O\textsubscript{(1+\textit{x})/2}F\textsubscript{(3--\textit{x})/2}}{$\mathrm{Co}_{\sfrac{1-x}{2}}\mathrm{Fe}_{\sfrac{1+x}{2}}\mathrm{O}_{\sfrac{1+x}{2}}\mathrm{F}_{\sfrac{3-x}{2}}$}. 
Structural and thermal analyses confirm a Co\textsuperscript{2+}/Fe\textsuperscript{2+}
substitution in hydrated fluoride precursors, while those of calcined
samples indicate a complex thermal decomposition leading to Co--Fe
oxyfluorides. The OER electrocatalytic
performance of these oxyfluorinated materials shows that it is possible
to reduce the amount of cobalt by up to 20\% without affecting the
overall OER activity, namely an overpotential of 320 mV at
\tralicstex{10 mA\textperiodcentered{}}{10 mA${\cdot}$}cm\textsuperscript{--2} and a mass activity of 112 \tralicstex{A\textperiodcentered{}g\textsuperscript{--1}}{A${\cdot}$g$^{-1}$} at 1.55 V
vs. RHE, with high stability.
\end{abstract}

\begin{altabstract}
Dans un objectif de d\'{e}veloppement durable, il est n\'{e}cessaire
de prendre en consid\'{e}ration la pr\'{e}ciosit\'{e} des m\'{e}taux
utilis\'{e}s dans le d\'{e}veloppement de nouveaux catalyseurs pour
accro\^{i}tre leurs performances. Dans ce cadre, ce travail vise \`{a}
r\'{e}duire la quantit\'{e} de cobalt dans l'oxyfluorure Co--Fe
Co\textsubscript{0.5}Fe\textsubscript{0.5}O\textsubscript{0.5}F\textsubscript{1.5}, pr\'{e}par\'{e} par
d\'{e}gradation thermique du fluorure hydrat\'{e}
CoFeF\textsubscript{5}(H\textsubscript{2}O)\textsubscript{7}, tout en maintenant ses performances
\'{e}lectrocatalytiques comme \'{e}lectrocatalyseur de la r\'{e}action
d'oxydation de l'eau (Oxygen Evolution Reaction, OER) en milieu
alcalin. Pour cela, la strat\'{e}gie propos\'{e}e consiste \`{a}
substituer les ions Co\textsuperscript{2+} {par Fe}\textsuperscript{2+} dans
CoFeF\textsubscript{5}(H\textsubscript{2}O)\textsubscript{7}, choisi pour son abondance terrestre,
menant \`{a} la solution solide
(Co\textsubscript{1--\textit{x}}Fe\textsubscript{\textit{x}})\textsuperscript{2+}Fe\textsuperscript{3+}F\textsubscript{5}(H\textsubscript{2}O)\textsubscript{7} obtenue par copr\'{e}cipitation. Ensuite, le traitement thermique sous
air de cette s\'{e}rie de pr\'{e}curseurs hydrat\'{e}s a \'{e}t\'{e}
r\'{e}alis\'{e} dans l'objectif de pr\'{e}parer la solution solide
d'oxyfluorures correspondante
\tralicstex{Co\textsubscript{(1--\textit{x})/2}Fe\textsubscript{(1+\textit{x})/2}O\textsubscript{(1+\textit{x})/2}F\textsubscript{(3--\textit{x})/2}}{$\mathrm{Co}_{\sfrac{1-x}{2}}\mathrm{Fe}_{\sfrac{1+x}{2}}\mathrm{O}_{\sfrac{1+x}{2}}\mathrm{F}_{\sfrac{3-x}{2}}$}. 
Les analyses structurales et thermiques ont confirm\'{e} une
substitution Co\textsuperscript{2+}/Fe\textsuperscript{2+} dans les pr\'{e}curseurs
hydrat\'{e}s, alors que celles des \'{e}chantillons calcin\'{e}s
indiquent un processus de d\'{e}composition thermique complexe qui
m\`{e}ne \`{a} des oxyfluorures Co--Fe de formulations qui restent \`{a}
pr\'{e}ciser. Les performances \'{e}lectrocatalytiques OER de ces
mat\'{e}riaux oxyfluor\'{e}s montrent qu'il est possible de r\'{e}duire
jusqu'\`{a} 20\% la quantit\'{e} de cobalt sans impacter l'activit\'{e}
globale OER, \`{a} savoir un surpotentiel de 320 mV \`{a} \tralicstex{10 mA\textperiodcentered{}}{10 mA${\cdot}$}cm\textsuperscript{--2} et une activit\'{e} massique de 112 \tralicstex{A\textperiodcentered{}g\textsuperscript{--1}}{A${\cdot}$g$^{-1}$} \`{a} 1.55 V vs. RHE, avec une excellente stabilit\'{e}.
\end{altabstract}

\maketitle

\clearpage

\twocolumngrid

\end{noXML}

\section{Introduction\label{mark-1}}

In order to meet the growing demand for energy, it is essential to
promote renewable means of production and transformation. Hydrogen is a
promising candidate that can be sustainably produced from
water~\cite{1} via a green electricity--driven water splitting
reaction~\cite{2,3} and used as an energy vector~\cite{4,5}.
Unfortunately, the cost competitiveness of these systems suffers in
part from low energy efficiency due to the high overpotential required
to initiate the Oxygen Evolution Reaction (OER), that oxidizes water to
oxygen~\cite{6,7}. Reducing the overpotential requires the use of an
electrocatalyst to lower the energy barrier associated with the
formation of reaction intermediates~\cite{8,9,10}. Noble metal
catalysts such as RuO$_{2}$ and IrO$_{2}$ are the reference materials
as OER electrocatalysts but the scarcity of these metals and their
prohibitive cost make large-scale industrialization impossible in the
future.

In this way, the scientific community has focused on the development of
3{d}-metal-based catalysts and this research has highlighted
some iron-based materials as the best OER catalysts in alkaline
electrolytes reported in the literature, such as Ni--Co--Fe layered
double hydroxides (LDH)~\cite{11,12,13}. Although the role of iron in
the OER mechanism is not yet clear, its presence seems essential to
further improve the electrochemical performance~\cite{14,15,16}.

