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\DOI{10.5802/crchim.297}
\datereceived{2023-09-29}
\daterevised{2023-12-15}
\datererevised{2024-01-30}
\dateaccepted{2024-02-02}
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%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{GDR Prom\'eth\'ee - French Research Network on \textit{Hydrometallurgical Processes for Primary and Secondary Resources}}
%\def\cdr@specialtitle@french{GDR Prom\'eth\'ee - Groupement de Recherche sur les Proc\'ed\'es Hydrom\'etallurgiques pour la Gestion Int\'egr\'ee des Ressources Primaires ...}
%\makeatother

\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Propylene glycol-based deep eutectic solvent as an alternative
to Ethaline for electrometallurgy}

\alttitle{Solvant eutectique profond \`{a} base de propyl\`{e}ne glycol
comme alternative \`{a} l'Ethaline pour l'\'{e}lectrom\'{e}tallurgie}

\author{\firstname{Calogera} \lastname{Bertoloni}\CDRorcid{0000-0001-7275-4109}}
\address{Institut Jean Lamour, Groupe Chimie et Electrochimie des
Mat\'{e}riaux, UMR CNRS 7198---Universit\'{e} de Lorraine, 1 Boulevard
Arago, BP 95823, 57078 Metz Cedex 3, France}

\author{\firstname{Vitalys} \lastname{Mba Ekomo}}
\address{Laboratoire Syst\`{e}mes Mol\'{e}culaires et nanoMat\'{e}riaux
pour l'\'{E}nergie et la Sant\'{e} (SyMMES), Commissariat \`{a}
l'\'{e}nergie atomique site de Grenoble, France}

\author{\firstname{Beno\^{i}t} \lastname{Villemejeanne}}
\address{Laboratoire des technologies de Valorisation des
proc\'{e}d\'{e}s et Mat\'{e}riaux pour les EnR (LVME), Commissariat
\`{a} l'\'{e}nergie atomique site de Grenoble, France}

\author{\firstname{Charly} \lastname{Lemoine}}
\addressSameAs{3}{Laboratoire des technologies de Valorisation des
proc\'{e}d\'{e}s et Mat\'{e}riaux pour les EnR (LVME), Commissariat
\`{a} l'\'{e}nergie atomique site de Grenoble, France}

\author{\firstname{Romain} \lastname{Duwald}\CDRorcid{0000-0002-9857-1340}}
\addressSameAs{3}{Laboratoire des technologies de Valorisation des
proc\'{e}d\'{e}s et Mat\'{e}riaux pour les EnR (LVME), Commissariat
\`{a} l'\'{e}nergie atomique site de Grenoble, France}

\author{\firstname{Emmanuel} \lastname{Billy}\CDRorcid{0000-0002-0405-1118}}
\addressSameAs{3}{Laboratoire des technologies de Valorisation des
proc\'{e}d\'{e}s et Mat\'{e}riaux pour les EnR (LVME), Commissariat
\`{a} l'\'{e}nergie atomique site de Grenoble, France}

\author{\firstname{Hakima} \lastname{Mendil-Jakani}\CDRorcid{0000-0001-5471-4365}}
\addressSameAs{2}{Laboratoire Syst\`{e}mes Mol\'{e}culaires et
nanoMat\'{e}riaux pour l'\'{E}nergie et la Sant\'{e} (SyMMES),
Commissariat \`{a} l'\'{e}nergie atomique site de Grenoble, France}

\author{\firstname{Sophie} \lastname{~Legeai}\CDRorcid{0000-0002-0901-1799}\IsCorresp}
\addressSameAs{1}{Institut Jean Lamour, Groupe Chimie et Electrochimie
des Mat\'{e}riaux, UMR CNRS 7198---Universit\'{e} de Lorraine, 1
Boulevard Arago, BP 95823, 57078 Metz Cedex 3, France}
\email[S. Legeai]{sophie.legeai@univ-lorraine.fr}

\keywords{\kwd{Electrometallurgy}\kwd{Deep eutectic
solvent}\kwd{Precious metals}\kwd{Propeline}\kwd{Transport properties}}

\altkeywords{\kwd{Electrom\'{e}tallurgie}\kwd{Solvant eutectique
profond}\kwd{M\'{e}taux
pr\'{e}cieux}\kwd{Propeline}\kwd{Propri\'{e}t\'{e}s de transport}}

\begin{abstract}
Deep eutectic solvents (DESs) are of particular interest for
electrometallurgy processes, since intrinsically conductive and
electrochemically stable in a wide range of potentials. Cheaper and
greener than conventional ionic liquids, DESs are often bio-sourced and
exhibit a higher biodegradability. Ethaline, a DES composed of choline
chloride (ChCl) as hydrogen bond acceptor and ethylene glycol (EG) as
hydrogen bond donor in 1:2 molar proportions (Et 1:2), is commonly used
in electrometallurgy thanks to its good transport properties. However,
if ChCl can be considered as a ``green'' reactant, this is not the case
for EG. A DES with a lower toxicity can be obtained by replacing EG by
propylene glycol (PG), widely used in cosmetics and pharmacology,
yielding a DES called Propeline. The present paper explores the
potential of this lesser-known DES in the electrometallurgy of precious
metals. Because changing the hydrogen bond donor leads to a
modification in the DES bulk properties, the first part of this work
deals with the determination of PG-based DESs' density, viscosity, and
conductivity, which are properties of interest for electrochemical
processes. The influence of water and PG content is presented and
values are compared to those of Ethaline. It appears that ChCl:PG in a
molar ratio 1:3 (Pr 1:3) presents the best transport properties. The
potentiality of this solvent for the electrometallurgy of precious
metals is then discussed: electrochemical stability and electrochemical
systems of Ag, Pd, and Au are compared in Pr 1:3 and Et 1:2. Finally,
diffusion coefficients of the metallic species and the DES components
are given, determined by electrochemical and NMR techniques,
respectively.
\end{abstract}

