Enriched Fe Doped on Amorphous Shell Enable Crystalline@Amorphous Core-Shell Nanorod Highly Efficient Electrochemical Water Oxidation

Crystalline Ru-Decorated MOF-Derived Amorphous CoMo-LDH Nanosheet Arrays as Bifunctional Catalysts for Overall Natural Seawater Electrolysis

The construction of an amorphous/crystalline heterostructure of MOF-derived electrocatalysts offers an intriguing pathway to improve hydrogen production efficiency, but it has received little attention. Here, we report crystalline Ru-decorated MOF-derived amorphous CoMo-LDH nanosheet arrays as highly active and robust bifunctional electrocatalysts for natural seawater electrolysis. Benefiting from the abundant interfaces, the Ru-CoMo-LDH catalyst exhibits excellent activity toward OER under fresh and natural seawater, in particular; it requires only 257 and 406 mV overpotential at 10 and 500 mA cm-2 in 1 M KOH + Seawater, outperforming the benchmark RuO2. In addition, this electrocatalyst is an eminently active and stable HER in various electrolytes, emphasizing its outstanding bifunctional capability. Strikingly, in full-cell overall water splitting in natural seawater test, Ru-CoMo-LDH ∥ Pt/C exhibits superior electrochemical behavior (i.e., overpotential of 1.5545 and 1.731 V to obtain the current density of 10 and 200 mA cm-2, respectively) and high stability. These excellent electrocatalytic activities highlighted the synergistic effects of intimated amorphous/crystalline junctions, which provide a rich population of exposed active sites and enhance electron transport. This, in turn, lowers the adsorption energy barrier of intermediates, leading to improved performance. Our work proves that designing an amorphous/crystalline heterointerface is a promising platform for further enhancing the hydrogen generation efficiency.

Bifunctional Amorphous Transition-Metal Phospho-Boride Electrocatalysts for Selective Alkaline Seawater Splitting at a Current Density of 2A cm-2

Hydrogen production by direct seawater electrolysis is an alternative technology to conventional freshwater electrolysis, mainly owing to the vast abundance of seawater reserves on earth. However, the lack of robust, active, and selective electrocatalysts that can withstand the harsh and corrosive saline conditions of seawater greatly hinders its industrial viability. Herein, a series of amorphous transition-metal phospho-borides, namely Co-P-B, Ni-P-B, and Fe-P-B are prepared by simple chemical reduction method and screened for overall alkaline seawater electrolysis. Co-P-B is found to be the best of the lot, requiring low overpotentials of ≈270 mV for hydrogen evolution reaction (HER), ≈410 mV for oxygen evolution reaction (OER), and an overall voltage of 2.50 V to reach a current density of 2 A cm-2 in highly alkaline natural seawater. Furthermore, the optimized electrocatalyst shows formidable stability after 10,000 cycles and 30 h of chronoamperometric measurements in alkaline natural seawater without any chlorine evolution, even at higher current densities. A detailed understanding of not only HER and OER but also chlorine evolution reaction (ClER) on the Co-P-B surface is obtained by computational analysis, which also sheds light on the selectivity and stability of the catalyst at high current densities.

A Coordination-Derived Cerium-Based Amorphous-Crystalline Heterostructure with High Electrocatalytic Oxygen Evolution Activity

The rational design of heterogeneous catalysts is crucial for achieving optimal physicochemical properties and high electrochemical activity. However, the development of new amorphous-crystalline heterostructures is significantly more challenging than that of the existing crystalline-crystalline heterostructures. To overcome these issues, a coordination-assisted strategy that can help fabricate an amorphous NiO/crystalline NiCeOx (a-NiO/c-NiCeOx) heterostructure is reported herein. The coordination geometry of the organic ligands plays a pivotal role in permitting the formation of coordination polymers with high Ni contents. This consequently provides an opportunity for enabling the supersaturation of Ni in the NiCeOx structure during annealing, leading to the endogenous spillover of Ni from the depths of NiCeOx to its surface. The resulting heterostructure, featuring strongly coupled amorphous NiO and crystalline NiCeOx, exhibits harmonious interactions in addition to low overpotentials and high catalytic stability in the oxygen evolution reaction (OER). Theoretical calculations prove that the amorphous-crystalline interfaces facilitate charge transfer, which plays a critical role in regulating the local electron density of the Ni sites, thereby promoting the adsorption of oxygen-based intermediates on the Ni sites and lowering the dissociation-related energy barriers. Overall, this study underscores the potential of coordinating different metal ions at the molecular level to advance amorphous-crystalline heterostructure design.

Defective ferrocene-based metal-organic frameworks for efficient solar-powered water oxidation via the ligand competition and etching effect

Two-dimensional metal-organic frameworks are considered to be promising electrocatalytic materials due to their ultrathin lamellar structure, ultrahigh porosity and large surface area, but there are still many challenges such as the embedding of organic ligands leading to low density of active sites and poor conductivity. Herein, we synthesize two-dimensional ferrocene-based metal-organic frameworks nanosheet electrocatalysts via the one-step hydrothermal hydrogen peroxide etching method. The prepared FcNi-BDC-H2O2/NF exhibits excellent oxygen evolution reaction performance with a current density of 100 mA·cm-2 at only 258 mV and a small driving potential of 1.542 V (10 mA·cm-2) is required to achieve overall water splitting. Significantly, an overall water-cracked cell using a solar cell assembly achieves the solar hydrogen conversion efficiency of 19.5%. The introduction of high electronegativity ferrocene and the etching of H2O2 increase the Ni3+ content of FcNi-BDC-H2O2, and expose more unsaturated active sites, which improve the intrinsic activity of the catalysts and the mass transfer rate during the catalytic process. Moreover, the FcNi-BDC-H2O2/NF demonstrates significant urea oxidation reaction performance, achieving a potential of 1.35 V and producing 10 mA·cm-2. This study presents a viable approach to investigating highly efficient electrocatalysts for oxygen evolution reaction and urea oxidation reaction using MOF-based bifunctional catalysts.

A moderate method for in situ growing Fe-based LDHs on Ni foam for catalyzing the oxygen evolution reaction

Fe-based LDHs have been proven to be an excellent class of catalysts for the oxygen evolution reaction (OER). To achieve industrial applications of water splitting, it is critical to develop a cost-effective and simple strategy to achieve large-area catalytic electrodes. Herein, we present a moderate in situ method for growing Fe-based layered double hydroxide nanosheets on a Ni foam (LDH@NF) substrate at room temperature. Through systematic experimental design characterization, it is found that this in situ growth process is mainly driven by moderate oxidation of Fe2+ in an O2-dissolved solution, the consequent local alkaline environment, and abundant TM2+ ions (Ni2+, Co2+, Ni2+/Co2+). Compared with other in situ methods, this method is not accompanied by violent redox reactions and is favorable for the uniform growth of LDHs, and the composition of the catalyst can be easily regulated. Specifically, the optimized NiFe-LDH@NF catalyst demonstrates excellent catalytic performance in the alkaline water oxidation reaction with a low overpotential of 206/239 mV at a current density of 10/100 mA cm-2, respectively.