While many works in the literature focus on the determination of the
best cationic composition, which seems to lean towards the Co/Ni/Fe
association~\cite{17}, few studies are devoted on the modification of
the anionic part, which is nevertheless of interest~\cite{18,19}.
Fluorine, the most electronegative element in the periodic table, is of
particular interest to modulate the electronic structure of neighboring
metals thanks to its strong inductive effect. As a result, the metal
active site is electron-depleted and easily hydroxylated in highly
alkaline electrolytes~\cite{20}, which is the first step of the OER
mechanism~\cite{9,21}. The first transition-metal fluoride reported in
the literature as an OER catalyst in alkaline media is a nanostructured
mixed Co--Ni--F material, which exhibits a low overpotential of 300~mV at
10~mA${\cdot}$cm$^{-2}$, outperforming the reference
material IrO$_{2}$~\cite{22}. Pei et~al.{}\ reported the
fluorination of Fe--Co Prussian blue analog nanocubes, which resulted
into 3D porous Fe--Co--F nanocubes reaching an overpotential of 250~mV at
10~mA${\cdot}$cm$^{-2}$ with a Tafel slope of
39~mV${\cdot}$dec$^{-1}$~\cite{23}. Subsequently, Pei
et~al.{}\ fluorinated NiFe-LDH to further enhance its OER
activity~\cite{24}. This study shows that a suitable fluorination
temperature changes the initial morphology of NiFe-LDH and leads to the
formation of fluorides, resulting in an excellent OER activity with an
overpotential of 225~mV for a Co-free material, close to the best
performance obtained with trimetallic\break NiCoFe-LDH.

In the same direction, some studies aim to combine oxygen and fluorine
anions in 3{d}-materials and conclude that fluorine
incorporation into transition metal oxides improves OER
performance~\cite{25,26,27}. Recently, a mixed Co/Fe oxyfluoride with
the formulation Co$_{0.5}$Fe$_{0.5}$O$_{0.5}$F$_{1.5}$ has been
synthesized and further patented by us~\cite{28}. 
This material, synthesized by thermal
decomposition of the hydrated fluoride CoFeF$_{5}$(H$_{2}$O)$_{7}$,
exhibits excellent OER performance and long-time stability in highly
alkaline media~\cite{29}. Substitution of the Co$^{2+}$ ions by
Ni$^{2+}$ resulted in better OER performance and stability, with an
optimum for the Co$_{0.25}$Ni$_{0.25}$Fe$_{0.5}$O$_{0.5}$F$_{1.5}$
composition~\cite{30}.

In a sustainable framework, this work presents the reduction of
Co$^{2+}$ amount in Co$_{0.5}$Fe$_{0.5}$O$_{0.5}$F$_{1.5}$ by
substitution with Fe$^{2+}$ following the same method as
before~\cite{30}. Different
(Co$_{1-x}$Fe$_{x}$)$^{2+}$FeF$_{5}$(H$_{2}$O)$_{7}$ compositions 
($0 \leq x_{\mathrm{theo}}\leq 0.75$) have been
synthesized and structurally characterized. The thermal behavior of
these materials is studied by High Temperature X-ray Diffraction
(HT-XRD) and Thermogravimetric Analysis (TGA) to understand the
decomposition mechanism leading to Fe-enriched oxyfluorides. The phases
obtained from the calcination of
(Co$_{1-x}$Fe$_{x}$)$^{2+}$FeF$_{5}$(H$_{2}$O)$_{7}$ were tested as
catalysts for OER to evaluate the cationic Co$^{2+}$/Fe$^{2+}$
substitution on the electrocatalytic performance.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig01}
{\vspace*{-2pt}}
\caption{(a) (Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$  PXRD
diagrams with the corresponding [100] projection of the structure,  
(b) SEM images of the microcrystalline powder of
(Co$_{0.75}$Fe$_{0.25}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$  and
elemental mapping of each metal belonging to the hydrated fluoride
phase, (c) $^{57}$Fe M\"{o}ssbauer spectra at 300~K and
77~K of (Co$_{0.75}$Fe$_{0.25}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$, (d)
Rietveld refinement of
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$
structure.\label{fig1}}
{\vspace*{-2pt}}
\end{figure*}

\section{Results and discussion\label{mark-2}}

\subsection{Synthesis and characterization of\hfill\break
$(\mathrm{Co}_{1-x}\mathrm{Fe}_{x})^{2+}\mathrm{Fe}^{3+}
\mathrm{F}_{5}(\mathrm{H}_{2}\mathrm{O})_{7}\hfill\break 
(0\leq x_{\mathrm{theo}} \leq 0.75)$ precursors\label{mark-2.1}}

Iron-rich hydrated fluorides
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}${\ubreak}(H$_{2}$O)$_{7}$ with
theoretical compositions $x_{\mathrm{theo}} = 0$, 0.25, 0.5 and 0.75
have been synthesized by coprecipitation with aqueous hydrofluoric acid
(HF) as fluorinating agent. The substitution of Co$^{2+}$ ions by
Fe$^{2+}$ is possible only by preventing the oxidation of Fe$^{2+}$ to
Fe$^{3+}$; it is achieved by dissolving Co$^{2+}$ and Fe$^{3+}$ salts
together in HF and Fe$^{2+}$ salt separately in methanol. The obtained
polycrystalline powders, which turn yellow depending on the Fe$^{2+}$
amount (Figure~S1), were first analyzed by powder X-ray
diffraction (PXRD). PXRD diagrams of the
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ series show
that the structures are isostructural with the reference
(Co$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$, $x=0$), and
indexed in the triclinic $P-1$ space group (Figure~\ref{fig1}a). 
Their crystalline structure consists of isolated
{[}Fe$^{3+}$F$_{5}$(H$_{2}$O){]}$^{2-}$ (brown) and
{[}(Co/Fe)$^{2+}$(H$_{2}$O)$_{6}${]}$^{2+}$ octahedra (beige)
(Figure~\ref{fig1}a,~inset). As the radii of the Co$^{2+}$
and Fe$^{2+}$ ions are close, 0.742~\AA{} and 0.781~\AA{},
respectively~\cite{31}, only a subtle shift of X-ray peaks is
observed.