\begin{altabstract}
Les solvants eutectiques profonds (DES) pr\'{e}sentent un
int\'{e}r\^{e}t particulier dans les proc\'{e}d\'{e}s
\'{e}lectrom\'{e}tallurgiques de par leur conductivit\'{e} et leur
stabilit\'{e} \'{e}lectrochimique dans une large gamme de potentiel.
Ils sont moins chers et plus \'{e}cologiques que les liquides ioniques
conventionnels, et souvent biosourc\'{e}s et biod\'{e}gradables.
L'Ethaline, un DES compos\'{e} de chlorure de choline (ChCl) et
d'\'{e}thyl\`{e}ne glycol (EG) en proportions molaires 1:2 (Et 1:2),
est couramment utilis\'{e}e en \'{e}lectrom\'{e}tallurgie pour ses
propri\'{e}t\'{e}s de transport. Cependant, si le ChCl peut \^{e}tre
consid\'{e}r\'{e} comme un r\'{e}actif \og{} vert \fg, ce n'est pas le
cas de l'EG. Il est possible de formuler un nouveau DES appel\'{e}
Propeline, moins toxique, en rempla\c{c}ant l'EG par du propyl\`{e}ne
glycol (PG), ce dernier \'{e}tant largement utilis\'{e} en
cosm\'{e}tique et en pharmacologie. L'article pr\'{e}sent\'{e}
s'emploie \`{a} \'{e}tudier la potentielle utilisation de ce DES moins
connu dans le domaine de l'\'{e}lectrom\'{e}tallurgie des m\'{e}taux
pr\'{e}cieux. \'{E}tant donn\'{e} que le changement de donneur de
liaison hydrog\`{e}ne entra\^{i}ne une modification des
propri\'{e}t\'{e}s globales des DES, la premi\`{e}re partie de ce
travail porte sur la d\'{e}termination de la densit\'{e}, de la
viscosit\'{e} et de la conductivit\'{e} des DES \`{a} base de PG,
propri\'{e}t\'{e}s importantes dans les processus \'{e}lectrochimiques.
L'influence de la teneur en eau et en PG est \'{e}tudi\'{e}e et les
valeurs sont compar\'{e}es \`{a} celles obtenues pour l'Ethaline. Cette
\'{e}tude montre que le m\'{e}lange ChCl:PG 1:3 (Pr 1:3) pr\'{e}sente
les propri\'{e}t\'{e}s de transport les plus favorables. Les DES Pr 1:3
et Et 1:2 sont ensuite compar\'{e}s sur diff\'{e}rents crit\`{e}res
li\'{e}s \`{a} l'application \'{e}lectrom\'{e}tallurgique : leur
stabilit\'{e} \'{e}lectrochimique, ainsi que le comportement des
syst\`{e}mes \'{e}lectrochimiques de Ag, Pd et Au. Enfin, les
coefficients de diffusion des esp\`{e}ces m\'{e}talliques et des
composants des DES sont respectivement d\'{e}termin\'{e}s par des
techniques \'{e}lectrochimiques et par RMN.
\end{altabstract}

\thanks{Agence Nationale de la Recherche (EE4Precious project, 
ANR-20-CE08-0035-01)}

\shortrunauthors

\maketitle

\pagebreak

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

The discovery of ionic liquids in 1914 by Paul Walden~\cite{1} with
beneficial properties (vapor pressure, flammability, modulation,
viscosity, density, refractive index, electrical conductivity, and
thermal stability) led to applications such as electrochemistry,
extraction, synthesis, analytical chemistry discussed by Kaur
et~al.~\cite{2}. These ionic liquids have also been used as additives
in cosmetic products or as corrosion inhibitors~\cite{2,3}. An ongoing
need to improve existing properties has led to the emergence of new
so-called deep eutectic solvents (DES) based on the mixture of a proton
acceptor (HBA) and a proton donor (HBD), first presented by Abbott
et~al.~\cite{4}. The first DES studied was a mixture of choline chloride
as HBA and urea as HBD, and the resulting solvent was named Reline.
Reline has properties similar to ionic liquids, which intended it for
the same applications, but with a better control over the properties
mentioned above and a reduced cost due to easier synthesis and
composition modulation. As for ionic liquids, the conductivity of DESs
and their complexing properties intend them for applications in
electrochemical processes of metals. In the field of metal recycling,
electrometallurgy in DES can be an alternative to toxic aqueous
electrolytes (cyanides) for the recovery of precious metals for
example, by consecutive electrochemical leaching and electrodeposition.
The most popular DES for electrometallurgy applications is Ethaline
which is based on the eutectic mixture of choline chloride (ChCl) and
ethylene glycol (EG), i.e., in the 1:2 molar proportion~\cite{5,6,7,8}.
Ethaline has more suitable properties for electrometallurgy (for
example, low viscosity, better conductivity) than other DESs, even
Reline. Most of the studies on DESs have thus been devoted to Ethaline.
To date, a more ecological approach and a need to associate it with a
reduction in costs guide research towards DESs that are more respectful
of the environment and less expensive. Although the toxicity of
eutectic mixtures is weakly investigated, databases are full of
information on the toxicity of HBD. Several studies were published that
call into question the harmlessness of some DESs considered as natural,
thus showing that natural is not synonymous with green for
environment~\cite{9}. The conclusion of such studies ultimately shows a
certain unexpected toxicity of some DESs~\cite{10}. These studies also
highlight the in-vivo toxicity of DESs compared to the individual
elements (HBA and HBD) and the influence of their molar
ratio~\cite{10,11}.  In summary, studies have shown that DESs based on
choline chloride as HBA have rather low toxicity~\cite{12,13} but that
their toxicity increases with the toxicity of the HBD and thus with its
molar ratio~\cite{10,11,14}. It is then reasonable to expect that the
less toxic a HBD is, the less toxic the resulting DES will be.

