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Using extended X-ray absorption fine structure (EXAFS) spectroscopy and cyclic voltammetry (CV) in combination, the fraction of surface Pt atoms poisoned by the phosphoric acid adsorbed (θPA) on Pt/C was analyzed for various electrode potentials (0.35, 0.60, and 0.8 V). The θPA values at 0.35 V, 0.6 V, and 0.8 V were determined to be 0.32, 0.45, an...
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... XAS measurements were conducted at beamline 10C of the Pohang Accelerator Laboratory using a laboratory-made in-situ XAS cell ( Figure 1). The XAS spectra were recorded under potential control at voltages of 0.35 V, 0.60 V, 0.80 V, and 1.00 V at room temperature (20 • C-25 • C). ...
Context 2
... fitting the curve of the RDF obtained using PA, the fitting parameters were CN Pt , CN OB , CN OS , R OS , and E OS ; the values of R Pt , E Pt , R OB , and E OB were obtained from the parameters determined without using PA. Using the same process, the curves of the EXAFS spectra measured at 0.60 V, 0.80 V, and 1.00 V were fitted ( Figure S1 and S2). Figure 5 shows the CN OS values determined from the EXAFS spectra measured in 0.1 M HClO 4 and 0.1 M HClO 4 + 10 mM H 3 PO 4 . When 0.1 M HClO 4 was used as the electrolyte, no CN OS value was detected at 0.35 and 0.6 V; this indicated that a negligibly small number of Pt-OH bonds formed on the Pt particles. ...
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Citations
... Over the past few decades, people have studied diverse phosphoric acid resistance electrocatalysts toward ORR in HT-PEMFCs [28][29][30][31][32][33]. You et al., [28] studied the effect of alloy core for ORR in HT-PEMFCs through preparation of PdNi, PdCu, and PdNiCu based core-shell alloy catalysts. ...
... Unfortunately, phosphoric acid brings along two serious drawbacks as well. Phosphate ions adsorb strongly to platinum [11] and this makes the catalytic sites on the platinum surface much less available to oxygen and the oxygen overpotential is much larger in HT-PEMFC than in LT-PEMFC for that reason. Additionally, the oxygen solubility in phosphoric acid is very low, and if the catalyst is covered by a phosphoric acid film, the penetration of oxygen is set back. ...
An introduction to high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC) is given. The rationale behind the increased temperature is outlined in terms of tolerance to carbon monoxide, less critical water management, cooling issues, and quality of waste heat. Additionally, elevated temperature might imply a shorter route to platinum-free oxygen reduction catalysts. The means for making operation possible at temperatures between 100 and 200 °C is to dope a thermally stable polymer, typically polybenzimidazole, with proton conductive phosphoric acid. Fuel cells based on this membrane system show remarkable stability when operated at 160 °C. The different materials and cell components used are reviewed with comparison to conventional low-temperature polymer fuel cells and phosphoric acid fuel cells all along. The role of nanostructured carbon materials is mostly in relation to composite membranes and catalysts. For the membranes, carbon nanotubes and graphene have been applied as structural fillers to improve mechanical properties. For catalysts, carbon black, carbon nanotubes, and graphene have been used as catalyst support for the catalytic platinum nanoparticles. Moreover, the main trend within development of alternatives to platinum as oxygen reduction catalyst is iron–nitrogen–carbon structures. These materials and their possible application in HT-PEMFC are discussed.
Phosphoric acid used as a proton-conductive medium in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) poisons the Pt surface and prevents oxygen transport in the cathode catalyst layer. The hydrophobic binders in the catalyst not only maintain the catalyst layer structure but also control the phosphoric acid distribution. In this study, polytetrafluoroethylene (PTFE)/carbon black (Vulcan XC-72R) added to the catalyst layer generates an oxygen transport channel. The catalyst layers coated on the gas diffusion layer by the bar-coating method serve as the cathode. High PTFE content causes hydrophobicity in the catalyst layer. The membrane electrode assembly (MEA) with 6 wt% PTFE/Vulcan results in the highest peak power density (0.347 W cm⁻²) and voltage (0.653 V) at 0.2 A cm⁻². A critical reason for its high performance is having the lowest Rct + Rmt values measured at 0.6 V and 0.4 V. These results could contribute to improving the MEA performance for HT-PEMFCs.
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The development of high-performance polymer electrolyte membrane fuel cells that operate at elevated temperatures is urgently required to overcome the technical problems associated with operation at low temperatures. Here, we report an effective way to enhance electrode performance via simple Pt pulse electrodeposition at room temperature. This electrodeposition process enables both the formation of additional Pt electrocatalysts with extremely low loading (i.e., ~0.05 mg cm−2 for 100 pulse cycles) as a function of pulse number and control of the wetting properties of commercial Pt-based electrodes. Following optimization of the electrode conditions and configurations, the controlled hydrophilicity of the anode enhances the phosphoric acid distribution and the formation of a triple phase boundary in the catalyst layer, resulting in lowered ohmic and charge transfer resistances, respectively. The mass activities of the membrane electrode assembly with the anode modified by Pt pulse electrodeposition is 437.2 mW mgPt−1 (H2/O2), which is approximately 1.36 times higher than that of the pristine membrane electrode assembly. The controlled hydrophilicity allows moderate improvement of the performance, even without additional Pt. The results presented herein demonstrate the importance of surface property control for electrode preparation to achieve enhanced performance of high-performance polymer electrolyte membrane fuel cells.
In this paper, we have shown that an increase in the pressure of oxygen on the cathode side in HT-PEM fuel cells leads first to an increase and then a decrease in the potential of the cell and again at even higher pressures to an increase. The most likely explanation for this peculiar phenomenon is a competition between oxygen and phosphoric acid on the surface of the platinum catalyst in the three phase boundary. An increase in the oxygen pressure will in this way remove phosphoric acid from the platinum giving more space for oxygen but at higher pressure creating mass transport problems due to lack of phosphoric acid whereas at even higher pressures oxygen can orient itself in a different way on the platinum surface giving more space to the acid.
