Electrochemical Investigation of the Hydrogen Evolution Reaction on Electrodeposited Films of Cr(OH)3 and Cr2O3 in Mild Alkaline Solutions
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The hydrogen evolution reaction (HER) from water reduction is the main cathodic reaction in the sodium chlorate process. The reaction typically takes place on electrodes covered with a Cr(III) oxide-like film formed in situ by reduction of sodium dichromate in order to avoid reduction of hypochlorite and thereby increase the selectivity for the HER. However, the chemical structure of the Cr(III) oxide-like film is still under debate. In the present work, the kinetics of the HER were studied using titanium electrodes covered with electrodeposited Cr(OH)3 or Cr2O3, which were characterized by means of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. A clear difference in the morphology of the deposited surfaces was obtained, and the structure could be revealed with Raman spectroscopy. The kinetics for the HER were investigated using potentiodynamic and potentiostatic techniques. The results show that the first electron transfer is rate limiting and that the activity decreases in the order Cr2O3@Ti > bare Ti > Cr(OH)3@Ti. The low activity obtained for Cr(OH)3@Ti is discussed in terms of the involvement of structural water in the HER and the slow ligand exchange rate for water in Cr(III) complexes, while the high activity obtained for Cr2O3@Ti is rationalized by a surface area effect in combination with reduction of surface water and water in solution.
KeywordsElectrocatalysis Raman spectroscopy Sodium chlorate Electrodeposition
The use of sodium dichromate, even though optimum from the process point of view, is undesirable owing to the presence of Cr in the hexavalent form (Cr(VI)), which is poisonous, carcinogenic, reprotoxic, and mutagenic. Comprehensive reviews on the toxicity of Cr(VI) can be found in [21, 22]. The removal of chromium(VI) from chlorate electrolytes has long been pursued, and is even more motivated now, since Cr(VI) compounds were included in the REACH list Annex XIV to be banned for use in industrial processes in Europe , unless a special authorization is given.
Replacing sodium dichromate, however, is not an easy and straightforward task, and considerable efforts have been spent to find a suitable and more environmental friendly chemical to replace Cr(VI). Rare earth metal salts presented promising results in bench-scale experiments [24, 25, 26], but their solubility in a chlorate electrolyte  and/or low efficiency in long-term experiments  limit their application in real processes. Another option is the use of molybdate in the electrolyte. Molybdate is also reduced in situ and favors water reduction over chlorate and hypochlorite reduction. But molybdate ions interfere with the anodic reactions increasing the O2 concentration in the cell gas , thus creating a safety issue. As an alternative to soluble molybdate, MoO2 nanoparticles have been immobilized onto Ti and Fe electrodes in a Cr2O3 matrix, yielding a surface very selective towards water reduction in the presence of hypochlorite . However, it was shown elsewhere that electrodeposited Cr2O3 completely prevents hypochlorite reduction on Ti electrodes  and, hence, the effects of MoO2 particles as presented in  cannot be precisely evaluated.
It turns out that to find a suitable replacement for sodium dichromate, a deeper knowledge of the selective mechanism is necessary. In this context, a recent paper focusing on Cr(OH)3 and Cr2O3 surfaces has shown that both are capable of blocking hypochlorite reduction, probably due to the p-type semi-conducting properties of Cr(III) oxide-like sur-faces . However, studies regarding water reduction on such films were not deeply investigated. With the aim to fill this gap, the present paper provides a comprehensive investigation of the mechanism and kinetics of water reduction on Cr(OH)3- and Cr2O3-electrodeposited films. Notably, the idea of depositing Cr(III) oxide-like films offers the opportunity to characterize them prior to electrochemical analysis, and thus interpret the results based on the films’ structure and composition.
Working rotating disk electrodes were home-made by molding Ti disks, with a 1-cm2 geometrical surface area, in epoxy, referred in the text as Ti. The electrodes were used as the substrate for electrodeposition of Cr(OH)3 and Cr2O3 films, henceforth referred to as Cr(OH)3@Ti and Cr2O3@Ti, respectively.
Prior to electrodeposition, Ti substrates were polished with SiC paper (Struers) and later with alumina slurries (OP-AN Struers) until mirror-like surfaces were obtained. To remove any contamination from the polishing steps, the electrodes were ultrasonicated in Milli-Q water for at least 10 min. The Ti electrodes were etched in a HF solution (3.5 mM in H2O, BASF) for 10 s, rinsed, and then directly immersed into the deposition solution.
Composition of the electrolyte used for the deposition of Cr(III) oxide and hydroxide films 
All electrochemical measurements were performed using a Gamry600™ potentiostat. The three-electrode glassy cell configuration was used, where a high-surface area platinum mesh was used as counter-electrode and the reference electrode was a double-junction Ag/AgCl (KCl 3 M, Metrohm, E = +0.210 V vs nhe). Potential sweeps were recorded between 0.0 and − 2.2 V (vs Ag/AgCl) at 50 mV s−1 to investigate surface stability. Capacitive currents were measured between 0.0 and − 0.8 V (vs (Ag/AgCl) to estimate the capacitance of Cr(OH)3- and Cr2O3-electrodeposited films.
