OER remains the dominant bottleneck in electrochemical water splitting because it is a sluggish four-electron process that demands high energy input. Although precious-metal oxides such as RuO₂ and IrO₂ perform well, their scarcity and cost block large-scale deployment. NiFe-based catalysts have therefore been the target of intense research, yet the atomic-level role of anionic species like phosphorus in catalyst reconstruction has been unclear.

The researchers introduce a phosphorus-driven, multi-step engineered catalyst that starts from Ni precursor prisms, forms a hollow NiFe cyanide framework, and is then phosphidated at 350 °C under argon to yield hollow NiFeP prisms. Under anodic OER, these hollow structures undergo dynamic reconstruction into ultrathin, defect-rich NiFe (oxy)hydroxide nanosheets—captured in real time using identical-location TEM and supported by spectroscopy and electrochemical measurements.

Crucially, phosphorus does not merely leave the lattice. Residual phosphate oxyanions (PO₄³⁻) remain and cooperate with iron to act as an intrinsic redox buffer. This synergy suppresses Fe dissolution and stabilizes the key oxygenated intermediates that control reaction kinetics, while also preventing detrimental nickel over-oxidation.

Density functional theory calculations reveal that PO₄³⁻ modifies the reconstructed NiFe (oxy)hydroxide electronic environment. Together with Fe, phosphate narrows the bandgap dramatically (from ~0.85 eV to ~0.15 eV), enhancing charge delocalization and conductivity. Free-energy analysis shows a compressed energy span among OH, O, and *OOH, leading to a significant reduction in theoretical overpotential.

Experimentally, the reconstructed NiFeP catalyst delivers a low overpotential of 225 mV at 10 mA cm⁻² in alkaline media. Its performance surpasses pre-NiFe, NiFeO, Ni₂P, and even commercial RuO₂ under comparable conditions. Kinetic analysis yields a Tafel slope of 31 mV dec⁻¹, and electrochemical impedance indicates fast charge transfer.

The catalyst maintains stability for over 100 hours even at high current densities, reaching up to 500 mA cm⁻² without meaningful degradation. In two-electrode overall water splitting paired with Pt/C, the system reaches 10 mA cm⁻² at 1.51 V and sustains operation for more than 100 hours, outperforming a Pt/C || RuO₂ reference.

More broadly, the work reframes phosphorus as an active, synergistic promoter rather than a disposable additive. By revealing how residual anions buffer redox states and stabilize intermediates during reconstruction, it provides a blueprint for designing next-generation, anion-engineered OER catalysts suited to scalable green hydrogen production.

Subject of Research: Oxygen evolution reaction (OER) electrocatalysis for alkaline water splitting
Article Title: Unlocking the Synergistic Promoter Role of Phosphorus in Evolving NiFe Phosphides for Enhanced Water Oxidation
News Publication Date: 11-Jun-2026
Web References: http://dx.doi.org/10.1007/s40820-026-02238-0
References: 10.1007/s40820-026-02238-0
Image Credits: Ningning Shi, Mingcheng Gao, M. Maneesha, C. S. Praveen, Panpan Liu, Shengnan Yue, Wangjing Xie, Dechao Chen, Yu Tang, Yuanqing Wang, Hua Fan, Xing Huang*.

Keywords

Oxygen evolution reaction; NiFe phosphides; phosphorus promotion; phosphate anion synergy; redox buffering; catalyst reconstruction; alkaline water splitting; water oxidation; electrochemical energy conversion.

The isolation of this long-lived NiOOH phase marks a pivotal advancement. What sets this phase apart is its unusually high concentration of nickel in the +4 oxidation state, a chemical characteristic rarely stable under normal conditions. This finding challenges prevailing assumptions wherein Ni^3+ species were thought to dominate catalytically active states. By carefully controlling electrochemical environments and employing sophisticated spectroscopic techniques, the research team revealed a stable, bulk Ni–O–O–Ni_2 configuration within the NiOOH matrix. This unique structural motif is not merely a transient intermediate but persists throughout the oxygen evolution process.

