Microfluidic flow cells for the analysis of electrocatalytic reactions

Abstract

English summary

Microfluidic flow cells for the analysis of electrocatalytic reactions

The electro-oxidation of small organic molecules such as methanol could play an important role in the transition from fossil fuels to renewable energy sources. For real applications the direct methanol fuel cell has been limited by the sluggish reactions, limited durability and high cost, and further development is needed for the DMFC to be cost-effective compared to other technologies.

Microfluidic flow cells are flow cells that can manipulate small volumes of fluids, and typically have channel dimensions on the micrometer scale. The microfluidic flow cell could be a very useful tool in the investigations of electrochemical reactions. While microfluidic flow cells have been extensively used in other fields of research, such as biology and analytical chemistry, their application in the electroanalytical toolbox has so far been limited. The attractive features of performing electrochemical experiments in a microchannel is that the laminar flow profile gives well-described mass transport processes, the ability to work with very small electrolyte volumes, and the ability to switch between electrolytes much faster than conventional cells.

In this thesis we demonstrate a method to reproducibly fabricate microfluidic flow electrochemical cells with high quality noble metal thin-film electrodes using photolithographic methods. The flexibility of these methods means that a cell design can go from concept to fully operational cell in less than a week. The design of the electrodes can be tailored for different mass transport properties, such as high collection efficiency and fast mass transit between electrodes. Working-sense electrode mass transit times, i.e. the time it takes for a species produced at the working electrode to reach the downstream sense electrode, down to 3 ms were demonstrated.

The integrated palladium hydride thin-film electrode is demonstrated to be a suitable reference electrode for the microfluidic electrodynamics cell. Fabricated by the same method as the other thin-film electrodes and charged in situ, the main advantages of this reference electrode are the simplicity of the design and operation and that it does not contaminate the analyte. While the longevity of the PdH reference electrode is lower than the conventional alternatives, it was found to provide stable potentials for at least 5 hours. This, and the ability to recharge the electrode in a short time, means that it was suitable for all the electrochemical measurements in this thesis. Being able to place the reference electrode upstream of the other electrodes meant that the potential distribution in the microchannel, which was found to be a problem when using an external reference electrode, could be much better managed.

A reduction of the width of the electrodes and the distance between them resulted in short transit times, and enabled novel measurement techniques to be attempted. By applying a periodic perturbation at the working electrode, the same signal could be detected and isolated at the sense electrode through the changes in the concentration of the species. This method may have the potential to be developed further to filter out both periodic and steady noise from the target signal.

The microfluidic flow electrochemical cell was applied to the study of the methanol oxidation reaction through measurement of the soluble intermediates formic acid and formaldehyde, using an

electrode design with a platinum working electrode located upstream of a palladium sense electrode. First, the oxidation reactions of formic acid and formaldehyde on palladium were investigated, with the goal of using this electrode for in situ quantitative measurements. Palladium does not oxidize methanol in acidic electrolytes, and is a good catalyst for formic acid oxidation. Using a fast potential step technique, oxidation of formic acid at rates close to the mass transport limited rate, with negligible contribution to the current from formaldehyde, were demonstrated. This technique was then applied to in situ measurement of formic acid released from the upstream platinum electrode, showing that a substantial fraction of the methanol oxidized at the smooth Pt electrode is released downstream as formic acid.

Norsk sammendrag

Nytt forskningsverktøy kan gi bedre brenselceller

Vi har utviklet en mikro-versjon av elektrokjemiske celler. Denne gir en ny innfallsvinkel når vi studerer elektrokjemiske utfordringer, og kan hjelpe videre forskning på å gjøre brenselsceller mer effektive.

Brenselsceller er elektrokjemiske celler som omdanner kjemisk energi i et drivstoff direkte til elektrisk energi, og er en teknologi som kan brukes i stedet for eller sammen med batterier i elbiler. Det enkleste drivstoffet for brenselceller er hydrogen, men metanol eller andre organiske stoffer er attraktive alternativer fordi det tar mindre plass å lagre. Metanol kan for eksempel lages fra biomasse, og det kan også bli mulig å lage metanol gjennom CO2-fangst.

For å konkurrere med alternativene krever metanol‑brenselsceller mer forskning for å bli mer energieffektive og kostnadseffektive. Metanoloksidasjon, den halvdelen av reaksjonen der metanol og vann omdannes til CO2, protoner og elektroner, trenger en katalysator som øker hastigheten til reaksjonen. En økt forståelse av oksidasjonsprosessen kan hjelpe oss å utvikle bedre katalysatorer til fremtidens brenselsceller.

Mikrofluidiske celler er definert ved at minst en av dimensjonene er på mikrometer-skalaen, slik at volumet måles i nanoliter eller mindre enheter. Mikrofluidceller er også kjent som «lab-på-en-brikke», siden flere eksperimentelle metoder kan integreres i en enhet.

I denne oppgaven har mikrofluidiske strømningsceller blitt utviklet som en del av den elektrokjemiske verktøykassen. Ved å bruke produksjonsteknologi lånt fra mikroelektronikk, kan man få plass til flere elektroder av forskjellige materialer på et areal mindre enn et hårstrå. Dette gjør at transport av molekyler til og mellom elektrodene går mye raskere enn med større konvensjonelle elektroder. Sammen med en økt kontroll over elektrolytten, der både strømningshastighet og konsen­trasjoner kan endres raskt, åpner dette opp for nye eksperimentelle muligheter. Ved å ta i bruk en integrert referanseelektrode basert på palladiumhydrid, kan elektrodepotensialer måles nøyaktig og pålitelig.

Under metanoloksidasjon dannes formaldehyd og maursyre som produkter av ufullstendig omforming av metanol, og å studere mengden av disse kan gi et innblikk i reaksjonsmekanismen. En metode for elektrokjemisk kvantifisering av maursyre som produkt av metanoloksidasjon på platina presenteres.

Mikrofluidceller er et lovende verktøy som kan styrke videre forskning, både innen metanol‑brenselsceller og andre elektrokjemiske utfordringer.