When the electrochemical somoto method prepares the catalyst, it can first prepare the precursor of the catalyst by simple and easy impregnation reduction method, but because it is difficult to control the load of metal nanoparticles during the preparation process, the dispersion of the metal nanoparticles of the prepared catalyst is poor, and the concentration of metal nanoparticles will affect the performance of the catalyst. In order to avoid this problem, the catalyst precursor prepared by impregnated reduction method can be placed in the electrochemical system, and electrochemical circulation can be carried out within a certain potential range, so that the catalytic ally metal in the precursor body can be partially dissolved and dissolved to achieve the dissolved balance . Electrochemical solutation can reduce the aggregation of metal nanoparticles in the catalyst and release the covered active sites to improve the comprehensive performance of the catalyst.
Research background on Pd-based anode catalysts
Experimental and theoretical studies show that the suitably designed high-performance ethanol electrooxidation reaction catalyst should include:
- C-C bond fracture of the active surface site;
- suitable surface composition to increase the selectivity of CO2 formation,
- promote water adsorption and activation to remove CO and -CHx species. Using these design guidelines, researchers have developed many new catalysts that are highly selective, active, and tolerable .
These advances have significantly contributed to the development of direct ethanol fuel cell technology. Despite these advances, the development of catalysts is theoretical and practical, and much of the work is still very challenging. The mechanism of ethanol electrooxidation reaction has not yet been determined [57, 58], it is necessary to determine the key evidence of ethanol electrooxidation reaction pathway, to clarify the essence of intermediate products and the adsorption mode on the pd surface. From a practical point of view, the reported method of synthesizing high-performance catalysts is still not feasible for large-scale commercialization. To this end, from the theoretical and practical aspects of joint efforts, make full use of the Pd-based catalyst in alkaline media, direct ethanol catalytic oxidation activity of high characteristics, through structural regulation and catalytic element coupling, the development of high-activity, high-stability direct ethanol fuel cell anode catalyst is imperative.
In order to fully explore the potential of Pd-based catalysts and improve the activity and stability of catalysts, domestic and foreign researchers mainly study the catalyst structure and the coupling of catalytic elements . The structural aspects of the catalyst are mainly metal nanoalloy catalysts and load catalysts. By growing Ni (OH)2 pieces on the surface of graphene oxide in a two-dimensional mesh structure, Ni (OH)2 pieces are fixed on the graphene oxide surface, and then the metal Pd (Ni(OH)2/rGO catalyst is reduced on both surfaces, which effectively avoids the aggregation of catalysts in the catalytic process and ensures the long-term stability of the catalysts. By reducing the distance of the Pd-Ni alloy active site in the Pd-Ni-P catalyst, the COads adsorption on the Pd surface can be oxidized more quickly by OHads on the Ni surface, improving the activity and stability of the catalyst. Using NaBH4 as a reducing agent, the one-step synthesis of the polyporous three-dimensional layered structure Pd-PEDOT nanocatalyst with graphene as the carrier shows that the electrochemical activity surface area of the catalyst of the system is high, and has good electrocatalytic activity and stability for ethanol oxidation. It can be seen that special catalyst structures can be constructed to improve the long-term stability of the catalyst.
In terms of catalyst element coupling, the long-term stability of the catalyst is promoted by the introduction of other metal elements (e.g. Ru, Ni, Sn, etc.) to form PdM binary or teraconia catalysts, or by doping non-metallic elements (e.g. P, N, etc.) to promote the electrocatalytic oxidation of ethanol [60, 61]. The PdNi alloy catalyst prepared by co-reduction method has good catalytic properties, which is due to the synergy between Ni and Pd to reduce the formation of toxic intermediates, and Ni is also an effective OHads attractor to facilitate the further oxidation of COads. By constructing NixPd100-x/rGO load catalyst, the pro-oxygen metal Ni was introduced to the Pd-based catalyst, the formation of OHads group in the ethanol oxidation process was promoted, and COads was oxidized into CO2, which enhanced the anti-CO poisoning resistance of the catalytic system and the long-term stability of the catalyst . The self-supported PdSn catalyst was synthesized by sacrificial template method, and the research shows that the modification of Sn to Pd inhibits the destructing adsorption process of alcohol, which is conducive to promoting the dual function mechanism, and greatly improves the catalytic performance of the catalyst and the ability to resist CO-poisoning . PdSn/CNT catalyst was prepared by microwave-assisted polyol method, and the introduction of pro-oxygen metal Sn greatly inhibited the adsorption of COads on the Pd surface and improved the long-term stability of the catalyst . PdRu/C catalyst was prepared by impregnated reduction method, which helped the catalytic metal Ru itself to have good catalytic ethanol capacity, and was able to promote the formation of OHads groups at lower potential, improving the catalytic activity and long-term stability of the catalyst . Through doping P to PdNi binary system, the electron density of Pd surface is changed, and the electrochemical activity surface area and electrocatalytic properties of ethanol are greatly improved under alkaline conditions . The above studies show that the introduction of pro-oxygen metals (Ru, Ni, Sn, etc.) in Pd-based catalysts can promote the formation of OHades groups, so that the adsorption species COads are oxidized, improve the anti-CO poisoning capacity of catalysts, and improve the atomic utilization of Pd.
