Amino Acid-assisted Synthesis Of High-performance Fuel Cell Catalysts

Considering the unfriendly effects on environment of relying on fossil fuels,an environmentally friendly alternative to realize clean and efficient energy conversion and storage is highly desired.Fuel cells are identified as promising and clean energy conversion technique because they can be operated at low temperature,and their energy conversion efficiency is high while carbon release is low.A major problem that hindering advances of fuel cells is the lack of cost-effective,active,and durable catalysts.The advancement of synthetic chemistry,electrochemistry,and computational chemistry has greatly facilitated the understanding of the synthesis of electrocatalysts,catalytic mechanisms,and structure-property relationships.Amino acids,containing a range of functional groups（e.g.,-NH₂,-SH,-COOH and-OH）,can coordinate with metal cations effectively.The amino acids-M²⁺（M=Pd,Cu）complexes were favorable for morphological and structural control due to the regulated reduction kinetics.On the other hand,the introduction of N heteroatoms can also regulate the electronic state of the active center of the catalyst or the intrinsic activity of the carbon support materials.In this dissertation,four typical amino acids,including glutamic acid,arginine,proline,and histidine,were used to assist the controlled synthesis of electrocatalysts.Furthermore,the influence of the shape,composition and structure of the catalysts on their electrocatalytic performances has been also investigated.The main results are summarized as follows:1.Pt Pd@Pt core/satellite nanoassemblies have been successfully synthesized by using glutamic acid-M^Ⅱ（M=Pd,Pt）complex as precursors.It can be found that the introduction of glutamic acid is crucial for the formation of the Pt Pd@Pt core/satellite nanoassemblies.The prepared Pt Pd@Pt core/satellite nanoassemblies exhibit higher electrocatalytic activity and stability toward oxygen reduction reaction（ORR）compared with commercial Pt black.More specifically,the Pt Pd@Pt core/satellite nanoassemblies exhibit more positive onset potential(E_onset)and half-wave potential(E_1/2)than those of the commercial Pt black,30 m V and 60 m V,respectively.After1000 potential cycles,the Pt Pd@Pt core/satellite nanoassemblies remain 93.7%of the original electrochemically active surface area（ECSA）value and show only 4.2 m V degradation in E_1/2,while the Pt black catalyst undergoes a 33.4%loss in ECSA and21 m V degradation in E_1/2.The porous self-assembly structure of Pt Pd@Pt core/satellite nanoassemblies can immensely restrain the Ostwald ripening effect and the undesirable agglomeration of nanoparticles,which avoids the activity loss.In addition,the introduction of Pd to form alloy with Pt can prevent electrochemical corrosion/dissolution by sacrificing Pd or increase the dissolution potential of Pt,thus promoting the enhancement of the electrocatalytic stability.2.Based on the previous chapter,transition metals were introduced to form alloy with Pd and further decrease the cost of catalysts.Pd₃M（M=Co,Ni）alloy nanoassemblies have been successfully synthesized by using arginine-M^Ⅱ（M=Pd,Ni and Co）complex as precursors.Self-assembly properties between arginine effectively determine the formation of self-assembly structure,and the coordination interaction between guanidine and metal precursors is significant for the Pd₃M-alloy formation.The resulting Pd₃M（M=Co,Ni）alloy nanoassemblies exhibit considerably enhanced catalytic activity and durability for the ORR compared with those of commercial Pd black,which is mainly attributed to the unique assembled structure and synergetic effects deriving from alloyed composition.It is evident that both Pd₃Co and Pd₃Ni catalysts exhibit much more positive E_onset（Pd₃Co:1.045 V;Pd₃Ni:1.022 V）and E_1/2（Pd₃Co:0.871 V;Pd₃Ni:0.864 V）than those of commercial Pd black(E_onset=1.001 V,E_1/2=0.833 V).The long-term stability of all catalysts was explored by chronoamperometry for over 200 min.After the continuous operation,the Pd₃Co and Pd₃Ni only decayed 21%and 31%of the initial current,whereas the commercial Pd black showed much more current loss of 45%.3.Concave Cu Pd@Pd tetrahedra have been successfully synthesized by using proline-M^Ⅱ（M=Pd,Cu）complex as precursors.It is demonstrated that the strong coordination ability of C=O,O-H and N-H groups of proline with metal ions is important in tuning the nucleation and growth of concave Cu Pd@Pd tetrahedra.Sparked by unique concave tetrahedral structure and remarkably electronic effect,the obtained catalyst exhibited the better catalytic activity,stability and CO_ads poisoning tolerance for the formic acid oxidation reaction（FAOR）than those of commercial Pd black catalyst.The mass activity of the concave Cu Pd@Pd tetrahedra exhibits a larger current density(501.8 A g _Pd^-1)than that of commercial Pd black(120.6 A g _Pd^-1).As further supported by CO-stripping experiment,it is clearly observed that the onset oxidation potential and oxidation peak potential of the Cu Pd@Pd tetrahedra possess a negative shift of 143 m V and 39 m V compared to those of the Pd black catalyst,respectively.The chronoamperometry tests manifest the concave Cu Pd@Pd tetrahedra exhibit a lower current decay than that of the commercial Pd black catalyst.4.Single atom catalysts（SACs）have sparked tremendous interests for their maximum atom-utilization efficiency and fascinating electrocatalytic properties.Currently,the development of effective synthetic methodology toward SACs remains challenging.Herein,we elaborately designed a reliable Si O₂-templated strategy to synthesize atomically dispersed Fe atoms anchoring on N-doped carbon nanospheres（denoted as Fe-N-C HNSs）using the cheap histidine to coordinate with metal precusors and as the N and C precursor.By virtue of numerous atomically dispersed Fe-N₄ moieties and unique spherical hollow architecture,the as-fabricated Fe-N-C HNSs exhibited excellent ORR performance in alkaline medium with outstanding activity,high long-term stability and superior tolerance to methanol crossover,exceeding commercial Pt/C catalyst.The E_onset and E_1/2of Fe-N-C HNSs are measured to be 1.046 and 0.87 V,respectively,which surpass the values of Pt/C(E_onset=1.03 V;E_1/2=0.84 V).Chronoamperometric tests for over 20000 s manifest that the Fe-N-C HNSs exhibit a 91%retention of the initial current density,while only 78%retention can be found on the commercial Pt/C.This present synthetic strategy will provide new inspiration to the fabrication of various high-efficiency single atom catalysts for diverse applications.