\begin{table*}
\caption{\label{tab1}Theoretical and experimental $n$(Fe$^{2+}$/Fe$^{3+}$)
and $n$(Co$^{2+}$/Fe$_{\mathrm{tot}}$) ratios determined by $^{57}$Fe
M\"{o}ssbauer spectrometry (M\"{o}ss.)\ and EDX-coupled SEM analysis
(EDX). The value of $x$ in
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ were
adjusted in accordance to these results ($x_{\mathrm{exp}}$).\vspace*{-2pt}}
\begin{tabular}{cccccccc}
\thead
\multicolumn{3}{c}{Theoretical} &amp;&amp; \multicolumn{4}{c}{Experimental} \\ 
\cline{1-3}\cline{5-8}
$x_{\mathrm{theo}}$ &amp; $n$(Fe$^{2+}$/Fe$^{3+}$)$_{\mathrm{theo}}$ &amp;
$n$(Co$^{2+}$/Fe$_{\mathrm{tot}}$)$_{\mathrm{theo}}$ &amp;&amp;
\parbox[t]{2cm}{\centering $n$(Fe$^{2+}$/Fe$^{3+}$)  M\"{o}ss.} 
&amp; \parbox[t]{2cm}{\centering $n$(Co$^{2+}$/Fe$_{\mathrm{tot}}$) M\"{o}ss.} &amp;
\parbox[t]{2cm}{\centering $n$(Co$^{2+}$/Fe$_{\mathrm{tot}}$) EDX}\vspace*{2pt} &amp;
$x_{\mathrm{exp}}$\\
\endthead
0 &amp; 0 &amp; 1 &amp;&amp; 0 &amp; 1 &amp; 1 &amp; 0 \\ 
0.25 &amp; 0.25 &amp; 0.60 &amp;&amp; 0.20 &amp; 0.67 &amp; 0.70 &amp; 0.20 \\ 
0.5\0 &amp; 0.50 &amp; 0.33 &amp;&amp; 0.45 &amp; 0.38 &amp; 0.43 &amp; 0.45 \\ 
0.75 &amp; 0.75 &amp; 0.14 &amp;&amp; 0.72 &amp; 0.16 &amp; 0.17 &amp; 0.72
\botline
\end{tabular}
\vspace*{-2pt}
\end{table*}

EDX-coupled SEM confirms the crystallinity of particles with sizes in
the range of tens of micrometers, together with a homogeneous
distribution of 3{d} cations (Figures~S2--S5), as illustrated
in Figure~\ref{fig1}b, corresponding to the composition
(Co$_{0.75}$Fe$_{0.25}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$. The
experimental values of the $n$(Co/Fe$_{\mathrm{tot}}$) ratio are
consistent with the substitution of Co$^{2+}$ ions by Fe$^{2+}$ ions in
the triclinic phase
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$. 
However, the experimental $n$(Co/Fe$_{\mathrm{tot}}$) ratio
for compositions containing Fe$^{2+}$ is
systematically higher than the theoretical ones, indicating a lower
Fe$^{2+}$ content, in contrast to Fe$^{2+}$-free
Co$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ where the value fits
perfectly. Thus, despite the precautions taken during synthesis, a
small amount of Fe$^{2+}$ is probably oxidized to Fe$^{3+}$ in
solution.

$^{57}$Fe M\"{o}ssbauer experiments were then performed to gain insight
into the iron environment and on its valence state. All M\"{o}ssbauer
spectra of the
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ series were
collected at 300~K, while one was obtained at 77~K ({Figure~S6}), shown
here in Figure~\ref{fig1}c for a theoretical value of
$x_{\mathrm{theo}}=0.25$. At 300~K they
{consist of a quadrupolar structure resulting
from two different doublets, which can be unambiguously attributed to
the presence of Fe}$^{2+}$ {and Fe}$^{3+}$
{species, except for the composition}
$x_{\mathrm{exp}}=0$ where the quadrupolar doublet is attributed to Fe$^{3+}$
{species only. Different fitting models with 2, 3 and 4 quadrupolar
components were considered. The table in} Figure~S6 {shows the mean
refined values of the hyperfine parameters: the isomer shift and
quadrupolar splitting values are quite independent of the cationic
composition, but their respective absorption regions are strongly
dependent on the cationic composition. It is important to note that
these values are the same at 77~K, suggesting that the
Lamb--M\"{o}ssbauer factors are similar for the two Fe species. In
addition, the values of isomer shift suggest that the Fe}$^{3+}$ {ions
are on average located in octahedral FeF}$_{5}${O units, while the
Fe}$^{2+}$ {ions are located in octahedral FeO}$_{6}$ {units. The}
$^{57}$Fe {M\"{o}ssbauer data exploitation} allowed quantifying the
proportion of Fe$^{2+}$/Fe$^{3+}$ cations, and then, identifying the 
real formulations. The latter are in agreement with the Co/Fe$_{\mathrm{tot}}$
ratios determined by EDX-SEM (Table~\ref{tab1}), revealing an
incomplete substitution of Co$^{2+}$ by Fe$^{2+}$. In the following,
the experimental values of $x$ are used (i.e., $x_{\mathrm{exp}}$ in
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$).

The PXRD diagrams of the
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}${\ubreak}(H$_{2}$O)$_{7}$ series were
refined by the Rietveld method ({Figure~S7}), as shown for the
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}${\ubreak}(H$_{2}$O)$_{7}$ series in
Figure~\ref{fig1}d. The details of the structure refinement are summarized
in~Table~S1, and atomic coordinates (except for hydrogen atoms, which
were not refined) together with interatomic distances are gathered in
Tables~S2--S9. The Co$^{2+}$/Fe$^{2+}$ occupancy ratios in the
{1e} and {1f} Wyckoff positions were fixed considering
$x_{\mathrm{exp}}$ and assuming a statistical distribution on both sites. Good
reliability factors confirmed the crystalline purity for all samples.
The substitution of Co$^{2+}$ ions by Fe$^{2+}$ in the whole
composition range ($0\leq x_{\mathrm{exp}}\leq 0.72$) is confirmed by
the linear increase of the unit cell volume as a function of the Fe
content ({Figure~S8}).

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig02}
\caption{(a) Evolution of the X-ray diffractograms of
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ with
temperature under ambient air. (b) TGA analyses of
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ performed
under ambient air. (c)~Temperature range of stability domains determined
by HT-XRD for
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$. 
(d)~Corresponding TGA analyses performed under ambient air.\label{fig2}}
\vspace*{-3pt}
\end{figure*}

\subsection{Thermal behavior of
$(\mathrm{Co}_{1-x}\mathrm{Fe}_{x})^{2+}\mathrm{Fe}^{3+}
\ubreak \mathrm{F}_{5}(\mathrm{H}_{2}\mathrm{O})_{7}$
precursors\label{mark-2.2}}

