Researchers pioneer new solar steam method for water purification

As the global population grows, fresh water supplies are more precious than ever. And while scientist and engineers have largely addressed the issue of how to purify water, much work remains in making those methods sustainable and energy efficient.

One promising approach is solar-driven distillation, or solar steam generation, which can help us get fresh water from wastewater or seawater. Researchers have been able to use this method to successfully distill small batches of purified water, but they are still searching for a way to do this on a large scale.

That’s where researchers at the Pritzker School of Molecular Engineering (PME) and Argonne National Laboratory come in. These scientists and engineers were part of a team that developed a new method of solar steam generation that could help bring this technology into the real world.

“Solar steam generation techniques are still mostly focused on lab use now,” said Zijing Xia, a graduate student at Pritzker Molecular Engineering and lead author of the research. “We want to find an easy way to fabricate solar steam generators at relatively low cost.”

The results of their work were published in the June 7 issue of the journal Advanced Materials Interfaces.

Making water purification sustainable

Researchers have already used various photothermal materials, which convert light to heat, for solar steam generation, such as carbon materials, plasmonic metals, and semiconductors. But many of these options have relatively low efficiency, among other challenges.

“Most existing methods cannot be easily engineered to produce steam-generating devices with both arbitrary control over the shape and high photothermal efficiency,” Xia explained.

A high-performance solar steam generator will ideally combine several characteristics, such as buoyancy on water, broad-spectrum light absorption, high light-to-heat conversion efficiency, and heat-to-water transfer. Unfortunately, many previously studied methods lack the porous structure needed to facilitate the heat transfer to water.

A POF-based strategy

What sets Xia’s method apart is the use of a porphyrin covalent organic framework (POF). POFs, a newly discovered class of materials, have the advantage of growing uniformly on the surface of a variety of materials with different levels of porosity, and they show high performance for water evaporation. POFs also have unique light-harvesting characteristics beneficial for new applications.

In the lab, POFs successfully grew on the inner and outer surfaces of every tested material. And every template showed favorable photothermal properties, indicating that POF-based materials are promising candidates for solar steam generation. The POF membrane also demonstrated broad-spectrum light absorption—above 95 percent over the wavelength range of 300-1300 nm, capturing the majority of the spectrum of sunlight irradiation.

The most promising result of the research, Xia said, was the POFs’ ability to grow at the surface of many different kinds of materials, including membranes, fabrics, sponges, and wood. The wood showed particularly strong performance, with researchers measuring roughly 80% light-to-steam conversion efficiency.

The ability of POFs to grow on many types of materials makes them easily adaptable for use with locally-available materials. This versatility, coupled with the easy, one-step fabrication process, could make the method practical for large-scale production.

Solar steam generation technology in action

The POF-based approach proved highly effective in a lab setting, and the research team plans to conduct further experiments outside the lab to observe the practical performance of POFs.

So far, the research suggests POFs could help drive the sustainable water purification systems of the future.

“POF-based interface engineering design shows promise for large-scale purification methods, and it could also be used for desalination, wastewater treatment, and beyond,” Xia said.

Other authors of the paper include PME graduate students Ruben Z. Waldman and Chao Zhang; PME professor and Argonne scientist Shrayesh Patel; Argonne scientist, director of its Center for Molecular Engineering, and PME fellow Seth B. Darling; Zhaowei Chen of Argonne; Hao-Cheng Yang of Sun Yat-sen University; and Yusen Zhao of UCLA.

Citation: “Porphyrin Covalent Organic Framework (POF)-Based Interface Engineering for Solar Steam Generation.” Zijing Xia et al. Advanced Materials Interfaces, doi: 10.1002/admi.201900254

Funding: The Advanced Materials for Energy-Water Systems (AMEWS) Center.