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α- and γ-FeOOH: stability, reversibility and nature of the active phase under hydrogen evolutionMartina Fracchiaa,‡, Alberto Visibileb,‡, Elisabet Ahlbergc, Alberto Vertovab,d, Alessandro Minguzzib,d,*, Paolo Ghignaa,d,* and Sandra Rondininib,d
a. Dipartimento di Chimica, Università degli Studi di Pavia, Viale Taramelli 13, 27100, Pavia, Italy b. Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milan, Italyc. Department of Chemistry and Molecular Biology, University of Gothenburg, Kemigården 4, SE-412 96 Gothenburg, Swedend. INSTM, Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali, Via Giusti 9, Firenze.‡M.F. and A.V. share the first authorship; * A.M. and P.G. share the role of corresponding authors.
KEYWORDS: FeOOH, goethite, lepidocrocite, XAS, FEXRAV, hydrogen, stability
ABSTRACT: -FeOOH (goethite) and -FeOOH (lepidocrocite) were found to be the main corrosion products of the steel cathode in the sodium chlorate process; the identification of the phases formed under reducing potentials, along with the study of the electrodes during the re-oxidation, is fundamental to understand their role in this process. In this paper, FeOOH-based electrodes were investigated through in situ and operando X-ray Absorption Spectroscopy (XAS), combined to electrochemical measurements (e.g. voltammetry and chronoamperometry). At sufficiently negative potentials (below -0.4 V vs. RHE ca.) and under hydrogen evolution conditions an unknown iron (II)-containing phase is formed. A comprehensive analysis of the whole XAS spectrum allowed proposing a structure bearing relation with that of green rust (Space Group P3 1 m¿. This phase occurs independently of the nature of the starting electrode ( or -FeOOH). During electrochemical re-oxidation, however, the original phase is restored, meaning that the reduced phase brings some memory of the structure of the starting material. Spontaneous re-oxidation in air suppresses the memory effect, producing a mixture of alpha and gamma phase.
INTRODUCTIONThe Fe-O-H phase diagram shows outstanding complexity, the oxides, hydroxides and oxide-hydroxides counting at least sixteen different compounds and crystal structures1, 2. Iron oxides are widespread compounds in nature as well, occurring in lithosphere, biosphere and hydrosphere. Fe-O-H compounds are also of extreme importance for technological applications: indeed, they are used as pigments3, catalysts for various reactions like oxygen evolution reaction (OER)4–7, adsorbants8 and, of course, are ubiquitously found as steel corrosion products9. Steel is used as cathode for a number of important industrial electrochemical processes, such as for the NaClO3 production as a precursor for ClO2, a bleaching agent with a huge worldwide market. ClO2 is one of the most effective agents in Elemental Chlorine Free (ECF) bleaching of pulp. This process is environmentally friendly and economically feasible. For practical and safety reasons the storage of ClO2 has to be limited; consequently, ClO2 must be produced in situ from
its precursor, sodium chlorate (NaClO3), which has no safety issue, by reduction with H2O2 or methanol under acid condition10. The electrochemical route to NaClO3, however, is a highly power demanding process, which starts with the following reaction, at the anode:
2Cl- → Cl2 + 2e- (1)and is completed by the following reactions taking place in the bulk electrolyte:
Cl2 + 2OH- → ClO- +Cl- + H2O (2)
ClO- +H+ → HClO (3) 2HClO + ClO- → ClO3- +2Cl- + 2H+
2At the cathode, the hydrogen evolution reaction (HER) occurs:
2H2O + 2e- → H2 + 2OH- (5)
Many efforts have been done in order to improve the feasibility of these reactions, including the improvement of cathode performances, which strongly depend on the nature of the electrode material. Nowadays, the worldwide used material for commercial cathodes is mild steel such as EN 10277-2-2008, which is a low-alloyed steel with low amount of carbon. A good balance between low cost and current efficiency is provided by this material. Nevertheless, its main drawback is high corrosion rates that lead, in some circumstances, to lower current efficiency and to a dramatic loss of activity.Cathodes from different plants were examined in previous works mainly by X-ray diffraction, detecting the formation of a thin layer of -FeOOH (goethite) or -FeOOH (lepidocrocite) as corrosion products11-13. Since water reduction on those corroded iron surfaces is the desired cathodic reaction, there is a strong interest in understanding the nature of the phases formed under reducing potentials; in this sense, many efforts have been made to determine the species formed by oxyhydroxides under strong cathodic polarization in alkaline and neutral conditions, and to elucidate the mechanism of the reduction. The characterization has been attempted with many techniques, but the conclusions reached are often controversial and the nature of the Fe(II)-species formed under reducing potentials is not