Beneficial effect of Fe addition on the catalytic activity of electrodeposited MnO
xfilms in the water oxidation reaction.
Marco Etzi Coller Pascuzzi,a,‡,1 Elizabeth Selinger,b,‡ Adriano Sacco,c Micaela Castellino,c Paola Rivolo,a Simelys Hernández,a,c Gregory Lopinski,d Isaac Tamblyn,b,d Roberto Nasi,a Serena Esposito,e Maela Manzoli,f Barbara Bonelli,a and Marco Armandia,*
aDepartment of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24,
10129 Turin, Italy.
b Department of Physics, University of Ontario Institute of Technology, Canada, L1H7K4 c Center for Sustainable Future Technologies, Istituto Italiano di Tecnologia, Via Livorno 60, 10144 Turin,
Italy.
d Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada
e Department of Civil and Mechanical Engineering and INSTM Research Unit, Università degli Studi di
Cassino e del Lazio Meridionale, Via G. Di Biasio 43, 03043 Cassino, FR, Italy.
f Department of Drug Science and Technology and NIS Interdepartmental Centre, University of Torino, Via
Pietro Giuria 9, 10125 Torino, Italy.
‡ The Authors equally contributed to the work.
1 Present address: Department of Chemical Engineering and Chemistry, Eindhoven University of
Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands * Corresponding Author: [email protected]
Abstract
We report on a fast and simple protocol for the electrodeposition of Fe-MnOx films used as catalysts
for the water oxidation (WO) reaction at neutral pH, and showing the beneficial effect of iron in
terms of both activity and stability of the catalyst. While most electrodeposited MnOx WO catalysts
are obtained starting from Mn(II) precursors, the proposed protocol consists in the galvanostatic
cathodic deposition of Fe-MnOx onto conductive FTO glasses, using KMnO4 and Fe(NO3)3 as Mn
and Fe precursors, respectively. In the absence of Fe, the Tafel slope drastically increases from 103
to 270 mV dec-1 when passing from low to high overpotentials. The slope change, instead, is progressively reduced when the Fe precursor is added to the deposition solution and a constant
slope of 105 mV dec-1 is obtained in the whole overpotential range with an optimal Fe concentration of 1.00 mM, Accordingly, Electrochemical Impedance Spectroscopy (EIS) shows that Fe addition
improves both charge transfer and transport properties of the electrodeposited films. In particular, a
five-fold decrease in charge transfer resistance at the catalyst/electrolyte interface was observed,
suggesting a more facile oxygen evolving kinetics for Fe containing samples. Furthermore, the
lower the iron content, the lower the film stability, as pointed out by chronopotentiometric
measurements, and confimed by FESEM analysis and EIS as measured both before and after water
electrolysis experiments. To establish structure activity relationships, an extended characterization
of the electrodeposited films was carried out by means of Raman Microscopy, Transmission
Electron Microscopy, UV-vis and X-ray Photoelectron Spectroscopies. The ensemble of the
characterization results suggests that Fe3+ ions are actually incorporated within the electrodeposited film, with limited effects on the final Fe-MnOx structure, consisting in a defective MnO2
birnessite-type structure with significant fraction of surface Mn3+ species.
