Javier Parrondo

Degradation Mitigation in Polymer Electrolyte Membranes Using Cerium Oxide as a Regenerative Free-Radical Scavenger

The efficacy of CeO 2 nanoparticles in mitigating free-radical-induced polymer electrolyte membrane (PEM) degradation is investigated. Commercially obtained CeO 2 and nanoparticles synthesized in-house were incorporated within a recast Nafion membrane. Membrane electrode assemblies were prepared using Nafion and Nafion-CeO 2 composite membranes (0.5, 1, and 3 wt % CeO 2 ). The composite membranes exhibited very similar proton conductivities (∼35 mS/cm) and hydrogen crossover (∼ 1 mA/cm 2 ) as Nafion. However, the fluoride emission rate (from accelerated tests) was lowered by more than 1 order of magnitude upon addition of CeO 2 into the Nafion membrane, suggesting that CeO 2 nanoparticles have tremendous potential to greatly enhance membrane durability.

Assessing the influence of different cation chemistries on ionic conductivity and alkaline stability of anion exchange membranes

Polysulfone (PSF) backbones were functionalized with reactive chloromethyl groups for preparing thin film anion exchange membranes (AEMs) with fixed benzyl quaternary cations. Three different cation chemistries of varying basicity were evaluated: 1,4-dimethylpiperazinium (DMP+), trimethylammonium (TMA+), and trimethylphosphonium (TMP+). The water uptake, ionic conductivity, and stability in alkaline media of these AEMs were assessed with both chloride and hydroxide counteranions. The results obtained revealed that the basicity value of the free base conjugate of the functionalized quaternary cations correlated well with gains in ionic conductivity. Cation basicity also correlated well with the alkaline stability of cations with the same inorganic atom, but was not an appropriate heuristic for comparing alkaline stability across cations with different inorganic atoms. The alkaline stability studies indicated that the primal degradation pathway of the TMA+ cation differed from that of the TMP+ cation (direct nucleophilic attack versus ylide formation). PSF with TMA+ and DMP+ cations were demonstrated to show alkaline fuel cell performance that reflected their respective ionic conductivity values.

CeO2 surface oxygen vacancy concentration governs in situ free radical scavenging efficacy in polymer electrolytes.

Nonstoichiometric CeO(2) and Ce(0.25)Zr(0.75)O(2) nanoparticles with varying surface concentrations of Ce(3+) were synthesized. Their surface Ce(3+) concentration was measured by XPS, and their surface oxygen vacancy concentrations and grain size were estimated using Raman spectroscopy. The surface oxygen vacancy concentration was found to correlate well with grain size and surface Ce(3+) concentration. When incorporated into a Nafion polymer electrolyte membrane (PEM), the added nonstoichiometric ceria nanoparticles effectively scavenged PEM-degradation-inducing free radical reactive oxygen species (ROS) formed during fuel cell operation. A 3-fold increase in the surface oxygen vacancy concentration resulted in an order of magnitude enhancement in the efficacy of free radical ROS scavenging by the nanoparticles. Overall, the macroscopic PEM degradation mitigation rate was lowered by up to 2 orders of magnitude using nonstoichiometric ceria nanoparticles with high surface oxygen vacancy concentrations.

Platinum supported on titanium–ruthenium oxide is a remarkably stable electrocatayst for hydrogen fuel cell vehicles