fully understood. In a work by Stratmann and Hoffmann14, the reduction of -FeOOH at pH 6 was investigated by in situ Mössbauer Spectroscopy. The reduced phase showed a Mossbauer signal similar to that of bulk Fe(OH)2, but not exactly coincident. As a result, this new phase was identified as “Fe(OH)2”, and it was described as a conductive gel, deposited on -FeOOH, with a structure similar but not equal to iron (II) hydroxide. The authors proposed for this reaction a solid-state mechanism, probably induced by proton hopping. This reaction was also investigated through in situ X-ray Absorption Spectroscopy (XAS) and X-ray diffraction (XRD) by Monnier et al.15 The measurements were acquired on both -FeOOH and ferrihydrite-based electrodes at neutral and basic pH (7.5 and 9 respectively). In this case, the authors hypothesized the formation of a mixture of magnetite and Fe(OH)2 under reducing potentials, in variable proportions depending on the pH. Contrary to what was claimed by Stratmann, the authors proposed a mechanism that proceeds via dissolution and re-precipitation. Recently, an in-
situ Raman spectroscopy study was performed16, clearly showing the reduction of - and γ-FeOOH at negative potential. The Raman signal for the starting materials disappeared but no other signals appeared. These results, together with re-oxidation experiments involving -FeOOH as an intermediate, indirectly show that the reduced form is related to Fe(II) hydroxide.While the overall process of reduction and re-oxidation is still under debate, there is a general consensus that the usage of in-situ techniques is mandatory to investigate this reaction. In fact, an immediate re-oxidation occurs as soon as the material is not biased, thus preventing any reliable ex-situ structural characterization17. The coupling of spectroscopic and electrochemical techniques is therefore crucial for a correct identification of the phases formed under various potentials. In the present paper, electrodes of -FeOOH and -FeOOH were investigated at pH 11 by in situ and operando XAS with the purpose of identifying the structure of the reduced phase and clarifying the subsequent process of re-oxidation. XAS represents an ideal probe, thanks to i) its ability to give element-specific information, thus avoiding interference with the electrolytic solution and with the support: this is a massive advantage with respect to XRD, for example, where the electrolyte and the support are expected to give a large contribution with diffuse scattering and diffraction peaks, respectively. ii) its sensitivity to the oxidation state, to the coordination geometry and to the radial distances of a given atom; iii) the easiness and effectiveness of in situ measurements performed with a properly-designed electrochemical cell18. Moreover, the use of FEXRAV (Fixed Energy X-ray Absorption Voltammetry) enables a real time screening of the XAS response in a desired potential window. Coupling this with cyclic voltammetry gives unique information with unprecedented sensibility and allows to select the best conditions for a detailed XAS investigation19-22. Finally, it should be noted that iron oxides and oxyhydroxides are increasingly studied as materials for the water splitting reaction23: therefore, a study on the phase stability and structure of the active phase under hydrogen evolution condition is a pre-requisite for a complete understanding of the reaction mechanisms of these materials.
EXPERIMENTAL-FeOOH and -FeOOH were prepared using electrochemical deposition in order to obtain a homogeneous layer of the desired thickness for XAS study. Two different substrates, FTO and Ti foil, were employed as support and current collector.Electrodeposited samples. FeOOH electrodes were prepared on FTO (Fluorine-doped Tin Oxide, Sigma-Aldrich 8 Ω 1x1.6 cm) by potentiostatic
3deposition in 0.01 M Fe(NH4)2(SO4)2 * 6H2O (Sigma Aldrich® 99%) + 0.04 M CH3COOK (Baker analyzed Reagent ® 99.0%) at 90 °C in N2-saturated solution under stirring. The FTO was previously washed in an ultrasound bath for 30 minutes in acetone, 30 minutes in ethanol and 30 minutes with water. This long procedure allows to remove any trace of greases and dust that can affect the deposition. The electrodes were then dried under N2 flux before being used; otherwise, they were kept in distilled water to avoid any type of contamination. All electrochemical techniques were carried out using a CHI 633D potentiostat/galvanostat. The linear sweep voltammetry (LSV) of the deposition bath is reported in Figure S1 (Supporting Information) in comparison with the same material deposited on Ti foil (Alfa Aesar® 99% metal basis), previously etched in oxalic acid 10% at 80°C for 1 h. A platinum foil (2x1 cm) was used as counter electrode and the reference was a Saturated Calomel Electrode (SCE) in a double bridge filled with KNO3 0.5 M to avoid Cl- interferences. All potentials in this paper are referred to the Reversible Hydrogen Electrode (RHE). A reflux condenser allowed to