Effects of Incorporated Iron or Cobalt on the Ethanol Oxidation Activity of Nickel (Oxy)Hydroxides in Alkaline Media
Nickel (oxy)hydroxides (NiOxHy) are promising cost-effective materials that exhibit a fair catalytic activity for the ethanol oxidation reaction (EOR) and could be used for sustainable energy conversion. Doping the NiOxHy structure with other metals could lead to enhanced catalytic properties but more research needs to be done to understand the role of the doping metal on the EOR. We prepared NiOxHy films doped with Fe or Co with different metallic ratios by electrodeposition and evaluated the EOR. We found a positive and negative effect on the catalytic activity after the incorporation of Co and Fe, respectively. Our results suggest that Ni atoms are the active sites for the EOR since Tafel slopes were similar on the binary and pristine nickel (oxy)hydroxides and that the formal potential of the Ni(II)/Ni(III) redox couple is a good descriptor for the EOR activity. This work also highlights the importance of controlled metal doping on catalysts and may help in the design and development of improved materials for the EOR.
KeywordsEthanol oxidation reaction Nickel catalyst Bimetallic catalysts Electrocatalysis Energy conversion
In the last years, much effort has gone into the study of the electrochemical oxidation of alcohols for energy-related applications such as fuel cells  or hydrogen generation . Ethanol is very appropriate due to its lower toxicity and higher energy density compared to methanol and to the possibility of renewable production from biomass . Precious metal catalysts, especially Pt or Pd, show high electrocatalytic activity towards alcohol oxidation at low potentials. However, these materials have several drawbacks, such as limited global availability, high cost and deactivation issues by irreversible oxidation , or adsorption of poisoning species . Therefore, the search for alternative earth-abundant electrocatalysts is still a constant concern. Nickel-based materials have been employed for the electrocatalytic ethanol oxidation [6, 7, 8, 9] due to their low cost and high stability in alkaline media. Some strategies to enhance the overall catalytic activity for alcohol oxidation involve the exfoliation of the nickel layered hydroxide  in order to generate more reactive sites and expose a higher number of them to the solution, the synthesis of nanostructured materials with controlled shape [10, 11] or the use of effective catalyst supports such as carbon nanoflakes [12, 13] or nanofibers . The incorporation of other metal atoms to the nickel (oxy)hydroxide (NiOxHy) structure is also a known method to change the catalytic properties of the material .
For instance, bimetallic Co/Ni materials have been reported for the ethanol oxidation reaction (EOR) [16, 17, 18]. Carbon nanofibers modified with NiCo alloyed nanoparticles showed increased catalytic activity for ethanol oxidation  compared to the same material formed only by nickel nanoparticles. These alloyed nanoparticles were synthesized at high temperatures in a reducing atmosphere leading to elemental NiCo, which needed electrochemical activation to generate active nickel hydroxides for ethanol oxidation. Interestingly, the oxidation activity was different for different metal ratios. Similar conclusions were reached using graphene as the carbon support for the CoNi alloyed nanoparticles . Mixed Ni-Co oxides with different structural properties were also reported for the ethanol oxidation in alkaline media [18, 19]. For instance, carbon nanotubes-supported NiCo2O4 nanocomposite aerogels were used for this purpose . The fibrous network of the nanotubes enabled the preparation of a material with uniform dispersion of NiCo2O4 nanoparticles that showed excellent activity for ethanol oxidation. Materials with different geometric structures such as mesoporous NiCo2O4 fibers also showed enhanced activity compared to NiO and Co3O4 materials , demonstrating the positive synergistic effect of combining both materials.
Incorporation of Fe to NiOxHy has been widely employed for the oxygen evolution reaction (OER) as it has appeared as one of the best catalysts for this reaction , but just a few reports have been published using NiFe or Fe-based (oxy)hydroxides for alcohol oxidation. For instance, a multi-component NiFe hydroxide nanocatalyst was evaluated for oxygen evolution and methanol oxidation . A slightly enhanced response was found using the bimetallic NiFe catalyst compared to a Ni material, and the material only composed of Fe showed a very low activity for methanol oxidation. In contrast, FeOOH nanorods modified with fluorine atoms was demonstrated as a great catalyst for both OER and EOR . In summary, as suggested by the different reported materials, there is enough evidence that nickel-based catalysts modified with other metal and non-metal atoms can enhance the EOR activity and it is a good strategy for designing improved materials. Thus, a systematic fundamental study of the incorporation of different metals to NiOxHy materials for the ethanol oxidation is needed to understand the role of metal doping, which would facilitate the design of new materials with enhanced properties.
In this work, we evaluate the effect of Fe or Co incorporation on the ethanol oxidation activity of electrodeposited NiOxHy films in alkaline solution. Metallic (oxy)hydroxide catalytic films with different metal ratios were prepared by a simple electrodeposition method. Analytical and electrochemical characterization of the catalysts was performed to gain a deeper understanding of the effect of metal doping on the nickel (oxy)hydroxides for the ethanol oxidation reaction. We conclude that the ethanol oxidation follows the same mechanism in the binary materials and the pristine nickel films, suggesting that only the nickel atoms are the active sites.
