NiFe Foam as an Electrocatalyst After Harsh Anodization

By Surbhi JainSep 19 2022Reviewed by Susha Cheriyedath, M.Sc.

In an article recently published in the journal ACS Applied Energy Materials, researchers discussed the development of an efficient and stable electrode for oxygen-evolution reaction (OER) through the anodization of a NiFe foam.

Study: Anodization of a NiFe Foam: An Efficient and Stable Electrode for Oxygen-Evolution Reaction. Image Credit: Party people studio/Shutterstock.com

Background

Fossil fuels are scarce, and their harm to the environment makes it necessary for people to employ clean, sustainable energy sources. A promising method for storing various energy sources is to split water to create molecular hydrogen on a massive scale.

However, a bottleneck for water splitting is the OER through the water-oxidation reaction. Ru4 and Ir5 compounds are effective catalysts for the OER, but they are too expensive and scarce to be used widely. The high costs of Ru and Ir are not major issues because thin IrO₂- or RuO₂-based catalysts are readily accessible and inexpensive. However, due to their dearth in the crust of the Earth, they should be replenished.

NiFe (hydr)oxides function well as OER catalysts in alkaline solutions. NiFe and other metal (hydr)oxides have been synthesized toward the OER using a variety of techniques. According to certain studies, enhancing the metal-metal oxide contact can significantly increase catalytic activity. The interfaces created between metals and metal oxides are beneficial for strain production, electron transfer, oxygen adsorption energy modification, and transfer of electrons to metal oxides.

About the Study

In this study, the authors discussed the development of a NiFe foam as an effective and stable electrocatalyst following severe anodization at 60 V in a two-electrode system. Several techniques were used to characterize the NiFe oxide that formed on the surface of NiFe foam. These techniques demonstrated the existence of several NiFe (hydr)oxides on the surface of the NiFe foam, including NiO, Ni(OH)₂, and NiO(OH).

The team demonstrated that the overpotential for the beginning of the OER was 220 mV for the prepared electrode in the KOH solution (1.0 M). The overpotentials for the 1, 10, and 100 mA/cm² activities were observed at, respectively, 290, 346, and 500 mV. The bare foam was shielded from further oxidation by a stable NiFe-oxide layer resulting in an OER electrocatalyst that was stable.

The researchers used anodization as a quick and easy way to create a surface of NiFe foam that was nanostructured using NiFe (hydr)oxide. The ratio of Fe to Ni was 1:50, which was low enough to prevent a significant amount of Fe oxide from forming. However, the ratio was substantial enough to prevent Fe depletion from occurring on the anodized NiFe foam's surface. An increase in Fe oxide synthesis or a significant reduction in Fe resulted in a decrease in the OER for NiFe (hydr)oxide.

Observations

The OER moved quickly at a current density of 50 mA/cm², and gas bubbles developed without noticeably accumulating on the electrode's surface. At overpotentials of 290, 346, and 500 mV, respectively, the current densities of 1, 10, and 100 mA/cm² were seen. The inductively coupled plasma mass spectrometry (ICP-MS) demonstrated that no Fe or Ni was found in the electrolyte after 10 hours of chronoamperometry at 1.61 V. The anodized foam's OER activity was four times more than that of the fresh foam at 1.55 V. The total specific surfaces of the fresh and anodized electrodes, according to the Brunauer, Emmett, and Teller (BET) method, were calculated to be 3.7 and 10.3 m².g^-1, respectively.

The fresh and anodized electrodes' particle pore sizes ranged from 3.2 to 30 nm. A fresh NiFe foam measured by chronoamperometric analysis at 1.61 V had a current density value of 11.4 mA/cm², which increased to 17.9 mA/cm² when the solution was stirred. Without stirring, a current density of 17 mA/cm² was seen for 24 hours. The oxygen metric experiment and chronoamperometry were combined, and the evolved oxygen for the anodized NiFe foam was directly sensed using fluorescence. This revealed a Faradaic efficiency of 75% for the first 500 s, but more than 90% after that.

The anodized electrode's Tafel plot at pH 14 demonstrated that the Log(j) vs. overpotential was linear, with a slope as low as 56.8 mV per decade. According to corrected cyclic voltammetry (CV) plots based on electrochemical active surface area (ECSA) or BET methods, the number of active sites rather than the amount of activity at each site increased for the anodized foam. However, at low overpotentials in the range 220–250 mV, adjusted CVs based on both BET and ECSA demonstrated higher activity for the anodized electrode than a fresh foam due to the higher concentration of Fe on the surface of the anodized electrode compared to a fresh electrode.

Conclusions

In conclusion, this study elucidated that the creation of reliable and effective OER catalysts is essential to putting green energy technology on the market. By use of anodization, in a simple, one-step process, nanostructured NiFe oxides were created on the surface of the NiFe foam.

NiFe oxide was characterized using several techniques, which demonstrated that a variety of NiFe oxides were produced during the anodization process.

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References

Hashemi, N., Nandy, S., Chae, K. H., et al. Anodization of a NiFe Foam: An Efficient and Stable Electrode for Oxygen-Evolution Reaction. ACS Applied Energy Materials, (2022). https://pubs.acs.org/doi/10.1021/acsaem.2c01707

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