Elucidating the Discrepancy between the Intrinsic Structural Instability and the Apparent Catalytic Steadiness of M-N-C Catalysts toward Oxygen Evolution Reaction.

Despite the widespread investigations on the M-N-C type single atom catalysts (SACs) for oxygen evolution reaction (OER), an internal conflict between its intrinsic thermodynamically structural instability and apparent catalytic steadiness has long been ignored. Clearly unfolding this contradiction is necessary and meaningful for understanding the real structure-property relation of SACs. Herein, by using the well-designed pH-dependent metal leaching experiments and X-ray absorption spectroscopy, an unconventional structure reconstruction of M-N-C catalyst during OER process was observed. Combining with density functional theory calculations, the initial Ni-N coordination is easily broken in the presence of adsorbed OH*, leading to favorable formation of Ni-O coordination. The formed Ni-O works stably as the real active center for OER catalysis in alkaline media but unstably in acid, which clearly explains the existing conflict. Unveiling the internal contradiction between structural instability and catalytic steadiness provides valuable insights for rational design of single atom OER catalysts.

Identifying Fe as OER Active Sites and Ultralow-Cost Bifunctional Electrocatalysts for Overall Water Splitting.

Electrocatalysts based on Fe and other transition metals are regarded as most promising candidates for accelerating the oxygen evolution reaction (OER), whereas whether Fe is the catalytic active site for OER is still under debate. Here, unary Fe- and binary FeNi- based catalysts, FeOOH and FeNi(OH)x , are produced by self-reconstruction. The former is a dual-phased FeOOH, possessing abundant oxygen vacancies (VO ) and mixed-valence states, delivering the highest OER performance among all the unary iron oxides- and hydroxides- based powder catalysts reported to date, supporting Fe can be catalytically active for OER. As to binary catalyst, FeNi(OH)x is fabricated featuring 1) an equal molar content of Fe and Ni and 2) rich VO , both of which are found essential to enable abundant stabilized reactive centers (FeOOHNi) for high OER performance. Fe is found to be oxidized to 3.5+ during the *OOH process, thus, Fe is identified to be the active site in this new layered double hydroxide (LDH) structure with Fe:Ni = 1:1. Furthermore, the maximized catalytic centers enable FeNi(OH)x @NF (nickel foam) as low-cost bifunctional electrodes for overall water-splitting, delivering excellent performance comparable to commercial electrodes based on precious metals, which overcomes a major obstacle to the commercialization of bifunctional electrodes: prohibitive cost.

Tailoring the d-Band Centers Enables Co4 N Nanosheets To Be Highly Active for Hydrogen Evolution Catalysis.

Endowing materials with specific functions that are not readily available is always of great importance, but extremely challenging. Co4 N, with its beneficial metallic characteristics, has been proved to be highly active for the oxidation of water, while it is notoriously poor for catalyzing the hydrogen evolution reaction (HER), because of its unfavorable d-band energy level. Herein, we successfully endow Co4 N with prominent HER catalytic capability by tailoring the positions of the d-band center through transition-metal doping. The V-doped Co4 N nanosheets display an overpotential of 37 mV at 10 mA cm-2 , which is substantially better than Co4 N and even close to the benchmark Pt/C catalysts. XANES, UPS, and DFT calculations consistently reveal the enhanced performance is attributed to the downshift of the d-band center, which helps facilitate the H desorption. This concept could provide valuable insights into the design of other catalysts for HER and beyond.

Pb Single Atoms Enable Unprecedented Catalytic Behavior for the Combustion of Energetic Materials.

Manipulating the thermal decomposition behavior of energetic materials is the key to further pushing the combustion performance of solid rocket propellants. Herein, atomically dispersed Pb single atoms on polydopamine (PDA-Pb) are demonstrated, which display unprecedented catalytic activity toward the thermal decomposition of cyclotrimethylenetrinitramine (RDX). Impressively, RDX-based propellants with the addition of PDA-Pb catalyst exhibit substantially enhanced burning rates (14.98 mm s-1 at 2 MPa), which is 4.8 times faster than that without PDA-Pb and represents the best catalytic performance among Pb-based catalysts. Moreover, it also possesses low-pressure exponents in broad pressure ranges, which can enable more stable and safer combustion in solid rocket engines. Theoretical calculation unravels the efficient catalytic activity is stemmed from the enhanced interfacial electronic coupling between RDX and PDA-Pb via orbital level engineering. More importantly, PDA-Pb also presents similar catalytic behavior toward the decomposition of nitrocellulose, suggesting its broad catalytic generality. This work can open up new opportunities in the field of energetic compound combustion by exploring Pb-based single atom catalysts and beyond.

N-induced lattice contraction generally boosts the hydrogen evolution catalysis of P-rich metal phosphides.

P-rich transition metal phosphides (TMPs) with abundant P sites have been predicted to be more favorable for hydrogen evolution reaction (HER) catalysis. However, the actual activities of P-rich TMPs do not behave as expected, and the underlying essence especially at the atomic level is also ambiguous. Our structural analysis reveals the inferior activity could stem from the reduced overlap of atomic wave functions between metal and P with the increase in P contents, which consequently results in too strong P-H interaction. To this end, we used N-induced lattice contraction to generally boost the HER catalysis of P-rich TMPs including CoP2, FeP2, NiP2, and MoP2. Refined structural characterization and theoretical analysis indicate the N-P strong interaction could increase the atomic wave function overlap and eventually modulate the H adsorption strength. The concept of lattice engineering offers a new vision for tuning the catalytic activities of P-rich TMPs and beyond.

