The growing population of Earth requires more and more energy. However, a large part of this energy is produced from fossil fuels that are non–renewable resources and their amount is shrinking rapidly. In order to address this energy consumption problem, many researchers investigate alternative ways that can help in our future. One promising way is to improve fuel cell systems, because, in general, these work with hydrogen, methanol, ethanol etc. instead of fossil fuels. Moreover, the extraction of energy stored in chemical bonds is more efficient using fuel cells, because – in contrast with conventional engines– there is no mechanical energy conversion step involved. One of the main components of fuel cells is the catalyst layer.
Nowadays, many researchers work on developing cheaper and more durable catalysts with better tolerance against envinronmental impurities, because these could help in solving problems that hinder the spreading of fuel cells.
During my doctoral work, we tried to contribute to the spreading of fuel cell systems by developing and investigating catalysts that promote the oxygen reduction reaction on the fuel cell cathode side.
In the first part of my work, we prepared platinum decorated nitrogen doped graphene composites by annealing the mixture of the precursors in ammonia atmosphere. The nanoparticles and the nitrogen doped graphene were formed simultaneously during the synthesis. The as–prepared composites contained the same amount of precious metal, but they were annealed at different temperatures. In the next step, the catalysts were characterized by transmission electron microscopy, and it was found that the average particle size increased with the annealing temperature. Even so, it still remained close to the optimum diameter range determined for the oxygen reduction reaction by theoretical and experimental works.
X-ray photoelectron spectroscopy results revealed the presence of platinum in +2 and +4 oxidation states besides the zero-valent form. One of the possible explanations for this is the high percentage of surface atoms that can interact with the oxygen molecules of the air. Another possible explanation is that graphene has higher electronaffinity close to the moieties formed during the doping process. This can make electron donation from the platinum nanoparticles more efficient, which in turn results in a stronger interaction between the support and the particles.
Oxygen reduction activity was tested in a three electrode system in acidic and also in alkaline media. It was found, that the composites have higher electrocatalytic activity at high pH because lower overpotential was needed for the reduction of oxygen and higher current densities were measured compared to the data measured in low pH electrolyte. According to the electron transfer numbers, reduction current densities and onset potentials, higher activity was reached in case of the composites prepared at 500 and 600 °C when 0.1 M perchloric acid was used, while better performance was reached on the composite annealed at 700 °C when the electrolyte was 0.1 M potassium hydroxide.
During the second part of my PhD work, we prepared non-noble metal catalysts using a nitrogen doped graphene support decorated with cobalt- or iron nitride particles. As with the previous catalysts, the precursor mixture was annealed in ammonia atmosphere in order to prepare the composites. The annealing temperature was constant and the transition metal content was varied.
According to X-ray diffraction analysis, Co4N, and a mixture of FeN and Fe2N phases was formed during thermal annealing at 600 °C in ammonia atmosphere. Transmission electron microscopy results showed that the nanoparticles dispersed well, without aggregation on the surface of the two-dimensional support up to a certain cobalt and iron content. It was also found that the size of the particles increased with increasing transition metal amount. The average diameter of cobalt nitride particles was 14.3 ± 7.1 and 43.1 ± 17.4 nm in case of 5 and 10 wt%
of cobalt content, while aggregated particles with the size of 205.2 ± 165.9 nm were found in case of 20 wt%. In case of composites containing iron nitride, the average particle size was determined to be 23.4 ± 9.2, 78.2 ± 33.6, 105.1 ± 56.4, and 127.0 ± 41.8 nm in case of 5, 10, 20 and 50 wt%, respectively. The aggregation was also discernible but only in the 50 wt% case.
X-ray photoelectron spectroscopy results evidenced that the nitrogen/carbon ratio in the support was increased from 0.062 to 0.086 upon increasing the cobalt content up to 10 wt%, whereas higher transition metal contents did not cause any significant changes in the ratio.
Higher nitrogen/carbon atom ratios could be achieved in the composites in which iron nitride particles were formed. The N/C ratio increased from 0.098 to 0.122 when increasing the iron content from 5 to 20 wt%. Further increase in the transition metal content did not improve the N/C ratio any further.
