The paradox centers on two classic tools of the synthetic chemist: Lewis acids and Lewis bases. A Lewis base is a molecule that donates an electron pair, and it naturally seeks out positively charged, or cationic, defects on the perovskite surface. A Lewis acid, which accepts an electron pair, does the complementary job on negatively charged, or anionic, defects. In principle, adding both to a solar cell should heal both classes of defect at once. In practice, the strategy has repeatedly failed for the most fundamental of reasons: acids and bases react with each other. When mixed, they form stable adducts, quenching one another’s reactivity before either molecule ever reaches the defect it was designed to neutralize. The result is a passivation cocktail that neutralizes itself.

The new study, published in Nature Chemistry on 23 September 2026 and led by Donghoon Shin and Shuta Kitade under the supervision of Bin Chen and Edward H. Sargent, with Chad A. Mirkin and Mercouri G. Kanatzidis among the senior co-authors, sidesteps this self-defeating chemistry by making the two passivators chemically orthogonal. Orthogonality here means that each molecule reacts with its intended target on the perovskite surface but essentially ignores its partner in the solution. The key insight is geometric rather than electronic: the researchers exploited steric hindrance, the physical bulk of molecular groups, to keep the acid and base from getting close enough to share electrons.

The molecular design hinges on a family of bulky sp3-phosphine Lewis bases. Phosphines, compounds built around a trivalent phosphorus atom, are workhorse Lewis bases in coordination chemistry. By decorating the phosphorus center with bulky substituents arranged in tetrahedral, sp3-hybridized geometry, the team created bases whose lone electron pair is shielded by a thicket of atoms. When paired with fluorinated aromatic Lewis acids, these sterically congested phosphines simply cannot achieve the orbital overlap needed to form a conventional acid-base adduct. The fluorinated aromatic acids, for their part, are relatively weak and geometrically restrained acceptors, so the mutual attraction that normally dooms mixed acid-base formulations is suppressed at its source.

Crucially, the strategy works because of an asymmetry in accessibility. The defect sites on the perovskite surface, such as undercoordinated lead ions and halide vacancies, protrude from the crystal lattice and are more sterically exposed than the crowded acid-base pair is to itself. In other words, each passivator can still reach its target defect even though the two passivators cannot reach each other. The researchers describe this as a sterically gated arrangement, in which molecular bulk acts as a selective filter: small, exposed defect sites pass through the gate, while the bulky acid-base encounter does not. Spectroscopic measurements, including nuclear magnetic resonance and infrared techniques, along with defect-profiling methods such as drive-level capacitance profiling, support the picture that the two additives suppress trap states additively rather than destructively.

The photophysical consequences are exactly what defect passivation should deliver. Photoluminescence quantum yield measurements, which track how many absorbed photons are re-emitted rather than lost as heat, improve markedly when the orthogonal pair is applied, indicating that fewer photoexcited carriers are falling into trap states. Time-resolved photoluminescence shows longer carrier lifetimes, and transient absorption spectroscopy corroborates the reduction in non-radiative recombination pathways. Density functional theory calculations, performed with the VASP and ORCA packages, provide a molecular-level account of the binding energies involved, showing that each passivator binds effectively to its intended ionic defect while the acid-base interaction between the two molecules remains weak.

Translated into devices, the dual-passivation strategy produced impressive numbers. Solar cells fabricated with the sterically gated acid-base pair achieved power conversion efficiencies up to 27.4 percent, with an independently certified steady-state efficiency of 25.8 percent, placing the devices among the leading perovskite cells reported to date. Beyond raw efficiency, the team reported gains in operational stability, an equally important metric for a technology hoping to displace silicon in commercial panels. Because the passivation suppresses the charged surface defects that also serve as nucleation points for ion migration and chemical degradation, the molecular repair kit addresses both efficiency loss and instability with a single intervention.

The work also showcases an unusually broad collaboration across chemistry subdisciplines. Mirkin’s group contributed high-throughput spray-based screening methods for rapidly evaluating candidate passivators, developed with Jaewon Lee, while Kanatzidis brought deep expertise in halide perovskite materials chemistry. The team’s computational and spectroscopic characterization spanned X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and microscopy performed at Northwestern’s shared facilities. The underlying datasets have been deposited openly on Zenodo, and the authors have filed an invention disclosure with Northwestern University covering the molecular mutual passivation strategy, signaling interest in translating the chemistry toward commercial fabrication.

