CHEMICAL SCIENCE

New principles for catalyst design for direct ethanol fuel cells


On March 15, 2023, Beijing time, Professor Yang Yang’s team from the University of Central Florida published a research result entitled “Rational design of septenary high-entropy alloy for direct ethanol fuel cells” in the journal Joule.

In this study, the design strategy of seven-membered high-entropy alloy electrocatalyst with PtPd-rich surface structure based on direct ethanol fuel cell is reported, and the catalytic function and role of each element in the alloy are systematically studied. This work provides a feasible method for the development of structural alloy catalysts with low precious metal loading, high precious metal utilization, high selectivity, high catalytic activity and stability.

The corresponding author of the paper is Professor Yang Yang, and the first authors are Dr. Chang Jinfa and Wang Guanzhi.

Direct ethanol fuel cells (DEFCs) offer greater advantages over hydrogen fuel cells due to lower fuel prices, mature ethanol production infrastructure and industry, and ease of storage and transportation. However, the slow reaction kinetics of electrocatalytic ethanol oxidation (EOR) at the anode significantly jeopardizes the actual performance of DEFCs and restricts their large-scale commercial application. Since complete oxidation of ethanol requires cleavage of carbon-carbon bonds by transferring 12 electrons (i.e., C1-12e pathway), it is necessary to have high-loading platinum group metals (PGMs), such as Pt and Pd-based electrocatalysts, to facilitate complete reactions, but the CO intermediates produced on the surface of such catalysts cannot be further oxidized to CO2 at low potentials due to the lack of oxidants (OH* and O*), thereby poisoning the active site and reducing the activity and stability of PGMs for EOR. The high potential required to produce the oxidant inhibits the cleavage of C-C bonds, resulting in an incomplete reaction process. Therefore, even on the surface of PGMs, EOR tends to be carried out through incomplete 4e or 2e incomplete oxidation pathways, producing acetic acid/acetate or acetaldehyde as products, respectively, resulting in poor output performance and cell efficiency of DEFCs. Therefore, the development of low PGM content EOR and ORR catalysts with high activity and stability is an urgent task. High-entropy alloys (HEAs) have attracted much attention in recent years, such as multiple active sites and stable extreme conditions. HEAs contain more than five different elements, and their surface atomic arrangements are diverse, and the active sites are numerous and complex, making it difficult to effectively identify the role played by each element in HEAs. Establishing universal design principles for high-performance HEAs for electrocatalytic reactions and the role of effective identification of each element remains challenging.

In view of this, Professor Yang Yang’s team reasonably designed a seven-membered high-entropy alloy (PtPd HEA) composed of platinum, palladium, iron, cobalt, nickel, tin and manganese, of which platinum and palladium are the most active materials for EOR, and the choice of abundant and inexpensive iron, cobalt, nickel and manganese as their solid solutions is because they are easy to form a solid solution with platinum/palladium. After synthesis, the active site-rich PtPd enriched surface and stable single-phase solid solution structure were formed by relatively low temperature heat treatment, maximizing the utilization rate of the precious metal. The authors examined and identified the functional role of each element in a catalytic reaction in which both Pt and Pd act as active sites to catalyze EOR. In PtPd HEA, the C-C bond dissociation reaction barrier at the Pd site is greatly reduced due to synergistic and electron effects with other elements. Compared with Pt(111) and Pd(111), the adsorption of CO at the platinum and palladium sites in HEA is weakened, which makes PtPd HEA have better CO poisoning resistance and ensure its high activity and stability. Fe, Co, and Mn are easier to adsorb water molecules, so that the Pt/Pd site does not react with H2O to form Pt/Pd-OH*, so the reaction passes through the complete 12-electron pathway. Due to the appropriate water and ethanol adsorption energy, Ni and Sn can significantly enhance the activity and kinetic reaction rate of EOR.

Figure 1: Physical characterization of PtPd HEA.

Figure 2: Electrocatalytic EOR performance and functional role of each element.

Figure 3: EOR mechanism and DFT calculation.

In addition, these five reasonably selected non-noble metal elements can adjust the electronic structure of Pt and Pd, making them beneficial for both EOR and ORR. The PtPd(111)-rich structural surface of PtPd HEA maximizes the electrochemically active surface area (ECSA), which greatly improves the utilization efficiency of PGMs and optimizes the activity of EOR and ORR. EOR of the complete 12-electron pathway with mass and specific activity of 24.3 A mgPGMs-1 and 21.2 A cm-2 was achieved at 0.81 VRHE. In addition, the ORR half-wave potential of PtPd HEA is as high as 0.95 VRHE, which is about 100 mV higher than Pt/C. At 0.9 VRHE, the mass and specific activity of PtPd HEA were 17.7 A mgPGMs-1 and 15.5 mA cm-2. Assembled in DEFC, PtPd HEA achieves a peak power density of up to 0.72 W cm-2 and can operate stably for more than 1200 hours, achieving performance comparable to hydrogen fuel cells and far exceeding other DEFC catalysts in its class.

Figure 4: Electrocatalytic ORR performance.

Figure 5: DEFC performance.

This work will serve as a design principle for nanostructured alloys to develop renewable energy and sustainable applications. (Source: Science Network)

Related paper information:https://doi.org/10.1016/j.joule.2023.02.011



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