Potentiostatic sweeps from − 0.2 to − 2.2 V (vs Ag/AgCl) in potential steps of − 0.1 V were also performed. At the same time, electrochemical impedance spectroscopy (EIS) measurements were carried out at every potential. EIS frequencies varied between 100 kHz and 10 mHz. For impedance experiments, the working electrode was placed in the centre of a round platinum mesh to improve the electrochemical response.
Cyclic voltammetry (CV), potentiostatic sweeps, and EIS were performed in de-aerated 0.2 M Na2SO4 at pH 11. The pH adjustment was made with addition of NaOH; N2 gas flow was maintained in the top of the cell to prevent dissolution of O2 in the electrolyte during experiments.
The surface morphology and composition were investigated with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) in a LEO Ultra 55 FEG SEM equipped with Oxford Inca EDX system. The molecular structure of the films was analyzed with Raman spectroscopy using an InVia Reflex spectrometer from Renishaw. The excitation source was the 532-nm line of a diode laser, which together with a grating of 2400 lines per millimeter gives a spectral resolution better than 1 cm−1. The presented Raman spectra are the result of 10 scans with a duration of 10 s each, if not otherwise stated. X-ray diffraction (XRD) was performed using a Siemens D5000 Diffractometer with a Bragg-Brentano setup equipped with a Cu kα X-ray source and a scintillation detector from 20° to 63° at 0.100° intervals of 2θ, and cumulated count time of 4 s for each step.
Results and Discussion
In electrochemistry, the determination of surface composition and surface area is a prerequisite for a complete investigation of mechanisms and kinetic parameters. However, the investigation of Cr(III) films formed during the chlorate electrolysis was always limited by the uncertainty of the surface composition. For instance, for a long time it was believed that the reduction of chromate yields a hydrated Cr(OH)3 film on platinum and gold  electrodes, but recently Hatch and Gewirth  determined with in situ Raman spectroscopy and atomic force microscopy (AFM) that the structure of chromate electrochemically reduced on gold is in fact a Cr2O3 thin film. In addition, a dual composition of the film had also been reported, with an inner Cr2O3-like structure covered by an outer Cr(OH)3 film [9, 30].
Chemical composition of Ti electrodes covered with electrodeposited Cr(OH)3 and Cr2O3
37 ± 5
47 ± 10
2 ± 1
20 ± 1
55 ± 8
17 ± 7
Figure 3 presents a collection of spectra recorded at increasing laser power from an electrode covered with Cr(OH)3. With the lowest laser power (i.e., 0.35 mW), a weak signature is observed at ≈ 850 cm−1, which is characteristic for amorphous Cr(OH)3 [33, 34]. This peak is still visible when a higher laser power is used, although other peaks also appear whose presence is attributed to the in situ formation of Cr2O3 as a result of local heating . Compared to the Raman spectra previously recorded for nanoparticles of amorphous Cr(OH)3 (see, e.g., Fig. 6 in ), the spectra recorded from Cr(OH)3@Ti appear more noisy (at comparable experimental conditions), which we attribute to the deposited film being very thin. In addition, the spectra recorded at low laser power (e.g., 0.35 and 0.70 mW) display a weak or inexistent peak at 530 cm−1 as compared to the intensity at 850 cm−1, a feature that, based on the assignment proposed in , suggests films of Cr(OH)3 with partially condensed –OH groups. Moreover, Raman spectra recorded at different spots but using the same laser power (3.50 mW) show some spatial inhomogeneities with a variable proportion of Cr2O3 versus C(OH)3 (see Fig. S1 in Supporting Information). We believe that these differences can arise from a variable thickness of the film, and thus different responses to the local heating induced by the laser.
Figure 4 shows the Raman spectra for Ti covered with Cr2O3 recorded at the same experimental conditions used in Fig. 3. For increasing laser power, the peaks characteristic of Cr2O3 [34, 35, 36, 37, 38] gain intensity, as already observed for nanoparticles of Cr2O3 . However, differently from the case of nanoparticles  the Raman spectrum of Cr2O3@Ti recorded with a laser power of 0.7 mW looks featureless, which could be attributed to the film here investigated being amorphous (see x-ray diffractogram in Supporting Information, Fig. S2) and consisting in elongated crystals with a length of ~ 600 nm and a width of less than 100 nm (different from the case of ~ 50-nm-sized nanoparticles grouped together into large particles studied in ).
It is known that the electrodeposition of so-called black chromium yields amorphous Cr(III) oxide films/deposits [29, 32, 39], whose actual composition is difficult to determine . XPS has thrown some light towards characterizing this type of films [29, 32, 39, 40], showing either a pure Cr2O3 phase [29, 39], a mixture of Cr2O3 and Cr(OH)3 , or a much more complex matrix of different Cr(III) oxy-hydroxides . The XPS investigation made in the present work (Fig. S3) has revealed that the surfaces correspond to Cr(III) oxide-like materials, with some percentage of Cr metal. The complexity of the C1s reference peaks, however, hinders a precise peak positioning and a distinction between Cr(OH)3 and Cr2O3 was not possible due to the proximity of the corresponding binding energies [41, 42]. Thus, we assume that the temperature increase induced by light absorption, in this case, promotes amorphous-Cr2O3 (or a mixture of oxides) to transform into the more thermodynamically stable crystalline-Cr2O3.
