class="head no_bottom_margin" id="sec1title">IntroductionSolar energy is expected to have a profound impact on human future development as well as be the key to tackle the serious environmental and energy challenges the world faces today (). Besides the strong urge to develop more eco-friendly and cost-effective energy harvesting technologies (, , , , , ), the pursuit of utilizing this abundant energy source in addressing some of the environmental and social issues like water crisis has also gained immense research interest in recent times (, ). Enormous efforts are made in desalination of seawater to mitigate the ill effects of water deficit emerging in various parts of the globe. Among all the available approaches to desalinate water, solar-driven steam generation has drawn tremendous attention, and exciting advances in this technology were witnessed in recent years (, , , , , , href="#bib46" rid="bib46" class=" bibr popnode">Zhu et al., 2017). Apart from desalination and the associated applications, such as water purification and wastewater treatment, steam generation has immense applications in burgeoning fields like power generators and actuators (href="#bib40" rid="bib40" class=" bibr popnode">Yang et al., 2017b, href="#bib27" rid="bib27" class=" bibr popnode">Ma et al., 2013, href="#bib5" rid="bib5" class=" bibr popnode">Chen et al., 2015, href="#bib3" rid="bib3" class=" bibr popnode">Cavusoglu et al., 2017, href="#bib42" rid="bib42" class=" bibr popnode">Zhang et al., 2015a, href="#bib1" rid="bib1" class=" bibr popnode">Arazoe et al., 2016). The concept of using water vapor and environmental moisture to generate electricity and transduce mechanical energy is patently rising. To catch the tide, devising effective ways of steam generation will be critical. Of late, this idea has gained momentum with numerous new solar absorber materials being reported. However, improving scalability and reducing the overall cost is still a major concern. For instance, plasmonic nanomaterials-based absorbers are known to be an effective way to localize solar heat. The traditionally employed noble metals in the system, although effective in solar absorption and heat localization, have a limitation of high material cost. However, the use of plasmonic aluminum nanoparticles is promising thanks to the low cost and is believed to be a disruptive innovation for future in this field (href="#bib45" rid="bib45" class=" bibr popnode">Zhou et al., 2016, href="#bib20" rid="bib20" class=" bibr popnode">Knight et al., 2013). On the other hand, graphene-based absorbers show promising prospect and practical value by cutting down the material cost, and limitations regarding scalability can be overcome by simplifying the fabrication process (href="#bib14" rid="bib14" class=" bibr popnode">Hu et al., 2017, href="#bib21" rid="bib21" class=" bibr popnode">Li et al., 2017, href="#bib18" rid="bib18" class=" bibr popnode">Jiang et al., 2016). Besides this, photoreceivers developed with fully reusable materials have offered a distinctive way to enhance holistic practicability and reduce overall cost (href="#bib23" rid="bib23" class=" bibr popnode">Liu et al., 2017a, href="#bib36" rid="bib36" class=" bibr popnode">Wang et al., 2017).In light of this, the focus has recently been shifted from those cost- and energy-intensive material systems to naturally abundant materials to minimize fabrication cost and achieve easy scalability (href="#bib46" rid="bib46" class=" bibr popnode">Zhu et al., 2017, href="#bib37" rid="bib37" class=" bibr popnode">Xu et al., 2017, href="#bib38" rid="bib38" class=" bibr popnode">Xue et al., 2017, href="#bib4" rid="bib4" class=" bibr popnode">Chen et al., 2017, href="#bib24" rid="bib24" class=" bibr popnode">Liu et al., 2017b, href="#bib25" rid="bib25" class=" bibr popnode">Liu et al., 2017c, href="#bib17" rid="bib17" class=" bibr popnode">Jia et al., 2017). Two crucial requirements for such a material system would be (1) to have a broad range of optical absorption and (2) to render highly efficient heat localization. To achieve this, natural materials are typically subjected to carbonization process before putting into service. As described in previous studies and based on general experience (href="#bib46" rid="bib46" class=" bibr popnode">Zhu et al., 2017, href="#bib37" rid="bib37" class=" bibr popnode">Xu et al., 2017), fabrication of a single carbonized layer (e.g., wooden surface) is typically much simpler than that of a bulk material such as mushroom, which requires tube furnace and protective gas. Nonetheless, it has been found that heat-treated wood shows better decay resistance compared with non-treated wood (href="#bib19" rid="bib19" class=" bibr popnode">Kamdem et al., 2002, href="#bib35" rid="bib35" class=" bibr popnode">Vukas et al., 2010, href="#bib9" rid="bib9" class=" bibr popnode">Esteves and Pereira, 2008), which would be very likely to limit the practical sustainability and endurance of wood-based device without a carbonized “root” when permanently soaked in complex water environment, e.g., seawater (href="#bib11" rid="bib11" class=" bibr popnode">Fojutowski et al., 2014). The use of expensive and energy-intensive bulk carbonization procedures, though, can aid in the development and study of a laboratory-scale product; for better scalability it is also crucial to lower down the production cost by devising more productive and cost-effective process routes, even if the raw material employed is cheap and abundant. Herein, mimicking the outdoor barbecue cooking, we propose a modified heating approach by using charcoal as the heat source and a tailored furnace, which is made of aluminum foil, to shield the loads from being burnt into ashes. Moreover, we have selected one of the most common household food wastes, moldy bread, as the material of interest (href="#bib22" rid="bib22" class=" bibr popnode">Lin et al., 2013). It is for the first time that a material system developed from food waste is introduced in this field, and our findings suggest that the unique porous structure of the moldy bread makes itself an ideal candidate as steam generator (href="#bib41" rid="bib41" class=" bibr popnode">Yuan et al., 2016). The overall solar steam generation efficiency of the carbonized bread reached as high as 71.4% under 1 sun illumination as its surface temperature stabilizes, and it is noteworthy that the achieved efficiency was obtained under a high relative humidity (RH) of around 70%, which is way higher than in most previous studies (href="#bib14" rid="bib14" class=" bibr popnode">Hu et al., 2017, href="#bib37" rid="bib37" class=" bibr popnode">Xu et al., 2017, href="#bib38" rid="bib38" class=" bibr popnode">Xue et al., 2017, href="#bib39" rid="bib39" class=" bibr popnode">Yang et al., 2017a, href="#bib16" rid="bib16" class=" bibr popnode">Ito et al., 2015, href="#bib43" rid="bib43" class=" bibr popnode">Zhang et al., 2015b). It is also noteworthy that by using moldy bread and cost-effective carbonization approach, the final cost is lowered greatly. In short, the use of moldy bread, which is one of the commonly generated food waste, and a facile and cost-effective carbonization approach has greatly cut down the overall cost both at the material and process levels.
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