In this work, to look for a DES with a lower toxic profile than
Ethaline, ethylene glycol was switched by propylene glycol as HBD. The
choice fell on propylene glycol because it shows the advantage of being
a lot less toxic than ethylene glycol, known to be lethal for humans as
well as terrestrial and aquatic animals by drinking water
contamination. The structure of propylene glycol derives from ethylene
glycol based on substitution of a proton by a methyl functional group.
In addition, the use of propylene glycol instead of ethylene glycol is
still economically viable, as the price of both solvents is similar 
(78~{\texteuro}/L, December
2023)\footnote{\url{https://www.sigmaaldrich.com/FR/fr/product/sigald/102466}.}
to propylene glycol price (50 {\texteuro}/L, December 
2023)\footnote{\url{https://www.sigmaaldrich.com/FR/fr/substance/760957556}.}
according to Sigma Aldrich for a same \textit{reagent grade}
(${>}$99\%). Aware that the change of HBD leads to a modification of
DES properties, and that these properties condition the applications of
DESs, the first part of this work will deal with the investigation of
relevant transport properties such as density, viscosity, and
conductivity depending on the HBD choice, either ethylene glycol or
propylene glycol. In addition, the possibility of using the prepared
PG-based DESs for electrometallurgy applications will be presented by
comparing their electrochemical properties to those of Ethaline and
studying the electrochemical behavior of precious metals (Pd, Au, Ag)
in these two DESs.

\section{Experimental section}\label{sec2}
\subsection{Chemicals and materials}\label{ssec21}

The choline chloride (ChCl, 98\%), ethylene glycol (EG, 99\%),
propylene glycol (PG, 99\%) used for DES preparation, as well as
ferrocene (Fe[C\tsub{5}H\tsub{5}]\tsub{2}${\cdot}$6H\tsub{2}O, 98\%),
gold chloride  (AuCl\tsub{3}, ${>}$99.99\%) and silver chloride (AgCl,
${>}$99\%) were purchased from Sigma-Aldrich, France. Ethanol (EtOH,
96\%) and palladium chloride (PdCl\tsub{2}, 59\%) were purchased from
Merck France. Gold wire (99.99\%), silver wire (99.99\%) and palladium
wire (99.95\%) were purchased at M\'{e}talor Technologies France.
{\vspace*{-6pt}}

\subsection{DES preparation}\label{ssec22}

ChCl:EG (Et) and ChCl:PG (Pr) DESs were prepared respectively by mixing
ChCl with EG, and ChCl with PG at 60~\textdegree C in a closed
container under stirring. DESs were prepared at different molar ratios
written down: HBA:HBD. When the influence of water content is studied,
a third molar ratio is added in the name of the sample:
HBA:HBD:H\tsub{2}O. This ratio is expressed relative to HBA set at~1.
{\vspace*{-6pt}}

\subsection{Physico-chemical properties measurements}\label{ssec23}

All experiments were performed without any atmosphere control. The
water content was measured by Karl Fischer coulometric titration using
a Metrohm Karl Fischer 899 Coulometer with a HYDRANAL Coulomat E
solution. Density was measured in a temperature range from 25 to
65~\textdegree C with 5~\textdegree C intervals, using an Anton Paar
DMA 4100~M density meter, with a measurement accuracy of 
10$^{-4}$~g${\cdot}$cm$^{-3}$. Viscosity was determined using an Anton Paar AMVn
viscometer with a repeatability of 0.1\%. Conductivity was evaluated by
potentiostatic electrochemical impedance spectroscopy (PEIS) using a
Biologic Lab Potentiostat VSP 300. The conductivity cell was
constituted of two parallel platinum plates, and the cell constant was
determined using calibration samples (1.413; 5.0 and 12.8
mS${\cdot}$cm$^{-1}$ at 25~\textdegree C). The sine wave amplitude was
10~mV, the studied frequency range extends from 1~MHz to 100 mHz with 6
measuring points per decade. The viscosity and the conductivity were
measured in a temperature range of 25--60~\textdegree C  with a step of
5~\textdegree C.{\vspace*{-6pt}}

\subsection{Electrochemical characterizations}\label{ssec24}

All experiments were performed at 40~\textdegree C without any
atmosphere control, using a Biologic Lab Potentiostat VSP 300. Au, Pd,
and Ag were dissolved by anodic leaching of metallic wires, applying 
1~mA${\cdot}$cm$^{-2}$ in galvanostatic mode at 313~K in Et 1:2 and 333~K
in Pr 1:3. The coulombic charge was adapted to the targeted
concentration. Final concentrations were determined by the weight loss
of the anode. Counter electrode (CE) was separated from the bulk
electrolyte by a fritted glass to avoid electrodeposition of leached
metals on the CE. Resulting electrochemical systems were studied by
cyclic voltammetry. Electrochemical stability and electrochemical
systems of metallic species were studied by cyclic voltammetry (CV) at
a scan rate of 20~mV${\cdot}$s$^{-1}$. For CV experiments, the working
electrode (WE) was a Pt disk of 2~mm in diameter. The CE and the
reference electrode (RE) used in all electrochemical experiments were
respectively a glassy carbon rod and an Ag$^{\mathrm{I}}$/Ag electrode (Ag
wire immersed in the studied DES containing 10~mM AgCl). The RE was
calibrated against Fc\tsup{+}/Fc ($E^{0}$ (Fc\tsup{+}/Fc) $=$\ 0.63
V/SHE). All potential values measured in this study are therefore
relative to SHE (standard hydrogen electrode).{\vspace*{-2pt}}

\subsection{NMR measurements}\label{ssec25}

All NMR spectra were recorded with a Bruker Neo 400~MHz NMR
spectrometer, equipped with a BBFO smartprobe. DOSY experiments were
performed with Bipolar Pulsed Pair Stimulated Echo (BPSTE) pulse
sequence. Each DOSY-NMR was based on 16 BPSTE-recorded spectra with 8~K
data points. The diffusion time was 750~ms. The duration of the pulse
field gradient was adjusted in a 600--2000~ms range in order to obtain
2--5\% residual signal with the maximum gradient strength. The gradient
strength was incremented in 16 steps from 5 to 95\% of its maximum
value in a linear ramp. Data processing was performed using Topspin
4.0.9 software.