keep the electrolyte concentration stable during the deposition. The electrode area, 1 cm2, was controlled using a Teflon® tape. The LSV, recorded at a low scan rate (1 mVs-1), allows the identification of the different potential regions for the deposition of the two iron oxy-hydroxides of interest. Indeed, it is possible to notice on the LSV a shoulder and two distinct peaks. According to Martinez et al.24 the shoulder (I) at about 0.3 V, is attributed to the formation of Fe3O4, while, at higher potentials, peak II, centred at 0.7-0.8 V is due to the formation of -FeOOH while formation of -FeOOH gives rise to peak III, at E>1.0 V. The deposition was then performed at +0.45 V vs. RHE for -FeOOH and +1.60 V vs. RHE for Fe. The selected values should ensure high purity of the two materials. Using a Ti substrate the characteristic peaks are more evident, but even on FTO it is clearly possible to identify each specific region. Deposition was stopped after 0.11 C of charge had passed to ensure a 0.1 mg cm-2 loading on the electrode. This value guarantees a sufficient X-ray absorption, yet is low enough to reduce the possibility of self-absorption effects in the XAS experiments. -FeOOH and -FeOOH were also deposited on etched titanium foil (Alfa Aesar® 99% metal basis) using the same potential. A Teflon® tape ensured that the deposited surface was 1 cm2. The loading on this type of electrode is 0.2 C cm-2 in order to have the same amount of material for each face of the foil. The so prepared electrodes were rinsed with 18.2 MΩ MilliQ water to remove any trace of the deposition solution.The materials were initially characterized by cyclic voltammetry (CV) in 0.200 M Na2SO4 (aq)
(Sigma Aldrich ReagentPlus) with pH 11 set with NaOH (Sigma Aldrich ReagentPlus). This basic pH was selected in order to simulate the basic condition present during strong hydrogen evolution on industrial plant electrodes.In-situ/operando XAS. All experiments were performed in a specifically designed cell. The backbone of the cell is made of silicon rubber while the walls are composed by a polyethylene terephthalate layer with a Mylar® window in correspondence of the working electrode position. The working electrode is placed with the deposited layer faced towards the X-rays source. A platinum wire acts as counter electrode while Ag/AgCl electrode is used as reference in a double bridge obtained from a bended glass pipette filled with the electrolyte. To avoid contamination of the main cell chamber with Cl- ions, the tip of the pipette is filled with pressed cotton. The thickness of the cell is slightly higher than the FTO one, in order to have a small layer of electrolyte (in the order of tens of micrometers) in front of the electrode thus optimizing the X-ray beam loss from electrolyte absorption. We expect that this setup could lead to a low conductivity of the solution layer and thus to high ohmic drops. Therefore, all XAS experiment were carried out impinging the beam on the electrode area closer to the counter electrode.XAS measurements were performed at the Fe K-edge (7112 eV), in the fluorescence mode at the LISA (Linea Italiana Spettroscopia Atomica)25
beamline at the ESRF (European Synchrotron Radiation Facility). A Si(311) double crystal monochromator was used; the harmonic rejection was realized by Pd mirrors with a cut-off energy of 20 keV, and a High Purity Germanium fluorescence detector array (13 elements) was used. The energy calibration was performed by measuring the absorption spectrum of metallic iron foil at the Fe K-edge. The energy stability of the monochromator was checked by measuring the absorption spectrum of the Fe foil several times during the experiment. All data were obtained at room temperature. Spectra of Fe2O3, Fe3O4 and FeO were acquired in the transmission mode and used as standards. For those measurements, a proper amount of sample (as to give a unit jump in the absorption coefficient) was mixed with cellulose and pressed into a pellet. The signal extraction was performed by means of the ATHENA code26,27. The EXAFS (Extended X-ray Absorption Fine Structure) data analysis was performed by using the EXCURVE code, using a k2
weighing scheme and full multiple scattering calculations. The goodness of fit (GOF) is given by the F-factor:
F = 100 ∑i
N [ χ¿¿ i , exp− χ i ,calc] ²σ i
For the X-ray Absorption Near Edge Structure (XANES) analysis, the raw spectra where first background subtracted using a straight line fitting the pre-edge, and then normalized to unit absorption at 800 eV above the edge energy, where the EXAFS oscillations are not visible anymore. XANES calculations were performed by means of EXCURVE software; the theoretical signals were generated through a Full Multiple Scattering (FMS) approach28,29.For the fitting of the pre-edge peaks30 the background due to the rising edge was simulated through a second-order polynomial function, and then subtracted to the experimental signal. The resulting curve was modelled by Gaussian functions.
