Operando Raman characterization of oxygen evolution reaction (OER) catalysts

Significance of the topic

Understanding the formation and stability of active catalyst phases during the oxygen evolution reaction (OER) is essential for the rational design of efficient, durable electrocatalysts. Operando Raman spectroelectrochemistry provides molecular-level, time-resolved insight into potential-dependent structural transformations of nickel-based materials that are commonly used in alkaline OER. Correlating vibrational fingerprints with electrochemical activity allows identification of active species (e.g., NiOOH) and clarifies mechanistic steps that govern performance and durability, informing development of improved catalysts for water splitting and related oxidation processes.

Aim and overview of the study

This application note demonstrates the use of a compact SPELEC RAMAN spectroelectrochemical system to track electrocatalyst evolution during OER-relevant electrochemical protocols. The goal was to show how synchronized Raman and electrochemical measurements reveal the electrochemical conversion of Ni(OH)2 to the active oxyhydroxide NiOOH and to correlate the onset of spectroscopic markers with applied potential.

Methodology

• Electrode preparation: Toray carbon paper (TCP) modified with Ni(OH)2 at a loading of 0.5 mg cm−2 served as the working electrode. Ag/AgCl and Pt wire were used as reference and counter electrodes, respectively.
• Electrolyte: 1.0 mol L−1 KOH prepared with ultrapure water.
• Preconditioning: 15 cyclic voltammetric cycles between 0.00 V and +0.70 V at 0.01 V s−1 to improve electrode–electrolyte contact and stabilize the film.
• Operando measurement protocol: Multipulsed Amperometric Detection (MAD) with 600 s potential steps from 0.00 V to +0.60 V in 0.10 V increments. Amperometric pulses were synchronized with Raman acquisition.
• Raman acquisition: 785 nm excitation, 30 s integration per spectrum, enabling direct attribution of each spectrum to a defined applied potential.

Used instrumentation

• SPELEC RAMAN spectroelectrochemical instrument: portable integrated unit combining a 785 nm class 3B laser, spectrometer (wavelength range ~787–1027 nm; Raman shifts ~35–3000 cm−1), and bipotentiostat/galvanostat.
• Spectroelectrochemical cell: RAMANCELL-M configured for electrode surface interrogation (laser focus on working electrode).
• Cable/connectivity: mStat cable connector and CABSTAT connection cable to interface electrodes and potentiostat.
• Software: DropView SPELEC for synchronized acquisition of electrochemical and Raman data and for subsequent data treatment.

Main results and discussion

• Potential-dependent spectral evolution: Two Raman bands at ca. 475 cm−1 and 550 cm−1 appeared when the applied potential reached and exceeded approximately +0.50 V vs Ag/AgCl.
• Band assignment: These features were attributed to NiOOH — specifically, the Eg bending vibration (~475 cm−1) and the A1g Ni–O stretching vibration (~550 cm−1). Their emergence marks the electrochemical oxidation of Ni(OH)2 to the active oxyhydroxide phase.
• Correlation with electrochemistry: The simultaneous acquisition established a clear potential threshold for active-phase formation and allowed temporal tracking of its evolution and stability under OER-relevant potentials. This links the spectroscopic signature directly to catalytic onset and behavior.
• Implications for mechanism: Detection of NiOOH formation supports mechanistic models where the oxyhydroxide phase is the OER-active state in alkaline Ni-based catalysts. Monitoring its formation and persistence provides insight into activation, deactivation, and possible structural changes during prolonged operation.

Benefits and practical applications of the method

• Real-time molecular-level observation: Operando Raman enables direct detection of active phases as they form under electrochemical control, overcoming ambiguity from ex situ techniques that may miss transient or unstable states.
• Rapid diagnostics for catalyst development: The technique can rapidly screen catalyst compositions and treatments to determine which modifications favor early, robust formation of active oxyhydroxide species.
• Transferability to related reactions: The same approach can be extended to study other oxidation electrocatalysis processes where Ni-based materials participate, such as urea oxidation or ammonia oxidation, aiding cross-application catalyst optimization.

Future trends and opportunities

• Higher time resolution and faster synchronization to capture transient intermediates during dynamic potential protocols.
• Combination with complementary operando probes (e.g., X-ray absorption, mass spectrometry, or electrochemical impedance) for a multi-modal mechanistic picture linking electronic, structural, and kinetic information.
• Integration with advanced data analysis (multivariate spectral decomposition, machine learning) to extract subtle signatures of mixed-metal or doped catalysts and to quantify phase fractions under operation.
• Miniaturized and field-deployable spectroelectrochemical platforms for in situ diagnostics in applied electrolyzers and industrial reactors.

Conclusion

This study demonstrates that a compact, fully integrated Raman spectroelectrochemical system can unambiguously identify the electrochemical conversion of Ni(OH)2 to NiOOH and determine the potential at which active oxyhydroxide forms during OER-relevant conditions. Operando Raman spectroscopy, synchronized with controlled electrochemical protocols, provides actionable mechanistic information that accelerates rational design and screening of Ni-based OER catalysts and can be extended to other oxidation electrocatalysis problems.

Authors

The application note was prepared in collaboration with the Composite Materials Group at INCAR-CSIC (Oviedo, Spain): María González-Rocha, Zoraida González Arias, and Victoria García Ingelmo.

References

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