Efficient water splitting requires highly active, earth-abundant, and robust catalysts. Monometallic phosphides such as NiP have been shown to be active toward water splitting. Our theoretical analysis has suggested that their performance can be further enhanced by substitution with extrinsic metals, though very little work has been conducted in this area. Here we present for the first time a novel PH plasma-assisted approach to convert NiCo hydroxides into ternary NiCoP. The obtained NiCoP nanostructure supported on Ni foam shows superior catalytic activity toward the hydrogen evolution reaction (HER) with a low overpotential of 32 mV at -10 mA cm in alkaline media. Moreover, it is also capable of catalyzing the oxygen evolution reaction (OER) with high efficiency though the real active sites are surface oxides in situ formed during the catalysis. Specifically, a current density of 10 mA cm is achieved at overpotential of 280 mV. These overpotentials are among the best reported values for non-noble metal catalysts. Most importantly, when used as both the cathode and anode for overall water splitting, a current density of 10 mA cm is achieved at a cell voltage as low as 1.58 V, making NiCoP among the most efficient earth-abundant catalysts for water splitting. Moreover, our new synthetic approach can serve as a versatile route to synthesize various bimetallic or even more complex phosphides for various applications.

Water splitting driven by electricity or sunlight is one of the most promising ways to address the global terawatt energy needs of future societies; however, its large-scale application is limited by the sluggish kinetics of the oxygen evolution reaction (OER). NiFe-based compounds, mainly oxides and hydroxides, are well-known OER catalysts and have been intensively studied; however, the utilization of the synergistic effect between two different NiFe-based materials to further boost the OER performance has not been achieved to date. Here, we report the rapid conversion of NiFe double hydroxide into metallic NiFeP using PH 3 plasma treatment and further construction of amorphous NiFe hydroxide/NiFeP/Ni foam as efficient and stable oxygen-evolving anodes. The strong electronic interactions between NiFe hydroxide and NiFeP significantly lower the adsorption energy of H 2 O on the hybrid and thus lead to enhanced OER performance. As a result, the hybrid catalyst can deliver a geometrical current density of 300 mA cm −2 at an extremely low overpotential (258 mV, after ohmic-drop correction), along with a small Tafel slope of 39 mV decade −1 and outstanding long-term durability in alkaline media.

No abstract

NiCo 2 S 4 is a technologically important electrode material that has recently achieved remarkable performance in pseudocapacitor, catalysis, and dye-synthesized solar cell applications. 1−5 Essentially, all reports on this material have presumed it to be semiconducting, like many of the chalcogenides, with a reported band gap in the range of 1.2−1.7 eV. 6,7 In this report, we have conducted detailed experimental and theoretical studies, most of which done for the first time, which overwhelmingly show that NiCo 2 S 4 is in fact a metal. We have also calculated the Raman spectrum of this material and experimentally verified it for the first time, hence clarifying inconsistent Raman spectra reports. Some of the key results that support our conclusions include: (1) the measured carrier density in NiCo 2 S 4 is 3.18 × 10 22 cm −3 , (2) NiCo 2 S 4 has a room temperature resistivity of around 10 3 μΩ cm which increases with temperature, (3) NiCo 2 S 4 exhibits a quadratic dependence of the magnetoresistance on magnetic field, (4) thermopower measurements show an extremely low Seebeck coefficient of 5 μV K −1 , (5) first-principles calculations confirm that NiCo 2 S 4 is a metal. These results suggest that it is time to rethink the presumed semiconducting nature of this promising material. They also suggest that the metallic conductivity is another reason (besides the known significant redox activity) behind the excellent performance reported for this material.

We present the first report in which the thermoelectric properties of two-dimensional MXenes are calculated by considering both the electron and phonon transport. Specifically, we solve the transport equations of the electrons and phonons for three MXenes, M 2 CO 2 , where M = Ti, Zr, or Hf, in order to evaluate the effect of the metal M on the thermoelectric performance. The lattice contribution to the thermal conductivity, obtained from the phonon life times, is found to be lowest in Ti 2 CO 2 and highest in Hf 2 CO 2 in the temperature range from 300 K to 700 K. The highest figure of merit is predicted for Ti 2 CO 2 . The heavy mass of the electrons due to flat conduction bands results in a larger thermopower in the case of n-doping in these compounds.