RESULTS AND DISCUSSIONPreliminary characterizations of both materials were performed in 0.2 M Na2SO4. As mentioned before, the pH was adjusted to 11 by adding a proper amount of 0.1 M NaOH, in order to mimic the operation conditions found at the industrial plants of chlorate production under hydrogen evolution. Slow cyclic voltammetries (5 mVs-1) from 0.9 to 0.1 V vs. RHE are shown in Fig. 1. The CV shapes for the gamma and alpha phases nicely agree with what reported in literature in the selected potential window11.
Figure 1. Comparison of the cyclic voltammograms of -FeOOH electrode (black) and -FeOOH (red) in 0.2 M Na2SO4 + NaOH, pH 11. Scan rate 5 mVs-1.
The XANES spectra of the two phases are shown in Fig. 2. Subtle differences are detected, reflecting the different crystal structure of the two phases. In particular, the alpha phase gives rise
to a more intense and broader White Line (WL). However, the most effective way to distinguish the two phases is through the comparison of their EXAFS Fourier Transforms, as it will be shown later. The spectra obtained for -FeOOH deposited on Ti, a commonly adopted substrate for studying this material, and for -FeOOH on FTO show a complete similarity (see SI, Fig. S2): this proves the goodness and reproducibility of the synthesis performed on different supports. However, electrodes deposited on Ti show a remarkably higher HER activity, and this leads to FeOOH detachment under hydrogen evolution condition12. Therefore, the results described in the following all refer to electrodes deposited on FTO. No differences were recorded for dried electrode and for the electrode immersed in the electrolyte at open circuit potential, (OCP) value is 0.9 V vs. RHE in both cases. We can then state that the spectra are not influenced by the presence of the electrolyte (see Fig. S2 in SI).
Figure 2. Comparison of the XANES spectra of (black line) and -FeOOH (cyan line) on FTO.
FEXRAV (Fixed Energy X-ray Absorption Spectroscopy)19 is a recently introduced technique where the X-ray energy is kept constant at a value which grants the maximum contrast between the absorption coefficients of two oxidation states of the selected atom. While the energy is constant, the applied potential is swept in the range of interest. FEXRAV experiments were carried out in dark to avoid any possible influence of light on this semiconductor material. The FEXRAV experiments were performed fixing the X-ray energy at 7125 eV; as shown in Figure S3 in the SI, this energy gives a good contrast between the absorption spectra of Fe(II) and Fe(III). In this case, an increase in the absorption coefficient indicates a reduction of the material
5from Fe(III) to Fe(II). The electrode potential was swept in the range given in figures captions, with a scan rate of 1 mV s-1. Figure 3 shows the cyclic voltammetries (a and c) and the FEXRAV (b and d) for and -FeOOH, respectively. During the first half-cycle the two FEXRAV signals (orange lines in Fig. 3b and 3d) are similar and they show an increase in the absorption coefficient, thus indicating a reduction from Fe(III) to Fe(II). From the second half of the cycle the trend of the FEXRAV signals (red line) becomes markedly different depending on the type of the starting electrode. In the curve corresponding to -FeOOH there is only a slight decrease of the absorption coefficient, thus demonstrating that at the end of the cycles the electrode is substantially not re-oxidized. The reversibility of -FeOOH is therefore scarce, at least in the timescale considered in this experiment. Indeed, in the potential window studied, the CV shows just a small oxidation peak at 0.7 V. On the contrary, in the curve corresponding to the -FeOOH, the initial increase of the absorption coefficient is almost fully compensated by a decrease at the beginning of the second cycle; -FeOOH shows therefore almost a total reversibility, and the fraction of iron that is reduced is almost completely re-oxidized after the second cycle. In addition, the CV presents an additional oxidation peak which is absent in the alpha phase. Indeed, the spectrum acquired at 0.41 V (pink line in Fig. 3e) shows a mixed Fe(III)/Fe(II) oxidation state, while the full re-oxidation is obtained only at 0.76 V. For the This will be further explained in the following. In order to identify the species formed during the process, XANES spectra were acquired for and -FeOOH in correspondence of the reduction and the oxidation peaks in the relevant cyclic voltammograms. Before doing this, reference spectra were acquired at 0.32 V (insets in Fig. 3e and 3f, dark yellow lines), where, as evident from the first FEXRAV cycles, no faradaic process occurs. These spectra are undistinguishable from those of the pristine materials either in air or in solution. XANES spectra were acquired in correspondence of the second reduction peak (-0.55 V for -FeOOH and -0.4 V for -FeOOH, blue lines in figure 3e and 3f), where much higher currents were recorded. The reduction for γ-FeOOH appearing at less negative potentials compared with the reduction of -FeOOH16 may be correlated to the different morphology of the electrodeposited layers, with well-defined elongated crystals for γ-FeOOH and more equally sized crystallites for -FeOOH, lumped together in a porous layer12. This could influence the reduction rate since the elongated crystals for γ-FeOOH provide a large surface area and less solid state transport. The XANES spectra show a large shift to lower energy and an impressive change of
