Operando Raman characterization of oxygen evolution reaction (OER) catalysts

Significance of the topic

Operando Raman spectroelectrochemistry enables direct, time-resolved observation of structural and chemical changes at electrocatalyst surfaces during applied potentials. For nickel-based materials used in the oxygen evolution reaction (OER), identifying the formation and stability of active oxyhydroxide phases is essential to understand catalytic mechanisms, optimize activity and durability, and guide rational catalyst design. The approach therefore bridges spectroscopy and electrochemistry to deliver mechanistic insight that is immediately relevant to energy conversion technologies and industrial electrolysis.

Objectives and study overview

The application note demonstrates the use of an integrated Raman spectroelectrochemical instrument to track potential-dependent transformations of a nickel hydroxide catalyst during OER-relevant conditions. The goals were to (i) synchronize electrochemical control and Raman acquisition, (ii) detect the electrochemical conversion of Ni(OH)2 into its active NiOOH oxyhydroxide phase, and (iii) correlate the emergence of vibrational signatures with applied potential to inform catalytic behaviour.

Methodology

Samples and electrolyte:

• Working electrode: Toray carbon paper (TCP) loaded with Ni(OH)2 at 0.5 mg cm−2.
• Reference electrode: Ag/AgCl.
• Counter electrode: platinum wire.
• Electrolyte: 1.0 mol L−1 KOH prepared with ultrapure water.

Electrochemical conditioning and measurement protocol:

• Initial conditioning: 15 cyclic voltammetry cycles from 0.00 V to +0.70 V at 0.01 V s−1 in 1 M KOH to improve electrode/electrolyte contact.
• Operando measurement: Multipulsed Amperometric Detection (MAD) sequence applying 600 s potential steps from 0.00 V to +0.60 V in +0.10 V increments.
• Raman acquisition: spectra recorded concurrently with electrochemical pulses; integration time 30 s per spectrum. The precise synchronization of optical and electrochemical signals ensured each spectrum could be assigned to its applied potential.

Instrumentation used

The study employed a compact, integrated spectroelectrochemical system combining optical and electrochemical modules. Key hardware and software components included:

• SPELEC RAMAN instrument: integrated 785 nm laser (class 3B), spectrometer and bipotentiostat/galvanostat in a single portable enclosure; spectrometer covers 787–1027 nm (Raman shift ~35–3000 cm−1).
• DropView SPELEC software: controls instrument, synchronizes Raman and electrochemical acquisition, and provides basic data processing tools.
• Raman spectroelectrochemical cell compatible with microscope focusing to the working electrode surface.
• Accessories: connection cable (CABSTAT / mStat connector) for conventional electrodes.

Main results and discussion

Operando Raman measurements revealed clear potential-dependent spectral changes associated with activation of the Ni catalyst. Two Raman bands at approximately 475 cm−1 and 550 cm−1 were observed to appear from potentials of +0.50 V onward. These bands are assigned to the Eg bending vibration and the A1g stretching vibration of Ni–O in NiOOH, respectively, demonstrating the electrochemical formation of the NiOOH oxyhydroxide active phase from the initial Ni(OH)2.

The synchronized MAD protocol allowed unambiguous correlation between applied potential and spectral signatures, confirming that Ni(OH)2 → NiOOH conversion occurs under the studied alkaline conditions and at potentials relevant to OER activity. Detection of these vibrational modes provides qualitative evidence of active phase generation and offers information about the formation onset and relative stability of that phase during extended polarization steps.

Implications of the findings include direct evidence linking structural evolution at the catalyst surface with its electrochemical state, validating the use of operando Raman to probe activation processes that are often transient or potential-dependent. The data support that the NiOOH phase is central to OER catalysis on Ni-based materials and that monitoring its formation can serve as a diagnostic of catalyst readiness and performance.

Benefits and practical applications of the method

Operando Raman spectroelectrochemistry as described delivers several practical advantages:

• Real-time identification of active phases and phase transitions under working conditions, which is more informative than ex situ characterization.
• Ability to correlate spectroscopic markers with electrochemical parameters (potential, current), informing mechanistic interpretations and performance optimization.
• Applicability to a range of oxidation processes beyond OER, including urea oxidation, ammonia oxidation, and other reactions where Ni-based catalysts are relevant.
• Compact, integrated instrumentation reduces experimental complexity and improves reproducibility by eliminating the need to separately align electrochemical and optical setups.

Future trends and potential applications

Opportunities to extend and enhance operando Raman spectroelectrochemistry include:

• Combining operando Raman with complementary in situ techniques (e.g., X-ray absorption, electrochemical impedance spectroscopy, mass spectrometry) for a multi-modal mechanistic picture.
• Applying surface-enhanced Raman scattering (SERS) strategies to increase sensitivity and enable detection of low-concentration intermediates at interfaces.
• Time-resolved and faster-scan Raman protocols to capture short-lived intermediates during transient electrochemical events.
• High-throughput operando screening of catalyst libraries to accelerate materials discovery and optimization.
• Integration with data science and machine learning for automated spectral deconvolution, phase identification and correlation with performance metrics.

Conclusion

This application note demonstrates that synchronized Raman spectroelectrochemistry is an effective and practical tool to observe the potential-driven formation of active NiOOH phases from Ni(OH)2 under alkaline OER conditions. The technique provides direct vibrational evidence of activation processes, links structural evolution to electrochemical behavior, and offers a pathway to better understand and design Ni-based electrocatalysts for water oxidation and related electrochemical oxidations.

Reference

1. Yang, X.; Zhang, H.; Yu, B.; et al. An Unveiled Electrocatalysis Essence of NiCo Hydroxides through in Situ Raman Spectroscopy for Urea Oxidation. Energy Technology 2022, 10 (5), 2101010. DOI: 10.1002/ente.202101010
2. Yan, Z.; Sun, H.; Chen, X.; et al. Anion Insertion Enhanced Electrodeposition of Robust Metal Hydroxide/Oxide Electrodes for Oxygen Evolution. Nature Communications 2018, 9 (1), 2373. DOI: 10.1038/s41467-018-04788-3