| Sep 22, 2023 |
Graphene membranes improve fuel cell efficiency by blocking fuel crossover
(Nanowerk News) Researchers have made significant progress in developing advanced fuel cell membranes that could enable greener technologies like direct methanol and direct formic acid fuel cells. These fuel cells can convert carbon dioxide into useful fuels using renewable electricity, potentially revolutionizing sustainable energy systems. However, a major challenge has been preventing the fuel from crossing through the membranes, decreasing efficiency.
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Now, scientists at the University of Tsukuba have engineered graphene-based membranes that block this fuel crossover while still allowing high proton conductivity, marking a leap forward for carbon-neutral fuel cell viability.
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The researchers focused on modifying graphene, a one-atom thick sheet of carbon with remarkable properties. While plain graphene sheets can reduce fuel crossover, they also inhibit proton conductivity, which is critical for fuel cell performance. By introducing nanoscale holes and sulfanilic acid functional groups to the graphene, the scientists struck an optimal balance.
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This sulfanilic-functionalized holey graphene allowed proton conductivity comparable to standard fuel cell membranes while reducing methanol and formic acid crossover by around 80% and 60%, respectively. According to the researchers, the nanoholes enable selective passage of protons over fuel molecules, while the sulfanilic groups facilitate rapid proton transfer between holes via a "Grotthuss mechanism." Computational models revealed that the functional groups increased energy barriers for fuel crossover events.
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"These synergistic effects of introducing holes and sulfanilic-functionalized groups into graphene play a crucial role in balancing selective proton transfer and suppressing the crossover of fuel molecules," said Dr. Yoshikazu Ito of the University of Tsukuba, senior author of the study published in Advanced Science ("Suppression of Methanol and Formate Crossover through Sulfanilic-Functionalized Holey Graphene as Proton Exchange Membranes").
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Direct formic acid and methanol fuel cells could provide clean power for portable electronics and electric vehicles by oxidizing liquid fuels electrochemically without combustion. But fuel crossover has been a key limitation, lowering efficiency and output. By sandwiching functionalized graphene between standard Nafion polymer membranes, Ito and his team reduced methanol and formic acid crossover to levels sufficient for practical applications.
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While previous work has modified Nafion membranes with materials like graphene oxide to decrease crossover, this often came at the cost of reduced proton conduction. The extremely thin functionalized graphene layers minimally impact Nafion's high proton conductivity according to the researchers.
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Ito's group systematically investigated the number of graphene layers and found that two to three produced optimal results. "Our findings should contribute to the development of electrosynthetic cells for electrochemical CO2 reduction and advanced fuel cells such as DMFCs and DFAFCs," said Ito, referring to direct methanol and direct formic acid fuel cells.
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The researchers suggest that specialized fuel cell membranes integrating functionalized graphene could also be applied to other chemical fuels like ammonia and hydrogen peroxide. By providing a simple method to balance proton conduction versus fuel crossover, this advance removes a major roadblock for fuel cell systems that can utilize renewable resources and reduce dependence on fossil fuels. Ito's team is already pursuing ways to scale up fabrication.
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According to Dr. Samuel Jeong, first author of the study, "this graphene-based technology is a simple yet powerful method for the rational design of fuel membranes." With further development, sulfanilic-functionalized graphene membranes may soon become a key enabling component for sustainable energy innovations.
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