shape in the XANES manifold. The potential of -0.7 V for -FeOOH and -0.65 V for -FeOOH was also tested to identify any possible variation of oxidation states hidden by the hydrogen evolution signal (dark green lines in figure 3e and 3f). Under these conditions, the spectra are almost identical to those at -0.55 and -0.4 V, thus indicating that under H2 evolution the materials are not further reduced and their structure is preserved. Moreover, after the reduction, neglecting marginal differences, the spectra of the two electrodes are almost identical, pointing towards the fact that the structure of the reduced material is substantially the same independently of the starting phase. Therefore, we can conclude that, below -0.55 V ca., a single spectrum (i.e. at -0.4 V starting from the -FeOOH phase, or at -0.7 V starting from the -FeOOH phase) can be selected as being representative of both the local atomic and electronic structure of Fe in all the experimental conditions here considered).The spectrum of the reduced phase is shifted towards lower energies with respect to the spectrum of FeOOH, and the energy edge position is typical of Fe(II). DFT calculations of surface stability and termination for Fe(OH)2 show that at potentials more negative than 0 V vs. RHE, there is a tendency to form clusters of iron31. However, the considerable intensity of the White Line excludes the presence of metallic iron. As a preliminary step in the analysis, XANES was used for phase fingerprinting. The spectrum was thus compared to various spectra available in literature, and it was found to be similar to those of Fe(OH)215,32 and sulfate green rust33. In the present case, however, the intensity of the White Line (WL) is far larger. A possible explanation of this huge intensity is the
presence of Fe(II) in a high spin state; in fact, it is known in literature that the WL amplitude increases after a Fe(II) low spin high spin transition34,35. The WL is due to the 1s-4p transition and the passage from low spin to high spin causes an increase in the distances between iron and the first neighboring shells, leading to a larger localization of the 4p orbitals of iron. As XAS probes the local density of empty states, a larger localization of the 4p orbitals implies that the density of accessible states increases and thus the WL intensity increases as well. More accurate information about the valence, the coordination geometry and the spin state of iron can be obtained by the so-called pre-edge peak. This peak results from the formally dipole forbidden 1s-3d transition and the position,
intensity and shape are highly sensitive to the local atomic and electronic structure of iron. The pre-edge peak was analyzed as explained in the methods section. The peak shows a characteristic profile consistent with the presence of high spin Fe(II) in an octahedral environment30, in agreement with the WL shape and intensity. According to Tanabe and Sugano36 the electronic configuration of octahedrally coordinated Fe(II) in a high spin configuration is expected to give three multiplets. Therefore, three Gaussian functions were used to fit the signal, obtaining a good agreement with the experimental, as shown in Fig. 4. Repeating this procedure for the four spectra representative of the “reduced” materials provides the values of intensity (calculated as total integrated area) and of the centroid
7(intensity-weighted mean of the energy
Figure 4. Fe-K edge pre-edge peak for -FeOOH (panel a) at -0.4 V and -FeOOH (panel b) at -0.7 V: the black and blue dots represent the experimental signal, while the red and orange curves are the fit. The green, blue and pink curves are the Gaussian functions used for the fitting.positions), listed in Table S1 in SI. According to Winkle et al.37, the centroid is located at 7113.7 eV in minerals where the iron oxidation state is (II). This value coincides with that of reduced - FeOOH, while the values resulting from the reduced -FeOOH are shifted of 0.4 eV towards lower energy. The cause of this shift has not yet been identified; at any rate, it can be safely stated that in all cases the iron is substantially present as Fe(II). In addition, the pre-edge peak in all conditions has a very low intensity, thus indicating that the 1s-3d transitions are strictly forbidden by the dipolar selection rule. This is consistent with a non-distorted octahedral coordination which, presenting the inversion symmetry, prevents the p-d mixing. All this preliminary information can be exploited for a combined fit of the XANES and the EXAFS region that was performed in the attempt of identifying the structure of the reduced phase.