\section{Results and discussion}\label{sec3}

\subsection{Density}\label{ssec31}

Density ($\rho$) and its temperature dependence are important physical
properties, which are useful for designing industrial processes.
Densities of ChCl:EG and ChCl:PG DESs with HBA:HBD molar ratio 1:2 and
1:3 were measured in a 293--333~K temperature range at low and similar
water content. The results are reported in Figure~\ref{fig1}. The
density of both DESs decreases with increasing temperature. This
\mbox{phenomenon} is due to thermal expansion associated with hydrogen
bonding, and follows a linear temperature dependence.

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Density comparison of ChCl:EG and ChCl:PG DESs at
different HBA:HBD ratios as a function of temperature.}
\end{figure}

Values were not measured under 318~K for Pr 1:2 (Propeline) because it
is solid at lower temperatures. Moreover, there is a decrease in DES
density with increasing HBD molar ratios, with much more impact on
ChCl:EG than on ChCl:PG DESs given the difference observed. In the
cases where only density matters, this would mean that the properties
of ChCl:PG DESs would be more easily modulated than the properties of
ChCl:EG DESs. The density of ChCl:PG DESs is lower than the density of
ChCl:EG DESs whatever the HBA:HBD ratio. The values measured in this
study are in accordance with existing literature~\cite{15,16}. Knowing
that the density of ethylene glycol is higher than that of propylene
glycol, it can be concluded that the density of the resulting DES is
directly related to the density of the HBD. Wang et~al.\ had already
made the same observation by comparing the density of a larger number
of DESs and concluded that the molar mass as well as the density of the
HBD had a very large impact on the density of the resulting
DES~\cite{17}. The density partially reflects the interaction strength
between HBA and HBD components, which is ascribed to the strong
hydrogen bond ability. This notable difference of density might be
attributed to a different molecular organization or packing of the DES,
changing the size of the HBA/HBD association spheres and leading to
more free volume in ChCl:PG than in ChCl:EG DESs~\cite{18}. The extra
methyl group of propylene glycol could be responsible for this by
steric hindrance.

\subsection{Viscosity}\label{ssec32}

Viscosity is a very important information to relate the macroscopic
properties linked to matter transport and hydrodynamic flow to the
microscopic phenomena from which these macroscopic properties most
often find their explanation.

The impact of temperature (Figure~\ref{fig2}a) on ChCl:EG and ChCl:PG
DESs' viscosity has been measured for 1:2 and 1:3 ratio. Increasing the
HBD fraction clearly makes viscosity decrease for both DESs. Besides,
viscosity of Pr 1:3 is higher than that of Et 1:3. This seems logical
as propylene glycol alone is more viscous than ethylene glycol (56 and
21 mPa${\cdot}$s at 293~K, respectively). Several studies in the literature
have already corroborated this observation: the decrease can be related
to the formation or strengthening of hydrogen bonds, resulting in loss
of molecular mobility as it closes free volume, on which the viscosity
depends~\cite{15,16,19}. As for all liquids, the viscosity of ChCl:EG
and ChCl:PG DESs decreases when temperature increases, due to thermal
dilatation. The higher viscosity of ChCl:PG DESs suggests that they
would be less interesting than ChCl:EG DESs, however it has to be
noticed that both viscosities tend to get closer as temperature
increases. Giving the fact that Pr 1:2 (Propeline) is solid under
318~K, and that it is very viscous compared to Et 1:2 (Ethaline) and Pr
1:3, further characterizations reported in this work will only concern
Pr 1:3 in comparison to\break Et~1:2.

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig02}
{\vspace*{-4pt}}
\caption{\label{fig2}Viscosity of the DESs depending on (a) temperature
and (b) water content at 313~K.} 
{\vspace*{-3pt}}
\end{figure*}

The influence of water content was studied by adding variable amounts
of water from 600 to 47,000~ppm, which corresponds to
ChCl:PG:H\tsub{2}O molar ratios from 1:3:0.01 to 1:3:0.818. Viscosity
measurements of Et 1:2 at similar water contents were added for
comparison purpose (Figure~\ref{fig2}b).

The increase in water content leads to a decrease in viscosity of both
Et 1:2 and Pr 1:3. Water content therefore seems to be another
interesting way to modulate the transport properties of Pr
1:3~\cite{20}.  Tuning temperature and water content of Pr 1:3 could
then allow this DES to remain in the range of viscosity considered for
ionometallurgical recycling applications, however keeping in mind that
a high water content could destabilize the chloro-metallic \mbox{complexes}
or even lead to the formation of insoluble metallic species.

\subsection{Conductivity}\label{ssec33}

Figures~\ref{fig3}a and~\ref{fig3}b show the conductivity values of
Et 1:2 and Pr 1:3 normalized by the ChCl molar ratio, as a function of
temperature and water content, respectively. Ch\tsup{+} and Cl$^{-}$
being the conductive species of the DESs, this representation allows to
free oneself from the difference in ChCl proportion between the two
considered DESs.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Conductivity of the DESs depending on (a)
temperature and (b) water molar ratio at 298~K.}
\end{figure*}

The conductivity of both DESs increases with temperature
(Figure~\ref{fig3}a). This is linked to the decrease in viscosity
allowing faster ion diffusion. Indeed, the Walden plot ($\log(\Lambda)$
vs $\log(1/\eta)$) based on the Walden equation
($\Lambda\cdot\eta^{\alpha}=C$, $\Lambda$ being the molar conductivity,
$\eta$ the viscosity and $\alpha$ and $C$ constant parameters)
presented in SI as Figure~SI1 clearly shows that a linear fit can be
obtained, meaning that only the viscosity impacts the molar
conductivity. 

