Development of electrocatalysts and membranes for the cogeneration of electricity and valuable
A lot of economically valuable chemicals are obtained in industry through oxidation and reduction reactions. While many of these processes are highly exothermic, liberating energy as heat, they generally do not reach high energy efficiencies because most of this liberated energy cannot be recovered efficiently. Fuel cells offer the possibility to produce these chemicals through electrochemical reactions while converting the released energy into electricity, thus offering a clear advantage over the conventional production process. Another drawback of many of the current industrial processes is a rather low selectivity and as a consequence, a lot of raw materials are lost and an extensive downstream processing is required to purify the product stream. By controlling the potential in a fuel cell, the electrochemical approach offers a possibility to drive the reaction to a desired product and thus to increase the selectivity. The ultimate aim of this Ph.D. thesis was to develop suitable electrocatalysts and proton-exchange membranes for this electrochemical cogeneration approach, which would allow the use of fuel cells for a simultaneous and efficient production of electricity and industrially important chemicals. Since the currently applied electrocatalysts (typically based on noble metals) and proton-exchange membranes (Nafion®) are too expensive to allow a wide-spread commercialisation, cheaper alternatives were investigated in this work. As alternative to the noble metals, two types of non-noble metal-containing or even metal-free electrocatalysts were developed: (1) non-noble, abundant transition metals (Cu, Co or Fe) supported on N-doped carbons and (2) metal-free ordered mesoporous carbons doped with nitrogen, boron or phosphorus. Polyvinylidene membranes grafted with sulphonated polystyrene (PVDF-g-PSSA) were selected and investigated as alternative to the commercial Nafion® membrane. The selected electrocatalysts were tested for three target reactions, each producing a relevant product, (1) reduction of oxygen to hydrogen peroxide, (2) reduction of nitric oxide to hydroxylamine and (3) reduction of nitrobenzene to aniline.
In a first phase of the research, ordered mesoporous carbons doped with nitrogen (NOMCs) were studied as electrocatalysts for the oxygen reduction reaction (ORR) because of two main reasons, (1) N-doped carbons have already been reported in the literature as efficient metal-free electrocatalysts for the reduction of oxygen to water in an alkaline environment and (2) ordered mesoporous carbons have the additional advantage of resulting in a high surface area combined with easy accessible active sites compared to microporous materials as graphene. The NOMCs were synthesised according to a novel nanocasting method from two low cost precursors. They were thoroughly characterised and their electrochemical performance was evaluated in a half-cell setup. In alkaline environment, the best NOMC electrocatalyst achieved a much superior kinetic current density in the ORR compared to previously reported N doped carbon materials, and displayed high selectivity for a two-electron reduction process. Furthermore, long-term chronoamperometric tests revealed that the synthesised electrocatalysts also exhibit an excellent long-term stability. The best NOMC was further selected as electrocatalyst to perform a thorough investigation of the influence of the ink composition and the applied amount on the electrocatalytic performance for the oxygen reduction reaction. This study was deemed necessary because the contribution of the electrode composition and preparation on the ORR performance is generally underestimated. It was found that the factor with the largest impact was the catalyst loading: by increasing it an increased selectivity towards water was observed. The results of this study allowed to determine the optimal electrode composition to get the best ORR performance for electricity generation. Using this procedure as standard for the preparation of electrodes would be of enormous help as it would enable a meaningful comparison between different literature results, which is thus far hindered by the wide variety in electrode compositions that can be found throughout the literature.