Significance In this study, we showcase titanium–ruthenium oxide (TRO)—a remarkably stable support material, and Pt/TRO, a derivative electrocatalyst that is also extremely stable. These materials have been tested to establish their catalytic activity and stability using accelerated tests that mimic conditions and degradation modes encountered during long-term fuel cell operation. We have evaluated Pt/TRO using these protocols, and provide concrete evidence that this material is far more stable than Pt/C and, would meet the requirements for use in an automotive fuel cell stack. Our cost analysis indicates the use of ruthenium is not a factor given that >90% of catalyst cost resides in the platinum metal; moreover, the exceptional stability of Pt/TRO removes the needs for overdesign or replacement. We report a unique and highly stable electrocatalyst—platinum (Pt) supported on titanium–ruthenium oxide (TRO)—for hydrogen fuel cell vehicles. The Pt/TRO electrocatalyst was exposed to stringent accelerated test protocols designed to induce degradation and failure mechanisms identical to those seen during extended normal operation of a fuel cell automobile—namely, support corrosion during vehicle startup and shutdown, and platinum dissolution during vehicle acceleration and deceleration. These experiments were performed both ex situ (on supports and catalysts deposited onto a glassy carbon rotating disk electrode) and in situ (in a membrane electrode assembly). The Pt/TRO was compared against a state-of-the-art benchmark catalyst—Pt supported on high surface-area carbon (Pt/HSAC). In ex situ tests, Pt/TRO lost only 18% of its initial oxygen reduction reaction mass activity and 3% of its oxygen reduction reaction-specific activity, whereas the corresponding losses for Pt/HSAC were 52% and 22%. In in situ-accelerated degradation tests performed on membrane electrode assemblies, the loss in cell voltage at 1 A · cm−2 at 100% RH was a negligible 15 mV for Pt/TRO, whereas the loss was too high to permit operation at 1 A · cm−2 for Pt/HSAC. We clearly show that electrocatalyst support corrosion induced during fuel cell startup and shutdown is a far more potent failure mode than platinum dissolution during fuel cell operation. Hence, we posit that the need for a highly stable support (such as TRO) is paramount. Finally, we demonstrate that the corrosion of carbon present in the gas diffusion layer of the fuel cell is only of minor concern.

Polysulfone-based anion exchange membranes demonstrate excellent chemical stability and performance for the all-vanadium redox flow battery

A polysulfone-based anion exchange membrane functionalized with quaternary benzyl trimethylammonium groups (PSF-TMA+) demonstrated a 40-fold reduction in vanadium(IV) permeability when compared to a Nafion® membrane. Comprehensive 2D NMR (COSY and heteronuclear quantum correlation spectroscopy) studies verified that PSF-TMA+ remained chemically stable even after exposure to a 1.5 M vanadium(V) solution for 90 days. Excellent energy efficiencies (85%) were attained and sustained over several charge–discharge cycles for a vanadium redox flow battery prepared using the PSF-TMA+ separator.

Derivatized cardo-polyetherketone anion exchange membranes for all-vanadium redox flow batteries

Cardo-polyetherketone (PEK-C) based anion exchange membranes (AEMs) were synthesized by chloromethylation of PEK-C followed by quaternization using trimethylamine (TMA). The ion exchange capacity (IEC) of the AEM was 1.4 ± 0.1 mmol g−1. The sulfate ion conductivity and vanadium(IV) permeability were 5.6 ± 0.5 mS cm−1 and 8.2 ± 0.2 × 10−9 cm2 s−1 at 30 °C. The chemical stability and mechanical integrity of the AEMs were investigated upon exposure to 1.5 M VO2+ solution by monitoring the ionic conductivity, ultimate tensile strength, elongation at break, and chemical structure over 1500 hours. 1-D and 2-D NMR spectroscopy confirmed the chemical stability of the AEM over this period. The ionic conductivity of the AEM decreased from 5.6 to 4.4 mS cm−1 over the first 48 hours but subsequently stabilized and was reversible, while the ultimate tensile strength and the elongation at break were reduced by ca. 35%. The PEK-C based AEMs were stable during operation in a vanadium redox flow battery (VRFB) for 100 hours of testing. The coulombic and energy efficiencies of the VRFB were 98% and 80%, respectively. Post-mortem analysis of the AEM using 1-D and 2-D NMR spectroscopy showed a 15% reduction in the number of quaternary ammonium groups in the AEM.

Degradation of anion exchange membranes used for hydrogen production by ultrapure water electrolysis

Solid-state alkaline water electrolysis using a pure water feed offers several distinct advantages over liquid alkaline electrolyte water electrolysis and proton exchange membrane water electrolysis. These advantages include a larger array of electrocatalyst available for oxygen evolution, no electrolyte management, and the ability to apply differential pressure. To date, there have been only a handful of reports on solid-state alkaline water electrolyzers using anion exchange membranes (AEMs), and there have been no reports that investigate loss in system performance over time. In this work, a solid-state alkaline water electrolyzer was successfully demonstrated with several types of polysulfone-based AEMs using a relatively expensive but highly active lead ruthenate pyrochlore electrocatalyst for the oxygen evolution reaction. The electrolysis of ultrapure water at 50 °C resulted in a current density of 400 mA cm−2 at 1.80 V. We demonstrated that the short-term degradation of water electrolyzer performance over time was largely a consequence of carbon dioxide intrusion into the system and could be easily remedied, while long-term deterioration was a consequence of irreversible AEM polymer degradation.

