High Valence State Sites as Favorable Reductive Centers for High-Current-Density Water Splitting

Deliberate Amorphization of Co-MOF for Constructing Crystalline-Amorphous Heterostructures Toward High-Performance Water Electrolysis

The endowment of metal organic frameworks (MOF) with superior electrocatalytic performance without compromising their structural/compositional superiorities is of great significance for the development of renewable energy devices, yet remains a grand challenge. Herein, a deliberate partial amorphization strategy is developed to construct a heterostructured electrocatalyst consisting of crystalline Co-MOF and amorphous Co-S nanoflake arrays aligned on the carbon cloth (CC) substrate (abbreviated as Co-MOF/Co-S@CC hereafter) through a rapid sulfuration method. The simultaneous implement of crystalline-amorphous (c-a) heterostructure and nanoflake arrayed architecture on CC substrate renders the Co-MOF/Co-S@CC with abundant and tight active sites, accelerated charge transfer rate, regulated electronic structures, and reinforced structural stability. As such, the obtained Co-MOF/Co-S@CC electrode demonstrates outstanding electrochemical hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances with the overpotentials of 64 and 217 mV at 10 mA cm-2, respectively. Moreover, a two-electrode electrolyzer assembled by Co-MOF/Co-S@CC electrodes exhibits the lower cell voltages and larger current densities than those of Pt/C and RuO2 counterparts, excellent reversibility and prominent long-term stability, representing a great prospect for feasible H2 production. This adopted concept of c-a heterostructure for electronic regulation may bring about insightful inspiration for designing high-performance electrocatalysts for sustainable energy systems.

Confined growth of Ultrathin, nanometer-sized FeOOH/CoP heterojunction nanosheet arrays as efficient self-supported electrode for oxygen evolution reaction

Self-supported electrodes, featuring abundant active species and rapid mass transfer, are promising for practical applications in water electrolysis. However, constructing efficient self-supported electrodes with a strong affinity between the catalytic components and the substrate is of great challenge. In this study, by combining the ideas of in-situ construction and space-confined growth, we designed a novel self-supported FeOOH/cobalt phosphide (CoP) heterojunctions grown on a carefully modified commercial Ni foam (NF) with three-dimensional (3D) hierarchically porous Ni skeleton (FeOOH/CoP/3D NF). The specific porous structure of 3D NF directs the confined growth of FeOOH/CoP catalyst into ultra-thin and small-sized nanosheet arrays with abundant edge active sites. The active FeOOH/CoP component is stably anchored on the rough pore wall of 3D NF support, leading to superior stability and improved conductivity. These structural advantages contributed to a highly facilitated oxygen evolution reaction (OER) activity and enhanced durability of the FeOOH/CoP/3D NF electrode. Herein, the FeOOH/CoP/3D NF electrode afforded a low overpotential of 234 mV at 10 mA cm-2 (41 mV smaller than FeOOH/CoP grown on unmodified Ni foam) and high stability for over 90 h, which is among the top reported OER catalysts. Our study provides an effective idea and technique for the construction of active and robust self-supported electrodes for water electrolysis.

Engineering interfacial sulfur migration in transition-metal sulfide enables low overpotential for durable hydrogen evolution in seawater

Hydrogen production from seawater remains challenging due to the deactivation of the hydrogen evolution reaction (HER) electrode under high current density. To overcome the activity-stability trade-offs in transition-metal sulfides, we propose a strategy to engineer sulfur migration by constructing a nickel-cobalt sulfides heterostructure with nitrogen-doped carbon shell encapsulation (CN@NiCoS) electrocatalyst. State-of-the-art ex situ/in situ characterizations and density functional theory calculations reveal the restructuring of the CN@NiCoS interface, clearly identifying dynamic sulfur migration. The NiCoS heterostructure stimulates sulfur migration by creating sulfur vacancies at the Ni3S2-Co9S8 heterointerface, while the migrated sulfur atoms are subsequently captured by the CN shell via strong C-S bond, preventing sulfide dissolution into alkaline electrolyte. Remarkably, the dynamically formed sulfur-doped CN shell and sulfur vacancies pairing sites significantly enhances HER activity by altering the d-band center near Fermi level, resulting in a low overpotential of 4.6 and 8 mV at 10 mA cm-2 in alkaline freshwater and seawater media, and long-term stability up to 1000 h. This work thus provides a guidance for the design of high-performance HER electrocatalyst by engineering interfacial atomic migration.