Keywords
Manganese oxides; Electrocatalysis; Oxygen evolution reaction; Water electrolysis; Iron addition
1. Introduction
Water Splitting (WS) represents a promising means to store solar energy [1,2]. Out of the
two half-reactions involved, Water Oxidation (WO) is the most challenging one and is
considered as the bottleneck of the whole WS process [3-5], as four electrons must be
scavenged from two water molecules allowing the stabilization of oxidized intermediates
and the formation of an O–O bond. Manganese Oxides (MnOx) are earth-abundant and
low-toxicity materials and have been extensively investigated as WO catalysts by means of both
sacrificial reagents [6-8] and electrochemical analysis [9-16]. The latter evidenced that
MnOx are effective electrocatalysts for WO in alkaline conditions [9-11] whereas, at neutral
become overwhelming [12]. Direct electrodeposition of MnOx on conductive substrates
represent an attractive option for the fast and simple preparation of WO anodes, but the
catalytic activity of electrodeposited MnOx films is strongly affected by the deposition
method. Indeed, common anodic deposition from Mn2+ sources results in the formation of films that are scarcely active at neutral pH [13], and complex protocols are generally
required to obtain active MnOx films. Among those, voltage-cycling deposition [13,16 ] and
cathodization of a pre-existing MnOx film [15] were shown to be particularly effective in the
formation of active films exhibiting Tafel slopes in the 70 -110 mV/dec range at pH =7. In
this context, here we firstly report on a fast and simple deposition protocol for the
electrodeposition of iron-containing MnOx films as WO catalysts at neutral pH, showing the
beneficial effects of Fe in terms of both activity and stability of the catalyst. Interestingly, Fe
addition was shown to be an effective strategy to enhance the activity of Ni and Co based
catalysts [17-19], whereas, to the best of our knowledge, a beneficial effect on Mn based
catalysts has never been observed, so far. In this regard, the development of WO catalysts
that are entirely based on earth-abundant and non-toxic elements, i.e. Fe and Mn, is crucial.
As for the adopted Mn source, while most of electrodeposited MnOx WO catalysts are obtained starting from Mn(II) precursors [13-16,20-22], the present protocol envisages the
electro-reduction of permanganate ions, according to eq. (1):
MnO4- + H2O + 3e- → MnO2 + 4OH- (1)
Electro-reduction of MnO4- ions was exploited in the production of different Mn-based energy storage material [23,24], whereas there is a lack of report describing the synthesis of
WO anodes by this procedure. Finally, it is worth noticing that the proposed protocol does not
envisage any post-deposition thermal treatment for film activation, a feature that makes it
interesting from the technological point of view.
2.1 Materials
Fluorine-doped Tin Oxide (FTO) glasses (15 Ω/sq - Solaronix) were used as substrate. FTO
serves as a good substrate because it exhibits high Tafel slopes and current densities for
oxygen evolution reaction of at least an order of magnitude lower than that of MnOx catalyst
films [15]. FTO slides were sonicated in acetone and rinsed with Milli-Q water before use.
Deposition areas of 1 cm2 were created by masking the FTO with insulating tape.
Fe(NO3)3· 9H2O (≥ 99,999 % trace metals basis) , KMnO4 (99.0%), NaH2PO4· 2H2O (≥ 99,0
%) and Na2HPO4· H2O (≥ 99,5 %), FeCl3· 6H2O (≥99.99% trace metals basis), KNO3 (≥
99,0 %) were purchased from Sigma-Aldrich and used without further purification. All the
aqueous solutions were prepared with Milli-Q water with a resistance of 18 MΩ·cm at 25
°C.
2.2 Deposition protocol
The electrodeposition protocol consists in the cathodic deposition of Fe-MnOx onto conductive FTO glasses at constant current density of 0.25 mA cm-2 for 165 s, using 50 mL of KMnO4 1.5 mM solutions containing different Fe(NO3)3 · 9H2O (FeNit) amounts, i.e.