Perhaps the most astonishing discovery is the spontaneous release of oxygen molecules at room temperature and in pure water from this Ni–O–O–Ni_2 containing NiOOH phase, all happening without any external applied potential. This unprecedented phenomenon implies that the catalyst itself stores enough oxidative potential to drive oxygen evolution autonomously. Such self-driven catalysis defies traditional electrochemical paradigms and opens the door to new energy-efficient strategies for water splitting and beyond.

To precisely characterize the oxygen evolution dynamics, the team utilized online mass spectrometry, an advanced technique that allows real-time detection and quantification of evolved gases. This enabled them to dissect the reaction sequence, evidencing that lattice oxygen atoms within the catalyst actively engage in O–O bond coupling, a critical step forming the nascent oxygen molecule. Following this initial lattice oxygen involvement, sustained oxygen generation proceeds via continued water oxidation at surface-active sites enriched with Ni^4+ ions, confirming a layered, dual-pathway catalytic mechanism.

This dual mechanism—initiated by lattice oxygen coupling followed by ongoing surface oxidation—is groundbreaking. It implies a synergy between bulk and surface phenomena within the catalyst, where bulk-stored charges in high-valence nickel centers effectively migrate to surface active sites. This charge mobility not only sustains catalysis but also points to an intrinsic reservoir of oxidative potential embedded in the catalyst’s interior. Understanding this charge transfer conduit reshapes conventional models of catalytic oxygen evolution, highlighting the importance of ‘reserved charges’ in long-lived active phases.

From a materials chemistry perspective, stabilizing Ni^4+ centers in NiOOH under operational conditions has been a formidable challenge due to their tendency toward reduction or structural destabilization. The researchers’ success in isolating and maintaining these centers opens avenues for engineering catalysts with finely tuned electronic structures. Such precision could drastically improve catalytic efficiency and durability by preventing degradation pathways that currently limit catalyst lifetimes.

Furthermore, this work underscores the significance of lattice oxygen participation in water oxidation, a mechanism often overshadowed by classical surface adsorption and desorption models. The direct coupling of oxygen atoms within the catalyst lattice shifts the paradigm toward ‘lattice oxygen redox’ contributions in catalytic cycles, which may be leveraged to design catalysts that exploit similar stored oxygen species for enhanced performance.

The implications extend beyond the realm of catalysis alone. The ability to drive spontaneous oxygen evolution in neutral pH and ambient conditions points to potential applications in decentralized and low-energy water-splitting devices. This could democratize access to hydrogen fuel production, mitigating reliance on expensive, high-energy input electrolysis systems and advancing sustainable energy infrastructure worldwide.

At the fundamental level, the study provides molecular-scale insights into the intricate interplay between oxidation states, structural motifs, and charge dynamics in transition metal oxyhydroxides. It bridges gaps in our mechanistic comprehension, offering a blueprint to reconcile discrepancies observed in previous experimental and theoretical studies of Ni-based catalysts, where active species identification was ambiguous or debated.

Moreover, the experimental strategy employed sets a new benchmark for catalyst characterization. Combining rigorous electrochemical isolation, spectroscopic identification, and mass spectrometric real-time analysis allowed the researchers to capture transient species and catalytic intermediates that are often lost in conventional ex situ studies. Such methodological advancements will likely become standard in future investigations of complex catalytic systems.

Notably, this work revitalizes interest in NiOOH derivatives and their applications beyond traditional OER. The concepts of stored charges and lattice oxygen redox may be relevant for other electrocatalytic reactions, such as oxygen reduction, carbon dioxide reduction, and nitrogen fixation, stimulating cross-disciplinary innovations in energy conversion and storage technologies.

As water oxidation remains a bottleneck in overall water splitting schemes, the discovery of a long-lived, highly oxidative NiOOH phase with spontaneous oxygen evolution marks a significant leap forward. It challenges scientists to rethink catalyst design philosophies, focusing not just on surface active centers but on bulk properties and charge reservoirs that can drive continuous catalytic turnover.