{\fontsize{9.8}{12}\selectfont The structural evolution of (Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$} 
precursors
($0 \leq x_{\mathrm{exp}} \leq 1$) was studied as a function of
the temperature by HT-XRD and TGA under ambient air from room
temperature to 600~\textdegree C ({Figures~S9} and {S10}). First,
hydrated precursors are stable up to 80--100~\textdegree C for all
compositions, as exemplified by
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ in
Figure~\ref{fig2}a (in blue). Above 100~\textdegree C,
the weight losses are consistent with the departure of
five water molecules, leading to mixtures of MFeF$_{5}$(H$_{2}$O)$_{2}$
and MFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ with M~${=}$~Co$^{2+}$/Fe$^{2+}$
(Figure~\ref{fig2}b and {Figure~S11}). For
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$, an
additional loss of two water molecules is then observed due to
progressive thermal degradation of MFeF$_{5}$(H$_{2}$O)$_{2}$ and
MFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ (${\Delta}m_{2}$ (exp./th.):~10.7\%;
{10\%}) leading to quasi amorphous fluorides. After complete
dehydration, oxyfluorinated phases with a rutile-type structure appear,
resulting from the hydrolysis of amorphous phases
(${\Delta}m_{3}$ (exp./th.): 6.7\%; 6.8\%) with air humidity 
(220~\textdegree C--300~\textdegree C, in black), as shown in our previous
studies on the thermal decomposition of
CoFeF$_{5}$(H$_{2}$O)$_{2}$~\cite{29} and
CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$~\cite{27}. It is important to note
that in the CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ study, the oxyfluoride
phase was considered to be amorphous. However, a closer inspection of
the thermal decomposition of CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ reveals
a poorly crystallized rutile oxyfluoride with unit cell parameters
close to the rutile oxyfluoride obtained from
CoFeF$_{5}$(H$_{2}$O)$_{2}$, as the XRD peak positions are similar
(Figure~S12). For Fe$^{2+}$-rich compositions ($x_{\mathrm{exp}}=0.72$ and 1), 
FeF$_{3}$-type oxyfluorides with anionic vacancies (referred to as
vac-FeF$_{3}$ in this work) are observed on HT-XRD diagrams, in the
temperature range 190--240~\textdegree C and 210--310~\textdegree C,
respectively  (Figure~S9 and Figure~\ref{fig2}c), identified as
FeF$_{2.5}$O$_{0.25}\Box_{0.25}$ and
FeF$_{2.66}$O$_{0.17}\Box_{0.17}$, which have been identified as
the result of the thermal treatment of Fe$_{2}$F$_{5}$(H$_{2}$O)$_{2}$
and Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$, respectively~\cite{32}. For all
compositions, spinel-type $\mathrm{M}^{2+}\mathrm{M}^{3+}_{2}\mathrm{O}_{4}$ and/or
$\upalpha$-Fe$_{2}$O$_{3}$ oxides (for $x_{\mathrm{exp}}$~${\geq}$~0.45) are observed at
temperatures above 300~\textdegree C (Figure~S13), resulting from the
slow \mbox{hydrolysis} of oxyfluorinated rutile phases and vac-FeF$_{3}$
phases, respectively.

The HT-XRD results, summarized for all the compositions in Figure~\ref{fig2}c,
show that the temperature range for obtaining pure oxyfluoride(s) with
rutile structure (in black) shrinks as the amount of Fe$^{2+}$
increases, until it disappears completely for
Fe$_{2}$F$_{5}$(H$_{2}$O)$_{7}$ ($x_{\mathrm{exp}}=1$). This trend indicates
that Co$^{2+}$ is required to produce the rutile oxyfluorinated phase.
In order to better understand the differences between the thermal
behavior of the 
(Co$_{1-x}$Fe$_{x}$)$^{2+}$FeF$_{5}$(H$_{2}$O)$_{7}$ compounds, the
PXRD diagrams at 120~\textdegree C where MFeF$_{5}$(H$_{2}$O)$_{2}$ and
MFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ phases are produced from the thermal
degradation of the initial precursors, have been studied. The data
obtained from Rietveld refinements through two hypotheses
(Figures~S14--16 and Tables~S10--12) showed that
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ is
decomposed into
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{2}$ and
Co$^{2+}$Fe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$, and  additionally to
Fe$_{2}$F$_{5}$(H$_{2}$O)$_{2}$ and Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$ for
$x_{\mathrm{exp}}\geq 0.45$. In fact, the linear increase in unit cell volume of
the M$^{2+}$FeF$_{5}$(H$_{2}$O)$_{2}$-type phase as a function of
$x_{\mathrm{exp}}$ indicates the existence of a solid solution for
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{2}$ while the
invariant volume for the M$^{2+}$Fe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$-type
phase proves that the
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ solid
solution does not seem to exist ({Figure~S16}). The formation of pure
Fe phases, Fe$_{2}$F$_{5}$(H$_{2}$O)$_{2}$ and
Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$, manifested by the presence of
$\upalpha$-Fe$_{2}$O$_{3}$ on PXRD up to 600~\textdegree C for $x_{\mathrm{exp}} \geq 0.45$
compositions, could be explained by the mass balance. Indeed, for all
$0\leq x_{\mathrm{exp}} \leq 0.72$ compositions, a spinel-type phase indexed as
CoFe$_{2}$O$_{4}$ is observed at 600~\textdegree C, with a ratio
$n$(Fe)/$n$(Co) equal to 2. For $x_{\mathrm{exp}} \geq 0.45$ compositions, the ratio
$n$(Fe)/$n$(Co)~is superior to~2 and consequently, an iron-based oxide is
required to achieve the mass balance, which is $\upalpha$-Fe$_{2}$O$_{3}$, as
observed in Figure~S13. On the other hand, for $x_{\mathrm{exp}} \leq 0.2$
compositions, only spinel-type CoFe$_{2}$O$_{4}$ is identified although
$n$(Fe)/$n$(Co)~${&lt;}$~2 so a Co-rich material is needed to equilibrate the
mass balance. In our previous work on CoFeF$_{5}$(H$_{2}$O)$_{7}$, the
PXRD pattern at 600~\textdegree C was identified as a mixture of
CoFe$_{2}$O$_{4}$ and CoO~\cite{29,30}, but as shown in {Figure~S17},
it's more likely to be a mixture of two spinel-type oxides with the
formulations Co$^{2+}$(Co$_{0.24}$Fe$_{1.76}$)$^{3+}$O$_{4}$ and
Co$^{2+}$(Co$_{1.17}$Fe$_{0.83}$)$^{3+}$O$_{4}$. These formulations
were calculated according to their unit cell volumes determined by
Le~Bail refinements, 583.1 and 549.2~\AA{}$^{3}$, respectively, and
used in the formula extracted from Vegard's law for the solid solution
Co$^{2+}$(Co$_{2-x}$Fe$_{x}$)$^{3+}$O$_{4}$ ({Figure~S18}). The formula
Co$^{2+}$(Co$_{0.24}$Fe$_{1.76}$)$^{3+}$O$_{4}$ and
Co$^{2+}$(Co$_{1.17}$Fe$_{0.83}$)$^{3+}$O$_{4}$ indicate a partial
oxidation of Co$^{2+}$ to Co$^{3+}$, which has already been observed in
the literature for the thermal treatment of CoF$_{2}$ under similar
thermal treatment conditions~\cite{33}. By plotting the respective
spinel volume as a function of $x_{\mathrm{exp}}$ obtained by Le~Bail
refinements for the different compositions ({Figure~S19}), a gradual
increase in unit cell volume is observed, indicating a progressive
increase in the amount of Fe in the spinel up to a limit dictated by
CoFe$_{2}$O$_{4}$. Finally, the reason for the appearance of
CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ and Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$
phases is probably related to a partial oxidation of Fe$^{2+}$ ions
under such thermal decomposition conditions (high temperature, ambient
air), leading to Fe$^{3+}$-rich phases, which is not obviously observed
for Fe$^{2+}$-free Co$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$\break
(Figure~S11).

