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Structural Engineering of 2D Atomic Planes towards Task-Specific, Freestanding Superstructures through Combined Physical-Chemical Pathway

Periodic Reporting for period 1 - CHEPHYTSSU (Structural Engineering of 2D Atomic Planes towards Task-Specific, Freestanding Superstructures through Combined Physical-Chemical Pathway)

Reporting period: 2016-07-25 to 2018-07-24

Proton exchange fuel cell technology was regarded as a promising energy conversion system, which can be operated in a more environmentally benign manner to convert chemical energy of sustainable fuels into electrical energy. The central core of the fuel cell system is the membrane electrode assembly (MEA) which is composed of two parts in terms of fuel electrodes (anode and cathode) and the electrolyte membrane sandwiched between them. For electrolyte membrane, its proton conductivity and selectivity are utmost parameters that not only link to membrane efficacy but also determine the final performance of fuel cell systems. The importance for innovation of highly selective and conductive proton exchange membranes is ever increasing. Currently, a fundamental and technological bottleneck is that most proton conductive materials still show much lower conductivity, below the general target of <10−2 S/cm for practical applications. The Manchester host group has first reported the 2D crystal-based proton transport properties, and pioneered the development of 2D proton conductive membrane. On this basis, we aim to innovate a new class of 2D polymer crystal based fast proton conductor, in which well aligned 3 Å channels were incorporated into in the 2D polymer crystals along the c-axis (as free proton transport pathway). Furthermore, we challenged ourselves to develop technologically feasible techniques that can integrate wafer scale CVD graphene or hBN with commercial MEA, and showcased them in a working fuel cell system. This can serve as the prototype devices that applied the state-of-the-art 2D proton conductors to overcome the “fuel crossover bottleneck issue” while negligible affecting proton conductivity. Further scale-up from this level should be straightforward and could enable significant new technologies for fuel cell related applications.
Two work packages have been thoroughly investigated towards the objectives of this project listed above. In work package 1, a new family of 2D polymer crystal proton conductor was created, and the fast and selective proton transport features enabled by the aligned transport channels were highlighted, which reflects a new transport mechanism. In this work package, bulk MOFs single crystals have been grown. Towards few layer 2D crystals, exfoliation and transfer techniques have been developed. Furthermore, proton transport devices were fabricated in cleanrooms. In the following, the proton conductivity of the 2D MOFs crystals was investigated. The role of ultramicropore channels was found effective in enabling a low-energy-barrier Grotthuss hopping mechanism for proton conductance. Due to intriguing structural properties and high-performances as fast proton conductor, a research paper is expected to be published in a high-profile journal. In work package 2, a technical innovation of proton exchange membrane was presented, in which single layer proton selective conductor including the commercial CVD graphene and hBN was integrated with commercial proton exchange membranes. Dimensionally, the new type of graphene strengthened proton exchange membrane can be scale up to wafer-scale. In a working fuel cell, the fuel crossover was suppressed by up to 90%, while negligibly affected the proton conductivity. The dissemination of the work has been undertaken through presentations at Carbon international conference 2017 held in Melbourne, Australia.
Regarding the state of the art, the progress in package 1 and 2 was evaluated scientifically and technologically below. First, a new family of 2D polymer proton conductor with aligned 3 Å channels has been demonstrated. The record high proton conductivity (among reported crystalline polymer materials) was achieved with the value around 74.8 S cm-2, which is in the same order of that of the state-of-the-art Nafion. Imparting ultramicroporous into the 2D polymer crystals enables proton selectively while blocking fuels. This is a promising direction for future work can be pursued to achieve high-rate proton conduction and prevent fuel crossover issues. This may motivate researchers from different disciplines including chemistry, materials, chemical technology to further optimize and scale up such type of 2D polymer as fast proton conductors. Considering the fact that the most advanced polymeric proton conductor is still perfluorosulfonic acid polymer (Nafion) which was invented by DuPont half a century ago, the work in package 1 would be highly valuable in research community in devising a new type of proton conductor for task-specific applications. In the view of long term run, once the scaling up based on this principle becomes possible, it would boost prosperity of the proton exchange fuel cell and related applications.
The work in package 2 represents an important technical advance following on the ground breaking new finding by the host group that proton transfer could occur directly though pristine single layers of hexagonal boron nitride and graphene with reasonably low energy input. The wafer-size graphene integrated PEM enables a 30-40% increase in peak power density (up to 45.2 mW cm-2) compared to the conventional PEM even with 10 M concentrated methanol. It could be implemented in actual fuel cells in future when the stability is improved for long-term run.