Introduction

Hydrogen is a key energy vector for transitioning to sustainable energy systems, with applications in energy storage, power-to-gas systems, fuel cells, and transportation. Water electrolysis powered by renewable energy enables hydrogen production without direct CO₂ emissions, supporting the European Union’s climate neutrality goals [1]. Among technologies, alkaline water electrolysis (AWE) stands out for its maturity, robustness, scalability, and use of non-noble metal materials [2,3]. Regarding electrodes, nickel is a reference material for AWE because of its chemical stability and ability to form catalytically active phases such as nickel hydroxides and oxyhydroxides (β- and α-NiOOH) at higher potentials, enhancing reaction kinetics and lowering overpotentials for both HER and OER [2–4].

Furthermore, photovoltaic (PV) energy is becoming a promising renewable energy source. However, the intermittent availability and the fluctuations in the offer–demand market create the need for incorporating energy carriers. Based on this, integrating AWE with renewable sources introduces dynamic operation challenges: load variations and on/off cycles may affect electrode behavior and long-term durability [5].

In this work, we explored the formation of different active catalytic species of Ni under continuous and intermittent regimes simulating realistic photovoltaic (PV) generation profiles, focusing on their potential application in green hydrogen production [2,3].

Methods

This study investigates the electrochemical behavior of smooth Nickel 200 (Ni200, Electrocell, Denmark) electrodes as cathodes and anodes under alkaline electrolysis with renewable energy profiles. Experiments were conducted in 30 wt % KOH at 70 °C and atmospheric pressure, applying two polarization regimes: long-term polarization (LP) and intermittent polarization (IP).

Electrochemical characterization and LP tests used three-electrode cells with a 0.95 cm² active area, Pt counter electrodes, and a reversible hydrogen electrode (RHE), connected to an Autolab PGSTAT302N potentiostat.

For hydrogen evolution reaction (HER), a current density of −150 mA cm⁻² was applied for up to 100 h to assess surface activation and long-term stability. The formation of active species was monitored by cyclic voltammetry at 25, 50, and 100 h of testing. For oxygen evolution reaction (OER), +150 mA cm⁻² was applied, evaluating the formation of β-NiOOH and anodic activation via cyclic voltammetry, polarization curves, and chronopotentiometry.

IP tests were performed in 10 cm² AWE cells on a CNH2-designed test bench, following the criteria defined by the JRC (Joint Research Centre) and using open-access data from the Belgian electricity grid. A simulated PV profile was applied over 100 h to replicate daily cycles consisting of 11 h of daylight (0.01–1.5 A) and 13 h of night (0 A). After testing, electrodes were characterized through cyclic voltammetry, morphological analysis, and X-ray photoelectron spectroscopy (XPS) to identify surface species (nickel hydroxides, oxyhydroxides, and oxides) associated with performance improvements.

Results and Discussion

Results show progressive electrochemical activation of Ni200 electrodes under both regimes. In cathodic conditions, LP reduced the HER overpotential by up to 20 mV, associated with α-Ni(OH)₂ formation, as seen in cyclic voltammetry [2,4]. IP produced similar improvements after several cycles without degradation, showing that surface activation persists under dynamic conditions [5].

Anodic tests showed an increase in the redox peak corresponding to β-Ni(OH)₂ → β-NiOOH after 100 h of LP, with OER potential decreasing to ~1.4 V vs. RHE [3,4]. IP led to a potential decrease of ~300 mV after one day, reducing activation overpotential [5]. Polarization curves revealed a passivation region between 1.0 and 1.5 V vs. RHE, linked to Ni³⁺ species (β-NiOOH) formation, which likely stabilizes the surface and improves OER durability [3,4]. Additionally, iron traces originating from KOH impurities were detected by ICP analysis, contributing to an enhanced electrode performance, as confirmed by long-term testing.

Conclusions

This study highlights the effects of constant and intermittent polarization on Ni electrodes in alkaline water electrolysis. At the anode, polarization promotes β-NiOOH formation, reducing the OER overpotential and improving electrocatalytic performance [3]. At the cathode, surface activation of Ni200 via α-Ni(OH)₂ lowers the HER overpotential, although phase stability may influence long-term durability [2,4]. These results demonstrate that simple polarization strategies can effectively activate Ni electrodes under dynamic conditions, advancing the understanding of Ni electrode behaviour for renewable energy applications [2,3,5].

References

[1] Yue, M.; Lambert, H.; Pahon, E.; Roche, R.; Jemei, S.; Hissel, D. Hydrogen energy systems: A critical review of technologies, applications, trends and challenges. Renew. Sustain. Energy Rev. 2021, 146, 111180.

[2] Xue, S.; Liang, Y.; Hou, S.; Zhang, Y.; Jiang, H. Alpha-Nickel Hydroxide Coating of Metallic Nickel for Enhanced Alkaline Hydrogen Evolution. ChemSusChem 2022, 15, e202201072.

[3] Danilovic, N.; Subbaraman, R.; Strmcnik, D.; Chang, K.C.; Paulikas, A.P.; Stamenkovic, V.R.; Markovic, N.M. Enhancing the alkaline hydrogen evolution reaction activity through the bifunctionality of Ni(OH)₂/metal catalysts. Angew. Chem. Int. Ed. 2012, 51, 12495–12498.

[4] De Groot, M.T. Curr. Opin. Chem. Eng. 2023, 42, 100981.

[5] Rocha, F.; Delmelle, R.; Georgiadis, C.; Proost, J. J. Environ. Chem. Eng. 2022, 10, 107648–107659.