The comparison of the TGA profiles in {Figure~\ref{fig2}d} shows an evolution
of the thermal behavior according to the proportion of Fe$^{2+}$ in the
initial precursor. First, the thermal behaviors of $x_{\mathrm{exp}}=0$ and
0.2 are very similar, which is not surprising since, from the previous
results at 120~\textdegree C, $x=0.2$ is mainly composed of
M$^{2+}$FeF$_{5}$(H$_{2}$O)$_{2}$. The few changes can be attributed to
the contribution of the thermal behavior of
CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$, which is quite similar to that of
CoFeF$_{5}$(H$_{2}$O)$_{2}$, since it decomposes into a
rutile oxyfluoride and then a spinel-type oxide (Figure~S20). For the
composition $x_{\mathrm{exp}}=0.45$ there is a significant change between
300--450~\textdegree C which could be attributed to the appearance of
pure Fe phases to the detriment of
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{2}$. For
$x_{\mathrm{exp}}=0.72$, the thermal behavior is very close to that of pure
Fe$_{2}$F$_{5}$(H$_{2}$O)$_{7}$, except that a clear distinction
between the two successive weight losses associated with the formation
of FeF$_{2.5}$O$_{0.25}\Box_{0.25}$
and FeF$_{2.66}$O$_{0.17}\Box_{0.17}$ cannot be made (Figure S10).

\subsection{Synthesis and characterization of the oxyfluoride(s)\label{mark-2.3}}

The oxyfluorides with the theoretical formulation
$(\mathrm{Co}_{\sfrac{1-x}{2}})^{2+}(\mathrm{Fe}_{\sfrac{1+x}{2}})^{3+}
\mathrm{O}_{\sfrac{1+x}{2}}\mathrm{F}_{\sfrac{3-x}{2}}$ assuming the
formation of rutile MX$_{2}$, were prepared by thermal treatment of the
corresponding hydrated fluorides
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$. For each
composition, the selected calcination conditions to obtain the pure
rutile oxyfluorides, according to XRD and theoretical weight loss
(about 44--45 wt\% for all compositions) are summarized in
Table~S13. More details on the determination of the thermal
treatment conditions are given for $x_{\mathrm{exp}}=0$ in our previous
study~\cite{30}.\looseness=-1

\begin{figure*}
\includegraphics{fig03}
\caption{(a) Calcined
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ PXRD
diagrams with the corresponding [001] projection of the rutile
structure for oxyfluorides, (b) Le Bail refinement of calcined
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$,
(c) $^{57}$Fe M\"{o}ssbauer spectra at 300~K and 77~K of
calcined (Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$,
(d) SEM images of the calcined
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ and
elemental mapping of each metal.\label{fig3}}
\vspace*{-2pt}
\end{figure*}

After calcination, all oxyfluorinated phases were indexed in the
tetragonal $P$4$_{2}$/\textit{mnm} space group (Figure~\ref{fig3}a).
The PXRD patterns were refined by the Le Bail refinement method
(Figures~S21--24), as exemplified for the calcined
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$ Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$
precursor in~Figure~\ref{fig3}b. According to the evolution of the volume
obtained for the different rutile oxyfluorides ({Figure~S25}), it seems
that from $x$~${\geq}$~0.45, Fe$^{3+}$ and O-enriched oxyfluorides are
formed, since the volume of the unit cell decreases slightly, in
agreement with the radii of Co$^{2+}$, Fe$^{3+}$, O$^{2-}$ and
F$^{-}$ (0.742~\AA{}, 0.649~\AA{}, 1.366~\AA{}
and~1.300~\AA{}, respectively)~\cite{31}. However, the volume of
$x_{\mathrm{exp}}=0.2$ is equivalent to $x_{\mathrm{exp}}=0$, indicating a similar
chemical composition, even though the substitution was previously
demonstrated in the
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{2}$ precursor. This
clearly shows the complexity of the decomposition mechanism, especially
for (Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{2}$ and
Co$^{2+}$Fe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ hydrates, which seem to give a
similar oxyfluoride for $x_{\mathrm{exp}}\leq  0.2$, while the metal ratio
is different. For the sample $x_{\mathrm{exp}}=0.72$, an impurity of
$\upalpha$-Fe$_{2}$O$_{3}$ is present, which is not surprising given the narrow
stability window of the oxyfluoride (Figure~\ref{fig2}c).
Rietveld refinement of the PXRD pattern shows an $\upalpha$-Fe$_{2}$O$_{3}$ mass
fraction close to 60 wt\%, but this could be
misestimated given the complexity of the refinement\break (Figure~S26).

The M\"{o}ssbauer spectra obtained at 300~K and 77~K are shown in
Figure~S27 and exemplified for calcined $x_{\mathrm{exp}}=0.2$ in
Figure~\ref{fig3}c. At 300~K, a significant change from a pure quadrupolar
hyperfine structure ($x_{\mathrm{exp}}\leq0.45$) to a mixed quadrupolar
and magnetic structure ($x_{\mathrm{exp}}=0.72$) is observed, while the
M\"{o}ssbauer spectra at 77~K consist only of a pure magnetic structure
for all compositions. From the XRD analysis, this magnetic structure at
300 K is probably related to the presence of $\upalpha$-Fe$_{2}$O$_{3}$ magnetic
impurity. In addition, as shown in {Figure~S27} ($x_{\mathrm{exp}}=0.20$ and
0.45), two quadrupolar components can be distinguished, which can be
clearly attributed to a large contribution of Fe$^{3+}$ species on the
one hand, and to a very small contribution of Fe$^{2+}$ species on the
other hand, which cannot be easily detected from the magnetic hyperfine
structures due to their complexity. The quadrupolar spectra were
described by similar models as described above, while the magnetic
spectra were described using a discrete distribution of the hyperfine
field linearly correlated with that of the isomer shift to account for
the asymmetry of the sextets. The mean refined values are listed in the
Table of {Figure~S27}. These results clearly show that calcination
leads, on the one hand, to the almost complete disappearance of
Fe$^{2+}$ ions and, on the other hand, to the appearance of magnetic
oxyfluorinated phases containing predominantly Fe$^{3+}$ species. It is
\mbox{important} to note that the large decrease in Fe$^{2+}$ content is not
related to the difficulty of stabilizing Fe$^{2+}$ in the
rutile-structure oxyfluoride as Brink et~al.\ demonstrated that it is
possible to solubilize up to 30\% of FeF$_{2}$ in rutile
FeOF~\cite{34}. This decrease is more likely related to Fe$^{2+}$
oxidation during thermal decomposition, as has been observed after
thermal treatment of the mixed-valent iron fluoride hydrates
Fe$_{2}$F$_{5}$(H$_{2}$O)$_{2}$ and
Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$~\cite{32}. However, the complexity and
lack of resolution of the magnetic hyperfine structures at 77~K make it
difficult to rule out the presence of ${\upalpha}$-Fe$_{2}$O$_{3}$ and
to estimate its precise content. The comparison of the quadrupolar
hyperfine structures at 300~K of the spectra $x_{\mathrm{exp}}=0$ and 0.2
shows three components for Fe$^{3+}$ species that are significantly
different (Figure~S28). This result shows that the cationic
distribution is modified by increasing the amount of Fe. However, the
mean values of the isomer shift are unchanged, suggesting that the mean
anionic environment remains similar, which can be roughly estimated to
be about 50:50 F:O. Furthermore, the 77~K spectra show a pure magnetic
structure with broadened lines: the mean isomer shift values are also
similar, while the distributions of the hyperfine fields are consistent
with different atomic environments of the Fe$^{3+}$ species, as
inferred from the 300~K spectra. 

