Preparation And Investigation Of High-performance Platinum-based Catalysts With High Platinum Loading For Fuel Cells

Fuel cell,the new energy conversion device,has attracted huge attention recently due to its advantages,such as high energy conversion efficiency,environmental friendliness,and quick start-up at conventional temperature,and it has been recognized to be the most promising power generation technology in the 21st century.Electrocatalyst,one of the key materials of fuel cells,plays a crucial role for the development of fuel cells and their large scale commercialization.At present,the most widely used catalyst is still carbon supported platinum particles catalyst（Pt/C）,but there are still problems existed,such as low activity,insufficient stability,lack of anti-reverse performance,and high cost etc.The use of high loading Pt-based catalysts makes the catalyst layer thinner,thereby reducing mass transfer resistance,which is of great significance for improving the output performance of the fuel cell;in actual working conditions,uneven gas distribution,frequently start-up or shutdown,etc.will cause the anode and cathode of fuel cell reversal,which will seriously damage the structure of the catalytic layer,causing irreversible harm;further,the durability of current fuel cells is far from the expected value.Therefore,the development of high-performance catalysts with high platinum loading,high stability（high durability）,and excellent anti-reverse ability is of great significance to the development and large-scale commercial application of fuel cells.Based on the above facts and knowledges,we attempt to design and prepare a new type of multiple functions electrocatalyst by using an improved microwave-assisted polyol method in this thesis.The stability/durability of the catalyst can be enhanced by pretreating the carbon support with titanium silicon oxynitride,and the anti-reverse performance of the catalyst is provided by the addition of iridium and ruthenium.The main research work and results are as follows:（1）Using the stabilized carbon black as the support and chloroplatinic acid as the precursor,we successfully prepared a Pt/C catalyst with Pt loadings up to 60 wt%by an improved microwave-assisted polyol method.The influences of heating method,reaction temperature,reaction time,solvent composition,carbon support type,complexing agent and other factors on the particle size distribution and performance of the catalyst have been intensively investigated.It is showed that microwave heating is simple and efficient,and the active components of the prepared Pt/C catalyst are uniformly distributed,small in particle size,and high in activity.The optimizing reaction temperature and reaction time are 120℃and 10 min,respectively,and the optimized solvent composition is 70 vol%ethylene glycol,25 vol%H₂O,and 5 vol%acetone.It is found that the carbon support has a great influence on the particle size and dispersibility of the supported platinum nanoparticles.the Pt/C catalyst prepared with Black Pearls 2000 as the support exhibited the highest electrochemical active area and oxygen reduction activity,which is better than commercial Pt/C catalyst.Meanwhile,adding citric acid as complexing agent can improve the distribution of Pt nanoparticle and the performance of the catalyst.The catalyst prepared with our optimized parameters exhibits excellent ORR activity.Furthermore,we tried to scale up the preparation in gram scale,the particle size of the scale up prepared catalyst is below 2.5 nm,the distribution is uniform,and it almost perfectly keeps the good electrocatalytic performance as mini scale preparation.The half-wave potential is 19 m V higher than that of commercial Pt/C（60 wt%）,and the mass activity is about 1.63 times that of the commercial Pt/C catalyst.（2）The stabilization of catalyst through modifying the carbon black with titanium silicon oxynitride by the impregnation-nitridation method has been investigated.The effects of nitriding conditions,nitride content,and titanium to silicon ratio on the stability/performance of the catalyst were studied.It is found that the titanium silicon oxynitride modification and nitridation of the carbon support can reduce the particle size of the supported platinum nanoparticles and improve the dispersibility of platinum.When the content of nitride in the support is 5%and the molar ratio of titanium to silicon is 1:2,the catalyst has the best activity and stability.The half-wave potential of the 60 wt%Pt/TiSi₂N_xO_y-NC catalyst prepared under the optimal conditions is 21 m V higher than that of the commercial Pt/C,and the mass activity is 77%higher than that of the commercial Pt/C catalyst.It is especially important that the 60wt%Pt/TiSi₂N_xO_y-NC catalyst shows good stability.After 20,000 cycles of accelerated durability testing,its half-wave potential only drops by 13 m V,and the mass activity after ADT can reach 2.92 times that of the commercial Pt/C catalyst（ADT under the same conditions）.It is revealed by XPS characterization results that the percentage of Pt（0）in the catalyst modified with silicon-titanium oxynitride on the support is increased by 9.3%compared with the commercial Pt/C catalyst,which may be an important reason for the enhanced stability/durability of the catalyst.（3）By adding a small amount of iridium and ruthenium to the catalyst,the catalyst exhibits good anti-reverse performance.The influence of the content of the anti-reverse component,the ratio of iridium and ruthenium,etc.on the anti-reverse performance of the catalyst has been investigated.It is showed that the addition of a small amount of iridium or/and ruthenium to the catalyst has almost no negative effect on the oxygen reduction activity of the catalyst,but can enhance the anti-reverse performance significantly.When the addition amount of the anti-reactive component iridium is lower to 3 wt%,the catalyst can exhibit desired anti-reverse performance.Furthermore,it is found that replacing iridium with ruthenium partly can further improve the anti-reverse performance of the catalyst.The Pt₅₀Ir₂Ru₁/TiSi₂N_xO_y-NC catalyst shows outstanding comprehensive performance of oxygen reduction and oxygen evolution,and exhibits a 5 times better durability under cell reversal conditions than that of Pt/TiSi₂N_xO_y-NC catalyst without Ir and Ru addition.It is revealed by XPS characterization results that the binding energy of Pt 4f with a small amount of iridium and ruthenium catalyst has a positive shift,and platinum transfers electrons to iridium or ruthenium,which may be an important reason why good anti-reverse performance can be obtained by adding a small amount of ruthenium and iridium.