In the second phase, N-doped carbons containing different contents of iron were studied as electrocatalysts for the electrochemical production of hydroxylamine in a NO-H2 fuel cell. They were selected based on an investigation of the state of the art, which made it possible to define the desirable features for the selective reduction of NO to NH2OH. First of all, the presence of isolated metal centres as active sites is an important prerequisite. Secondly, the isolated metal should have at least two accessible oxidation states, in order to successfully adsorb NO. Finally, the presence of an extended delocalised π-system in the electrocatalyst will grant high electrical conductivity. Moreover, our iron composite electrocatalyst containing N-doped carbon (Fe-PANI-AC) offers several advantages compared to iron phthalocyanine (FePc) supported on carbon materials: (1) the synthesis is straightforward and easily upscalable; (2) it makes use of inexpensive, available chemicals; and (3) several parameters (pyrolysis temperature, metal content and type) can be easily modified in order to optimise the electrochemical performance. The electrocatalysts were evaluated by chronoamperometry in a fuel cell with a 6 or 18% NO in N2 feed. While at low NO concentration the Fe-PANI-ACs could not outperform the most promising electrocatalyst reported so far, at higher NO concentration the performance of Fe-PANI-ACs was much superior to that of the reference material (33% increase in selectivity to hydroxylamine, 1.5 times more power generated and an almost three times larger hydroxylamine production rate). Furthermore, they displayed an excellent stability under the operating conditions. To speed up the screening of the electrocatalysts an attempt was also made to construct a setup to test the electrochemical performance of four electrocatalysts at the same time with a rotating disk setup for the NO reduction reaction. However, due to recurring problems with the setup this work was eventually stopped.
Next, based on recent reports proving the possibility to use Cu nanoparticles supported on multi-walled carbon nanotubes for the electrochemical reduction of nitrobenzene, non-noble metal-containing (Co, Fe or Cu) supported on N-doped carbons were investigated as electrocatalysts for the reduction of nitrobenzene to aniline. By replacing the undoped support with a N-doped carbon, it was attempted to increase the selectivity towards aniline: on one side, by creating extra active sites in the carbon framework as a consequence of the nitrogen doping; on the other hand, by incorporating N, different metal configurations (metal coordinated to nitrogen, oxides, …) can be expected. The desired selectivity increase was successfully achieved. Indeed, the best performing electrocatalyst (Cu-PANI-AC) exhibited a superior electrocatalytic behaviour compared to the literature electrocatalysts, since a higher selectivity to aniline was achieved and a lower overpotential was necessary to initiate the reduction reaction. Through chronoamperometry experiments a conversion of 54% and a selectivity of 82% towards aniline was achieved in an acidic environment at -0.75 V vs. Fc+/Fc. Since a possible role in the nitrobenzene reduction of the N-containing sites alone could not be completely excluded, also the metal-free ordered mesoporous carbons doped with B, P or N were tested for this purpose. These specific dopant elements were chosen because their incorporation into the carbon framework is facilitated by their similar size to carbon and, furthermore, because they have either a higher or a lower electronegativity than carbon, which will have an influence on the electronic properties of carbon and thus on its electrocatalytic performance. It was discovered that each element had its own advantage, boron lead to the highest kinetic currents, phosphorus resulted in a lower overpotential and nitrogen resulted in the highest selectivity towards aniline. Overall, the NOMC resulted in the best performance: a value of six for the electron transfer number, a kinetic current density of -33 mA cm-² and an onset potential of -0.31 V vs. Fc+/Fc, hereby outperforming the Cu-containing electrocatalyst. This clearly demonstrated that a metal is not strictly necessary to reduce nitrobenzene towards aniline.
Finally, the synthesis of PVDF-g-PSSA membranes was optimised by selecting and varying the parameters that were considered having the most impact on the proton conductivity, which is the most crucial factor for their application in fuel cells. Three different steps were optimised, (1) the dehydrofluorination reaction was optimised with respect to the number of double bonds that could be generated, (2) the grafting with polystyrene was enhanced and (3) the sulphonation reaction was maximised to incorporate a higher number of proton-exchange groups. According to the literature, these membranes already offer a competing value for the proton conductivity with respect to the commercial Nafion® membranes, especially at elevated temperatures, but at a lower cost. However, since no attempt was made to optimise the different steps in its synthesis method, we assumed that higher conductivities could be achieved. Indeed, it was discovered that a 4-fold increase in conductivity could be obtained. Compared to Nafion®, a three times higher proton conductivity was achieved and this difference increased to 6-fold at 80°C. This clearly demonstrates the benefits of this study and the applicability of these membranes as potential replacement for Nafion® in fuel cells. Furthermore, since PVDF membranes are known as chemically, physically and thermally stable membranes with possible implementations in various filtration applications, it was investigated if the grafting could enhance the separation properties of these membranes. Besides extending the applicability of these membranes to extremely alkaline pH’s, the grafting with sulphonated polystyrene also enhanced the rejection of salts (NaCl to 60% and MgSO4 to 75%).