Urine Treatment in a Stacked Membraneless Direct Urea Fuel Cell with Honeycomb-like Nickel–Molybdenum Bimetal Phosphide as the Anodic Electrocatalyst

The aim of this study was to synthesize a novel catalyst for urea oxidation and to test a stacked membraneless direct urea fuel cell (DUFC) with raw urine as fuel. The molybdenum nickel phosphides on nickel foam (MoNiP/NF) were synthesized using a combined hydrothermal and phosphating method. The honeycomb-like MoNiP/NF catalyst with a Mo/Ni molar ratio of 0.50 (i.e., MoNiP/NF-0.50) showed the highest electrocatalytic activity for urea oxidation among different catalysts. The stacked membraneless DUFC was constructed using the MoNiP/NF-0.50 electrode as anode and a gas diffusion cathode. With the electrode spacing of 5 mm and 6 electrode pairs, the stacked membraneless DUFC had a maximum voltage of 0.55 V and a power density of 0.115 mW cm–2 at the external resistance of 1000 Ω, at which the power output was 5.32 times higher than that in the individual membraneless DUFC. The urea removal reached 65.8% in the cell at the external resistance of 100 Ω within 192 h. The excellent performance of the cell could be attributed to the high activity of the MoNiP/NF-0.50 catalyst, small electrode spacing, and multiple electrode pairs. Our results should provide promising potential for scalable DUFC with efficient urine treatment

Observation of topology transition in Floquet non-Hermitian skin effects in silicon photonics

Non-Hermitian physics has greatly enriched our understanding of nonequilibrium phenomena and uncovered novel effects such as the non-Hermitian skin effect (NHSE) that has profoundly revolutionized the field. NHSE is typically predicted in systems with nonreciprocal couplings which, however, are difficult to realize in experiments. Without nonreciprocal couplings, the NHSE can also emerge in systems with coexisting gauge fields and loss or gain (e.g., in Floquet non-Hermitian systems). However, such Floquet NHSE remains largely unexplored in experiments. Here, we realize the Floquet NHSEs in periodically modulated optical waveguides integrated on a silicon photonics platform. By engineering the artificial gauge fields induced by the periodical modulation, we observe various Floquet NHSEs and unveil their rich topological transitions. Remarkably, we discover the transitions between the normal unipolar NHSEs and an unconventional bipolar NHSE which is accompanied by the directional reversal of the NHSEs. The underlying physics is revealed by the band winding in complex quasienergy space which undergoes a topology change from isolated loops with the same winding to linked loops with opposite windings. Our work unfolds a new route toward Floquet NHSEs originating from the interplay between gauge fields and dissipation effects and offers fundamentally new ways for steering light and other waves