Material and Methods
Reagents and Solutions
Ni(NO3)2, Co(NO3)2, FeCl3, K4[Fe(CN)6], NaNO3, ethanol absolute, and NaOH were purchased from Merck. For experiments in the absence of Fe, the NaOH electrolyte was cleaned to avoid impurities using Ni(OH)2 as an Fe absorbent following a previously reported method . Ultrapure water obtained with a Millipore DirectQ3 purification system from Millipore was used throughout this work.
All the electrochemical experiments were performed at room temperature (21 ± 1 °C). After electrodeposition of catalysts, one cyclic voltammetry between + 0.8 V and 2.15 V (vs RHE) at 10 mV/s was performed in 0.1 M NaOH just before carrying out the ethanol oxidation experiments. Data shown in the figures is the average of three independent measurements and the error bars are the standard deviation of those measurements.
Electrochemical Deposition of Catalytic Materials
Electrodeposition was conducted onto disk electrodes using an unstirred solution with 0.1 M total metal concentration. When using Fe(III) solutions, sodium nitrate was added to keep a constant concentration of nitrate at 0.1 M in all the solutions. Electrochemical deposition was performed in a two-electrode cell with a carbon rod as a counterelectrode by applying a cathodic current of − 5 mA cm−2 for 30 s. After the deposition, the electrodes were rinsed with ultrapure water. In order to simplify the nomenclature, the binary materials are named as Ni1-xMxOyHz, where the x is the atomic fraction of the metals as recorded by energy-dispersive X-ray spectroscopy (vide infra).
Estimation of the Electrochemical Surface Area
Compositional and Structural Characterization of Catalysts
The catalysts were electrodeposited as previously described and to avoid the effect of the electrode substrate in the characterization, the films were rinsed with ultrapure water, left to dry, and scraped carefully to get a powdered sample. This process was repeated several times in order to get enough material for the characterization. The compositional analysis and determination of the experimental metallic ratio of the catalyst films were carried out by energy-dispersive X-ray spectroscopy (EDS). The powdered samples were placed on carbon conductive tabs and analyzed on a JEOL JSM-7000F scanning electron microscope using the integrated EDS detector. An acceleration voltage of 15 kV was applied. The crystalline properties of the catalysts were analyzed using powder X-ray diffraction (XRD) recorded with a PANalytical PRO MPD diffractometer in Bragg-Brentano geometry with 1.5406 Å Cu Kα1 radiation, using a 2θ range of 8.0–70.0° and a step size of 0.033°. Samples for XRD analysis were carefully grinded to a homogenous powder and deposited with the aid of isopropanol on a zero-background sample holder based on a Si wafer for use in reflection geometry.
Results and Discussion
Analytical Characterization of Metallic Catalysts
Experimental metal ratio for the different catalysts obtained by EDS data compared to the initial metal ratio of the precursor solution
Precursor solution (Ni:M)
EDS data (Ni:Co)
EDS data (Ni:Fe)
Figure 2 (red line) also shows the XRD pattern of the monometallic cobalt material, CoOxHy. In this case, some wide peaks were observed at 2Θ of 10.8, 33.7, and 51.5, and around 60°, and narrower peaks, suggesting a more crystalline phase, were also observed at 2Θ of 19.1 and 38.0°. It may be challenging to assign some of these peaks to the correct structure, but it seems clear that the wide peak at 10.8° can be assigned to the (001) lattice plane of low-crystalline α-Co(OH)2  while the narrow peak at 19.1° can be definitely ascribed to the (001) lattice plane of crystalline β-Co(OH)2 . Therefore, this material seems to be a combination of the α and β phases of Co(OH)2. XRD patterns for the bimetallic materials (Ni0.69Co0.31OxHy and Ni0.77Fe0.23OxHy) only showed similar features to those observed for α-Ni(OH)2 with a general small shift of the XRD peaks. This suggests the successful incorporation of Co or Fe leading to a more disordered α-Ni(OH)2 structure.
Effects of Incorporated Metals on the Ethanol Oxidation
The voltammograms obtained for the bare Ni electrode are also shown in Fig. S5 for comparison. For the bare electrode, a significantly lower activity for the EOR is observed. This fact demonstrates that the electrodeposited nickel film is essential to achieve a high catalytic activity for the EOR.
Effects of the Metallic Ratio on the Catalytic Activity and Reaction Mechanism
Effect of the Electronic Properties of Nickel Atoms on the Activity
We evaluated Ni (oxy)hydroxide films with the incorporation of increasing amounts of Fe or Co for the EOR. Co doping led to enhanced EOR catalysis while Fe addition showed a negative effect. A Tafel slope analysis indicated that the EOR follows the same mechanism in pristine nickel hydroxide than in binary catalysts. The catalytic activity was well correlated with the electronic properties of the nickel atoms as given by the formal potential of the Ni(II)/Ni(III) redox couple. Both findings are consistent with the nickel atoms being the active sites for the EOR. These results help to understand the effects of metallic doping in nickel (oxy)hydroxides on the catalytic activity and mechanism for the EOR and may be significant for the development of improved materials.
The authors are grateful for the support from the Swedish Energy Agency (Ref. 2017-004908).
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