Fuel cells are a promising clean energy solution, converting fuels like hydrogen directly into electricity with water as the only byproduct. But compared to current methods of electricity production, fuel cells are inefficient and costly.
The remedy may the very thing humans use when they need a boost in performance. Researchers at Chiba University in Japan have discovered that coating the electrodes of a hydrogen fuel cell with caffeine protects them from degradation that normally occurs during the chemical reactions that power the device — improving lifespan and efficiency.
Clean energy, dirty cathodes
Fuel cells resemble batteries in that they have a positive and negative electrode, called the cathode and anode, respectively. Like a battery, ions and electrons move between these charged ends. However, fuel cells produce electricity rather than storing and releasing it.
To accomplish this, fuel like hydrogen is fed to the anode. Here, it is split into hydrogen ions and electrons. The positively charged ions move through an aqueous solution called an electrolyte toward the cathode while the electrons are directed through a circuit, producing electricity. When the ions and electrons meet again at the cathode, oxygen, an oxidizing molecule which accepts electrons, is waiting to react with them to form water. The cathode is also where the inefficiencies build up.
That water is the only byproduct is certainly a welcome environmental outcome, but the water negatively affects the platinum electrodes, which catalyze the oxygen reaction. Water and platinum react, producing a layer of platinum hydroxide on the electrode surface and reducing the efficiency of the oxidizing reaction and the fuel cell overall.
Caffeine boosted performance
Dealing with this unwanted reaction has proven costly and prevents hydrogen fuel cells from competing on the electricity market. To protect the electrodes, the team from Japan turned to a familiar molecule that also has hydrophobic or water-repelling properties: caffeine.
Experimental results found that platinum electrodes modified with caffeine can increase the activity of the oxygen reduction reaction at the cathode.
“Caffeine, one of the chemicals contained in coffee, enhances the activity of a fuel cell reaction 11-fold on a well-defined platinum electrode of which atomic arrangement has a hexagonal structure,” explained lead author Nagahiro Hoshi in a press release. Crucially, it is the structure of the platinum atoms that determines how well the caffeine works.
Atoms on the surface of platinum can be arranged in different orientations. Hoshi tested three platinum types with three atomic arrangements called Pt(111), Pt(110), and Pt(100). Caffeine protected the platinum and improved the reaction for Pt(111) and Pt(110), but not Pt(100).
On Pt(111) and Pt(110), the arrangement of platinum atoms allowed caffeine molecules to be absorbed perpendicular to electrode surface. This configuration led to an 11-fold and 2.5-fold increase in reaction activity, respectively. When the group visualized the interaction between caffeine and the atoms on the surface of Pt(100), they saw that caffeine attached in a tilted manner rather than perpendicular, negating the potential increases in reaction activity.
According to the scientists, this was due to steric hinderance — non-bonding interactions between molecules that changes their shape and reactivity — produced by the arrangement of platinum atoms in Pt(100). They also noticed a curious peak performance for caffeine modified Pt(111) at a molar concentration of 1 × 10−6, above which reaction activity declines. The team is unclear as to why there is a limit on performance and say they will continue to investigate.
Despite these unknowns, the current boost in performance from caffeine has the researchers optimistic that “the proposed method has the potential to improve the designs of fuel cells and lead to their widespread use”.
References: Nagahiro Hoshi, et al. Enhanced oxygen reduction reaction on caffeine-modified platinum single-crystal electrodes, Communications Chemistry (2024). DOI: 10.1038/s42004-024-01113-6
Feature image credit: Anastasia Zhenina on Unsplash
