Semiconductive, Flexible MnO₂ NWs/Chitosan Hydrogels for Efficient Solar Steam Generation

Abstract

Solar-driven steam generation is anticipated as one of the most promising and inventive technologies to address the primitive issues of water shortage. Although extensive attempts have been made to develop highly efficient solar steam generators, hindrances are faced to integrate all desired functions in a single evaporating system. Herein, we designed semiconductive in situpolymerized MnO 2 nanowires/chitosan (SPM-CH) hydrogels as flexible, built-in, vertically aligned, macropore-based water channels (∼0.5 μm pore size) for enhanced solar water generation (17.02 kg m −2 in 1 day). The nonradiative relaxation-dependent defect engineering of SPM-CH hydrogel promotes more lattice vibrations, and its polymeric network endorses the formation of enhanced intermediate water clusters for vapor generation. The self-floating and salt-resistant device possesses an excellent evaporation rate (1.78 kg m −2 h −1 ) during a single sunny day along with efficient solar energy conversion efficiency (90.6%) under 1 sun intensity, good solar absorption (94%), and good compressing flexibility (42% compressive strain). Moreover, COMSOL Multiphysics simulations of SPM-CH hydrogels under experimental conditions reveal its superior centralized heat accumulation within the top-interface matrix. The single-step execution for a freshwater supply purified from various contaminations including industrial wastewater and oil-emulsified water shows its potential as a reusable device toward real-life applications.

Intensifying Solar Interfacial Heat Accumulation for Clean Water Generation Excluding Heavy Metal Ions and Oil Emulsions

“…This typical hydrogen bonding also exists in the FW structure which is similar to bulk water. [ 23 ] In addition, the nonfreezing or bound water (BW) comprises the linkage of hydrogen bonds between water molecules and polar groups of polymers. In addition, the freezing BW, which is known as IW, comprises weak hydrogen bonding usually in water clusters.…”

“…However, these water cluster molecules are limited in small pores or contain the physical interaction with the matrix of the polymer backbone. [ 23 ] Consequently, the IW water state exhibits higher diffusivity than BW and FW. Indeed, the absorbed water or IW by the IPNF matrix and its interaction with a polymeric chain of PPy NPs promote accelerated water transportation to provide sufficient replenishment to the evaporation interface for efficient solar vapor generation.…”

“…The single water molecules or fragmented water clusters at the liquid–air interface facilitate spontaneous vapor generation and its different states under solar intensity. [ 23 ] The solar energy confinement at the liquid–vapor interface and its effective utilization performs a significant role in smooth vapor generation. As shown in Figure a, a semiconductive in situ‐polymerized IPNF evaporation structure was designed to capture broadband solar energy and transform it into confined thermal energy.…”

“…However, the bands at 3450 and 3613 cm −1 correspond to the loosely bound hydrogen bonds, conforming to symmetric and asymmetric –OH stretching modes of IW. [ 23 ] The strong IW peak demonstrates the existence of the maximum IW content in in situ‐polymerized PPy NPs and maximum polymeric water interaction. The observed phenomena will also provide new insights in solar‐driven water‐splitting applications.…”

“…Polypyrrole (PPy) is one of the most stable conductive polymers due to its heterocyclic structure, which exhibits diffuse reflection and efficient solar absorption that promotes its direct application in solar-driven devices. [23][24][25][26] Indeed, nickel foam substrate was selected to support the stability of PPy NPs due to its open porous structure and exploit the intrinsic defuse reflection of conductive PPy NPs, [27] high porosity, and intrinsic strength with better uniformity. For this, monomer solutions (A&B) were prepared, as described in the Experimental Section (2.2), and sprayed successively for IPNF.…”