The development of bifunctional and stable non-noble metal electrocatalysts for the high-performance hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is very important and challenging for renewable energy. Herein, for the first time, a nickel–chromium layered double hydroxide (NiCr-LDH) nanosheet array was developed as a bifunctional electrocatalyst towards overall water splitting. By tuning different Ni/Cr ratios of LDHs, the optimized Ni₂Cr₁-LDH shows extraordinary HER activity with an ultralow overpotential of 138 mV at 100 mA cm⁻², compared with all of the reported Ni-based LDHs (NiFe-LDH, NiCo-LDH, NiMn-LDH, NiTi-LDH, NiV-LDH etc.) and even outperforming Pt/C catalysts. The small overpotential of 319 mV at 100 mA cm⁻² for the OER and outstanding durability at 1.55 V (vs. RHE) for 30 hours can also be achieved for Ni₂Cr₁-LDH. Notably, a two-electrode electrolyzer with a Ni₂Cr₁-LDH bifunctional electrocatalyst as both the anode and the cathode can work for at least 30 hours with a cell voltage of merely 1.55 V at 10 mA cm⁻². Both experimental and density functional theoretical calculations show that the Cr³⁺ ions within the LDH layer serve as charge transfer sites and thus effectively boost the intrinsic electrochemical activity. Therefore, this work provides a new NiCr-LDH system as a more efficient metal hydroxide for bifunctional water splitting electrocatalyst.

Paramagnetic metals are frequently used to regulate fluorescence emissions in chemical and biological probes. Accurate quantum calculations offer the first regulation theory that quenching is through the competitive nonradiative decay of the mixed fluorophore/metal ³ππ*/dd state isoenergetic to the fluorophore-localized ¹ππ* state.