0.00, 0.35, 0.50, 0.75, 1.00, and 1.25 mmol L-1. The obtained samples are referred as 0.00, 0.35, 0.50, 0.75, 1.00 and 1.25, respectively. The pH of deposition solutions was 6.5, 3.6,
3.3, 3.1, 3.0 and 2.9, respectively (Table 1). Precursor solutions were freshly prepared
immediately before each film deposition. Electrodepositions were carried out without any
supporting electrolyte. After deposition, the films were washed with Milli-Q water to
remove any trace of metal precursors and immediately tested. Electrodepositions and
electrochemical measurements were run at 25°C with a standard three-electrode setup in a
lab-made glass cell with an Ag/AgCl (3 M KCl) electrode as reference electrode and Pt wire
as counter electrode. The measurements were recorded by using a multichannel VSP
potentiostat/galvanostat (BioLogic), equipped with EC-Lab v. 10.44 software for data
2.3 Electrochemical characterization
All the electrochemical tests were performed in 0.10 M Sodium Phosphate buffer (Na-Pi) at
pH = 7.0. i·R correction was applied to Linear sweep voltammetry LSV and
Chronoamperometry measurements. LSV measurements were recorded within a potential range of 0.8 – 1.3 V vs Ag/AgCl at a scan rate of 20 mVs-1. Multistep Chronoamperograms (11 steps of 2 min, from 1,00 to 1,25V , ΔE = 0,025 V) were used to obtain the Tafel plots
according to ref. [13]. The current values were recorded at 100 ms resolution and averaged
over 0.1 s. The solution was not stirred during the measurements. Overpotential was
calculated by η (V) = EAg/AgCl + 0.210 − (1.23 − 0.0592 V·pH) - j ·A·Rs where j (mA cm-2) is
the measured current density, A (cm2) is the film area, and R
s (Ω) is the cell resistance
(electrolyte and contacts resistances). This latter was determined by impedance spectroscopy
and was the same for all the experiments (ca. 50 Ω). Chronopotentiometric (CP)
measurements were performed under magnetic stirring at 500 rpm with a Teflon stir bar to
avoid bubbles accumulation onto electrodes surface. Overpotential was calculated by η (V)
= EAg/AgCl + 0.210 − (1.23 − 0.0592 V·pH). Electrochemical Impedance Spectroscopy (EIS)
was carried out at 1.25 V vs Ag/AgCl, with small signal amplitude of 10 mV and frequency
range of 10-1 - 104 Hz.
2.4 Physico-chemical Characterization
UV-vis Spectroscopy. UV-Visible absorbance spectra were recorded by using a UV-Vis Varian Cary 5000 spectrophotometer. In order to clarify the effect of Fe addition on
resultant spectra, the UV-vis spectrum of sample 0.00 (which include FTO contribute) was
subtracted to that of Fe-containing samples. X-ray Photoelectron Spectroscopy (XPS). XP
spectra were taken on a PHI 5000 Versaprobe Scanning X-ray Photoelectron Spectrometer
(monochromatic Al Kα X-ray source with 1486.6 eV energy). Both high resolution (Mn2p,
Mn3s, Fe2p, Sn 3d, C1s, O1s - Pass energy: 23.5 eV) and survey XP spectra (Pass energy:
an Ar ion gun was used as neutralizer system to compensate for positive charging effects
during analysis, due to not perfectly conductive surfaces. The curve-fitting procedure was
carried out by using the Multipak 9.6 dedicated software. All core level peak energies were
referenced to the C1s peak at 284.5 eV (C-C/C-H sp2 bonds). Besides Mn, O, Fe and adventitious C, XP survey spectra of the films showed the presence of Sn due to the FTO
support. The contribution of Sn 3p3/2 line (716 eV, partially overlapping the Fe 2p3/2 line at
ca. 711 eV) was subtracted to the integrated area of high resolution spectra in the Fe2p
region (used for calculating Mn/Fe atomic ratio). Such contribution was calculated from the
integrated area ASn3d of the Sn 3d line by the equation ASn3p/R.S.F.Sn3p = ASn3d/R.S.F.Sn3d where
R.S.F. are the Relative Sensitivity Factors of the two lines. Raman Microscopy (RM).
Raman analysis was carried out by means of a Renishaw InVia Reflex micro-Raman
spectrometer (Renishaw plc, Wottonunder-Edge, UK), equipped with a cooled CCD
camera. The Raman source was a diode laser (λex =514.5 nm), and samples inspection
occurred through a microscope objective (50X), in backscattering light collection. To avoid
sample denaturation, RM spectra were recorded using a low excitation power of 10 mW (10
s of exposure time and 3 accumulations were employed to collect each spectrum). Field
Emission Scanning Electron Microscopy (FESEM) micrographs were collected on a Zeiss Merlin microscope equipped with a Gemini II column. Transmission electron microscopy
(TEM) and high resolution (HR-) TEM measurements were performed using a side entry
Jeol JEM 3010 (300 kV) microscope equipped with a LaB6 filament and fitted with X-ray
EDS analysis by a Link ISIS 200 detector. For analyses, the film nanoparticles were scraped
off the FTO support and were deposited on a copper grid, coated with a porous carbon film.