While the EXAFS region gives specific information about the two-body correlation function, the XANES calculation is sensitive to higher order correlation functions, giving access to finer structural details (bond angles and coordination geometry), thus helping in the discrimination between very similar structures. Initially, the model used for the simulation was Fe(OH)2, since it satisfies all the conditions imposed by the pre-edge peak analysis; however, the goodness of fit of the EXAFS and especially of the XANES region was unsatisfactory (Fig. 5a, green line). A second attempt was then performed by considering the green rust structure33, which is built up by brucite-like layers alternatively containing Fe(II) and Fe(III); sulfate anions are incorporated in the structure to compensate the positive charges. Since in our case the oxidation state of iron is (II), the structure used for the fit represents a reduced green rust, where Fe(III) and sulfate anions are not present. As show in Fig. 5, both XANES and EXAFS regions are well fitted by this model, and their goodness of fit F is 0.54% and 7.55%, respectively; the parameters obtained after refinement are shown in Table S2 in SI. The refinement was also performed including the hydrogen atoms in the hydroxyl groups, but this did not influence the goodness of fit. The distance between Fe and the oxygen nearest neighbors is larger than the crystallographic value: this is consistent with the presence of high spin iron (II). For both the XANES and EXAFS regions, the clusters were adjusted to a minimum size accounting for the experimental. Figure S4 in SI shows the cluster used for the EXAFS analysis: it is apparent that the Fe atoms in shell 2 produce a “lens” effect (bond angle equal to 180 °) with the Fe atoms in shell 6. This lens effect causes the far Fe atoms in shell 6 to give a significant contribution to the EXAFS. This is the reason why the dimension of the cluster used for EXAFS refinement is higher with respect to that used in XANES simulation (see Fig. 5b). In order to avoid unnecessary correlations, only few parameters were fitted.We point out that the structural model used in this fit is very similar to Fe(OH)2, since the connectivity within the layer is exactly the same, and the distances are comparable in the two cases. What changes between the two structures is the bond angles. This strongly affects the goodness of fit in the XANES region, since XANES is governed by the photoelectron multiple scattering paths within the different coordination shells. Indeed, the angle formed by two equatorial oxygens is 104.5° in the reduced green rust and 99° in Fe(OH)2, while the angle formed by an axial and an equatorial oxygen is respectively 75.5° and 81°. This shows how XANES is not only sensitive to the local
8coordination geometry but also to
Figure 5. a) XANES simulation of FeOOH at -0.4 V and FeOOH at -0.7 V. The black and blue dots represent the experimental signals for - and FeOOH, respectively, while the colored lines are the simulations with the model of reduced green rust (red and pink lines) and with Fe(OH)2 (green line). The bond angles corresponding to the two structures are also shown b) cluster of reduced green rust obtained after XANES simulation c) EXAFS signal and d) the corresponding Fourier Transform of γ-FeOOH at -0.4 V. The black line represents the experimental curve, while the red line is the fit with the reduced green rust model.
9Figure 6. Fourier transforms calculated from the corresponding EXAFS signals. a) Comparison between the FT of pristine -FeOOH (red line) and -FeOOH at 0.76 V (blue line) b) comparison between the FT of pristine -FeOOH (green line) and -FeOOH at 0.7 V (pink line). Clusters of -FeOOH (a) and of -FeOOH (b) are shown to help identifying the atoms which contribute to the peaks: the central iron is grey, oxygens are red, and all the others are iron atoms at various distances from the central one. For the sake of better visualization, only few representative atoms of the equivalent set are shown.