The oxygen reduction reaction activity was tested by using electrochemical methods. In case of the Co4N/NG composites, the onset potential of linear sweep voltammograms was shifted to more positive values by decreasing the average particle size, which refers to the improvement of the electrocatalytic activity. According to the current densities measured by linear sweep voltammetry by applying the same rotation rate and the electron transfer numbers calculated by using the Koutecky–Levich equation, it is the composite with 10 wt% cobalt content that has the highest activity towards oxygen reduction reaction, and not the one with 5 wt% cobalt content. The reason for this is that the nitrogen content of the support is higher in case of 10 wt%, and this could cause higher support activity and also higher composite activity compared to the 5 wt% sample, even if the particle size was bigger.
According to the electrochemical results, the properties of our composites are close to the electrocatalytic activity of the most commonly used amorphous carbon supported platinum nanoparticle systems.
The electrochemical properties of FeNx/NG samples were also tested. The optimal iron content was found to be 20 wt% based on the onset potentials, the reduction current densities of the measured LSV curves, and the electron transfer numbers determined from the K-L equation.
We suggest that the increased amount of nitrogen in the support resulted in higher composite catalytic activity here, even though the particle size of iron nitride was larger compared than that of the 5 and 10 wt% samples.
The catalytic activity of our non-noble metal catalysts was also tested in the presence of methanol. The chronoamperometric measurement of the most commonly used platinum catalyst showed that the current decreased to 40% of its initial value when methanol was added to the electrolyte in this refernce system. Notably, the measured current did not change when methanol was added to a system using our non-noble metal catalysts.
Köszönetnyilvánítás
Elsősorban köszönettel tartozom témavezetőimnek, Dr. Kónya Zoltán tanszékvezető egyetemi tanárnak és Dr. Haspel Henrik egyetemi tanársegédnek, hogy tanácsaikkal, biztató szavaikkal végigvezettek a doktori fokozat megszerzéséig vezető rögös úton. Köszönöm Dr.
Kukovecz Ákos egyetemi docensnek a publikációim megírásához adott tanácsait és tevékeny segítségét. Munkájával nagy mértékben hozzájárult írói készségem fejlesztéséhez ezáltal pedig publikációim színvonalának emeléséhez. Szeretném továbbá megköszönni Dr. Kónya Zoltán tanszékvezetőnek, hogy lehetőséget biztosított számomra az Alkalmazott és Környezeti Kémiai Tanszéken folytatott munkám megkezdéséhez.
Szeretném megköszönni Dr. Oszkó Albertnek a röntgen fotoelekton spektroszópiás mérérseket és a spektrumok kiértékelését, melyek nagyban hozzájárultak a disszertációm színvonalának emeléséhez.
Köszönöm Dr. Szűcs Árpádnak, hogy biztosította számomra az elektrokémiai vizsgálatokhoz elengedhetetlen forgó korongelektródos rendszert.
Köszönöm korábbi hallgatóimnak és jelenlegi kollégáimnak, Varga Ágnes Timeának, Vásárhelyi Líviának és Ballai Gergőnek, hogy munkájukkal hozzájárultak a disszertációhoz szükséges eredmények eléréséhez.
Szeretném megköszönni Nagy Lászlónak, hogy a kísérletek során felmerülő technikai problémák megoldása érdekében bátran fordulhattam hozzá tanácsokért. Szeretném megköszönni továbbá Buchholcz Balázsnak, Juhász Koppány Leventének, Dr. Pusztai Péternek, és az Alkalmazott és Környezeti Kémiai Tanszék összes volt és jelenlegi dolgozójának, akik valamilyen formában hozzájárultak a munkám eredményességéhez.
Végül, de nem utolsó sorban szeretnék nagy köszönetet mondani páromnak, Nagy Krisztinának, továbbá szüleimnek, és az egész családomnak, hogy hittek bennem, kitartottak mellettem, támogattak és biztattak még a legnehezebb időszakokban is. Nélkületek nem sikerült volna!
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