For the perovskite field, the significance of the result may lie less in the headline efficiency number than in the design principle behind it. Researchers have long known that single-function passivators leave half the defect population untouched, and that simply mixing acids and bases fails. The demonstration that steric gating can decouple two mutually reactive passivators gives the community a generalizable rule for building multi-component surface treatments: choose partners whose mutual reaction geometry is blocked but whose targets remain accessible. As perovskite technology pushes toward the detailed-balance efficiency limit set by Shockley and Queisser in 1961, every remaining percentage point must be wrested from non-radiative losses at surfaces and grain boundaries. Orthogonal, sterically engineered passivation offers a rational route to squeeze out those losses, and it suggests that the next generation of record-breaking cells may be designed as much by molecular architects thinking about shape and crowding as by device engineers optimizing layer thicknesses.

Subject of Research: Orthogonal Lewis acid-base defect passivation in perovskite solar cells

Article Title: Sterically gated Lewis acid and base pairs enable orthogonal defect passivation in perovskite solar cells

Article References: Sterically gated Lewis acid and base pairs enable orthogonal defect passivation in perovskite solar cells. (n.d.). https://doi.org/10.1038/s41557-026-02261-z

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02261-z

Keywords: perovskite solar cells, Lewis acids, Lewis bases, defect passivation, steric hindrance, phosphines, power conversion efficiency, non-radiative recombination, surface chemistry, Nature Chemistry, photovoltaics, trap states

The research team set out to answer a deceptively simple question: when hydrogen gas bubbles off a nickel electrode in an alkaline solution, which of the surface species that form on nickel is actually doing the catalytic work? Nickel electrodes in alkaline media are never chemically naked. Depending on the applied potential and local conditions, their surfaces can host nickel oxide, nickel hydroxide, and nickel hydride phases, each with distinct electronic structures and bonding geometries. For decades, researchers have debated whether these species are innocent bystanders, harmful blockers, or hidden active sites. Because real electrodes present a patchwork of all three, isolating their individual contributions has been extraordinarily difficult.

The breakthrough of the new work lies in its systematic approach to well-defined nickel surfaces. Rather than studying polycrystalline foams or nanoparticles, where grain boundaries and defects confound interpretation, the researchers prepared carefully controlled nickel surfaces and deliberately generated each candidate species under known conditions. By systematically varying the presence and coverage of nickel oxide, nickel hydride, and nickel hydroxide, and then measuring the hydrogen evolution activity of each configuration, they could assign catalytic roles with a clarity that previous ensemble measurements could not achieve. The strategy echoes a classic theme in surface electrochemistry: only when a surface is well defined can the link between structure and function be drawn unambiguously.

What emerged is a picture in which no single species can claim sole credit. Nickel oxide species, the team found, contribute to the reaction in ways that depend sensitively on their exact chemical state and distribution on the metallic surface. Nickel hydroxide, which forms readily in alkaline environments and has often been invoked as the key promoter of alkaline hydrogen evolution, plays a role that must be carefully separated from the contributions of the oxide and hydride phases. Meanwhile, nickel hydride, the species formed when adsorbed hydrogen penetrates the metal lattice or binds in hydride-like configurations, emerges as a central participant in the hydrogen formation chemistry itself. The study’s central achievement is showing how these three species interact and divide the labor of the reaction rather than acting in isolation.

The distinction matters enormously for the alkaline hydrogen evolution reaction because its mechanism differs fundamentally from the one that operates in acid. In acidic media, the reaction proceeds through adsorbed hydrogen atoms that form directly on the metal surface and combine into hydrogen molecules. In alkaline electrolytes, however, water itself is the proton source, so every hydrogen molecule produced requires a water molecule to dissociate on the surface, cleaving an O-H bond and liberating a hydroxide ion. This initial water dissociation step is widely regarded as the kinetic bottleneck on pure metals, and it is precisely where adjacent oxide or hydroxide species have been proposed to help, by offering oxygen-affine sites that cleave water while neighboring metallic sites assemble the hydrogen. The new results put this bifunctional picture on a much firmer experimental footing for nickel.