Enhancing Ni/Co Activity by Neighboring Pt Atoms in NiCoP/MXene Electrocatalyst for Alkaline Hydrogen Evolution

Density functional theory (DFT) calculations demonstrate neighboring Pt atoms can enhance the metal activity of NiCoP for hydrogen evolution reaction (HER). However, it remains a great challenge to link Pt and NiCoP. Herein, we introduced curvature of bowl-like structure to construct Pt/NiCoP interface by adding a minimal 1 ‰-molar-ratio Pt. The as-prepared sample only requires an overpotential of 26.5 and 181.6 mV to accordingly achieve the current density of 10 and 500 mA cm-2 in 1 M KOH. The water dissociation energy barrier (Ea) has a ~43 % decrease compared with NiCoP counterpart. It also shows an ultrahigh stability with a small degradation rate of 10.6 μV h-1 at harsh conditions (500 mA cm-2 and 50 °C) after 3000 hrs. X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and X-ray absorption fine structure (XAFS) verify the interface electron transfer lowers the valence state of Co/Ni and activates them. DFT calculations also confirm the catalytic transition step of NiCoP can change from Heyrovsky (2.71 eV) to Tafel step (0.51 eV) in the neighborhood of Pt, in accord with the result of the improved Hads at the interface disclosed by in situ electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) tests.

ZIF-derived oxygen vacancy-rich Co3O4 for constructing an efficient Z-scheme heterojunction to boost photocatalytic water splitting

With ZIF-67 as the precursor, oxygen vacancy-rich Co3O4 nanoparticles were derived and anchored on the surface of 2D polyimide (PI) to construct a Z-scheme hybrid heterojunction (20ZP) through a simultaneous solvothermal in situ crystallization and polymerization strategy. XRD, XPS and EPR confirmed that both Co(III) and oxygen vacancies are formed during the low temperature conversion of ZIF-67 to Co3O4 nanoparticles that in turn accelerate the polymerization of PI. Synchronous crystallization makes the interfacial architecture intermetal and compact, inducing a strong interfacial electronic interaction between Co3O4 nanoparticles and PI. UV-vis DRS spectra and transient photocurrent response demonstrate that the incorporation of Co3O4 on polyimide not only extends the light absorption in the visible range, but also enhances the charge transfer rate. EIS, TRPL techniques and DFT calculations have confirmed that the photoinduced interfacial charge transfer pathway of this hybrid heterojunction characterized the Z-scheme in which the photoinduced electrons transfer from the conduction band of Co3O4 to the valence band of PI, significantly inhibiting the recombination of electrons and holes within PI. More importantly, the oxygen vacancies located below the conductor band of Co3O4 can deepen the band bending, improve the charge separation efficiency and accelerate electron transfer between Co3O4 and PI. This Z-scheme hybrid heterojunction structure can not only maintain the high reducing capacity of photoinduced electrons on the conductor band of PI, but also enhance the oxidative capacity of the heterojunction composite material, thus promoting the overall progress of the photocatalytic hydrogen release reaction.

Opposite Electron Transfer Induced High Valence Mo Sites for Boosting the Water Splitting Performance of Ir Atoms

Developing highly efficient and low-cost bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting poses significant challenges. In this study, a novel bifunctional electrocatalyst, Irn-CoMoPOx, was achieved via incorporating low-loading Ir single atoms and clusters with the high-valence Mo6+ modified CoPOx nanosheets. The Irn-CoMoPOx catalyst demonstrates remarkable low overpotentials of 222 mV and 36 mV for the OER and HER, respectively, in delivering a current density of 10 mA cm-2. When employed as both the anode and cathode catalyst in overall water splitting, the Irn-CoMoPOx∥Irn-CoMoPOx configuration exhibits a superior cell voltage of 1.53 V, outperforming the benchmark Pt/C∥IrO2 electrolytic cell (1.60 V) for achieving the current density of 10 mA cm-2. Benefiting from the high-valence of Mo species, the metal-support interaction of Irn-CoMoPOx was greatly strengthened, resulting in an order of magnitude increase in the mass activity of Ir for the HER. The high valence of non-noble metals plays a crucial role in tuning the local electronic configurations and optimizing the adsorption energies of the intermediates, which synergistically improves the overall performance of Ir in water splitting. The study provides valuable insights for future research in the utilization of Ir-based bifunctional catalysts for overall water electrocatalysis applications.