All digital micrographs were acquired with an Ultrascan 1000 camera and the images were
processed by Gatan digital micrograph. For each sample, a representative number of images
3. Results and Discussion
The adopted deposition protocol led to the formation of light-brown films homogeneously
covering the FTO substrate, with the exception of the highest Fe3+ concentration (i.e. 1.25 mM), which yielded a non-uniform film (Figure S1). Electrochemical WO activity of the
obtained films was first evaluated by linear sweep voltammetry (LSV, Figure 1a). An
increase in the Fe precursor concentration from 0.00 to 0.75 mM results in the growth of
films showing increasing current densities. A further increase in Fe concentration (i.e. up to
1.00 mM) did not lead to higher current densities, especially at low potential. In contrast,
much higher Fe concentrations (i.e. 1.25 mM) led to a drastic decrease in activity, likely
related to the non-uniform deposition of the film (which was not considered further). Reason
for this could be the excessively acidic pH of the deposition solution, as discussed below.
Figure 1
Tafel plots as obtained from multistep chronoamperometry are reported in Figure 1b,
showing, as main feature, a broadening of the linearity range at increasing Fe concentration.
Indeed, in the absence of Fe, Tafel slope drastically increases from 103 to 270 mV dec-1 when passing from low to high overpotentials. Such slope change is progressively reduced
when the Fe precursor was added to the deposition solution and, with sample 1.00, a
constant slope of 105 mV dec-1 is obtained in the whole η range. Usually, the change in slope with increasing potential in WO Tafel plots is attributed either to a change in the Rate
Determining Step (RDS) within a given pathway [25,26] or to the influence of increasing
potential on the adsorption of reaction intermediates [27]. For MnO2 WO electrocatalysts,
Takashima et al. [28,29] suggested that the efficiency of charge accumulation, which must
occur prior to O–O bond formation, is inhibited by the instability of Mn3+ intermediates. Indeed, under neutral pH, such moieties disproportionate to Mn4+ and (soluble) Mn2+ species, and the regeneration process of Mn3+ from Mn2+ is expected to be the RDS. Similar considerations come from electrokinetic experiments at intermediate pH (i.e. 5.5 – 8.5) on
electrodeposited MnOx [30]. Two primary competing pathways yielding to a Tafel slope of
120 mV dec-1 at pH = 7.0 were identified: (i) a disproportionation mechanism, in which the RDS is a cross-site proton-coupled disproportionation, and (ii) a one-electron, one-proton
Proton Coupled Electron Transfer (PCET) mechanism, in which the RDS is O2 formation from the coupling of two adjacent terminal Mn(IV)=O oxo groups. Unfortunately, Tafel
plots were reported only over a narrow range of current density (8·10-4 - 5·10-3 mA cm-2), i.e. before plot bending occurs. Actually, the broadening of the linearity range towards higher current densities is desirable for practical application in water electrolysis. Based on
such findings, it is reasonable to assume that at high overpotentials the limiting factor for the
oxygen evolution reaction is the O-O bond formation, rather than the charge accumulation.
Thus, we hypothesize that the presence of adjacent Mn=O and Fe=O intermediates oxo
groups may favor the O-O bond formation and subsequent O2 evolution. Interestingly, Bush
et al. described a test rig for the evaluation of binuclear transition metal (TM) oxide sites
towards key steps in catalytic WO, assuming a reaction path made of two primary steps
[31]: the oxidation of two (TM)OH hydroxyl groups to form two (TM)=O oxo groups, and
the subsequent μ-peroxo bond formation between the M=O groups. The choice of TM was
shown to significantly affect the stability of the MO intermediates, suggesting that FeCo and
FeMn couples are the best choice.
It is important to note that increases in the Tafel slope do not depend only on mechanistic
aspects, but may be due also to reduction of the effective electrode surface area during the
chronoamperometric measurements or to ohmic effects. Actually, in our experiments,
evidences of charge transport limitations and film instability (e.g. dissolution and/or
detachment from the support) in Fe-free sample are confirmed by impedance spectroscopy
and FESEM analysis, respectively (see below).