Going forward, translating these findings into scalable, stable, and economically viable water oxidation electrodes will be crucial. Efforts to integrate such Ni^4+-rich NiOOH phases into device architectures, possibly through nanostructuring or hybridization with conductive supports, could yield next-generation electrolysers with unmatched efficiency and longevity.

The broader scientific community will undoubtedly be energized to explore related transition metal systems, searching for other long-lived, charge-reservoir phases that emulate or surpass the performance of the NiOOH catalyst described here. This could accelerate the arrival of a new class of catalysts designed with atomic-level precision and sustained catalytic autonomy.

In conclusion, the work by Cui, Ding, Zhang, and colleagues has reshaped our fundamental understanding of NiOOH as a water oxidation catalyst. The isolation of a long-lived Ni^4+-enriched phase, the identification of a lattice oxygen coupling mechanism, and the demonstration of spontaneous oxygen evolution collectively chart a promising course for future energy research. These insights herald a new era in catalyst science, where the orchestration of bulk redox states and surface chemistry is harnessed to unlock the full potential of sustainable water splitting technologies.

Subject of Research: Active phases and catalytic mechanisms of NiOOH for water oxidation

Article Title: Reserved charges in a long-lived NiOOH phase drive catalytic water oxidation

Article References:
Cui, X., Ding, Y., Zhang, F. et al. Reserved charges in a long-lived NiOOH phase drive catalytic water oxidation. Nat. Chem. (2025). https://doi.org/10.1038/s41557-025-01942-5

Image Credits: AI Generated

Electrocatalytic water oxidation, a cornerstone reaction for building a green hydrogen economy, demands catalysts that are both active and stable under harsh oxidative conditions. Transition-metal hydroxides, especially those incorporating iron, have garnered considerable attention for their robust catalytic properties. Yet, a persistent enigma has been the precise role of iron and the dynamic nature of the lattice oxygen species during the oxygen evolution reaction (OER). By employing advanced operando ¹⁸O-labeling spectroelectrochemistry combined with cutting-edge machine-learning-assisted global optimization, the researchers have mapped out how O_latt–O_latt moieties emerge and stabilize within the catalyst matrix during reaction conditions.

This methodological tour de force allowed the team to track the evolution of lattice oxygen species in real time, revealing that the Ni–Fe hydroxide precatalyst undergoes a profound rearrangement under anodic polarization. The study demonstrated that O_latt–O_latt ligands form robust superoxo-hydroxide structures, which substantially alter the electronic landscape of active iron sites. This modification is not a mere structural curiosity; it directly correlates with enhanced catalytic activity. By systematically analyzing a series of Fe-incorporated transition-metal hydroxides and oxides, the researchers established a compelling relationship between the concentration of these lattice oxygen ligands and the intrinsic activity of iron centers.

The implications of these findings are manifold. First and foremost, they refute the long-standing assumption that adsorbed intermediates alone govern catalytic reactivity, positing instead that lattice oxygen species play an active and indispensable role. The presence of O_latt–O_latt ligands near Fe sites triggers an activation mechanism that lowers the activation energy barrier for oxygen evolution, thereby accelerating reaction kinetics. This insight was reinforced through rigorous first-principles computational studies, which elucidated how electronic interactions within the newly formed superoxo-hydroxide framework facilitate oxygen liberation more efficiently than previously appreciated catalyst structures.

Understanding the distinct functionality of iron in these lattice oxygen configurations represents a significant leap forward in catalyst design. Iron, often considered an auxiliary dopant, emerges here as a central player whose activity is intimately tied to its local oxygen environment. The synergy between iron and lattice oxygen in the superoxo-hydroxide phase manifests as enhanced electronic conductivity and optimized binding energies for reaction intermediates, critical factors that collectively boost electrocatalytic performance. Such atomic-level insights empower materials scientists to rethink doping strategies and tailor catalyst morphology to exploit these beneficial lattice effects.