subtle variations in the bond angles. We here stress the fact that to best of our knowledge, this is the first example of a full structure determination based exclusively on XAS data. The coordinates obtained after XANES refinement are shown in SI, Table S3.It should be noted that our results are in good agreement with in-situ Mössbauer on -FeOOH14. In addition, in agreement with the in-situ Raman study16 but contrary to what was reported by Monnier et al.15 we exclude the presence of magnetite, which should result in a larger edge energy and a larger intensity of the pre-edge peak. It must be taken into account that the thickness of FeOOH layer can strongly influence the process of reduction; in our case, the small thickness (2.3 m ca.), together with the choice of strongly reducing potentials, should ensure the complete reduction of the material and, therefore, a correct identification of the reduced phase. In other words, under these experimental conditions we did not detect species at intermediate valence, indicating that the reduction was not complete. In addition, we do not exclude a strong influence of the pH in this reaction.The overall spectral profile obtained in all the experimental conditions below -0.4 V can be efficiently explained by our combined XANES and EXAFS analysis. However, the level of accuracy that can be reached is well beyond the subtler differences that are found in each case.In particular, the spectra obtained after reduction of the and - phases are dissimilar in the position of the pre-edge peak; this indicates that the 3d band is somehow different in the two cases. This may be the origin of the marked differences in the FEXRAV and CV curves obtained during re-oxidation (see Fig. 3). The cyclic voltammetry of -FeOOH shows two oxidation peaks, at 0.41 V and at 0.76 V vs. RHE; on the other hand, the CV of -FeOOH shows just one oxidation peak at 0.7 V vs. RHE. The first FEXRAV cycle (orange line in Fig. 3b and 3d) is quite similar in both cases but marked differences are detected in the second cycle. In particular, -FeOOH shows a slow re-oxidation when the potential is decreased from 0.9 to 0.6 V, and, at the end of the second cycle is almost completely re-oxidized. On the contrary, marked irreversibility is found for the -FeOOH phase, that, apparently, retains a mixed Fe(III)/Fe(II) oxidation state during the whole second cycle.
To get a better understanding of this complex behavior, also in this case XANES spectra were recorded in potentiostatic mode in correspondence of selected potentials. For the -FeOOH, the XANES at 0.41 and 0.76 V are shown in Fig. 3e as magenta and light blue lines, respectively. We consider first the spectrum at 0.41 V. The edge energy position is slightly higher but quite close to that of Fe(II), meaning that at this potential only a small fraction of Fe(III) is formed. In addition, this spectrum can be simulated as a linear combination of the spectra at -0.4 and 0.76 V (see SI, Fig. S5).More Fe(III) is formed at 0.76 V, but the reaction is very slow. Indeed, the spectrum acquired just after having set the potential al 0.76 V (light blue line in Fig. 3e) shows an edge energy position which is intermediate between the original spectrum of the phase and that of the reduced phase. A further spectrum was acquired just after the end of the previous one (i.e. after ca. 2 h), and it is shown as an orange line in Fig. 3e. It is well apparent that, neglecting a residual small shift towards lower energy, the spectrum is now coincident with that of pristine -FeOOH.Concerning the re-oxidation of the -FeOOH electrode, the spectrum at 0.7 V is shown in Fig. 3f as a light blue line. It is evident from the edge energy position that the re-oxidation process is largely incomplete and a mixed Fe(III)/Fe(II) oxidation state is detected. All the findings described above are in perfect agreement with both the FEXRAV and CV results. In particular, the -FeOOH is more likely to re-oxidation (even if the process is slow: the timescale is of the order of 1 h at 0.76 V, that is the time needed to obtain a full XAS spectrum). On the contrary, the marked irreversibility detected by FEXRAV for -FeOOH is confirmed by the detailed XANES analysis. A further confirmation can be obtained by integrating the oxidation peaks in the CV: the resulting quantity of charge for -FeOOH is nearly the double of that for the -FeOOH phase. This is rather surprising as, being the product of reduction the same independently of the starting phase, the process of re-oxidation is expected to proceed in a similar way in the two cases. An even more surprising result is found if the EXAFS Fourier Transform of the (first) spectra after re-oxidation at ca. 0.7 V are compared to those of pristine -FeOOH and -FeOOH (Fig. 6). -FeOOH and -FeOOH show some structural differences that influence their radial distribution function. Three main peaks can be identified in the region
10up to 5 Å; the atoms that contribute to every peak are schematized in Fig. 6, where the atom identified as “central iron” is colored in black. The intensity of the third peak represents the main difference between the two FTs, and it can be used as a fingerprint to distinguish the two phases. -FeOOH has a layered structure, so all the iron atoms belonging to other layers with respect to the central iron are too distant to give a significant signal. The third peak is therefore due to only two iron atoms (blue in Fig. 6a), which lie in the same plane of the central iron at a distance of 3.87 Å; as a result, this peak in -FeOOH is very weak. In -FeOOH, the third peak has two contributions: one from two iron atoms (yellow in Fig. 6b) that lie in the same layer as the central atom (these atoms contribute partially also to the second peak), and one from four iron atoms that serve as a connection between two consecutive layers (pink in Fig. 6b). As a result, the third peak