The Tafel plot of sample 1.00 showed lower exchange current density with respect to the
mM, and then drops to lower values with sample 1.00. It is probable that at high Fe3+ concentration, an inactive Fe-rich phase forms, partially hindering the active one. This
hypothesis is supported by UV-vis Spectroscopy and control experiment performed with the
sample deposited from pure iron nitrate solution. UV-vis spectra of the samples are reported
in Figure 2, showing a limited loss of optical transparency with respect to pure FTO.
Figure 2
Sample 0.00 displayed a broad absorption in the UV-vis light region similar to those
observed for layered MnO2 films [12,20] and assigned to band gap transition of the material
[32]. UV-vis absorption increases linearly with increasing Fe3+ concentration, at least up to 0.75 mM, without evidencing significant changes in the electronic state. This feature
suggests thicker (or denser) films formation, probably due to the decrease in pH of
deposition solution according to eq. (1). In contrast, the UV-vis spectrum of sample 1.00
shows a slightly different profile, with additional absorptions at 330, 370, 430 and 510 nm
(better resolved in the difference spectrum shown in the figure inset). Such absorption bands
are commonly observed in the electronic spectra of Fe3+ (oxy)hydroxides and correspond to ligand field transitions of Fe3+ [33]. Thus, UV-vis results suggest that some Fe-containing particles likely form at that Fe3+ concentration. Actually, the formation of OH- from eq (1) or from nitrate reduction could favor the precipitation of FeOOH particles.
In order to prove that the observed enhancement of the catalytic performance of MnOx films
was actually due to the addition of Fe3+ ions to the deposition solution, and not to NO3- ions nor to the acidic pH given by Fe3+ species, different control experiments were performed , as shown in Figure 3.
Figure 3
In particular, additional samples were deposited from KMnO4 1.5 mM solutions containing:
i) KNO3 3.00 mM (sample C1); ii) HNO3 added drop by drop up to a final pH = 3.0 (sample C2), i.e. the same pH of the solution used for sample 1.00; iii) FeCl3 0.35 mM (sample C3).
LSV and Tafel plots of sample C1 ruled out any possible role of NO3- ions in improving catalytic activity, being essentially overlapping to those of sample 0.00. As far as the effect
of pH is concerned, LSV and Tafel plots of sample C2 showed an improvement of the
catalytic activity with respect to the 0.00 sample, especially in the low overpotential region
where a Tafel slope of 83 mV dec-1 was observed. However, at higher overpotential, curvature of the Tafel plot to higher slopes was still observed and the performance of sample
1.00 was not achieved. The effect of pH on deposition is not straightforward. Indeed, a
further decrease in the pH of deposition solution (i.e. KMnO4 1.5 mM + HNO3 up to a final
pH = 2.5) completely hindered the formation of a MnOx film, leaving a “naked” FTO
surface. An excessively acidic environment likely favors the reduction of MnO2(s) to Mn2+ (aq) according to eq. (2),
MnO2(s) + 4H+ +2e- → Mn2+(aq) +2H2O (2) thus explaining the behavior of sample 1.25.
Finally, the beneficial effect of Fe3+ was observed independently from the Fe source. Actually, Fe(III) chloride was more effective than nitrate, and sample C3 showed similar
activity to that of sample 0.50. Although the roles of the Fe3+ precursor (nitrate, chloride, acetate etc.) on the resultant catalytic activity is currently under investigation, it is clear that
the addition of Fe3+ into the KMnO4 solution is responsible of the observed improvement in catalytic activity. The improvement is likely due to a synergistic effect between Mn and Fe,
since film deposited from pure 1.00 mM Fe(III) nitrate (sample C4 in Figure 3) showed very
low current densities.
In order to understand the different behavior observed in Tafel plots, charge transfer and
transport properties of the electrodeposited films were studied by means of impedance
spectroscopy. Electrochemical Impedance Spectra (EIS) were measured at 1.25 V vs
Ag/AgCl (1.87 V vs RHE), i.e. at an overpotential corresponding to upper part of the Tafel
Figure 4
All the curves exhibited a two-arcs behavior, though such feature is more evident in the
samples containing larger amount of iron. The first arc (at higher frequencies) is related to
the charge transport inside the catalyst, while the second one (larger and at lower
frequencies) accounts for the charge transfer at catalyst/electrolyte interface. Some of the
samples also exhibit a small tail in the very low frequency region, which can be attributed to
Nernstian diffusion into the electrolyte [34]. According to the literature [12,20,35], the
experimental data were fitted through the equivalent circuit reported in the Figure, where Rs represents the electrolyte resistance (with values comprised between 45 Ω and 55 Ω), the
charge transport is modeled through the parallel between the film resistance RCT and
capacitance Q1(here modeled with a constant phase element [36]), and the parallel RF//Q2
accounts for the charge transfer mechanism. The obtained Charge transfer resistance (RCT) and resistance of the catalyst film (RF) are reported in Figure 4c and Table 1.