The high viscosity of Pr 1:3 compared to Et 1:2 reflects a slower flow
rate related to van der Waals and hydrogen interactions in the network,
more important in Pr 1:3 than in Et 1:2. As well as the higher
viscosity, hydrogen interactions in Pr 1:3 also help to explain the
observed lower conductivity compared to Ethaline. Besides, Abbott
et~al.~\cite{21,22} put forward the theory of holes, which explains the
viscosity of DESs. In DESs, not only molecular interactions tend to
oppose the transport of ionic species, but also the holes generated by
these interactions which leave inter- or intramolecular spaces. The
size of these holes depends on hydrogen bond strength, temperature, and
water content. Smaller holes will increase the viscosity, while larger
holes will decrease it, and so hinder or enhance ion mobility,
respectively.

Figure~\ref{fig3}b shows that the conductivity of Ethaline remains
higher than that of Pr 1:3 regardless of the water content. The review
article published by Bryant et~al.~\cite{23} on the interfacial
nanostructure and the properties of ChCl:glycerol DES shows that water
can be considered as a co-HBD for molar content higher than 5\%, as a
structural modification of DES was observed above this value. Below
25\%, only the alignment of the liquid structure was modified. Alfurayj
et~al.~\cite{20} demonstrated that modest amounts of water addition
(1--10\%) to Ethaline are of little concern for practical use and can
even lead to performance improvements, such as accelerated solvation
and higher conductivity.

Knowing this behavior, Pr 1:3 therefore remains an interesting
electrolyte as an alternative to Et 1:2 with reduced cost and
environmental impact. Its similar physico-chemical properties can be
easily modulated by temperature and water addition.

\subsection{Electrochemical stability}\label{ssec34}

Electrochemical stability, commonly called electrochemical window (EW),
is one of the parameters of interest sought in DESs and ionic liquids,
because they often show at least similar or even higher stability
compared to common aqueous {electrolytes~\cite{24,25,26,27}.} For
electrometallurgy applications, this characteristic is of primary
importance, especially to avoid DES degradation, which is more critical
than water electrolysis using aqueous electrolytes, for cost and
environmental reasons. The EW of Pr 1:3 was determined at a temperature
of 313~K and compared to the EW of Et 1:2, widely used by the Abbott
et~al. group for electrometallurgy applications~\cite{28,29}. 
Figure~\ref{fig4} shows that the EW of Pr 1:3 ranges from ${-}$0.8 to 1.1
V vs SHE, very similar to the one of Et 1:2, as already observed by
Costa et~al. in the case of Pr 1:2~\cite{30}. The oxidation signal
recorded above 1.1~V vs SHE is usually attributed to EG/PG or chloride
oxidation, whereas the reduction signal around ${-}$0.8~V vs SHE is
associated to cholinium cation reduction~\cite{31}.\looseness=-1

\begin{figure}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}Electrochemical window of Et 1:2 and Pr 1:3
recorded by linear sweep voltammetry on a Pt working electrode at
20~mV${\cdot}$s$^{-1}$ and 313~K.}
\end{figure}

The low difference in stability between the two DESs suggests that Pr
1:3 could replace Ethaline in electrometallurgical recycling processes.
In addition, the potential ranges determined are comparable to those
defined in the literature and frame the redox potentials of numerous
electroactive metals of interest (gold, silver, palladium,
etc.)~\cite{8,32}.  The electrochemical stability of DESs can be
somewhat limited by the amount of water. Valverde et~al.~\cite{33}
showed that the electrochemical window of Ethaline can vary by around
0.2~V with varying water amount from 1~to~15\%.

\subsection{Electrochemical characterizations of\hfill\break precious
metals}\label{ssec35}

In electrometallurgy, the oxidation and reduction potentials of
metallic species are an indicator of their ability to be reduced (i.e.,
electrodeposited) or oxidized (i.e., electrochemically leached) in a
specific electrolyte. Corresponding potential values must be included
in the EW of the electrolyte. For that, Au, Pd, and Ag were dissolved
by anodic leaching of metallic wires in Et 1:2 and Pr 1:3, and
resulting electrochemical systems were studied by cyclic voltammetry.
The superposition of the voltammograms in Figures~\ref{fig5}a (Pr
1:3) and~\ref{fig5}b (Et 1:2) allowed to compare the redox
potentials of the chloro-metallic complexes formed in the two DESs, but
also to define the possibility to leach, deposit, and so
electrochemically recycle corresponding metals in both DESs.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig05}
{\vspace*{-2pt}}
\caption{\label{fig5}Cyclic voltammograms of Au(I), Pd(II) and Ag(I)
monometallic leachates in (a) Pr 1:3 ([Au(I)] $=$\ 50 mM, [Ag(I)] $=$\ 
16 mM, [Pd(II)] $=$\ 50 mM) and (b) Et 1:2 (([Au(I)] $=$\ 25 mM,
[Ag(I)] $=$\ 25 mM, [Pd(II)] $=$\ 25 mM) recorded on Pt at 313~K at a
scan rate of 20~mV${\cdot}$s$^{-1}$.}
{\vspace*{-2pt}}
\end{figure*}

Obviously, Pd(II)/)Pd(0) is a non-reversible system with a slow charge
transfer kinetics, as reduction and oxidation signals are separated by
almost 0.9~V. As can be observed on the figures SI2 and SI3, the peak
potential of Ag(I) and Au(I) reduction varies with the scan rate,
meaning that these systems are not fully reversible. Here the redox
system attribution is based on our previous work (Villemejeanne 2022)
and literature data~\cite{34,35} in Et 1:2.