For the electrolyser industry, the implications are immediate. Alkaline water electrolysis is the most mature and lowest-cost electrolysis technology on the market, but its cathodes and anodes still trail the performance achievable with platinum-group catalysts. Nickel is already the workhorse electrode material in commercial alkaline electrolysers, prized for its corrosion resistance in concentrated potassium hydroxide and its reasonable activity. If engineers can now identify which surface species to stabilize—and in what proportion—they can rationally tune electrode preparation protocols, whether through surface oxidation treatments, controlled potential cycling, or the deliberate engineering of oxide-hydride interfaces, instead of relying on empirical trial and error. The study effectively converts a long-standing controversy into an engineering design principle.

The findings also carry weight for the broader search for precious-metal-free catalysts. Earth-abundant transition metals such as nickel, cobalt, iron, and molybdenum have all been explored as hydrogen evolution catalysts, and many of the most successful candidates are not pure metals at all but composites in which metallic domains coexist with oxide or hydroxide phases. The nickel study provides a conceptual template for deconvoluting such systems: prepare well-defined versions of each phase, measure their individual kinetics, and then interrogate their combinations. Applied across the periodic table, this methodology could accelerate the discovery of catalysts that match platinum’s performance at a fraction of the cost, a goal that would ripple through green hydrogen production, fuel cells, and carbon-neutral synthesis of fuels and chemicals.

There are also cautionary lessons in the results. Because nickel hydride participates directly in the reaction, the subsurface and bulk hydride chemistry of nickel electrodes deserves renewed attention, particularly under the strongly reducing potentials of cathodic operation where hydride formation is thermodynamically favored. Hydride formation can induce lattice strain, alter electronic properties, and even degrade electrode morphology over time, so understanding its catalytic role may simultaneously illuminate pathways to more durable electrodes. Similarly, the finding that oxide and hydroxide species make separable, state-dependent contributions suggests that the dynamic potential-dependent evolution of surface chemistry during electrolyser start-up, shutdown, and fluctuating renewable power input could shift which species dominates, with consequences for both efficiency and lifetime.

The study arrives at a moment when green hydrogen is scaling from demonstration projects to gigawatt deployments, and every millivolt saved at the cathode translates into real energy and cost savings at industrial scale. By systematically dissecting the individual roles of nickel oxide, nickel hydride, and nickel hydroxide on well-defined nickel surfaces, the researchers have replaced a murky, contested narrative with a mechanistically grounded one. The work does not end the search for better alkaline hydrogen evolution catalysts, but it decisively sharpens it: the target is no longer simply nickel, or nickel oxide, or nickel hydroxide, but the precisely engineered coexistence of all three. For a field racing to strip precious metals out of the hydrogen economy, that clarity may prove as valuable as any single catalyst.

Subject of Research: The individual roles of nickel oxide, nickel hydride, and nickel hydroxide surface species in the alkaline hydrogen evolution reaction on nickel electrodes.

Article Title: The role of individual nickel surface species in the hydrogen evolution reaction on nickel in alkaline electrolytes

Article References: Kozlica, D. K., Finšgar, M., Farinazzo Bergamo Dias Martins, P., Huš, M., Genorio, B., Connell, J. G., Martins, M., Hývl, M., Žibert, T., Andrina Varda, K., Osmić, A., Bele, M., Likozar, B., Tomc, B., Gaberšček, M., & Strmčnik, D. (2026). The role of individual nickel surface species in the hydrogen evolution reaction on nickel in alkaline electrolytes. Nature Catalysis. https://doi.org/10.1038/s41929-026-01605-9

Image Credits: AI Generated

DOI: 10.1038/s41929-026-01605-9

Keywords: hydrogen evolution reaction, nickel, alkaline electrolytes, water electrolysis, nickel oxide, nickel hydride, nickel hydroxide, electrocatalysis, green hydrogen, surface chemistry, precious-metal-free catalysts, Nature Catalysis