Design Of Anode Catalysts Based On Pt,Co And The Study Of Their Catalytic Performance For Direct Methanol Fuel Cells

With the progress of human society and the increase of global energy demand,the issues of energy and environment become more prominent,people begin to worry about the energy security and environmental sustainability,which pushes people to explore new energy technologies.Direct methanol fuel cells(DMFCs)have attracted extensive attentions due to their extensive fuel sources,high energy density,high energy conversion efficiency and environmental friendliness.However,many technical bottlenecks of DMFCs hinder their commercialization.Among these bottlenecks,the development of anode catalysts is one of the most prominent.To date,the anodic catalysts of DMFCs mainly rely on Pt based noble metal catalysts,which have high cost,and the intermediates generated from the methanol oxidation reaction(MOR)will poison the Pt-based catalysts,leading to the slow kinetics of MOR,short life of the catalyst as well as other problems.Therefore,it is urgent to seek the catalysts with low noble metal content,even non-noble metal as well as high catalytic activity and stability.This paper aims to construct efficient and low-cost catalytic systems for methanol oxidation reaction.The design strategies are listed as follows:(1)Improve the catalytic performance of composite catalysts by introducing co-catalysts.(2)Constract non-noble metal catalytic system.(3)The photoassisted and electrocatalytic processes were cascaded to improve the performance of the catalyst under light irradiation.According to the above design strategies,the following works have been done:(1)Designing a Pt-Co₃O₄-CDs electrocatalyst with enhanced electrocatalytic performance and resistance to CO poisoning;(2)Constracting synergistic Cu@CoO_x core-cage structure on carbon layers as highly active and durable electrocatalysts for methanol oxidation;(3)Establishing a photoactive process cascaded electrocatalysis system for enhanced methanol oxidation over Pt-Ti₃C₂T_x-TiO₂ composite;(4)Improving the methanol oxidation property of Pt-CNTs/Ti₃C₂T_x composite catalyst with the assistance of visible light.The detailed work describes as follows:1.High efficient electrocatalysts remain huge challenges in direct methanol fuel cells(DMFCs).Here,the Pt-Co₃O₄-CDs/C composite was fabricated as anode electrocatalyst with low Pt content(12 wt%)by using carbon dots(CDs)and Co₃O₄ nanoparticles as building blocks.Pt-Co₃O₄-CDs/C composite catalyst shows a significantly enhanced electrocatalytic activity(1393.3 m A mg^-1 Pt),durability(over 4000 s)and CO-poisoning tolerance.The superior catalytic activity should be attributed to the synergistic effect of CDs,Pt and Co₃O₄.Furthermore,the Pt-Co₃O₄-CDs/C catalyst was also integrated into a single cell,which exhibits a maximum power density of 45.6 mW cm^-2,1.7 times of the cell based on commercial 20 wt%Pt/C catalyst.2.Active and inexpensive electrocatalysts for methanol oxidation reaction(MOR)are highly required for the practical application of direct methanol fuel cells(DMFCs).However,the efficient MOR is limited by using the expensive and rare noble metal-based catalysts.Here we report a Cu@CoO_x core-cage nanostructure on carbon layers(CLs)for the superior electrocatalysis of MOR in the alkaline media,which shows an excellent specific activity of 150.41 m A cm^-2 and a high mass activity of 467.94 mA mg^-1 at the potential of 0.8 V vs.SCE(1.85 V vs.RHE)in 1 M KOH+1 M CH₃OH.It represents the highest MOR activity ever reported for noble metal-free catalysts.Synchrotron radiation based in-situ X-ray absorption spectroscopy reveals that the outside CoO_x cage can form a high Co⁴⁺state to easily oxidize methanol,while the adsorption experiments indicate that Cu can act as the methanol adsorption center.The capture-catalysis process on the core-cage structure thus leads to the excellent MOR activity.The CLs can also anchor the Cu@CoO_x particles and accelerate the charge transport to enhance the performance.The Cu@CoO_x-CLs catalyst is economical,abundant,highly active and stable,which has the potential to act as a good alternate material for noble metal-based catalysts in DMFCs.3.Highly efficient photo-assisted electrocatalysis for methanol oxidation reaction(MOR)realizes the conversion of solar and chemical energy into electric energy simultaneously.Here we report a Pt-Ti₃C₂T_x-TiO₂ composite for highly efficient MOR via a photoactive cascaded electro-catalytic process.With light(UV and visible light)irradiation,Ti₃C₂T_x-TiO₂ serves as the photo active centre(photo induced hole)to activate the methanol molecules,while Pt particles are the active centre for the following electro-catalytic oxidation of those activated methanol molecules.Pt-Ti₃C₂T_x-TiO₂ catalyst exhibits a lower onset potential(0.33 V)and an impressive mass activity of 2750.42 mA mg_Pt^-1 under light illumination.It represents the highest MOR activity ever reported for photo-assisted electrocatalysts.Pt-Ti₃C₂T_x-TiO₂ also shows excellent CO tolerance ability and stability,in which,after long-term(5000 s)reaction,still keeps a high mass activity of 1269.81 mA mg_Pt^-1(62.66%of its initial activity).The photo-electro-catalytic system proposed in this work offers novel opportunities for exploiting photo-assisted enhancement of highly efficient and stable catalysts for MOR.4.The electrocatalytic methanol oxidation reaction with the assistance of visible light,which can not only realize photocatalytic and electrocatalytic processes occurring on the catalyst surface simultaneously,but also broad the application range of photocatalytic methanol oxidation reaction.Thus,it is completely important to design a reasonable and efficient photoassisted electrocatalytic system.This work provides a three-dimensional structure of TiO₂@Ti₃C₂T_x and then modified with Pt nanoparticles and carbon nanotubes to obtain the Pt-CNTs/TiO₂@Ti₃C₂T_x composite catalyst.The composite catalyst possesses a peak current density of 2901.53 mA mg_Pt^-1,which is 3.63times of that of the commercial Pt/C catalyst.The potential of the oxidation peak is also more negative than that of the commercial Pt/C catalyst.Besides,the composite catalyst also owns a better long-term stability.After 10800s,the current density is 1275.44 m A mg_Pt^-1,which is 63.90%of the initial value.In addition,the composite catalyst exhibits excellent CO tolerance.The excellent performance of the composite catalyst is attributed to the activation process of methanol molecules by the holes generated by photoexcitation of TiO₂@Ti₃C₂T_x component and the cascaded electrocatalytic process of Pt component.While the carbon nanotubes can connect each unit to balance the charge as well as improve the conductivity of the composite catalyst.With the synergistic effects of various components,the Pt-CNTs/TiO₂@Ti₃C₂T_x composite catalyst achieves the optimal activity and stability under photo illumination.