Table~\ref{tab1} and Figure~\ref{fig6} summarize the formal redox
potential $E_{\mathrm{app}}$ {(V vs SHE)} determined from cyclic
voltammograms according to Abbott et~al.~\cite{36,37} using the average
of onset potentials of the oxidation and reduction waves for each redox
couple. The values obtained are compared to those published by Abbott
et~al.\ in 2011 using Et 1:2~\cite{36,37}.

\begin{figure}
{\vspace*{-4pt}}
\includegraphics{fig06}
{\vspace*{-4pt}}
\caption{\label{fig6}$E_{\mathrm{app}}$ of Au(I)/Au(0), Pd(II)/Pd(0)
and Ag(I)/Ag(0) in Pr 1:3 vs $E_{\mathrm{app}}$ of Au(I)/Au(0),
Pd(II)/Pd(0) and Ag(I)/Ag(0) in Et 1:2. Black labels: this work, green
labels: Et1:2 values extracted from Abbott et~al.}
{\vspace*{-4pt}}
\end{figure}

%tab1
\begin{table}
\caption{\label{tab1}Formal redox potentials of Au(I)/Au(0),
Pd(II)/Pd(0) and Ag(I)/Ag(0) in Et 1:2 and Pr 1:3{\vspace*{-2pt}}}
\begin{tabular}{cccc}
\thead
\xmorerows{1}{Peaks} & \xmorerows{1}{Redox couple} & 
\multicolumn{2}{c}{\parbox[t]{7.5pc}{\centering $E_{\mathrm{app}}$ (V
vs SHE)\break \textit{In italics: Abbott 2011}}}\vspace*{2pt} \\\cline{3-4}
& & Pr 1:3 & Et 1:2 \\
\endthead
A/B & Au(I)/Au(0)  & 0.752 & \parbox[t]{2pc}{\centering 0.715 \textit{0.917}}\vspace*{2pt} \\
C/D & Pd(II)/Pd(0) & 0.130 & \parbox[t]{2pc}{\centering 0.211 \textit{0.367}}\vspace*{2pt} \\
E/F & Ag(I)/Ag(0)  & 0.132 & \parbox[t]{2pc}{\centering 0.117 \textit{0.283}}\vspace*{2pt}
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table}

It appears that all the redox potentials are included in the EW of Pr
1:3 determined in 3.4. The formal redox potential values are offset by
approximatively 150--210~mV compared to Abbott et~al.\ values, despite
Abbott values being corrected to SHE reference electrode. Nevertheless,
the oxidation and reduction peaks of metals determined in this work,
are very close for Et 1:2 and Pr 1:3. The difference for a same metal
does not exceed 100~mV. The distinct potentials for the reduction or
oxidation of considered metallic species show a possibility for some to
be leached and/or recovered. Depending on the targeted application,
values extracted from the voltammograms are useful to determine the
range of potential to apply. Indeed, to leach a metal, the applied
potential has to be on the ascending part of the oxidation peak or
superior to it, while for electrodeposition, the applied potential has
to be on the descending part of the reduction peak or inferior to it.
\mbox{However,} in case of a mix of metallic species, it would be possible to
leach or deposit some but not all. \mbox{Considering} increasing oxidation
potentials, Ag would be leached first, then Pd, and finally Au. In
opposition, considering decreasing reduction potentials, the deposition
would happen first with Au, then Ag, and finally Pd. It would not be
possible to leach and recover Pd alone for example.

The CVs presented above also allow to calculate the diffusion
coefficient of each metallic species leached in the DESs. The diffusion
coefficient is a key parameter in electrometallurgy, contributing to
the process efficiency. The deposition diffusion coefficients were
calculated using the Berzins--Delahay Equation (Equation~(\ref{eq1}))
which takes into account the changes at the surface of the working
electrode due to the metal deposition process.
{\begin{equation}\label{eq1}
I_{\mathrm{BD}} = 0.6105 AC(nF)^{3/2}\sqrt{D\nu/RT}.
\end{equation}}\unskip

$I_{\mathrm{BD}}$ is the current intensity of the cathodic peak, $A$ is the
electrode area, $C$ the concentration of the metallic ion, $n$ the
number of electrons transferred in the reduction reaction, $F$ the
Faraday constant, $D$ the diffusion coefficient of the metallic ion,
$\nu$ the scan rate, $R$ the gas constant and $T$ the temperature. In
order to verify that the diffusion of species is the driving force at
this scan rate value, scan rate $\nu$  was varied between 5 and
100~mV/s in the case of the Ag electrochemical system. Figure~SI4 shows
that the Ag(I) reduction peak current $i_{\mathrm{peak}}$ is
proportional to $\nu^{1/2}$ in the whole scan rate range tested for Pr
1:3 and Et 1:2. The diffusion laws can then be applied to calculate the
diffusion coefficient from CV runs at 20~mV/s for all metallic species.
The diffusion coefficients calculated at 313~K, for a concentration of
50~mM of salts in Pr 1:3 and 25 mM in Et 1:2, are displayed in
Table~\ref{tab2}.

%tab2
\begin{table}
\caption{\label{tab2}Diffusion coefficients of metallic species in Et
1:2 and Pr 1:3}
\tabcolsep=2.5pt
\begin{tabular}{cccc}
\thead
& Ag(I) & Pd(II) & Au(I) \\
\endthead
$D_{\mathrm{Pr\,1:3}}$ (m$^{2}{\cdot}$s$^{-1}$) & 2.5 $\times$ 10$^{-11}$ & 6.3 $\times$ 10$^{-12}$ & 5.2 $\times$ 10$^{-11}$ \\
$D_{\mathrm{Et\,1:2}}$ (m$^{2}{\cdot}$s$^{-1}$) & 1.9 $\times$ 10$^{-10}$ & 1.2 $\times$ 10$^{-11}$ & 3.9 $\times$ 10$^{-11}$
\botline
\end{tabular}
\end{table}

The diffusion coefficients of the considered metallic species in Pr 1:3
are almost the same or at mostone order of magnitude lower than in
Ethaline. Au(I) shows the fastest diffusion and Pd(II) the lowest for
both DESs. These values are comparable to those previously published in
Et 1:2 (Ag \cite{38,39}, Au \cite{39,40}, Pd \cite{36,37,41}) and are
higher than those determined in Reline~\cite{41,42,43}, a DES often
used in electrometallurgy.

