Hierarchical Ni2P@NiFeAlO x Nanosheet Arrays as Bifunctional Catalysts for Superior Overall Water Splitting.

Bifunctional electrocatalysts based on transition-metal phosphides are appealing for overall water splitting owing to their excellent electrical conductivity, low cost, and high stability. However, these specials are often restricted by some serious drawbacks such as its relatively poor activity for oxygen evolution reaction (OER) and its manufacture, which usually requires one to add additional large numbers of P sources and, consequently, inevitably leads to the release of flammable and detrimental PH3. Herein, we show an effective avenue to overcome these issues. For the first time, the in situ topological transformation of PO43--intercalated NiFeAl-layered double hydroxide nanosheet arrays upon calcination under a H2 atmosphere is developed to fabricate supported nickel phosphide without any additional P source. The resulting phase affords unique Ni2P@NiFeAlO x core-shell nanosheet arrays, which exhibit an excellent performance for OER and hydrogen evolution reaction in 1.0 M KOH, with low overpotentials of 210 and 105 mV at 10 mA cm-2, respectively. Impressively, it can also serve as both a cathode and an anode to drive water splitting in alkaline media, giving 10 and 100 mA cm-2 at cell voltages of only 1.52 and 1.62 V, respectively. This value is better than the commercial criterion of the Pt/C//IrO2 counterpart and also ranks at the top level in all established bifunctional electrocatalysts. The outstanding performance of Ni2P@NiFeAlO x is mainly attributed to the synergistic effect from a highly dispersed Ni2P core and a thin NiFeAlO x shell, as well as the efficient mass transport of a hierarchical nanoarray framework.

Ultralow-Content Iron-Decorated Ni-MOF-74 Fabricated by a Metal-Organic Framework Surface Reaction for Efficient Electrocatalytic Water Oxidation.

Transition-metal-organic frameworks (MOFs) have been regarded as one of the most intriguing electrocatalysts because of its low cost and diversity in functional organic groups and metal centers. Different from the common strategies of tuning the ratio of metal centers in multivariate MOFs, here, ultralow-content Fe2O3 is decorated on the surface of monometallic Ni-MOF-74 based on the fast "phenol-iron (Fe)" surface reaction between Fe2+ and the surface hydroxyl group in Ni-MOF-74. Benefiting from this flexible method, the Fe loading can be finely modulated and thus a series of Fe-decorated Ni-MOF-74 with different Fe contents are prepared. The optimized 0.6 wt % Fe2O3@Ni-MOF-74 with the Fe loading of 0.6 wt % only needs the overpotential of 264 mV to deliver 10 mA cm-2, which obviously outperforms Fe-free Ni-MOF-74 (323 mV) and other Fe2O3@Ni-MOF-74 and is even superior to the commercial IrO2 benchmark (300 mV). X-ray photoelectron spectroscopy results disclose that Fe decoration can obviously modulate the electronic structure of Ni center in Ni-MOF-74, thereby resulting in enhanced oxygen evolution reaction activity. This work opens up a new avenue to fabricate excellent MOF-based electrocatalysts for direct utilization in an electrocatalytic process.

Stable Iron Hydroxide Nanosheets@Cobalt-Metal-Organic-Framework Heterostructure for Efficient Electrocatalytic Oxygen Evolution.

Most studies are devoted to the use of metal-organic frameworks (MOFs) as templates to construct desirable electrocatalysts in situ by high-temperature pyrolysis. The emergence of heterostructures invokes new opportunities to use the full potential of pristine MOFs as efficient catalysts in the oxygen evolution reaction (OER). Here, a MOF surface-reaction strategy is developed to synthesize MOF-based heterostructures without pyrolysis. Uniform Fe(OH)3 nanosheets are grown controllably on the Co-MOF-74 surface by a fast "phenol-Fe" reaction that takes advantage of the hydroxyl sites in Co-MOF-74. The resulting Fe(OH)3 @Co-MOF-74 heterostructure delivers an excellent performance in the OER with a low overpotential of 292 mV at 10 mA cm-2 . Notably, the introduction of Fe can improve the intrinsic activity of the original Co atom significantly. The turnover frequency in Fe(OH)3 @Co-MOF-74 (1.209 s-1 ) is more than 25 times higher than that in Co-MOF-74 (0.048 s-1 ). This work presents a fresh concept for the fundamental design of advanced pure-MOF-based heterostructures and, thereby, provides a new avenue for the fabrication of other energy-conversion and -storage materials.