The resistance of the catalyst layer decreases at increasing Fe3+ concentration in deposition solution and reaches a minimum with sample 0.75, showing a halved RF with respect to
sample 0.00. Higher Fe concentrations led to a slight increase in RF, probably due to the
presence of Fe-containing particles, as evidenced by UV-vis spectroscopy. Notably,
resistivity of various FeOOH polymorphs was reported to be two order of magnitude larger
than that of MnO2 ones [37-39]. The same conclusions can be drawn by considering the
characteristic time constants associated to the high frequency process. Their values were
found to be of the order of milliseconds, with a clear dependence on the Fe content in the
MnOx film, similarly to what observed with film resistance. Sample 0.00 is characterized by
4 ms, while a minimum value of 0.4 ms was obtained for 0.75 mM sample and a slightly
larger value was found for sample 1.00.
On the other hand, the beneficial effect of iron on charge transfer resistance is much more
suggesting a more facile oxygen evolving kinetics for the latter. This observation matches
fairly well with the different behavior observed in the high overpotential region of Tafel
plots, where the slope change is more pronounced in the absence of Fe.
Table 1
Electrochemical stability of the MnOx films was assessed by chronopotentiometry (CP) at a
constant current density of 0.1 mA cm-2 (Figure 5).
Figure 5
The overpotentials recorded after 2 minutes are close to that recorded in chronoamperometry
and shown in Tafel plots. Sample 1.00 reached a constant operating overpotential η = 555
mV after 1 minute electrolysis. Such overpotential remains constant during the
chronopotentiometric measurement. In contrast, the lower the iron concentration in
deposition solution, the lower the film stability, as pointed out by a gradual increase in
overpotential over the entire measurement. Indeed, a progressive overpotential increase of
37, 65, 99 and 103 mV was observed with samples 0.75, 0.50, 0.35 and 0.00, respectively.
Notably, sample 1.00 showed a rather constant overpotential even during
chronopotentiometry at a current density of 0.5 and 1.0 mA cm-2 (Figure S2). Evidence of different stability toward prolonged electrolysis come from EIS and FESEM analysis, as
recorded for selected samples after the CP. Indeed, impedance spectroscopy (Figure 4b, 4c
and Table 1) evidenced a huge increase in RCT for sample 0.00, most likely due to the
massive loss of catalytic centres. The observed increase in RCT was inversely proportional to
Fe3+ concentration in the deposition solution, and was particularly limited for sample 1.00. This sample also showed the lowest increase in RF, confirming its remarkable stability.
Accordingly, comparison of FESEM pictures taken before and after CP measurements
evidenced the occurrence of film degradation for sample 0.00. FESEM pictures of selected
samples are reported in Figure 6.
As deposited sample 0.00 (Section a) shows a homogeneous “carpet” of nanoflakes covering
the FTO support. The FTO particles are still recognizable, suggesting a limited MnOx film thickness. Similar morphology is reported in the literature for different birnessite-like MnOx films [13,20,35]. At increasing Fe concentration, some flakes appear thicker and less
defined, being progressively embedded in a more amorphous matrix (Sections c, d and e).
This different morphology coexists with the original one in samples 0.50 and 0.75, whereas
it is predominant in sample 1.00. FESEM image of sample 0.00 taken after CP (Section b)
clearly shows the occurrence of film degradation, as remarked by the presence of “naked”
FTO particles. On the other hand, the same coverage and morphology were detected for
sample 1.00 before and after CP (Sections e and f, respectively). The reason for the observed
increase in film stability is not straightforward. However, we could infer that Fe3+ species favor re-oxidation of Mn2+ to Mn3+ (vide supra). Indeed, the high-spin nature of d5 Mn2+ ions imparts to them exceptional lability and accelerates the dissolution of the catalyst film [30].