EDX-SEM analysis revealed the presence and homogeneous distribution of
F, O, Co, and Fe within the calcined materials (Figures~S29--32) as
shown in {Figure~\ref{fig3}d}. The Co/Fe$_{\mathrm{tot}}$ ratio after calcination is
still in agreement with the respective precursor composition determined
by M\"{o}ssbauer spectrometry and EDX-SEM. Although a dominant impurity
of $\upalpha$-Fe$_{2}$O$_{3}$ is observed by PXRD for the sample $x_{\mathrm{exp}}=0.72$,
we are not able to distinguish two clearly separated phases by
elemental mapping (Figure~S32). This could indicate an intimate
mixture of both phases, divided into nanodomains visible only by High
Resolution Transmission Electronic Microscopy (HR-TEM), as already
observed in our previous study~\cite{30}. This is supported by the
broadening of the Bragg diffraction peaks observed after thermal
treatment ({Figure~\ref{fig3}a}), compared to those of the precursor,
indicating small coherent diffraction domains estimated in the range of
10~nm by the Laue--Scherrer relation. High specific surface areas
around 60--80~m$^{2}{\cdot}$g$^{-1}$ (Table~S13) were obtained with
such calcination conditions, as\break indicated by the grain size reduction
before and after calcination (Figure~S33).

The molar ratio $n$(Co)/$n$(Fe) in the calcined samples has been verified
by ICP-OES (Table~S14) and agrees with the formulation
previously determined for the initial hydrated precursors.  However,
for compositions with a higher amount of Fe$^{2+}$ (e.g., $x=0.45$
and 0.72), a slight difference between experimental and theoretical is
observed. This difference could be explained by an incomplete
dissolution of the material, since the experimental $n$(Co)/$n$(Fe) ratio
is still in agreement with the theoretical one, or by the presence of
amorphous impurities resulting from thermal decomposition of additional
Fe$_{2}$F$_{5}$(H$_{2}$O)$_{2}$ and Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$.

From this analysis, we can first conclude that the structure as well as
the chemical composition of the oxyfluoride derived from
(Co$_{0.8}$Fe$_{0.2}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ is
similar to that of the oxyfluoride derived from
Co$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$, which is surprising given
the difference in the metal ratios and the intermediates involved.
Thus, the excess Fe$^{3+}$ may be in the form of an amorphous phase not
visible by PXRD, or in another Fe enriched oxyfluoride with
indistinguishable unit cell parameters arising from
CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$. Oxyfluorides prepared from
(Co$_{0.55}$Fe$_{0.45}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ and
(Co$_{0.28}$Fe$_{0.72}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ are
found to be enriched in iron, but not as much as in the initial precursor
 value, as suggested by the high amount of ${\upalpha}$-Fe$_{2}$O$_{3}$
impurity observed for calcined $x_{\mathrm{exp}}=0.72$.

\begin{figure*}
{\vspace*{-1.5pt}}
\includegraphics{fig04}
\caption{(a) Cyclic voltammetry curves of calcined
(Co$_{1-x}$Fe$_x$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$  in 1M KOH,
the inset representing a zoom on the redox peaks area.  (b)
Overpotential at 10~mA${\cdot}$cm$^{-2}$ for the different
compositions.  (c) Tafel slope. (d) Mass activity at 1.55~V vs.\ RHE.
(e) Specific activity at 1.55~V vs.\ RHE.\label{fig4}}
{\vspace*{-2pt}}
\end{figure*}

\subsection{Electrochemical performance for oxygen evolution reaction
(OER)\label{mark-2.4}}

Cyclic voltammetry (CV) in 1M KOH electrolyte was performed to evaluate
the performance of the calcined
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ samples as\break
electrocatalysts for OER ({Figure~\ref{fig4}a}). For all compositions, the
Co$^{2+/3+}$ oxidation peak is clearly visible and varies
from 1.16 to 1.18~V vs.\ RHE depending on Fe$^{2+}$ amount. This shift
could be explained by a slight modification of the Co$^{2+}$
environment in the rutile phase, which is gradually enriched in Fe, and
consequently in O$^{2-}$ for $x_{\mathrm{exp}}=0.45$ and 0.72 samples. Both
position and intensity of the Co$^{2+/3+}$ oxidation peak
are identical between the composition $x_{\mathrm{exp}}=0$ and 0.2, the only
difference being a slight broadening of the oxidation peak. This could
be due to the contribution of a second oxyfluoride originating from
CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ which, according to the work of
Lemoine et~al.{}, shows a maximum intensity of Co$^{2+/3+}$
oxidation at a slightly higher potential of about
1.20~V~vs.~RHE~\cite{27} instead of 1.16~V~vs.~RHE for
Co$_{0.5}$Fe$_{0.5}$O$_{0.5}$F$_{1.5}$ ($x_{\mathrm{exp}}=0$)~\cite{30}. For
$x_{\mathrm{exp}}=0.45$ and 0.72, the intensity of the oxidation peak
decreases gradually and proportionally to the amount of Co$^{2+}$,
indicating a reduced activity for the main Co$^{2+}$ active site. In
terms of overpotential (Figure~\ref{fig4}b), Tafel slope (Figure~\ref{fig4}c), mass
activity (Figure~\ref{fig4}d), and specific activity (Figure~\ref{fig4}e), the
calcined $x_{\mathrm{exp}}=0$ and 0.2 samples show similar performance, which
are the best of the calcined
(Co$_{1-x}$Fe$_{x}$)$^{2+}$FeF$_{5}$(H$_{2}$O)$_{7}$ series, namely for
$x_{\mathrm{exp}}=0.2$ an overpotential of 320~mV~at 10~mA${\cdot}$cm$^{-2}$,
a low Tafel slope of 33~mV${\cdot}$dec$^{-1}$, a mass activity of
112~A${\cdot}$g$^{-1}$, and a specific activity of 1.67~A${\cdot}$m$^{-2}$. This
similar activity between calcined $x_{\mathrm{exp}}=0$ and 0.2 confirms that
both structures and chemical compositions are relatively close, and it
does not affect the global OER performance regardless of the presence
of an additional phase. On the other hand, the OER activity decreases
progressively for calcined $x_{\mathrm{exp}}=0.45$ and 0.72, which could be
attributed to the increase of less active Fe-enriched oxyfluorinated
phases to the detriment of more active Co-phases.
Chronopotentiometric measurements were carried out at a current density
of 125~mA${\cdot}$cm$^{-2}$ on calcined $x_{\mathrm{exp}}=0$ and 0.2 to study
the effect of Fe increase on the stability (Figure~S34). Both samples
are similar and reasonably stable, with only an increase of about
150~mV after 17 days at\break 125~mA${\cdot}$cm$^{-2}$.