\subsection{NMR characterization of Pr 1:3/metal salts
solutions}\label{ssec36}
\subsubsection{Identification of DES solution structure}\label{sssec361}

The solution structure of Pr 1:3 was investigated by \tsup{1}H NMR, and
compared to the solution structure of Et 1:2 as a reference. DESs were
characterized as pure solvents, without the use of a deuterated
diluent. Regarding the Pr 1:3 spectrum, the \tsup{1}H NMR signals
corresponding to ChCl are observed at 5.02, 3.68, 3.31, and 2.98 ppm.
The signals corresponding to PG are seen at 4.57, 3.45, 3.09, and 0.78
ppm (Figure~\ref{fig7}, top). The NMR spectra of Et 1:2 exhibit signals
at 5.24, 3.99, 3.61, and 3.30 ppm for ChCl, and 4.78 and 3.58 ppm for
EG. In both DESs, the most unshielded signals correspond to labile
protons $a$ of ChCl and $e$ and $h$ protons of PG or $f$ protons of EG
for Pr 1:3 or Et 1:2, respectively.

\begin{figure*}
\includegraphics{fig07}
{\vspace*{-1pt}}
\caption{\label{fig7}\tsup{1}H NMR spectra of Pr 1:3 (top) and Et 1:2
(bottom).}
\end{figure*}

The influence of metallic species was studied by addition of metallic
salts (AgCl, PdCl\tsub{2}, AuCl\tsub{3}). Interestingly, upon addition
of previously listed metals in Pr 1:3 (Figure~\ref{fig8}), it appears
that interactions between DESs and metallic species are not
systematically the same. In fact, no differences can be seen between Pr
1:3 with and without PdCl\tsub{2} or AgCl. Nevertheless, in presence of
AuCl\tsub{3}, it appears that all labile protons from OH groups are
fused (Figure~\ref{fig8}, green). The fusion of those signals is
explained by a fast exchange of the protons from the hydroxyl groups.

\begin{figure*}
{\vspace*{-.5pt}}
\includegraphics{fig08}
\caption{\label{fig8}\tsup{1}H NMR spectra of Pr 1:3 (blue), and Pr1:3
$+$ PdCl\tsub{2} (red), Pr~1:3 $+$ AuCl\tsub{3} (green) and Pr~1:3 $+$
AgCl (purple).}
{\vspace*{-1pt}}
\end{figure*}

As well as for Pr 1:3, there is no observable effect of PdCl\tsub{2} in
the solution structure of Et 1:2 in \tsup{1}H NMR (Figure~\ref{fig9}).
Nevertheless, a broadening of OH signals corresponding to a fast
exchange is observed in the presence of AuCl\tsub{3} and AgCl salts.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig09}
\caption{\label{fig9}\tsup{1}H NMR spectra of Et 1:2 (blue), and Et 1:2
$+$ PdCl\tsub{2} (red), Et 1:2 $+$ AuCl\tsub{3} (green) and Et 1:2 $+$
AgCl (purple).}
\end{figure*}

\subsubsection{PFG-NMR analyses}\label{sssec362}

Diffusion experiments carried by NMR allows an in-depth comprehension
of solvent/metals interactions. Diffusion-ordered spectroscopy (DOSY)
with a decreasing magnetic field gradient from 95 to 5\% were run at
313~K. The diffusion coefficient $D$ of each species in solution is
determined according to Debye--Einstein Equation~(\ref{eq2}), where the
molecule is approximated as a spherical particle of hydrodynamic radius
$r_{\mathrm{H}}$, $\eta$ is the viscosity of the solvent (mPa${\cdot}$s), $k_{\mathrm{B}}$
the Boltzmann constant, and  $T$ the temperature~(K).
{\begin{equation}\label{eq2}
D = \dfrac{k_{\mathrm{B}}T}{6\uppi \eta r_{\mathrm{H}}}.
\end{equation}}\unskip

The diffusion coefficients for choline chloride HBA and ethylene glycol
or propylene glycol HBD are listed in Tables~\ref{tab3} and
\ref{tab4} for Et 1:2 and Pr 1:3, respectively. The diffusivity
of aliphatic ($D_{\mathrm{Ch}+}$) and hydroxyl 
$(D_{\mathrm{OH}\ndash\mathrm{Ch}+})$ protons in the choline chloride moiety
of Et 1:2 are slightly different (Table~\ref{tab3}, entry~1). This is
due to a fast exchange between hydroxyl protons of ChCl and water.
Furthermore, the diffusivity of choline cations is lower than the
diffusivity of HBD. This is due to the higher molecular weight and
molecular size of the cation compared to those of the corresponding
HDB. The diffusivity of aliphatic protons of ChCl ($D_{\mathrm{Ch}+}$)
and ethylene glycol ($D_{\mathrm{EG}}$) in Et 1:2 are slightly
decreased after dissolution of Pd(II), Au(III) or Ag(I) metal ions
(Table~\ref{tab3}, entries 2--4). Despite the increase in water
fraction in the DES after metal salts dilution, which must be
responsible for an increase in diffusivity, the decreasing diffusivity
of both HBA and HBD species tends to indicate a strong interaction
between the metal ions and the DES. These observations are in agreement
with the study of Abbott et~al.~\cite{44}. The diffusivity difference
between aliphatic ($D_{\mathrm{EG}}$) and acidic protons
($D_{\mathrm{OH{\ndash}EG}}$) of ethylene glycol indicates that the acidic
protons are in fast exchange with some other species. This observation
is reinforced by the disappearance of the OH--Ch\tsup{+} protons in the
case of Au and Ag salts (Table~\ref{tab3}, entry 3--4). The high water
content of these solutions explains the rapid exchange between the
labile protons of all the components of the\break solutions.

