While a fair amount of graphene research is now centered around improving manufacturing techniques—and deservedly so—we are still at the point at which an equal amount of research is dedicated to just finding out all its properties. We know about its unparalleled conductivity in which electrons can move through it so that they behave almost like photons. And we’ve even been able to engineer its less desirable properties out of it, such as lacking a band gap, so that we can use it for stopping and starting the flow of electrons to enable its use in digital logic applications.
While many seem to measure the success of all nanomaterials—including the two-dimensional (2D) variety—by whether they are going to solve the looming dead end we’re approaching with Moore’s Law, there may be even more critical issues to man’s continued survival for which nanomaterials can offer a pretty viable solution.
Issues such as creating energy solutions that will be sustainable well into the future with improved photovoltaics, and, perhaps most importantly, access to clean drinking water through water desalination are some of the ways in which 2D materials are answering the call for big solutions to today’s biggest problems.
In this quarter’s newsletter, we cover work in which researchers used computer simulation in an attempt to rank the best 2D materials for water desalination. According to those calculations, molybdenum disulfide had some advantages over graphene. But that was just in computer models, engineering a filter medium is a different story.
A new start-up G2O Water is offering a graphene-based water membrane technology that can be applied to any filter medium used today and reduce their energy costs of those filters by 80-90%, which translates to saving $30 million per year for a water desalination plant that produces 50 million gallons a day, and thereby reducing water costs by 40%.
We took the opportunity to interview the man responsible for developing the technology behind this graphene-based membrane, Dr. Miao Yu, Assistant Professor of Chemical Engineering at the University of South Carolina, to get some insights of how this technology is different from other graphene-based membranes and what we can expect to see from this new company in the future.
Could you provide some context on how your approach to using graphene in membrane filters compares and differs to other approaches that use graphene in membranes, such as at MIT?
Dr. Miao - There are at least three ways of utilizing graphene-based material for membrane applications: i) create functional coatings, ii) create lamellar structures with nano-channels, and iii) allow selective permeation through structural defects of single-layer graphene or graphene oxide.
The group from MIT is working on selective permeation through defects. We are working on depositing graphene oxide functional coatings with optimized nanostructures on porous filters by facile and scalable processes for water purification, especially for antifouling oil/water separation.
We are also working on scalable fabrication of lamellar structures of graphene oxide with ultrathin thickness (<50 nm) for nanofiltration and organic dehydration. Our current goal is to develop scalable processes to prepare graphene-based membranes for separation applications while our longer-term goal is to prepare membranes for high flux desalination.
When did you have your “eureka moment” and realize that you had discovered a unique approach to using graphene oxide in membrane filters? What sort of potential implications did you recognize for the technology at that time?
Dr. Miao - While looking at the ultrathin thickness of single-layered graphene oxide and the highly defective structure resulting from the strong oxidation conditions in typical preparation process, our first question was what could we sieve using those defects?
Our first tests showed that H2 (kinetic diameter: 0.289 nm) can go through but CO2 (0.33 nm) can’t, which suggested that we may use it for highly selective water (0.26 nm) permeation. However, due to the swelling of the layered graphene oxide structure, it couldn’t be used directly for removing water from ions directly for desalination. Then we tried to use graphene oxide as a skin layer/coating on porous filters and found that it produced excellent antifouling performance in oil/water separation due to the high hydrophilicity and the optimized nanostructure that traps water and repels oil. Since this is a low-cost and simple coating process, we realized it must have large applications in oil/water purification to prolong membrane service times and lower operational costs.
On the G2O website, it says that the graphene oxide process you have developed can act "as a a functional coating for modifying the surface properties of existing filter media”. So this means that the coating process can be applied to any membrane technology currently out in the market, correct? Will that pose a large additional cost to the production of these membranes?
Dr. Miao - We have tested GO coatings on a variety of porous substrates for oil/water separation, including Anodic Aluminium Oxide (AAO), Polyvinylidene Fluoride (PVDF), Polysulfone (PS), Cellulose Nitrate (CN), Cellulose Acetate (CA) and Polyamide (PA), and found greatly improved antifouling performance, combined with increased water flux. So we believe this is a general way of significantly improving antifouling and throughput performance of filtration membranes. Since the graphene oxide coating thickness is very thin and the deposition process is simple, we do not expect significant increases in production costs.
The three keys to just about every membrane’s effectiveness is a precise control over pore sizes, being highly resistant to fouling and an ability to reduce energy costs. Could you discuss how your graphene oxide technology addresses these three criteria?
Dr. Miao - As a functional coating, graphene oxide plays a role of changing surface properties, especially underwater oleophobicity and/or low oil adhesion in water. In this case, we do not expect to modify the substrate pore size too much, although there is an option to fine tune sizes by varying the coating thickness. As a lamellar structure of graphene oxide, interlayer spacing controls the final pore size and the functional groups on graphene oxide determines surface properties for selective permeation of certain molecules, such as water. The challenge is how to precisely control the structure in a scalable way so no local defects or imperfections will exist. Currently, we believe we have found effective ways to do this and apply it for organic dehydration.
For single-layered graphene-based membranes with selective defects for high flux water permeation, there will still be a long way to go before large scale adoption.
Your technology seems to be applicable to both water desalination and oil separation. Could you please discuss how it is capable of doing both? And where do you see the most likely commercial applications for your filter technology and why so?
Dr. Miao - Our coating acts as an enhancer to existing membranes rather than replacing them. By adding the G2O coating, water transport through the membranes is significantly increased which makes the use of membranes either cheaper in terms of energy input or allows the use of membranes in applications where their current limitations would make them uneconomic.
Our initial target market is oil/water separation and wastewater where we were recently awarded $1m from Innovate UK to develop smart filters for the nuclear industry while we complete work on desalination.
Our lab work has demonstrated that the coating of ultrafiltration (UF) membranes with the G2O technology allows rejection of a high proportion of multivalent ions such as Mg2+ and Ca2+ and further development will allow us to create high flux / high recovery rate RO systems with excellent rejection of monovalent ions.
Could you discuss where G2O is currently in development, i.e. do you have a pilot plant, are you working with membrane producers in testing your process on their products, etc.?
Dr. Miao - The R&D element of the technology is largely behind us and we are concentrating on optimizing our coating for specific membrane types and specific applications. This involves optimizing the coatings, quantifying the performance improvements on commercially available membranes and then working with the manufacturers to integrate our technology into their mass production process. Besides the work with Innovate UK and the National Nuclear Laboratory we are discussing projects with a number of oil services companies for oil/water separation in the offshore industry and the treatment of produced water.
Could you provide a broader context of how this technology could impact issues such as water shortages and reducing costs associated with desalinating water?
Dr. Miao - We would not go as far as to claim the 99.9% energy savings that some have made, but the G2O technology should significantly decrease the cost of water for large desalination plants. Perhaps more significantly the reduction in energy required could make smaller, local desalination plants powered entirely by renewable sources economically viable.
In the short term, the high throughput combined with improved antifouling promises to have a significant environmental impact when applied to industrial waste streams. Many industries from oil production and paper making to textiles and the production of frozen french fries need to separate out large quantities of water from other materials, and the G2O technology allows us to both improve process throughput and leave behind cleaner water. This is particularly important now that global environmental standards are tightening and countries such as China are taking an increasingly hard line of environmental pollution.
Finally, we are working with a US based NGO and the World Economic Forum Global Agenda Councils to create a device for developing countries that can generate potable water with no energy input. This is especially important in rural areas where infrastructure is poor and water borne diseases are common.