This laboratory research will exploit some of the characteristic physical properties of the microscale, most notably laminar flow, to obtain membraneless microfluidic fuel cells that the PI and his coworkers recently introduced as a promising power source for portable applications. In these laminar flow-based fuel cells (LFFCs) a fuel containing stream (e.g. methanol, formic acid) and an oxidant containing stream (e.g. dissolved oxygen, permanganate) merge in a single microfluidic channel and proceed to flow laminarly in parallel due to lack of turbulent mixing at these small dimensions. Within this channel, these fuel and oxidant streams flow over and react at, respectively, the anode and cathode that line opposing sidewalls. The absence of a physical barrier eliminates issues such as fuel crossover, water management, and restrictions on media composition that are typically encountered in more common polymer electrolyte membrane (PEM, e.g. Nafion) based fuel cells. The performance of the membraneless LFFCs is dictated by well-understood microscale transport phenomena. Engineering of the mass transfer characteristics of the depletion boundary layers on the electrodes is one of the specific topics of study of this project. This work will focus on exploiting the opportunities of membraneless laminar flow-based fuel cells in the development and performance optimization of alkaline and bio-fuel cells, since the lack of a membrane overcomes many technical issues that to date have severely limited their promise. This will be accomplished as follows: a) Membraneless alkaline fuel cells are to be built that exploit the advantage of superior electrocatalytic activity at both the anode and the cathode in alkaline media while avoiding carbonate formation and membrane clogging issues that to date have hampered the development of PEM-type alkaline fuel cells with few exceptions. b) In membraneless biofuel cells the use of multistream laminar flow enables tailoring of the pH in the individual fuel and oxidant streams to maximize the stability and activity of the individual enzymes, whereas presently biofuel cells are operated using a certain compromise pH. c) Research to optimize the performance and fuel utilization of membraneless fuel cells is planned. Design rules will be derived to capture operation conditions (flow rates, fuel/oxidant flow rate ratio, fuel and oxidant concentrations, etc.) and design parameters (channel length, electrode to electrode distance, etc.) to maximize the performance of an individual LFFC. Introduction of an air-breathing gas diffusion electrode already overcomes mass transfer limitations at the cathode. The introduction of multiple inlets (or outlets) to periodically replenish (or remove) the depleted boundary layer is proposed to address the now arisen anode limitations.
Broad Impact The educational component of this CAREER development program will consist of (1) the development of modules for a course entitled Microchemical Systems; (2) a multidisciplinary Microchemical Systems lecture series; (3) a graduate program for the development of non-technical skills. The latter non-technical skills program, the core of the proposed educational program, will consist of four components: (i) a Workshop presenting the fundamentals and importance of non-technical skills, as well as the wide variety of available opportunities to improve those skills; (ii) a Personal Development Plan based on a skills assessment and implementation of that plan with the aid of mentors, typically chosen from alumni of the department; (iii) a Project Management Seminar in which typical situations in a corporate environment are simulated; and (iv) a Lectures Series by prominent alumni in leadership positions. The effectiveness of the program will be assessed by student feedback and third party evaluation. The PI will initiate and coordinate the development of this program and work closely with experts in the field in order to ensure the programs efficacy.
This grant sought to explore a new type of fuel cell which lacks the membrane to keep the two electrodes separated. Instead a liquid flowing electrolyte is used under so-called laminar flow conditions. This means that the only mechanism of mixing is by diffusion (a slow process) and not by turbulence (a fast process). Through the research performed under this grant we developed laminar flow fuel cells that can be operated using a range of different fuels (methanol, ethanol, hydrogen, hydrazine, borohydrate) and can be operated in both acidic and alkaline conditions. We focused very much on studying performance under alkaline conditions because (1) it is much easier in the absence of a membrane and (2) the reaction kinetics of both fuel oxidation and oxygen reduction are better under alkaline conditions (compared to acidic conditions). Indeed we were able to develop alkaline micro fuel cells that exhibited impressive performance. We also studied successfully how to avoid carbonate poisoning of the electrodes as a result of carbon dioxide (formed upon fuel oxidation) reacting with the hydroxyl ions present in alkaline media. We also were able to develop design rules on how to best configure multichannel microfluidic fuel cells; how many channels in parallel, best dimensions of each channel, best dimensions of each electrode, best flow rates etc. To maintain the required microscale fluid flow properties (laminar flow), one needs to arrange many single channels in parallel in order to obtain sufficient power for a (portable) application. Scaling up by increasing the size of the microfluidic channel would lead to loss in performance because one would enter the turbulent flow regime, which would lead to fuel crossover (fuel reacting at the wrong electrode), which is detrimental for performance. This fuel cell technology also was pursued for commercialization through a start-up company. Like with many other fuel cell technologies, lack of durability of the electrodes and system design are major hurdles still standing in the way of widespread commercialization. One of the microfluidic fuel cell platforms inspired by the results from this grant is now used by multiple research groups around the country for catalyst and electrode testing and characterization in order to speed up the development of more robust and durable electrodes. Towards the end of the grant period we successfully explored the reverse of a fuel cell process: with the same ‘fuel cell’ reactors by converting carbon dioxide back into fuel or into intermediates for chemical synthesis. This process would be driven by otherwise wasted renewable energy (too much sun, too much wind) to reduce carbon dioxide emissions and to reduce our dependence on fossil fuels as starting materials for chemical synthesis. This is the direction of our current research efforts, after this NSF CAREER project came to a conclusion. As part of this project also an elective course on microchemical systems was developed and a series of lectures on the importance of non-technical, soft skills was offered to graduate students. A total of 6 graduate students, more than 10 undergraduate students, and one postdoc received partial training through this grant. Several of these students were members of underrepresented groups in science and engineering.
