Revealing battery secrets with ECRaman solutions

Significance of the topic

Nickel-metal hydride batteries play a pivotal role in electric vehicles and renewable energy storage due to their balance of energy density, cycle life, and environmental impact.
Understanding their electrochemical behavior at the electrode interface is essential for optimizing performance and extending battery lifetime.
Electrochemical Raman spectroscopy (EC-Raman) offers real-time, in situ insight into the physicochemical transformations of active materials during charge and discharge.

Aims and Study Overview

This study demonstrates the use of a hyphenated EC-Raman approach to monitor the reversible oxidation of Ni(OH)₂ to NiOOH under simulated cycling conditions.
The objectives are to correlate electrochemical signatures with Raman spectral changes and to compare measurements in a sealed EC-Raman flow cell versus an open screen-printed electrode (SPE) format.

Methodology and Instrumentation

The workflow comprises three main steps:

• Electrochemical roughening of a gold working electrode to increase surface area and enhance Raman signal.
• Electrodeposition of Ni(OH)₂ by chronopotentiometry in Ni(NO₃)₂ solution.
• Simulated charge/discharge cycles via cyclic voltammetry (CV) between −0.4 and +1.5 V vs. Ag/AgCl while continuously acquiring Raman spectra.

Used instrumentation:

• RAMAN: i-Raman Prime 532H spectrometer with 50× objective and BWSpec software.
• POTENTIOSTAT: Metrohm Autolab PGSTAT302N.
• EC-Raman flow cell with Au working electrode, Pt counter, Ag/AgCl reference.
• Screen-printed electrode (220BT) with gold WE/CE and silver RE.

Main Results and Discussion

Cyclic voltammograms display reversible peaks near 0.50 V vs. Ag/AgCl, corresponding to the Ni(OH)₂/NiOOH redox couple.
Raman spectra collected at different potentials reveal:

• No detectable bands at 0 V due to low cross-section or thin deposit.
• Distinct NiOOH bands at 476 and 556 cm⁻¹ during oxidation above the redox potential.
• Reversion to baseline spectra upon reduction, confirming reversibility.

Comparative testing shows stronger Raman signals and slightly shifted redox peaks on the open SPE, attributed to differences in electrode geometry, reference behavior, and bubble management.

Benefits and Practical Applications

EC-Raman allows direct observation of structural changes in active battery materials, facilitating rapid screening of electrode formulations.
This approach supports advanced diagnostics for battery quality control and research into novel chemistries.
Closed cells offer better environmental control, while SPE formats enable cost-effective, high-throughput testing.

Future Trends and Potential Applications

Integration of EC-Raman with flow-through cells and automated sampling can accelerate combinatorial electrode discovery.
Expansion to other battery systems (Li-ion, solid-state) and coupling with complementary techniques (e.g., IR, mass spectrometry) will deepen mechanistic insights.
Advancements in probe design and multivariate analysis will further improve sensitivity and data interpretation.

Conclusion

The hyphenated EC-Raman methodology effectively tracks Ni(OH)₂/NiOOH transformations under cycling in both closed and open electrode configurations.
The correlation of electrochemical data with Raman spectroscopy validates EC-Raman as a powerful tool for battery material analysis and development.

References

1. O’Dea, S. Battery market size worldwide by technology 2018–2030. Statista (2023).
2. Yeo, B. S.; Bell, A. T. In situ Raman study of nickel oxide catalysts. J. Phys. Chem. C 2012, 116, 8394–8400.
3. Tian, Z.-Q.; Ren, B.; Wu, D.-Y. Surface-enhanced Raman scattering: from noble to transition metals and ordered nanostructures. J. Phys. Chem. B 2002, 106, 9463–9483.