Flash Joule Heating Equipment: Platinum-ruthenium bimetallic nanoparticle catalysts synthesized by direct Joule heating for methanol fuel cells

Background

Direct methanol fuel cells are photon exchange fuel cells fueled by liquid methanol, where methanol is oxidized at the anode and oxygen is reduced at the cathode. They produce fewer greenhouse gas emissions than conventional internal combustion engines. Methanol, as a liquid fuel, has advantages for storage, transportation and distribution Direct methanol fuel cells have a variety of applications ranging from transportation to stationary power generation, and have seen an annual growth rate of more than 10% over the past five years. However, the disadvantage of direct methanol fuel cells is the high cost and poor stability of the electrocatalysts used in their electrodes. Pt catalysts have high catalytic activity and are the most commonly used catalysts in methanol oxidation reactions (MOR)]. However, Pt metal has a strong adsorption effect on CO and other reaction intermediates produced by MOR, which can clog the catalytic active sites and reduce the catalyst lifetime. One potential solution is to produce bimetallic alloys with Pt using less costly metals, thus reducing the use of expensive Pt and potentially improving the stability of the catalysts Among the various metals, Ru is one of the better choices because Ru promotes the oxidation of CO adsorbed on Pt, scavenging the catalytically active sites and improving the catalytic performance. Various methods have been explored to synthesize PtRu bimetallic alloy catalysts, such as co-precipitation, impregnation, electrodeposition, and atomic layer deposition; however, a common problem of the current synthesis methods is the difficulty in achieving uniform elemental compositions and narrow nanoparticle size distributions of PtRu bimetallic alloy nanoparticles. Direct Joule heating, which can raise ultrafast temperatures up to 3000°C in microseconds (ms), has recently emerged as a new method for synthesizing alloy nanoparticles. Such high temperatures also allow the formation of various high-entropy alloys. In addition, the shorter heating time limits the nucleation of metal particles at high temperatures, leading to nanoparticles with a more uniform size distribution. We hypothesize that the use of the Joule heating method can produce homogeneous PtRu nanoparticles that can be used as high performance catalysts for the MOR required for direct methanol fuel cells.

The main point of this paper

Synthesis of PtRu catalysts by direct Joule heating method:

In this paper, PtRu catalysts for methanol oxidation reaction (MOR) were synthesized by direct Joule heating method and compared with PtRu catalysts synthesized by standard hydrothermal method and commercial Pt/C catalysts

Optimization of the parameters of the Joule heating method:

The performance of the catalysts was improved by optimizing the heating temperature and duration

Catalyst characterization and electrochemical testing:

Comprehensive catalyst characterization and electrochemical tests showed that PtRu alloy nanoparticles with narrow particle size distribution could be uniformly prepared on carbon black substrate under optimal synthesis conditions with large active surface area and high intrinsic catalytic activity

Performance of the optimized catalyst:

The optimized catalyst has better mass specific activity among the recently reported MOR catalysts

Experimental analysis and DFT calculation of catalytic activity:

Experimental analysis and density functional theory (DFT) calculations were performed to understand the improved catalytic activity

Further applications of the catalyst:

The catalyst was further fabricated into a two-electrode fuel cell and its excellent long-term stability was demonstrated

Full-cell performance:

Good rate capability and cycling stability were achieved using well-designed SS-Si/C anodes assembled with lithium cobaltate (LCO) or lithium iron phosphate (LFP) cathodes for full cells

Preparation of efficient PtRu catalysts by direct Joule heating: optimization of synthesis conditions and methanol oxidation reaction performance study

Catalyst performance optimization:

The heating temperature and duration of the Joule heating method were optimized to improve catalyst performance

Catalyst characterization and electrochemical testing:

Comprehensive catalyst characterization and electrochemical tests showed that PtRu alloy nanoparticles with narrow particle size distribution could be uniformly prepared on carbon black substrate under optimal synthesis conditions with large active surface area and high intrinsic catalytic activity

Comparison of catalyst performance:

The optimized catalyst has better mass specific activity among the recently reported MOR catalysts, with a peak mass activity of 705.9 mA mgPt-1 for MOR, which is 2.8 times higher than that of the 20 wt.% Pt/C catalysts, and much better than that of the standard hydrothermally synthesized PtRu catalysts

DFT calculations were performed to analyze the catalytic activity:

Theoretical calculations showed that the PtRu nanoparticles possessed strong methanol adsorption and weak carbon monoxide binding ability, which could be attributed to the modified Pt sites in the PtRu nanoparticles

Dual-electrode fuel cell stability:

The catalyst was further fabricated into a two-electrode fuel cell and demonstrated excellent long-term stability. 85.3% current density retention after 24 h and minimal oxidation on the Pt surface

Simplicity and safety of synthesis method:

This work uses a simple and safe synthesis method that enables rational design of hollow structures with unique properties

Summarize

Direct Joule heating over 50 ms at the optimum condition of 1000 °C allowed the formation of uniform PtRu alloy nanoparticles on carbon black substrate with an average size of 2.0 ± 0.5 nm (TEM) and mass loading of 6.32 wt.% (Pt) and 2.97 wt.% (Ru). The average particle size and particle size distribution of PtRu alloy nanoparticles prepared by Joule heating method are much smaller and narrower than those prepared by standard hydrothermal method. In addition, the optimization of Joule heating synthesis conditions showed that the formation of PtRu alloy nanoparticles was affected by the heating temperature and time. Higher temperatures and longer times resulted in larger nanoparticles, while lower temperatures and shorter times resulted in insufficient decomposition of the metal salt precursor. The optimized PtRu/C- jh -1000-50 exhibited the highest MOR activity at 705.9 mA mgPt-1, which is 2.8 times higher than that of the commercial 20 wt.% Pt/C catalyst and one of the best MOR catalysts reported recently. Its ECSA was 239 m2 g-1 (117 m2 g-1 for Pt/C) and its ECSA normalized specific activity was higher at 0.295 mA cm-2 (0.214 mA cm-2 for Pt/C) due to its uniform and small size of the PtRu alloy nanoparticles.DFT calculations showed that the Pt sites in the PtRu alloy nanoparticles have strong CH3OH adsorption and weak CO binding ability, resulting in a better MOR activity. A 24 h test in a two-electrode methanol fuel cell showed that the PtRu/C-JH-1000-50 also exhibited excellent stability, maintaining 85.3% of the initial current density.The XRD peaks of the PtRu alloy nanoparticles were well maintained, with minimal Pt oxidation on the surface. The fine structural control of the nanoparticles is crucial to provide excellent catalytic performance. This work demonstrates that the Joule heating method can produce high performance metal alloy nanoparticle catalysts for direct methanol fuel cells. Achieving precise heating temperatures and times is essential for the development of Joule heating based catalyst production techniques.