ABSTRACT
The electrochemical reaction kinetics, especially the oxygen reduction reaction (ORR) at the cathode, is crucial for the performance of a fuel cell. In this study, the electrochemical processes on a polycrystalline Pt electrode in the presence of protic ionic liquid (PIL) electrolyte diethylmethylammonium triflate [Dema][TfO] are investigated by means of cyclic voltammetry and electrochemical impedance spectroscopy. Since water is continually produced during fuel cell operation, the effect of the water content in the PIL has been intensively analyzed. In order to reveal the dependence of the interfacial reaction characteristics on the electrode potential, the impedance spectra were simulated by an equivalent circuit whose parameters can be related to both Faradaic and capacitive processes. Two interfacial resistances were identified, which differ by about 3 orders of magnitude. The larger one is a charge transfer resistance that can be associated with slow Faradaic processes like the ORR and platinum oxidation/oxide reduction. The smaller resistance is probably linked with fast processes that involve water molecules, such as hydrogen deposition and oxidation. The high- and midfrequency capacitive processes are attributed to "classical" double layer and pseudocapacitive behavior, similar to those identified under nitrogen atmosphere.
ABSTRACT
The cell performance and durability of polymer electrolyte membrane (PEM) water electrolyzers are limited by the surface passivation of titanium-based porous transport layers (PTLs). In order to ensure stable performance profiles over time, large amounts (≥1 mg·cm-2) of noble metals (Au, Pt, Ir) are most widely used to coat titanium-based PTLs. However, their high cost is still a major obstacle toward commercialization and widespread application. In this paper, we assess different loadings of iridium, ranging from 0.005 to 0.05 mg·cm-2 in titanium PTLs, that consequently affect the investment costs of PEM water electrolyzers. Concerning a reduction in the precious metal costs, we found that Ir as a protective layer with a loading of 0.025 mg·cm-2 on the PTLs would be sufficient to achieve the same cell performance as PTLs with a higher Ir loading. This Ir loading is a 40-fold reduction over the Au or Pt loading typically used for protective layers in current commercial PEM water electrolyzers. We show that the Ir protective layer here not only decreases the Ohmic resistance significantly, which is the largest part of the gain in performance, but moreover, the oxygen evolution reaction activity of the iridium layer makes it promising as a cost-effective catalyst layer. Our work also confirms that the proper construction of a multifunctional interface between a membrane and a PTL indeed plays a crucial role in guaranteeing the superior performance and efficiency of electrochemical devices.
ABSTRACT
Correction for 'Influence of residual water and cation acidity on the ionic transport mechanism in proton-conducting ionic liquids' by Jingjing Lin et al., Phys. Chem. Chem. Phys., 2020, 22, 1145-1153.
ABSTRACT
Proton-conducting ionic liquids (PILs) are discussed herein as potential new electrolytes for polymer membrane fuel cells, suitable for operation temperatures above 100 °C. During fuel cell operation, the presence of significant amounts of residual water is unavoidable, even at these elevated temperatures. By using electrochemical and NMR methods, the impact of residual water on 2-sulfoethylmethylammonium triflate [2-Sema][TfO], 1-ethylimidazolium triflate [1-EIm][TfO] and diethylmethylammonium triflate [Dema][TfO] is analyzed. The cationic acidity of these PILs varies by over ten orders of magnitude. Appropriate amounts of the PIL and H2O were mixed at various molar ratios to obtain compositions, varying from the neat PIL to H2O-excess conditions. The conductivity of [2-Sema][TfO] exponentially increases depending on the H2O concentration. The results from 1H-NMR spectroscopy and self-diffusion coefficient measurements by 1H field-gradient NMR indicate a fast proton exchange process between [2-Sema]+ and H2O. Conversely, [1-EIm][TfO] and [Dema][TfO] show only very slow or non-significant proton exchange, respectively, with H2O during the time-scale relevant for transport. The proton conduction follows a combination of vehicle and cooperative mechanisms in highly acidic PIL, while a mostly vehicle mechanism in medium and low acidic PIL occurs. Therefore, highly acidic ionic liquids are promising new candidates for polymer electrolyte fuel cells at an elevated temperature.
