Materials Today Volume 18, Number 10 December 2015
A shortcut to graphene aerogels?
A commercially viable material for use in energy applications, catalysis and environmental clean-up could be one step closer, thanks to researchers in the US. Graphene is rarely out of the headlines. The single layer of carbon atoms displays remarkable properties, including its superior electrical and thermal conductivity, and mechanical strength. In its native form, graphene has limited utility, so focus has shifted to integrating it into bulk-scale materials, to effectively ‘scale-up’ its properties. One option is graphene aerogels, which show potential for use in a wide range of applications. The time-consuming manufacturing processes involved in producing these aerogels have historically been a bottleneck to their use in commercial systems, but that may all be about to change. In a paper from the upcoming December issue of Carbon [M.B. Lim, et al. Carbon 95 (2015) 616–624],
researchers from the Pacific Northwest National Laboratory and the University of Washington have outlined an ultra-fast process for synthesising graphene-oxide (GO) aerogels. Carbon-based aerogels – often referred to RF aerogels after their main ingredients (resorcinol and formaldehyde) – have been in use since the 1990s. But most depend on a slow, water-based process at elevated temperatures that can take up to 72 hours to complete. Those based on sodium catalysts take even longer to cure – up to seven days at 858C. Pauzauskie and his team adapted the standard RF approach, using an acidcatalysed route, they produced a grapheneoxide (GO) laden aerogel in just two hours. Collaborating with an energy storage company, the researchers tested the aerogel’s performance as an electrode for supercapacitors. This work demonstrated that GOloaded aerogels exhibit a higher capacitance
and power capability than RF-aerogels, making them a material of interest for energy storage! As well as analysing the material’s electrochemical properties, the researchers also tested it as a possible sorbent for environmental toxins. Cyclohexane is a precursor for many industrial products – nylon being one of them. It was found that, despite having fewer pores, the GO-aerogels could absorb more than 3 times as much cyclohexane as RF-aerogels. The graphene’s hydrophobic nature was believed to be source of this behaviour. The team are now focused on finding an alternative, more environmentally friendly catalyst for the process. They believe that their approach will make it easier, and cheaper, to rapidly produce graphene aerogels on a large scale, and could open the door for their use in energy storage and environmental applications. Laurie Winkless
Energy [S.G. Hashmi, et al. Nano Energy 17 (2015) 206–215] goes further. Led by Michael Gra¨tzel, a team of Swiss and Finnish researchers have developed a low-cost, inkjet-printed dye sensitized solar cell that outperforms those already available. So how does it work? DSSCs are typically fabricated using standard thin film processes – first a layer of dye sensitized titanium dioxide is applied to a substrate and then topped with another electrode. Next, two holes are drilled in the top electrode and the liquid electrolyte inserted by suction. The holes are then sealed by topping the device with a foil and a glass cover. Gra¨tzel’s new approach is different – instead of being injected, the electrolyte is precisely printed onto the titanium layer before the second electrode is added. This removes the need for both hole-drilling and additional sealing, and reduces the amount of electrolyte required.
Beyond developing this new method, the researchers then compared the performance of the hole-free DSSC to a reference device, under full sun light intensity. The efficiency of the printed cell was found to be 6% higher than that of the reference (two-hole) cell. And significantly, the printed cell had a lower overall resistance, and maintained 100% of its performance over 1120 hours in an accelerated ageing test. These results surprised the researchers, as their main motivation was to make DSSC fabrication easier and quicker. The next step will be to determine the exact mechanism behind the improved performance. The team say that these results will accelerate the ‘‘production of cheaper, more robust, large area DSSC solar panels.’’ We’ll have to wait and see. Laurie Winkless
Inkjet printing of solar cells A new paper from the inventor of the dye sensitized solar cell suggests that inkjet printing may be the key to improving their performance. Back in the late 1980s, two Berkeley scientists invented the dye sensitized solar cell (DSSC). Formed by a thin layer of low-cost, dye-coated particles, sandwiched between two electrodes in an electrolyte, these cells can absorb a wide range of wavelengths. Now, one of those scientists has developed a new fabrication method that makes DSSCs even cheaper to produce, while retaining their performance. Silicon-based solar cells are a rapidly-growing technology, with improving efficiencies both at lab-scale and for commercial devices. For now, cost remains at a premium, but alternatives (such as thin film photovoltaics) are going some way towards changing that. But a paper in this month’s issue of Nano