Bicarbonate-Driven Two-Electron Oxygen Reduction on Nickel Disulfide

Hydrogen peroxide functions both as a widely used oxidant and as a short-lived reaction intermediate. It can be produced and consumed at low concentrations without the heavy infrastructure required for industrial-scale synthesis. This combination has sustained interest in pathways that generate hydrogen peroxide directly from dissolved oxygen under mild conditions, especially when electrical or photonic inputs are unnecessary. Metal sulfides from the pyrite family are central to this effort. They are abundant, redox-active materials and are already known to couple oxygen reduction with sacrificial lattice oxidation in aqueous systems. However, although spontaneous hydrogen peroxide formation on disulfide surfaces has been reported, its magnitude and stability vary sharply with solution chemistry. Alkalinity stabilizes peroxide once formed, but also alters surface speciation, electron transfer rates, and proton availability in ways that are not easily disentangled. Bicarbonate, ubiquitous in natural waters and alkaline process streams, complicates this picture further. It participates in acid–base equilibria, complexes with metal sites, and acts as a hydrogen donor in radical chemistry. Each of these roles could influence oxygen reduction, but their relative weight on disulfide surfaces has remained uncertain. Nickel disulfide can be considered an interesting case because if we compare it with iron disulfide, it shows higher peroxide yields under comparable conditions and slower peroxide decomposition by dissolved metal ions. Those differences mean altered surface intermediates not simple kinetic scaling. At the same time, prior work has emphasized hydroxyl and superoxide radicals in peroxide-forming systems, even though surface-bound oxygen species have increasingly been detected on sulfide minerals under alkaline conditions.

A recent research paper published in Langmuir and conducted by Yu-Le Wang, Song-Hai Wu and Professor Xu Han from the Tianjin University in collaboration with Dr. Cong Wang from the North China Institute of Science and Technology and Dr. Yong Liu from the Tianjin University of Technology, the researchers developed a mechanistic framework describing hydrogen peroxide formation on nickel disulfide under oxic, alkaline conditions. They identified surface nickel-superoxo and nickel-peroxo species as key intermediates and demonstrated bicarbonate-mediated hydrogen abstraction as the dominant pathway. They combined electrochemical analysis, surface spectroscopy, and density functional theory to link bicarbonate adsorption with two-electron oxygen reduction.

The researchers established early that dissolved oxygen governs peroxide formation in the nickel disulfide system. Under anoxic conditions, peroxide remained negligible across the examined pH range, which removed water–lattice reactions from consideration. When oxygen was present, peroxide appeared rapidly and reached higher transient levels at moderate alkalinity, a pattern that framed subsequent experiments at pH 9 where bicarbonate dominates carbonate speciation. The research team then examined bicarbonate concentration as an independent variable and found that increasing bicarbonate consistently raised the maximum peroxide accumulated, while sodium chloride at equivalent ionic strength produced no comparable effect. That comparison mattered, because it isolated bicarbonate chemistry from simple electrolyte screening. The investigators also contrasted nickel disulfide with iron disulfide under identical bicarbonate conditions and observed far lower peroxide levels on iron disulfide, which highlight that the response was not generic to pyrite-type materials.

The authors performed electrochemical measurements and recorded higher steady-state currents and altered polarization behavior as bicarbonate concentration increased, signaling faster interfacial electron transfer. Impedance analysis showed reduced charge-transfer resistance and thinner effective surface films. These changes implied that bicarbonate does more than coexist with the surface; it modifies the electrochemical accessibility of nickel sites that participate in oxygen reduction. To identify the reactive oxygen species involved, the researchers turned to spin trapping and selective fluorescence probes. They detected carbonate radical signals only when bicarbonate and oxygen were both present, while hydroxyl and superoxide signals remained weak. Quenching experiments reinforced this distinction. Suppressing carbonate radicals sharply reduced probe oxidation, whereas suppressing hydroxyl radicals had little effect. The logic was uncomfortable but clear: common solution radicals were not driving peroxide formation here.

The team also showed using modified peroxide assays, distinguished surface peroxo groups from dissolved peroxide, showing that the reactive intermediates resided primarily on the nickel disulfide surface. Plus, infrared spectra revealed bicarbonate adsorption accompanied by sulfur oxidation, and Raman spectra identified new bands consistent with nickel-superoxo and nickel-peroxo species under oxic bicarbonate conditions. When the authors chemically blocked surface nickel sites with a strong chelator, peroxide production collapsed, a result that tied all prior observations back to nickel-centered chemistry. The conducted simulations which showed oxygen binding end-on to surface nickel, followed by facile formation of a nickel-superoxo species. Hydrogen abstraction from bicarbonate to this intermediate required a substantially lower barrier than abstraction from water, and the same preference persisted for the subsequent peroxo step that yields hydrogen peroxide.

To summarize, the work of Professor Xu Han and colleagues identified bicarbonate as an active participant in oxygen reduction on nickel disulfide, and revealed how surface hydrogen abstraction pathways can govern spontaneous hydrogen peroxide formation without external energy input. The findings also complicate how bicarbonate is treated in alkaline aqueous chemistry. Often regarded as a passive buffer or background ion, bicarbonate here acts as an active hydrogen source that couples directly to surface oxygen intermediates. This role explains why peroxide accumulation rises with bicarbonate concentration even when pH and ionic strength remain fixed. It also clarifies why carbonate, dominant at higher pH, fails to substitute: the hydrogen donation step is lost. From a materials perspective, the contrast between nickel and iron disulfides highlights how little differences in metal–oxygen bonding propagate through an entire reaction sequence. Slower peroxide decomposition by nickel ions matters, but it is secondary to the altered surface pathway that favors two-electron oxygen reduction. Extending this reasoning to other transition-metal sulfides may help identify systems where peroxide generation and persistence can be balanced without external energy input. There are boundaries to these implications. The mechanism relies on alkaline conditions and sacrificial oxidation of the sulfide lattice, which limits long-term material stability. Peroxide concentrations remain in the micromolar range, suitable for disinfection or in situ oxidation but not bulk synthesis. Still, within those bounds, the study offers a coherent framework linking surface coordination, hydrogen donation, and electron transfer. That framework should translate to natural waters, corrosion environments, and engineered reactors where bicarbonate is unavoidable rather than optional. The new work is important to chemists and engineers because it clarifies how solution chemistry, specifically bicarbonate, directly controls surface reaction pathways. For chemists, we believe it provides a concrete mechanistic picture linking surface-bound superoxo and peroxo intermediates to hydrogen abstraction kinetics, which helps rationalize why certain aqueous environments favor two-electron oxygen reduction over radical-driven routes. For engineers, the work offers design guidance for low-energy peroxide generation systems by showing how modest changes in electrolyte composition can shift reaction selectivity, improve peroxide yield, and reduce reliance on external power, all while using earth-abundant materials under mild conditions.