To establish structure activity relationships, an extended characterization of the
electrodeposited films was carried out. X-ray diffraction patterns (not reported) showed only
the peaks of FTO substrate, reasonably due to the limited thickness of the obtained films.
The Raman spectra of selected samples (Figure S3), showed the typical features of
birnessite-type MnO2 [40-43], but did not evidence the formation of any Fe-rich segregated
phase (e.g. FeOOH) [44,45], probably because of the low incident laser power (adopted to
avoid sample denaturation). All Raman spectra were rather similar, showing two major
bands at 580 and 631 cm-1, and a shoulder at about 500 cm-1. The two bands at high wavenumbers are attributed to the ν3 (Mn–O) stretching vibration in the basal plane of MnO6
sheets (580 cm-1) and to the symmetric stretching vibration ν2(Mn–O) perpendicular to the chains of MnO6 octahedra (630 cm-1). The position, broadness and weakness of those bands are typical of defective MnO2 birnessite-type structures [40,46]. Accordingly, nanoflakes
made by nanowires with lengths of ∼100 nm and diameters of ∼5 nm were observed by
Transmission Electron Miscoscopy (TEM), as shown in Figure 7.
Figure 7
The distance between the diffraction fringes observed by High-resolution TEM (HRTEM)
on both 0.00 and 1.00 samples (Section c and d) was measured to be of ca. 2.4 Å, matching
with the lattice spacing of (100) planes in birnessite (PDF# 00-043-1456). The Fourier
Transform (FT) of the image puts in evidence diffraction spots corresponding to the same
plane and to the (001) plane, as well, with a lattice spacing of 7.20 Å, corresponding to the
interlayer distance. Such distance is not detected on sample 1.00, as Fe insertion may lead to
a very small number of randomly stacked sheets [47]. Notably, poorly crystalline birnessite
correlates with higher catalytic activity in different studies [46,48,49]. Some regions of TEM
pictures seem to reveal the presence of small nanoparticles (Fig S4) covering the nano-flakes of
sample 1.00, reasonably corresponding to the Fe-rich particles responsible for the UV-vis spectral
features above described.
Mn/Fe atomic ratios as measured from high-resolution X-ray photoelectron spectra of Fe 2p
and Mn 2p regions are reported in Table 1. The obtained values exceed the Mn/Fe molar
ratio used in deposition solution, as often observed with depositions obtained from mixed
salts solutions [50-52]. Mn/Fe molar ratio of sample 1.00 was also measured by means of
Energy dispersive X-ray spectroscopy (EDX) (Figure S5). In this case, a higher value was
obtained (i.e. 7.9 vs 5.1), suggesting that a larger fraction of Fe is present on the surface with
respect to the bulk.
The position of the Fe 2p3/2 and Fe 2p1/2 XPS lines (Figure S6a) agrees with the presence of
(mainly) Fe3+ species. Interestingly, the presence of Fe did not seem to affect the average Mn oxidation state of MnOx samples. Indeed, both the degree of Mn 3s peak splitting and the
binding energies of Mn 2p are the same with all the samples (Figure S6b and S6c). These
particular, the former is not affected by surface charging and does not depend on
spectrometer calibration. All the samples showed a Mn 3s peak splitting in the 5.6 – 5.5 eV
range (Table 1), i.e. larger than what observed for common birnessite samples [54],
suggesting the possible presence of a significant fraction of surface Mn3+ species. The latter have been shown to be crucial in enhancing the WO activity of MnOx catalysts
[14,15,46,55]. The ensemble of the characterization results suggests that Fe3+ ions are actually incorporated within the film during electrodeposition, with limited effects on the
final Fe-MnOx structure. This hypothesis is in agreement with previous studies showing that
the crystal structure of birnessite may incorporate Fe3+ ions, retaining almost the same layer structure and Mn local environment of the undoped compound [56,57]. Notably, ICP-MS
analysis of the solutions used in the CP measurements revealed the absence of any Fe
leaching with all Fe-containing samples, suggesting that Fe is strongly bound to the MnOx matrix, though the present results do not allow us to unveil the actual Fe distribution
between the interlayer region and the framework of the studied MnOx films. As a matter of
fact, Fe addition plays a crucial role in decreasing charge transport and transfer resistance
and improving stability of the MnOx films. We speculate that the presence of Fe3+ ions could stabilize surface Mn3+ species by inhibiting disproportionation to Mn4+ and soluble Mn2+ usually occurring in MnO2 electrocatalysts at pH < 9 [12,51].