Regulating the Interfacial Electronic Coupling of Fe2 N via Orbital Steering for Hydrogen Evolution Catalysis.

The capability of manipulating the interfacial electronic coupling is the key to achieving on-demand functionalities of catalysts. Herein, it is demonstrated that the electronic coupling of Fe2 N can be effectively regulated for hydrogen evolution reaction (HER) catalysis by vacancy-mediated orbital steering. Ex situ refined structural analysis reveals that the electronic and coordination states of Fe2 N can be well manipulated by nitrogen vacancies, which impressively exhibit strong correlation with the catalytic activities. Theoretical studies further indicate that the nitrogen vacancy can uniquely steer the orbital orientation of the active sites to tailor the electronic coupling and thus benefit the surface adsorption capability. This work sheds light on the understanding of the catalytic mechanism in real systems and could contribute to revolutionizing the current catalyst design for HER and beyond.

Carbon doping switching on the hydrogen adsorption activity of NiO for hydrogen evolution reaction.

Hydrogen evolution reaction (HER) is more sluggish in alkaline than in acidic media because of the additional energy required for water dissociation. Numerous catalysts, including NiO, that offer active sites for water dissociation have been extensively investigated. Yet, the overall HER performance of NiO is still limited by lacking favorable H adsorption sites. Here we show a strategy to activate NiO through carbon doping, which creates under-coordinated Ni sites favorable for H adsorption. DFT calculations reveal that carbon dopant decreases the energy barrier of Heyrovsky step from 1.17 eV to 0.81 eV, suggesting the carbon also serves as a hot-spot for the dissociation of water molecules in water-alkali HER. As a result, the carbon doped NiO catalyst achieves an ultralow overpotential of 27 mV at 10 mA cm-2, and a low Tafel slope of 36 mV dec-1, representing the best performance among the state-of-the-art NiO catalysts.

Boosting Water Dissociation Kinetics on Pt-Ni Nanowires by N-Induced Orbital Tuning.

Although it is commonly believed that the water-dissociation-related Volmer process is the rate-limiting step for alkaline hydrogen evolution reaction (HER) on Pt-based catalysts, the underlying essence, particularly on the atomic scale, still remains unclear. Herein, it is revealed that the sluggish water-dissociation behavior probably stems from unfavorable orbital orientation and the kinetic issue is successfully resolved via N-induced orbital tuning. Impressively, N modified Pt-Ni nanowires deliver an ultralow overpotential of 13 mV at 10 mA cm-2 , which represents a new benchmark for alkaline HER catalysis. Fine-structural characterization and density functional theory analysis illustrate that the introduced nitrogen can uniquely modulate the electron densities around the Ni sites, and further create empty dz 2 orbitals with superior orientation for water adsorption and activation. More importantly, it is demonstrated that N-induced orbital modulation can generally boost the alkaline HER activities of Pt-Co, Pt-Ni, and Pt-Cu, offering a new perspective for the design of HER catalysts and beyond.

Tuning orbital orientation endows molybdenum disulfide with exceptional alkaline hydrogen evolution capability.

Molybdenum disulfide is naturally inert for alkaline hydrogen evolution catalysis, due to its unfavorable water adsorption and dissociation feature originated from the unsuitable orbital orientation. Herein, we successfully endow molybdenum disulfide with exceptional alkaline hydrogen evolution capability by carbon-induced orbital modulation. The prepared carbon doped molybdenum disulfide displays an unprecedented overpotential of 45 mV at 10 mA cm-2, which is substantially lower than 228 mV of the molybdenum disulfide and also represents the best alkaline hydrogen evolution catalytic activity among the ever-reported molybdenum disulfide catalysts. Fine structural analysis indicates the electronic and coordination structures of molybdenum disulfide have been significantly changed with carbon incorporation. Moreover, theoretical calculation further reveals carbon doping could create empty 2p orbitals perpendicular to the basal plane, enabling energetically favorable water adsorption and dissociation. The concept of orbital modulation could offer a unique approach for the rational design of hydrogen evolution catalysts and beyond.

Electron density modulation of NiCo2S4 nanowires by nitrogen incorporation for highly efficient hydrogen evolution catalysis.

Metal sulfides for hydrogen evolution catalysis typically suffer from unfavorable hydrogen desorption properties due to the strong interaction between the adsorbed H and the intensely electronegative sulfur. Here, we demonstrate a general strategy to improve the hydrogen evolution catalysis of metal sulfides by modulating the surface electron densities. The N modulated NiCo2S4 nanowire arrays exhibit an overpotential of 41 mV at 10 mA cm-2 and a Tafel slope of 37 mV dec-1, which are very close to the performance of the benchmark Pt/C in alkaline condition. X-ray photoelectron spectroscopy, synchrotron-based X-ray absorption spectroscopy, and density functional theory studies consistently confirm the surface electron densities of NiCo2S4 have been effectively manipulated by N doping. The capability to modulate the electron densities of the catalytic sites could provide valuable insights for the rational design of highly efficient catalysts for hydrogen evolution and beyond.