\section{Conclusion\label{mark-3}}

In this work, the
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ solid
solution with 0~${\leq}$~$x_{\mathrm{exp}}$~${\leq}$~0.72 is synthesized by a\break
simple coprecipitation inspired by our previous work, as precursors of
Co/Fe oxyfluorides as OER catalysts. XRD, EDX-SEM and M\"{o}ssbauer
spectrometry analyses confirmed the Co$^{2+}$/Fe$^{2+}$ substitution in
the heptahydrated precursors, but the quantification of the Fe$^{2+}$
amount indicates that it is incomplete, due to a small part of
Fe$^{2+}$ oxidized to Fe$^{3+}$ during the synthesis. A detailed study
of the thermal behavior of these hydrates by coupling HT-XRD and TGA
analyses revealed for the first time a complex decomposition into a
mixture of (Co$_{1-x}$Fe$_{x}$)$^{2+}$FeF$_{5}$(H$_{2}$O)$_{2}$ and
CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ dihydrates for $x_{\mathrm{exp}}=0$ and 0.2,
and additionally into Fe$_{2}$F$_{5}$(H$_{2}$O)$_{2}$ and
Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$ for $x_{\mathrm{exp}}=0.45$~and~0.72. Then,
depending on the chemical composition of MFeF$_{5}$(H$_{2}$O)$_{2}$ and
MFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ (M~${=}$~Co$^{2+}$ and/or Fe$^{2+}$),
these materials are decomposed into various mixed Fe/Co or pure
amorphous Fe oxyfluorides. These oxyfluorinated materials, prepared by
appropriate thermal treatment in a muffle furnace, were tested for OER
in alkaline medium. The results show that it is possible to
replace 20\% of the cobalt by iron without affecting the
electrochemical performance. For higher substitution rates
($x_{\mathrm{exp}}$~${\geq}$~0.45), the proportion of
Fe$_{2}$F$_{5}$(H$_{2}$O)$_{2}$ and Fe$_{3}$F$_{8}$(H$_{2}$O)$_{2}$,
giving after thermal decomposition Fe-pure oxyfluorides, is increasing
at the detriment of CoFeF$_{5}$(H$_{2}$O)$_{2}$ and
CoFe$_{2}$F$_{8}$(H$_{2}$O)$_{2}$ leading to Co-containing oxyfluorides
with higher performance. These results seem to indicate that the best
electrochemical performance is obtained for a Co:Fe ratio of 1:1.
In order to avoid the formation of pure Fe phases, the synthesis of
$\mathrm{Co}_{\sfrac{1-x}{2}}\mathrm{Fe}_{\sfrac{1+x}{2}}\mathrm{O}_{\sfrac{1+x}{2}}\mathrm{F}_{\sfrac{3-x}{2}}$ 
oxyfluorides from the alternative hydrate solid solution 
Co$_{1-x}$Fe$_{x}$F$_{2}$(H$_{2}$O)$_4$, which does not induce the
formation of phases mixing during the thermal decomposition, is in
progress. In addition, EXAFS and PDF studies will be performed to
understand the local environment of transition metals in these
oxyfluorides. 

\section{Experimental section\label{mark-4}}

\subsection{Synthesis of fluorinated materials\label{mark-4.1}}

\paragraph*{Chemicals} The heptahydrate fluorides were synthesized using
Fe(NO$_3$)$_3${\textbullet}9H$_2$O (${\geq}$98\%, Alfa Aesar),
CoCl$_2${\textbullet}6H$_2$O (99.5\%, Acros Organics), and
FeCl$_2${\textbullet}4H$_2$O (98\% Alpha Aesar). Hydrofluoric acid solution
(HF 48~wt\%, Honeywell), methanol (MeOH, 99.8\% Honeywell), and
absolute ethanol (99.8\%, Carlo Erba Reagents) were used as received.
\textbf{\textit{Warning!}} \textit{HF is a hazardous chemical, toxic by
inhalation, contact with skin, and if swallowed}. All necessary
precautions were taken during our experiments.

\subsubsection{Synthesis of
$(\mathrm{Co}_{1-x}\mathrm{Fe}_x)^{2+}\mathrm{Fe}^{3+}\mathrm{F}_{5}(\mathrm{H}_{2}\mathrm{O})_{7}$~$(0\leq x_{\mathrm{theo}}\leq 0.75)$}

(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$
(0~${\leq}$~$x_{\mathrm{theo}}$~${\leq}$~0.75) precursors were synthesized by
coprecipitation at room temperature. Fe(NO$_3$)$_3${\textbullet}9H$_2$O and
CoCl$_2${\textbullet}6H$_2$O are dissolved in 10~mL HF solution
(HF$_{48~\mathrm{wt}\%}$) under stirring while
FeCl$_{2}${\textbullet}4H$_2$O is separately dissolved in 1~mL of MeOH.
Ongoing study aims to replace HF by solid sources as NH$_{4}$F or KF
for sustainability and security issues. This separate dissolution is
necessary because Fe$^{2+}$ is easily oxidized to Fe$^{3+}$ in HF or
EtOH solutions. After complete dissolution, 15~mL of EtOH is added to
the FeCl$_{2}${\textbullet}4H$_2$O solution and quickly poured into the HF
solution containing Fe$^{3+}$ and Co$^{2+}$ species, leading to the
precipitation of
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$. Stirring
is maintained for 5~min to ensure a complete reaction. The precipitate
is filtered, washed several times with EtOH, and dried under ambient
conditions. Polycrystalline powders ranging in color from pink to
yellow are obtained with good yields and high purity.