%tab3
\begin{table*}
\caption{\label{tab3}Diffusion coefficients for Et 1:2 solutions{\vspace*{2pt}}}
\begin{tabular}{ccccccc}
\thead
& Ethaline & 
\parbox[t]{6pc}{\centering Composition HBA/HBD/H\tsub{2}O} &
\parbox[t]{4.5pc}{\centering 10$^{-11}$\ $D_{\mathrm{Ch+}}$ (m\tsup{2}${\cdot}$s$^{-1}$)} &
\parbox[t]{6pc}{\centering 10$^{-11}$\ $D_{\mathrm{OH\ndash Ch+}}$ (m\tsup{2}${\cdot}$s$^{-1}$)} &
\parbox[t]{4.5pc}{\centering 10$^{-11}$\ $D_{\mathrm{EG}}$ (m\tsup{2}${\cdot}$s$^{-1}$)} &
\parbox[t]{5pc}{\centering 10$^{-11}$\ $D_{\mathrm{OH\ndash EG}}$ (m\tsup{2}${\cdot}$s$^{-1}$)}\vspace*{2pt} \\
\endthead
1 & Et 1:2        & 1/2/0.09 & 4.18 &  7.69 & 7.55 & 7.92 \\
2 & Et 1:2 $+$ Pd & 1/2/0.18 & 3.17 &  5.65 & 5.39 & 6.78 \\
3 & Et 1:2 $+$ Au & 1/2/0.09 & 2.84 &  -  & 5.89 & 6.44 \\
4 & Et 1:2 $+$ Ag & 1/2/0.18 & 2.41 &  -  & 4.66 & 5.47
\botline
\end{tabular}
{\vspace*{6pt}}
\end{table*}

%tab4
\begin{table*}
\caption{\label{tab4}Diffusion coefficients for Pr 1:3 solutions{\vspace*{2pt}}}
\begin{tabular}{ccccccc}
\thead
& Pr 1:3 & 
\parbox[t]{6pc}{\centering Composition HBA/HBD/H\tsub{2}O} & 
\parbox[t]{4.5pc}{\centering 10$^{-11}$\ $D_{\mathrm{Ch}+}$ (m\tsup{2}${\cdot}$s$^{-1}$)} &
\parbox[t]{6pc}{\centering 10$^{-11}$\ $D_{\mathrm{OH\ndash Ch}+}$ (m\tsup{2}${\cdot}$s$^{-1}$)} &
\parbox[t]{4.5pc}{\centering 10$^{-11}$\ $D_{\mathrm{PG}}$ (m\tsup{2}${\cdot}$s$^{-1}$)} &
\parbox[t]{5pc}{\centering 10$^{-11}$\ $D_{\mathrm{OH\ndash PG}}$ (m\tsup{2}${\cdot}$s$^{-1}$)}\vspace*{2pt} \\
\endthead
1 & Pr 1:3        & 1/3/0.01 & 1.69 & 1.91 & 2.50 & 2.41 \\ 
2 & Pr 1:3 $+$ Pd & 1/3/0.01 & 1.72 & 2.17 & 2.55 & 2.65 \\ 
3 & Pr 1:3 $+$ Au & 1/3/0.09 & 3.32 &  - & 5.17 & 5.06 \\ 
4 & Pr 1:3 $+$ Ag & 1/3/0.04 & 1.86 &  - & 2.72 & 3.30
\botline
\end{tabular}
{\vspace*{6pt}}
\end{table*}

On the contrary, the metal salts dissolution has no marked influence on
the diffusion coefficients of Ch\tsup{+} cation ($D_{\mathrm{Ch}+}$)
and propylene glycol ($D_{\mathrm{PG}}$, $D_{\mathrm{OH{\ndash}PG}}$) in Pr
1:3 (Table~\ref{tab4}). Two possibilities have to be considered (i) the
third equivalent of HBD in the DES composition interacts preferentially
with the metal ions rather than with the HBA/HBD association and 
(ii)~the metal salts are not dissociated in Pr 1:3 and keep a neutral form
instead of an ionic form. Indeed, as it was observed in the case of Et
1:2, the \mbox{formation} of a strong ionic pair between DES and metal ion
lead to a decrease in diffusivity of both HBA and HBD species, even if
the water ratio of the solution increases. The increase in diffusivity
of HBA and HBD species in the case of Au dissolution is attributed to
the increase in water content in the solvent (Table~\ref{tab4},
entry~3).

\section{Conclusions}\label{sec4}

{

From this work, it can be concluded that it is possible to consider Pr
1:3 as an alternative system to Ethaline for electroleaching/deposition
applications, with the advantage of much lower toxicity. The
electrochemical properties of Pr 1:3 and of precious metals studied
here as examples, are similar to Ethaline. The slightly lower transport
properties of ChCl-PG DESs could be circumvented by the modulation of
HBD molar ratio and a moderate increase in working temperature or water
content. Diffusion experiments carried by NMR showed that strong
interactions exists between Ethaline and metallic species, whereas the
dissolution of the same metals in Pr 1:3 had no influence on diffusion
coefficients, indicating a different state of solvation in this
solvent. The lower interaction between Pr 1:3 and metal ions should be
a strength in the durability of the solvent during cycling. This will
be a part of a future work.

}

\section*{Declaration of interests}

{\vspace*{-2pt}}

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

{\vspace*{-3pt}}

\section*{Funding}

{\vspace*{-2pt}}

This work has been supported by the Agence Nationale de la Recherche
(EE4Precious project, ANR-20-CE08-0035-01).

\CDRGrant[ANR]{ANR-20-CE08-0035-01}

{\vspace*{-3pt}}

\section*{Raw data}

{\vspace*{-2pt}}

Raw data corresponding to this research work are available in open
access at: \url{https://doi.org/10.12763/EGL57A}.

\back{}

{\vspace*{-3pt}}

\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-297-suppl.pdf}}

{\vspace*{-3pt}}

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