4. Conclusions
The development of WO catalysts that are entirely based on earth-abundant, low cost and
non-toxic elements, i.e. Fe and Mn, is crucial. The present study reports on a fast and
effective electrodeposition protocol for the preparation of Fe-MnOx films, demonstrating the
beneficial effect of Fe addition, both in terms of activity and stability of the catalyst. The
simplicity of the proposed protocol, which does not envisage any post-deposition thermal
transfer and transport properties of the MnOx films are improved at increasing Fe content,
with limited changes in MnOx structure. Furthermore, the lower the iron content, the lower
the film stability, as pointed out by chronopotentiometric measurements.
The development of new strategies for improving activity and stability of MnOx based
catalysts will benefit from these results. A next step forward will be the investigation of Fe3+ addition in other electrodeposition protocols [13-15], and the use of nanostructured
conductive supports to improve the resultant current densities, as well as the screening of
other Fe3+ sources with the aim of unravelling the mechanism(s) leading to the embedding of Fe ions within the MnOx structure.
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Table 1. Summary of synthesis conditions, XPS data and resistance of the catalyst films and charge
transfer resistances (as measured from EIS before and after CP) of samples deposited from 1.5 mM KMnO4 solutions containing Fe(NO3)3 in different concentrations.
a Sample name corresponds to Fe(NO
3)3 concentration (mM) in deposition solution. b As measured from curve fittings of the HR-XPS spectra in the Mn2p and Fe2p regions. c Values in brackets refer to post-CP measurements.
samplea pH of deposition Mn/Fe At. ratiob ΔE Mn3s [eV] RF [ohm]c RCT [ohm]c 0.00 6.5 0 5.7 72 (94) 263 (2034) 0.35 3.6 N/A N/A 43 (65) 241 (590) 0.50 3.3 16.5 5.6 37 (53) 159 (265) 0.75 3.1 7.8 5.5 32 (44) 66 (105) 1.00 3.0 5.1 5.6 43 (47) 55 (60)
Figure 1
Figure 1. LSV at 20 mV s-1 (a) and Tafel Plots (b) as measured in 0.1 M Na-Pi buffer (pH = 7.0) for samples deposited from 1.5 mM KMnO4 solutions containing Fe(NO3)3 in different concentrations.
Figure 2
Figure 2. UV-vis absorption spectra for as deposited films. Inset: difference spectra obtained by
subtracting the UV-vis spectrum of sample 0.00 (which includes also the FTO contribution) to those of Fe-containing samples.
Figure 3
Figure 3. Control experiments: LSV at 20 mV s-1 (a) and Tafel Plots (b) as measured in 0.1 M Na-Pi buffer (pH = 7.0) for samples deposited from pure 1.00 mM Fe(III) nitrate solution (C4) and from 1.5 mM KMnO4 solutions containing: 3.00 mM KNO3 (C1); HNO3 up to a final pH = 3.0 (C2); 0.35 mM FeCl3 (C3).
Figure 4
Figure 4. EIS Nyquist plots as measured before (a) and after (b) chronopotentiometry. The points
are experimental data, the lines are the curves obtained through a curve fitting procedure using the equivalent electrical circuit shown in the inset of (a). The calculated resistances of the catalyst films (RF) and charge transfer resistances (RCT) are reported in Section (c).
Figure 5
Figure 6
Figure 6. FESEM pictures of as deposited sample 0.00 (a), 0.50 (c), 0.75 (d) and 1.00 (e). FESEM
Figure 7
Figure 7. TEM image of sample 1.00 at 5000× (a) and 50000× (b) magnification. HR-TEM images