In addition, for load-type catalysts, different types of catalytic systems can be constructed by selecting different catalyst carriers to improve the long-term stability of catalysts, which mainly consist of carbon materials and oxide materials. Metal alloy nanoparticles are loaded on carbon materials by different methods, resulting in a variety of load-type catalysts, such as PdAg/C catalysts prepared by impregnated reduction method, PdAu/C catalysts prepared by chemical lymon seed growth method, which have a negative starting potential and higher current density for ethanol oxidation under alkaline conditions, but the long-term stability of the catalysts is insufficient. The electrocatalytic oxidation of ethanol in TiO2 surface load edified catalysts and used in alkaline media shows that the toxicity of metal surfaces is reduced, showing good electrocatalytic activity and long-term stability. PtRhSnO2/C catalysts and nanocrystaloxides CeO2, Co3O4, Mn3O4 and NiO-modified Pd/C catalysts were prepared with oxides and C materials as carriers, and the results showed that The OHads, which were easily absorbed ethanol decomposing on the surface of SnO2, were found to be easily absorbed ethanol decomposition Changing the electronic effect of the Pd surface promotes the adsorption of ethanol on the Pd surface, thus enhancing the whole ethanol oxidation dynamics and catalytic activity, PtRhSnO2/C catalyst is highly efficient for ethanol oxidation, and can accelerate the C-C bond in highly fractured ethanol at room temperature to accelerate its oxidation to CO2 at low potential, reducing the poisoning of catalysts . Improve the long-term stability of the catalyst. From the above studies, it can be seen that the introduction of oxide carriers in catalysts can change the electronic effects of catalytic metal surfaces, inhibit the adsorption of toxic intermediates on metal surfaces, and improve the long-term stability of catalysts.
Based on the structural design of the catalyst and the selection of catalytic elements, the researchers also explored the feasibility study direction of improving CO2 selectivity . Under different potential conditions, the process of catalyzing ethanol by Pd-based catalyst under alkaline conditions was analyzed, and the results showed that at -0.4 to -0.1 V, CH3COads were converted to CH3COOH (C2 pathway) and CO2 (C1 pathway); In addition, studies have shown that COads is the main toxic species produced in the catalytic ethanol process, and if the catalyst can be quickly oxidized to remove COads not only improves the long-term stability of the catalyst, it is also beneficial to improve the selectivity of the C1 pathway in the catalytic process. From the above results, it can be seen that it is of great significance to design and synthesize a pd-base catalyst of a direct ethanol fuel cell with high anti-CO poisoning capacity, which is beneficial to the complete oxidation of ethanol while improving the long-term stability of the catalyst. In the course of experimental exploration, the use of theoretical simulation calculation is not only beneficial to the researcher\'s initial design of catalyst, but also helps the researcher to explore the catalytic mechanism of the catalyst. From alkaline environment, the process of catalyzing ethanol oxidation reaction of Pd-based catalysts can be seen, and the key steps affecting the activity and stability of the catalyst are the rate control steps in the reaction process and the oxidation of poisoned intermediates . OHads are reactants in both reactions, so OHads\' adsorption on the surface of the catalyst is a key parameter for evaluating catalyst performance. Through the density general letter theory, the Pd-Ni-P catalytic system is simulated, and the adsorption energy of OHads on the surface of clusters is calculated, and the results show that the introduction of Ni and P in the Pd-based catalyst makes OHads adsorption energy on the surface of the catalyst decrease, which is conducive to promoting the rate control steps and oxidation of poisoned intermediates during the catalytic reaction process[35, 66]. Therefore, the combination of theoretical simulation calculation and experiment can better understand the role of the introduction of other elements and catalyst carriers in Pd-based catalysts, which can enhance the long-term stability of catalysts.
In 2017, in response to the long-term stability of catalysts, researchers began to target the design of a synthetic fuel cell anode catalysts with renewable properties. The Pd/Ni (OH)2/rGO catalyst after the 20000 s stability test was activated in NaOH solution, followed by a further stability test of 20000 s, and the performance of the Pd/Ni (OH)2/rGO catalyst was restored to the value of the previous test, so the catalyst was renewable. The catalytic properties of the PdSn/CNT catalyst prepared by microwave-assisted polyol method can also be restored to the value of the previous test after activation of the NaOH solution, thus realizing the renewable catalyst.
The development of direct ethanol fuel cell is restricted by the lack of long-term stability and high catalyst cost of methanol and ethanol fuel cell. Therefore, the preparation of catalysts with long-term stability, as well as reducing the cost of catalysts, is one of the key steps in commercializing for DMFCs and DEFCs. Starting with the reaction mechanism of ethanol electrocatalytic oxidation reaction, the long-term stability of catalysts, such as the introduction of pro-oxygen metals (Sn and Ag etc.), carriers (carbon XC72 and MWCNTs) in Pd-based catalysts, and the catalytic mechanism of catalysts are discussed, and the cost of catalysts is reduced on the basis of ensuring the long-term stability of catalysts. The main research contents are as follows:
- To investigate a new strategy for the synthesis of Pd-Sn/f-C electrocatalysts as latter characterized to insure their morphologies for EOR process.
- To develop Pd-Ag/MWCNTs nanocatalysts by simple and effective strategy under room temperature and surfactants free, for methanol and ethanol electrooxidation.
- To study and investigate advanced trimetallic [email protected] core-shell nanostructure towards ethanol oxidation in alkaline medium.
- This study is hopefully therefore contributing to advance development of Pd-Based catalysts, economic and efficient method and best catalytic activity and stability for DAFCs.