Concerns over climate change call for more innovative energy-conversion technologies to be developed and adopted to reduce the negative impacts associated with conventional fossil-fuel, combustion-based energy systems on the environment. Fuel cells have become one of the most promising energy-conversion technologies by virtue of their high efficiency and low or zero emission nature. However, they have yet to attain widespread appeal as their performance is still too low and their cost too high. This is particularly the case with proton exchange membrane fuel cells (PEMFCs). The performance and cost of a fuel cell are closely related to the design of the fuel cell electrodes, which have a complex micro/nanostructure involving interconnected electronic and ionic conducting phases, gas-phase porosity, and catalytically active surfaces and are extremely difficult to optimize. Studies have shown that only 15-20% of the catalyst loaded in state-of-the-art PEMFCs is catalytically active. Hence, there exists plenty of room for improving the cell performance and reducing precious metal catalyst loading. The primary objective of this research project was therefore to understand the coupled transport of protons, electrons, and mass species and explain the electrochemical reactions in nanostructured electrodes. An increased understanding of the nanoscale transport behaviour can lead to novel approaches for optimizing the electrode design, not to mention new classes of electrodes with a controlled, ordered structure that would revolutionarily boost the cell performance.

Methodology Used
We created an interdisciplinary approach that integrates the knowledge of electrochemistry and engineering thermo-physics, and with this unique approach we tackled the following scientific problems: i) the coupling mechanism of multiphase flow behaviours and electrochemical reactions; ii) the mutual interaction between heat and mass transport and electrochemical reactions; and iii) numerical modeling of the physical and chemical processes in fuel cells.

Key findings and implications
Our interdisciplinary research methodology and strategy have opened up entirely new avenues for the understanding of coupled multiphase heat/mass transport and electrochemical reactions and designing novel fuel cell electrode structures with the desired physical and chemical properties. The main academic achievements of the project are as follows:

1. We experimentally revealed the mechanism of interaction between multiphase heat/mass transport and reaction kinetics in fuel cells and found the intrinsic relationship between the flow behaviour in the flow field and current. This finding suggests that conventional models are incorrect because they make the assumption that the cell power output is independent of flow patterns. In particular, we discovered the transient capillary blocking phenomenon which occurs when the channel size becomes critically small and elucidated the bubble evolution in the flow field due to electrochemical reactions. The understanding of how the flow behaviour and the electrochemical reaction led to an analytical expression of the mass transfer coefficient that incorporates the effects of bubbles and channel ribs. This expression can be used to determine the mass transfer coefficient by measuring the limiting current. The theory represents an effort in bridging the gap between multiphase mass transport theory and electrochemistry.

A direct alcohol fuel cell

2. We experimentally demonstrated the intrinsic coupling between heat and mass transport in fuel cells. The fact that cell performance increases proportionally with fuel concentration in direct methanol fuel cells (DMFC) is well known. Researchers have attributed the better performance to an improved mass transport from the higher fuel concentration. We, however, found that the operating temperature also increased with an increase in fuel concentration, which accelerates the kinetics of electrochemical reactions. This discovery is important as it, for the first time, reveals the intrinsic coupling of heat and mass transport in fuel cells. The discovery not only led to the invention of an innovative asymmetric electrode architecture, but also enabled us to understand the mechanism of the coupled heat/mass transport and the kinetics of electrochemical reactions in fuel cells.

Working principle of a direct alcohol fuel cell

3. We developed a theoretical framework that describes the coupling of trans-scale, multiphase heat/mass transport and electrochemical reactions. The framework was developed on the basis of the newly gained understanding of coupled multiphase heat/mass transport and electrochemical reactions and the mechanism of improved electrochemical kinetics with nano-electro-catalysts, proton/electron transport in the nanoscale reaction layer, mass transport in the microscale diffusion layer and multiphase flow in the macroscale flow field. In addition, it provides a solid foundation for the theory of charge, heat and mass transport as well as the electrochemical reactions occurring in fuel cells and extends the application of classical heat/mass transport theory.

Prof Tianshou ZHAO
Department of Mechanical and
Aerospace Engineering
The Hong Kong University of Science and
Technology
metzhao@ust.hk