Beyond the immediate mechanistic revelations, this research exemplifies the growing power of operando spectroscopic techniques blended with machine learning for materials discovery. Traditional methods struggled to capture transient and dynamic catalyst states under working conditions, yet the ingenious use of ¹⁸O isotopic labeling unmasked the subtle but crucial transformations taking place within the lattice. Coupled with sophisticated global optimization algorithms capable of predicting energetically favorable structures, the study navigated the complex energy landscape of hydroxide catalysts with exceptional precision. This synergy marks a paradigm shift in how catalytic materials can be systematically understood and optimized.

The broader scientific community stands to benefit greatly from this work, as it highlights a previously overlooked class of active species—lattice oxygen ligands—as pivotal contributors to catalytic activity. This challenges the conventional adsorption-desorption-centric models and invites a reevaluation of ligand dynamics in transition-metal-based electrocatalysts. The concept that lattice oxygen can actively participate in bond formation and cleavage during the water oxidation cycle opens avenues for exploring other oxygen-containing functional lattices in diverse catalytic frameworks.

Such findings bear particular importance in the development of next-generation electrocatalysts for water splitting devices, where efficiency and durability are paramount. The newfound understanding of superoxo-hydroxide phases in Fe-incorporated systems suggests that catalyst formulations might be engineered to stabilize these active oxygen ligands, thereby prolonging catalytic lifetimes and boosting turnover frequencies. This could translate into more cost-effective and practical hydrogen production technologies, accelerating the transition toward clean energy economies.

Moreover, the insights derived from this study have significant ramifications for related energy conversion reactions involving oxygen species, such as fuel cell oxygen reduction and metal-air battery cathode processes. The mechanistic parallels invite cross-disciplinary applications of the observed superoxo-hydroxide lattice configurations, potentially inspiring novel material architectures to overcome kinetic bottlenecks and enhance catalytic specificity across electrochemical energy devices.

The study also elegantly bridges the gap between theoretical modeling and experimental validation, demonstrating how machine-learning-assisted structural predictions can be harnessed to decode complex catalytic phenomena that are not easily accessible through conventional characterization methods alone. This integrative approach not only expedites the identification of active sites and phases but also sets a new standard for catalyst research workflows, merging computational creativity with empirical rigor.

In the context of global efforts to combat climate change and reduce reliance on fossil fuels, the significance of catalytic water oxidation cannot be overstated. The ability to harness renewable electricity to split water into oxygen and hydrogen underpins the feasibility of green hydrogen as a sustainable energy carrier. Enhancements in catalytic performance, such as those enabled by the understanding of lattice O–O ligand dynamics, directly contribute to lowering energy input and operational costs, thereby accelerating commercial viability.

While this work constitutes a major conceptual advance, it naturally opens numerous questions for future research. Exploring the stability limits of superoxo-hydroxide phases under varying electrochemical potentials, investigating the universality of lattice oxygen activation across other transition metals, and devising scalable synthesis methods for these phases remain important pursuits. Furthermore, integrating these catalysts into complete electrolyzer systems will require addressing challenges related to interface engineering and mass transport.

In conclusion, the discovery of lattice O–O ligands as active participants in Fe-incorporated hydroxide electrocatalysts marks a transformative moment in the field of water oxidation catalysis. By illuminating the nuanced yet profound role of lattice oxygen species in activating iron centers and facilitating oxygen evolution, this study not only advances fundamental science but also charts a strategic course toward next-generation electrocatalyst design. It underscores the critical importance of ligand dynamics and offers a blueprint for harnessing atomic-scale phenomena to drive sustainable energy solutions.

Subject of Research:
In situ transformations and ligand dynamics in nickel–iron hydroxide electrocatalysts for enhanced oxygen evolution reaction (OER) activity.

Article Title:
Lattice O–O ligands in Fe-incorporated hydroxides enhance water oxidation electrocatalysis.

Article References:
Shi, G., Li, J., Lu, T. et al. Lattice O–O ligands in Fe-incorporated hydroxides enhance water oxidation electrocatalysis. Nat. Chem. (2025). https://doi.org/10.1038/s41557-025-01898-6

Image Credits: AI Generated