Pyrochlore electrocatalysts for efficient alkaline water electrolysis

A series of electrically conducting metal oxides with the pyrochlore structure (A2B2O7−y, with A = Pb or Bi and B = Ru, Ir or Os) were synthesized via precipitation/crystallization in alkaline medium and/or via solid-state reaction. The electrocatalytic activity for the oxygen evolution reaction (OER) in 0.1 M KOH was studied using a rotating disk electrode. Lead and bismuth ruthenate pyrochlores showed significantly lower overpotentials for the OER than the state-of-the-art IrO2 catalyst. Specific activities (at 1.5 V vs. RHE) of 3.0 ± 0.2 A m−2, 1.3 ± 0.2 A m−2 and 0.06 ± 0.01 A m−2 were obtained for Pb2Ru2O6.5, Bi2.4Ru1.6O7, and IrO2 respectively. Specific activities for iridate-based pyrochlores (0.3–0.5 A m−2) were 5–10 times lower than those for ruthenate-based pyrochlores. Lead osmate pyrochlore showed the lowest OER activity among all the pyrochlores evaluated, with a specific activity of 0.10 ± 0.07 A m−2. It is proposed that the reaction path for the OER involves several oxygen intermediate species (–O2−, –OOH, –OO2−, –OH) bonded to the B-site (Ru, Ir or Os) in the pyrochlore, and that the catalytic activity depends on the bonding strength between the B-cation site and the oxygen species. This hypothesis was supported by the fact that OER activity correlated with Ru concentration in lead-rich lead ruthenate pyrochlores. The decrease of the specific OER activity depended on the occupancy of the 3d orbitals and on the period in the periodic table occupied by the B-cation. The OER activity decreased for pyrochlores with B cations having more d electrons than Ru, and when the B cation occupied period 6. The observed trend in activity was similar to that observed for the oxygen reduction reaction on transition metals, and was related to the strength of the bonding between the adsorbed oxygen species and the B-cation. The exceptional OER activity and stability of lead ruthenate pyrochlore catalysts were evaluated in a solid-state alkaline water electrolyzer. The overpotentials obtained were 0.1–0.2 V lower than for IrO2 and the performance was stable for at least 200 h.

Degradation mitigation in PEM fuel cells using metal nanoparticle additives

The efficacy of added freestanding and silica-supported metal (Pt, Pd, Ag, and Au) nanoparticles in mitigating polymer electrolyte membrane (PEM) degradation in an operating fuel cell was investigated. The metal nanoparticles to be added were chosen based on their ability to scavenge free radicals, as confirmed through ex situ measurements using a model free radical 1,1-diphenyl-2-picrylhydrazyl as a test species. Composite membranes were prepared by adding 3 wt% of the freestanding or supported metal nanoparticles to Nafion®. Membrane electrode assemblies prepared using these membranes were subjected to accelerated degradation tests to determine the fluoride emission rate (FER), a key measure of the macroscopic rate of PEM degradation in an operating fuel cell. The results were as follows: the addition of Ag, Pt, Pd and Au freestanding nanoparticles lowered FER by 35%, 60%, 80% and 90%, respectively, when compared with the recast Nafion® membrane (control). In the case of silica-supported nanoparticles, more modest reductions were achieved: 40% and 60% in the case of Au on SiO2 and Pd on SiO2, respectively. Ag or Pt on SiO2 resulted in similar FER values as the control. These results suggest that the addition of selected metal nanoparticles with radical scavenging abilities is a promising approach to mitigate PEM degradation in an operating fuel cell.

Functionalized Silica Composite Membranes for Direct Methanol Fuel Cells

Introduction Methanol crossover through the polymer electrolyte membrane is a key issue limiting the performance of DMFCs. This leads to depolarization and mixed potential as well as poisoning of the Pt catalyst, all of which reduce performance. To resolve these issues, membranes with lower methanol permeability are required. The addition of inorganic additives to polymer membranes has been shown to lower methanol permeability through the membrane (1). However, these additives typically also reduce membrane proton conductivity. Hence, membrane selectivity (ratio of proton conductivity to methanol permeability) is usually unchanged, or even lowered. In an attempt to enhance membrane selectivity, a proton conducting inorganic additive (sulfonic acid functionalized silica aerogel) is synthesized and investigated. The effect of addition of sulfonated silica aerogel to hydrocarbon membranes is reported in terms of membrane selectivity.