Microfluidic flow cells in the service of fuel cell electrocatalysis

Electrochemical energy converting devices will be indispensable in the development of a future carbon neutral and sustainable society with their efficient conversion of hydrogen and biological derived fuels to electrical energy. Liquid hydrocarbons like formic acid, methanol and glycerol have high energy densities and can be fed directly to the anode side of a fuel cell and combined electrochemically with oxygen in air at the cathode. Although the complete oxidation products are CO2, water and electrical energy, the electrochemical oxidation processes involve various possible pathways and slow kinetics at low temperatures, inevitably leading to the existence of several intermediate surface species and partially soluble byproducts. This decreases the efficiency and reduces the fuel utilization. Gaining fundamental knowledge at the molecular level in general and also at conditions close to real conditions for operating units are needed to lay down guidelines for electrocatalyst development with the right properties to make the technology competitive. Improved microfabrication has made microfluidic flow cells more attractive, although mass transport in channel flow is mathematically challenging due to non-uniform accessibility. A novel on-chip, thin-film palladium hydride, PdH, reference electrode for microfluidic cells has been developed in the project. The PdH electrode is situated in a connecting side channel upstream allowing for accurate potential control of the channel electrodes at high current densities, and unaffected of species in the main channel. In close collaboration with University of Victoria (UVIC), a semianalytical method for simulating electrode reactions in channel flow has been developed. Furthermore, experimental and numerical solutions (Comsol Multiphysics®) of electrochemical impedance spectroscopy of a simple redox reaction were obtained and compared. The comparison was used as a validity test for common analytical approximations. A key objective in this project has been to exploit downstream electrodes as detector electrodes for solvable or partly solvable intermediates produced during methanol oxidation at a Pt electrode. Various detection methods have been attempted and the best detection of solvable intermediates (formic acid) was done through rapid potential stepping at a downstream Pd electrode. Pt is commonly used as catalyst material in low temperature fuel cells as it offers a stable and active surface for oxidation of hydrogen and hydrocarbon based fuels. However, Pt is easily deactivated by carbonaceous intermediates that adsorb strongly at the surface (CO) and impede further reaction. A relatively large overpotential is required for Pt electrodes to bind oxygen donating surface species that are essential in the removal of these strongly adsorbed intermediates and to reactivate the electrode once more for conversion of hydrocarbon fuel ("onset potential"). To increase the operating temperature is a well-known way to increase reaction kinetics and thus to lower the overpotential required for oxidation processes. In this project, we have established a method to perform aqueous electrochemical measurements using a self-pressurized autoclave. Conversion of methanol and glycerol were study at a thin Pt wire in sulfuric acid electrolyte from room temperature and up to 140°C. Experimental current-potential curves showed that the oxidation onset potentials are significantly reduced, and more than what would be expected by the improved kinetics of Pt oxide formation alone. This change is in fact several hundred millivolts in the case of glycerol, and it is clear that the onset potential depends on the surface activity of water. In this project we have established a general synthesis method to fabricate Ni-Pt nanoparticles with fine control of particle structure, size and composition. The goal with the catalyst development is to reduce the overpotential through increased catalytic and long-term activity and to reduce the consumption of noble metals. Extended potential cycling showed a continuous degradation of the Ni-Pt catalyst resulting in a surprisingly active Pt enriched catalyst towards CO oxidation and oxygen reduction reaction. The activity in this project has involved exchange of students and scientific staff from NTNU to UVIC and has been essential in establishing two new collaborative projects, with both UVIC and NTNU as formal partners.

Biomass-derived liquid fuels will play a key role in a future sustainable society that can no longer depend on non-renewable energy sources. Energy security and environmental concerns are the prime movers towards clean and efficient conversion technologie s based on renewable energy. Reducing the voltage loss resulting from introducing small organic molecules to the anode side of a PEM fuel cell, and simultaneously increasing the lifetime of the system are necessary for successful integration of the techno logy on a larger scale. The work proposed here is aimed at gaining new insight in the oxidation processes of small organic molecules (formic acid, methanol and ethanol) at model noble metal electrodes. To develop deeper understanding of the surface proce sses, techniques are required that quantify the reaction pathways at times scales that probe the kinetics. This project will employ microfluidic flow cells that offer a fast response time and couple with electrochemical and non-electrochemical methods tha t may prove useful to obtain important time-resolved kinetic data. Formation of soluble species, and their subsequent readsorption on or escape from the electrode during the oxidation processes must be correlated to surface and system properties. Advance d methods to analyse data collected with the microfluidic flow cell and affiliated techniques like dynamic impedance will be developed and used to maximize the utility of the data. Ultimately, a strong mix of analytical and kinetic methods will result in a detailed understanding of the reaction mechanisms for key liquid fuel reactions, and this will suggest ways to optimize the catalysis. Experiments at elevated temperatures based on the same selection of experimental methods will be pursued. The work wi thin this project is founded on a collaborative effort between the Department of Materials Science and Engineering at NTNU and the Department of Chemistry at the University of Victoria, Canada.