is much more intense in -FeOOH than in -FeOOH. As previously mentioned, the edge energy position of the spectra acquired at 0.76 V (-FeOOH) and 0.7 V (-FeOOH) shows that the valence state of iron is intermediate between Fe(II) and Fe(III); however, their FT signals, shown in Figure 6a and b, are similar to those of FeOOH, thus indicating that the structure is already substantially coincident with that of the oxidized electrode. In particular, the Fourier transform of the spectrum recorded at 0.7 V (pink line in Fig. 6b) shows the intensity of the third peak typical of the alpha phase. On the contrary, the Fourier transform of the spectrum acquired at 0.76 V presents a weak intensity of the third peak, perfectly comparable to that of -FeOOH. As a result, it can be stated that even if the reduced phase is essentially the same independently of the nature of the starting electrode, a different behavior arises during the electrochemically driven re-oxidation; the electrode presents therefore a “memory effect”, since it is able to preserve a memory of the initial crystal structure. This implies that the bond linkage of the starting phase is somehow preserved in the structure of the reduced phase. This aspect clearly needs some further investigation. As a working hypothesis, we can now suggest that the layers of octahedra of the reduced structure deriving from the gamma phase are completely independent, while weak correlations are still present for the reduced structure deriving from the alpha phase; if these correlations are effectively weak and randomly distributed, their contribution to the XAS spectra is expected to be negligible. However, the mere fact that the reversibility does exist allows excluding that the reaction proceeds via a dissolution/re-precipitation mechanism.The spontaneous re-oxidation of the material in air was also investigated. The electrode was
polarized at sufficiently negative potentials to guarantee HER (-0.7 V) and then left overnight in air at room temperature: the ex-situ XANES was then acquired. The resulting spectrum can be simulated by a linear combination of the spectra of pristine and -FeOOH electrodes, demonstrating the formation of 41% of the alpha phase and 59% of the gamma phase (see SI, figure S6). This shows how air oxidation destroys the reversibility of the reduced green rust to FeOOH, which probably re-oxidizes through another route that deletes the memory effect.
CONCLUSIONSIn the present paper, in situ X-ray Absorption Spectroscopy (XAS) was performed on -FeOOH and -FeOOH electrodes to obtain chemical and structural information about the species formed under cathodic condition and during the subsequent process of re-oxidation. The main conclusions after this investigation are: i) under strong reducing potentials (lower than -0.4 V vs. RHE) and under hydrogen evolution both and -FeOOH electrodes are reduced to an iron (II) species, which has been identified as reduced green rust. The spectra of the reduced electrodes show impressive similarity, meaning that the structure is substantially the same in all cases. However, some subtle differences are detected in the spectra, depending on the starting phase (alpha or gamma); this probably affects the process of re-oxidation. ii) the reduced materials present a memory effect, and during the electrochemically-driven re-oxidation the original phase (alpha or gamma) is restored. While for the phase the entire process of re-oxidation was followed, for the phase the complete re-oxidation could not be achieved in the selected range of potentials. Notwithstanding, the reversibility was clear, since the species with intermediate Fe(II) and Fe(III) oxidation state already show the structure of the pristine electrodes iii) when the re-oxidation occurs spontaneously in air, the memory effects is destroyed and a mixture of -FeOOH and -FeOOH is formed. These results are expected to be of interest in the development of cathodes for the chlorate process, in order to reach higher stability and a better selectivity for the hydrogen evolution reaction. In addition, this work can help in extending the knowledge in the wide field of iron corrosion products. Further studies are needed to clarify the mechanism of the reduction reaction, and to deeply understand the origins of the memory effect. As a final remark, in this paper we have demonstrated how an exhaustive analysis of the whole XAS spectrum allows to obtain precise structural information, even in absence of a previously existent structural model.
11Supporting Information. It contains XANES spectra of -FeOOH and -FeOOH deposed on titanium foil, under OCP conditions, and the reference spectra of FeO and Fe3O4 that justify the energy chosen for the FEXRAV. The cluster used for the EXAFS fitting and the coordinates of reduced green rust obtained after XANES refinement are also shown. It contains the linear combination fits related to the spectrum at 0.41 V and to the spectrum corresponding to the electrode spontaneously re-oxidized in air. “This material is available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATIONCorresponding Author* [email protected]* [email protected]
†If an author’s address is different than the one given in the affiliation line, this information may be included here.Author ContributionsThe manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / ‡These authors contributed equally. (match statement to author names with a symbol)Funding SourcesAny funds used to support the research of the manuscript should be placed here (per journal style).NotesAny additional relevant notes should be placed here.
ACKNOWLEDGMENT The authors thankfully acknowledge beamline BM08 “LISA” at the European Synchrotron Radiation Facility for provision of beamtime (experiment 08-01-1004) and Francesco D’Acapito for the kind support during the experiment.
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