Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel ...cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm
at 160 °C and 1,740 mW cm
at 240 °C under H
/O
conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.
The impact of the electrolyte’s pH on the catalytic activity of platinum group metal-free (PGM-free) catalysts toward the oxygen reduction reaction (ORR) was studied. The results indicate that the ...ORR mechanism is determined by the affinity of protons and hydroxyls toward multiple functional groups present on the surface of the PGM-free catalyst. It was shown that the ORR is limited by the proton-coupled electron transfer at pH values below 10.5. At higher pH values (>10.5), the reaction occurs in the outer Helmholtz plane (OHP), favoring hydrogen peroxide production. Using a novel approach, the changes in the surface chemistry of PGM-free catalyst in a full pH range were studied by X-ray photoelectron spectroscopy (XPS). The variations in the surface concentration of nitrogen and carbon species are correlated with the electron transfer process and overall kinetics. This study establishes the critical role of the multitude of surface functional groups, presented as moieties or defects in the carbonaceous “backbone” of the catalyst, in mechanism of oxygen reduction reaction. Understanding the pH-dependent mechanism of ORR provides the basis for rational design of PGM-free catalysts for operation across pH ranges or at a specific pH of interest. This investigation also provides the guidelines for developing and selecting ionomers used as “locally-confined electrolytes”, by taking into account affinities and possible interactions of specific functional groups of the PGM-free catalysts with protons or hydroxyls facilitating the overall ORR kinetics.
Commercial fuel cell electrocatalyst degradation results from carbon electrocatalyst support oxidation at high operating potential transients. Guided by density functional theory (DFT) calculations, ...Nb‐doped TiO2 (NTO) was synthesized, which exhibits a unique combination of high surface area, high electrical conductivity, and high porosity. This catalyst retained 78 % of its initial electrochemically active surface area compared with 57.6 % retained by Pt/C following the DOE/FCCJ protocol for accelerated stability test. Strong metal–support interactions, which were predicted by DFT calculations and confirmed experimentally by X‐ray photoelectron spectroscopy and kinetics measurements, resulted in 21 % higher oxygen reduction reaction mass activity (at 0.9 V vs. reversible hydrogen electrode) on Pt/NTO compared with commercial Pt/C. The ex situ activity and durability of Pt/NTO translated to a fuel cell. The rise in electrode ohmic resistance and non‐electrode concentration overpotential indicate that improving the conductivity of NTO and optimizing the catalyst ink formulation are critical next steps in the development of Pt/NTO‐catalyzed proton exchange membrane fuel cells.
Bettering the best: Pt supported on Nb‐doped TiO2 is an excellent fuel cell electrocatalyst that exhibits excellent durability (35 % more stable than the commercial state‐of‐the‐art catalyst) and activity (20 % more active than the commercial state‐of‐the‐art catalyst) after accelerated tests that simulated real‐world use.
The interaction energy of base–acid plays a key role in acid retention of phosphoric acid (PA)-doped polymer electrolytes under fuel cell operating conditions. Here, we investigate the energetics of ...proton-accepting and hydroxide-donating organic bases using density functional theory calculations. Because of their weak basicity, proton-accepting organic bases such as benzimidazole have relatively low interaction energy with the acid in the absence of water (15.3–28.0 kcal mol–1). Energetics of the proton-accepting base–PA complex increases by adding water, indicating that the interactions in the base–acid complex strengthen in the presence of water. On the other hand, hydroxide-donating organic bases, such as tetramethylammonium hydroxide, have high interaction energy with PA (∼110 kcal mol–1), which remains high in the presence of water. The chemical shifts of 31P NMR support the energetics of the base–acid complexes. This study further discusses the benefit of incorporating hydroxide-donating organic bases into the polymeric structure over proton-accepting bases as a way to increase acid retention.
Electrocatalytic reduction of waste nitrates (NO
) enables the synthesis of ammonia (NH
) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts ...demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO
reduction to NH
in neutral media, with a specific focus on deciphering the role of the NO
intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO
reduction activity and NO
reduction selectivity for NH
. Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO
evolution, over the normal M-N
sites and their oxo-form (O-M-N
) for oxyphilic metals. This work provides a platform for designing multi-element NO
RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.
The durability of alkaline anion exchange membrane (AEM) electrolyzers is a critical requirement for implementing this technology in cost-effective hydrogen production. Here, we report that the ...electrochemical oxidation of the adsorbed phenyl group (found in the ionomer) on oxygen evolution catalysts produces phenol, which may cause performance deterioration in AEM electrolyzers. In-line 1H NMR kinetic analyses of phenyl oxidation in a model organic cation electrolyte shows that catalyst type significantly impacts the phenyl oxidation rate at an oxygen evolution potential. Density functional theory calculations show that the phenyl adsorption is a critical factor determining the phenyl oxidation. This research provides a path for the development of more durable AEM electrolyzers with components that can minimize the adverse impact induced by the phenyl group oxidation, such as the development of novel ionomers with fewer phenyl moieties or catalysts with less phenyl-adsorbing character.
Stable nitroxyl radical-containing compounds, such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and its derivatives, are capable of electrocatalytically oxidizing a wide range of alcohols under ...mild and environmentally friendly conditions. Herein, we examine the structure–function relationships that determine the catalytic activity of a diverse range of water-soluble nitroxyl radical compounds. A strong correlation is described between the difference in the electrochemical oxidation potentials of a compound and its electrocatalytic activity. Additionally, we construct a simple computational model that is able to accurately predict the electrochemical potential and catalytic activity of a wide range of nitroxyl radical derivatives.
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