\subsubsection{Synthesis of
$\mathrm{Co}_{\sfrac{1-x}{2}}\mathrm{Fe}_{\sfrac{1+x}{2}}\mathrm{O}_{\sfrac{1+x}{2}}\mathrm{F}_{\sfrac{3-x}{2}}$\break
$(0\leq x_{\mathrm{exp}}\leq 0.72)$}

$\mathrm{Co}_{\sfrac{1-x}{2}}\mathrm{Fe}_{\sfrac{1+x}{2}}\mathrm{O}_{\sfrac{1+x}{2}}\mathrm{F}_{\sfrac{3-x}{2}}$ 
($0\leq x_{\mathrm{exp}} \leq 0.72$) samples were synthesized by calcination
under air of 200~mg of the corresponding hydrated fluoride
(Co$_{1-x}$Fe$_{x}$)$^{2+}$Fe$^{3+}$F$_{5}$(H$_{2}$O)$_{7}$ at various
temperatures for a short period of time in a muffle furnace (Nabertherm
LT 3/12). The temperature profile is represented in Figure~S35 and
the calcination conditions in Table~S13. The sample was
directly placed in the muffle furnace when the targeted temperature was
reached and heated for a suitable time. The sample was finally removed
and cooled to ambient temperature under ambient air.


\subsection{Characterization\label{mark-4.2}}

\paragraph*{X-ray diffraction (XRD)} X-ray diffraction patterns were
collected in the range of $10\text{\textdegree} \leq 2\theta \leq 120$\textdegree{} on
a PANalytical MPD-PRO diffractometer with a Bragg-Brentano geometry,
equipped with a Co K${\upalpha}$ source (k$_{\upalpha1} = 1.78901$~\AA{};
k$_{\upalpha 2}=1.79290$ \AA{}) and a linear X'Celerator detector.
Rietveld refinements were performed by using the Fullprof program.

\paragraph*{High-Temperature X-ray diffraction (HT-XRD)}
High-Temperature X-ray diffraction (HT-XRD) was performed under ambient
air in an Anton Paar XRK 900 high temperature furnace with the
diffractometer described above. The samples were heated from 30 to
600~\textdegree C at a heating rate of 3~\textdegree C${\cdot}$min$^{-1}$.
X-ray diffraction patterns were recorded in the [8--80\textdegree]
2${\theta}$ range with a scan time of 15~min at 20~\textdegree C
intervals from room temperature to 400~\textdegree C and at
50~\textdegree C intervals from 400 to 600~\textdegree C.

\paragraph*{Thermogravimetric analyses (TGA)} Thermogravimetric
analyses were carried out using a thermoanalyzer SETARAM TGA~92 under
ambient air flow at 100mL/min with a heating rate of 2~\textdegree
C${\cdot}$min$^{-1}$ from room temperature up to 600~\textdegree C.

\paragraph*{Scanning Electron Microscopy (SEM)} Scanning Electron
Microscopy images were obtained thanks to a Hitachi microscope (Hitachi
SU3800). The acceleration voltage used is 15~kV. Elemental quantitative
microanalyses were performed using an EDAX Energy Dispersive X-ray
Octane Elect Super EDS detector.

\paragraph*{N$_{2}$ sorption} The specific surface areas were measured
at 77~K using a TriStar II 3020 (Micrometrics). The samples were
degassed under secondary vacuum at 80~\textdegree C for 12~h prior to
measurement. The specific surface areas were calculated using the
Brunauer--Emmett--Teller (BET) method.

\paragraph*{$^{57}$Fe M\"{o}ssbauer spectrometry} M\"{o}ssbauer
measurements were obtained in transmission geometry with a 925~MBq
${\gamma}$-source of $^{57}$Co/Rh on a conventional constant
acceleration drive. The amount of substance for each sample weighed was
calculated to contain 5~mg of Fe per cm$^{2}$. Spectra were fitted using
the MOSFIT program~\cite{35} involving quadrupolar and/or magnetic
components with Lorentzian lines; the isomer shift values are referred
to that of ${\upalpha}$-Fe at room temperature. The velocity of the
source was calibrated using a ${\upalpha}$-Fe foil as a standard at
room temperature.

\paragraph*{Elemental analysis} From an initial solution containing
iron and cobalt (1000~ppm), standard solutions have been prepared to
obtain the concentrations: 0, 0.5, 1, 2, 5, and 10~ppm. The intensity
of the signal as a function of the element concentration, obtained for
each element (Fe, Co) is reported in (Figure~S36). The samples were
prepared by digestion of approximately 10 mg of samples dissolved first
in 5~wt\% HNO$_{3(\mathrm{aq})}$ followed by a dilution with
deionized water by a factor of 100 to be in the right range of\break
measurements. 

\paragraph*{Electrochemical measurements} A Biologic SP200
potentiostat and EC-lab software were used. Ag/AgCl reference (4M KCl
gel) and carbon rod counter electrodes were employed for the
measurements. Custom-built one-compartment glass electrochemical cells
were employed, and 1M KOH was used as the electrolyte. Working
electrodes were constructed from Toray carbon paper. A catalyst ink was
prepared by sonicating typically 10~mg of the catalyst and 0.1~mg of
carbon nanotubes in 1~mL of 2:1 ethanol:water (V:V) with 80~${\upmu}$L
of Nafion. The ink was dropped-cast onto the working electrodes and
allowed to dry under ambient conditions for 20~min prior to use.
Catalyst loadings on the carbon paper were 0.1~mg${\cdot}$cm$^{-2}$ and
1~mg${\cdot}$cm$^{-2}$ for cyclic voltammetry and chronopotentiometry,
respectively. Prior to electrochemical measurements, the impedance
between reference and working electrode was recorded in open circuit,
and the ohmic drop was subsequently corrected for at 95\% with the ZIR
function in the EC-lab software.

\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*{Acknowledgments}

The authors attached to IMMM gratefully acknowledge 
the technical platforms ``X-ray Diffusion and
Diffraction'' and ``Electron Microscopy'' of IMMM (Le Mans University)
as well as the French National Research Agency  (ANR OPIFCAT,
ANR-20-CE08-0026) and the Fondation Banque Populaire d'Entreprise de
l'Ouest for financial support. NK acknowledges NSERC for its
Discovery Grant RGPIN-2019-05927, and the International Emerging Action
IEA. All authors thank FRQNT Samuel de Champlain program, award 293526.

\CDRGrant[ANR]{ANR-20-CE08-0026}
\CDRGrant[NSERC]{RGPIN-2019-05927}
\CDRGrant[FRQNT]{293526}

\back{}

\section*{Supplementary data}

{\vspace*{-2pt}}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-371-suppl.pdf}}

{\vspace*{-2pt}}

\printbibliography
\refinput{crchim20240705-reference.tex}

\end{document}
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