To confirm that the transition from amorphous to crystalline Cr2O3 was caused by light adsorption at laser intensities higher than 0.7 mW, experiments using 0.7 mW of laser power were carried out at scan times of 50 s (data not shown). However, the Raman spectra showed no peaks related to Cr2O3, indicating that the temperature increase by Raman laser absorption at 0.7 mW is not enough to crystallize amorphous Cr2O3. Also, a pure temperature conversion was studied by electrodepositing Cr2O3 onto a Ti disk, which was further inserted in an oven and heated up to 900 °C for 1 h. The results are summarized in Fig. S4 (Supporting Information), and the Raman spectra obtained for the heat-treated sample show the Cr2O3 features already with 0.7 mW laser energy.
Electrode Surface Area Estimation
From Fig. 6, it is noticeable that the overpotential for water reduction increases in the order Cr2O3@Ti < Ti < Cr(OH)3@Ti. At -2 mA, the difference between Cr(OH)3@Ti and Cr2O3@Ti is approximately 0.36 V. Ti sits in the middle, indicating that the chromium oxide layer activates the electrode towards water reduction, while the chromium hydroxide deactivates it.
Tafel slopes and kinetic parameters could not be precisely evaluated based on the potential sweeps, due to the high capacitance. Therefore, potentiostatic measurements were used instead. For these experiments, freshly prepared Cr(OH)3@Ti and Cr2O3@Ti electrodes were used. The steady-state current densities (jss) were measured together with impedance (EIS), at potentials for which water reduction takes place.
Kinetic parameters for water reduction on both Cr (III) films electrodeposited
Number of repetitions
Tafel slopea (mV dec−1)
Charge transfer coef. (α)a
Reaction rate (k)b (mol cm-2 s−1)
− 239 (± 27)
0.25 (± 0.03)
2.3 (± 1.5) × 10−10
− 145 (± 28)
0.41 (± 0.06)
9.9 (± 12) × 10−13
8.0 × 10−12
Electrochemical Impedance Spectroscopy
The HER Mechanism
The rate-determining step (rds) for the HER on Cr(OH)3 and Cr2O3 films was determined by the Tafel slopes. The Tafel slopes obtained using Cr2O3@Ti and Cr(OH)3@Ti were, respectively, − 239 ± 27 and − 145 ± 28 mV dec−1 (Table 3), indicating that the first electron transfer is the rds at both surfaces. The low α values indicate that the transition state is shifted to the right on the reaction coordinate axis, and the structure of the activated complex is closer to Hads than H2O.
The α value for the Cr(OH)3 film is higher than for the Cr2O3 film, ~ 0.4 compared with ~ 0.25. A possible explanation for the difference in the transfer coefficient is that water is integrated as part of the Cr(OH)3 film and the film itself is probably involved in the reduction reaction. In this way, the activated complex has an intermediate state between water and Hads. For the Cr2O3 film, on the other hand, the reaction occurs at the outmost layer and surface water or water from the solution is involved in the reaction. Therefore, the activated complex becomes closer in structure to the fully reduced form, Hads.
The kinetics of the HER were investigated by potential sweeps and steady-state polarization (Figs. 6 and 7). Although the reaction mechanism follows the same path on oxide and hydroxide, the overpotential for the HER is the lowest in the presence of Cr2O3, and the highest when Ti is covered with Cr(OH)3. The lower overpotential and low α value observed on Cr2O3@Ti can be rationalized to be due to the larger surface area and the lack of water in the Cr2O3 crystallites. On the other hand, as water in the Cr(OH)3 structure is involved in the HER, water has to be replaced by water molecules from the solution, otherwise the film will be destroyed. However, this is hampered by the slow ligand exchange of Cr(OH)3 as described by Tang et al.  and Rustad and Casey . This is corroborated by the results presented by Larses et al. , who showed increased overpotentials for the HER with increasing Cr content in the Fe1-xCrxOOH structure.
The composition of the electrodeposited films was revealed by Raman spectroscopy.
Cr2O3 displays lower overpotentials towards water reduction compared to both Cr(OH)3 and bare Ti electrodes.
For Cr(OH)3, on the other hand, the HER kinetics are slower resulting in the largest overpotentials.
Structural water is involved in the HER reaction on Cr(OH)3, and the slow kinetics are explained by the slow water exchange rate for Cr(III) complexes.
On Cr2O3, only surface water or water from solution is involved in the reaction, and the low overpotential was rationalized to be due to the high surface area.
This study has shown that in the search for a replacement for Cr(VI) in the chlorate process, both the composition and the morphology of the deposited layer need to be considered to avoid increasing overpotentials for the HER and subsequent environmental issues related to the resulting increase in energy consumption.
Financial support from the Swedish research council, 621-2010-4035, is gratefully acknowledged.
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