High-Performance Anion Exchange Membrane Water Electrolyzers Enabled by Highly Gas Permeable and Dimensionally Stable Anion Exchange Ionomers.

Designing suitable anion exchange ionomers is critical to improving the performance and in situ durability of anion exchange membrane water electrolyzers (AEMWEs) as one of the promising devices for producing green hydrogen. Herein, highly gas-permeable and dimensionally stable anion exchange ionomers (QC6xBA and QC6xPA) are developed, in which bulky cyclohexyl (C6) groups are introduced into the polymer backbones. QC650BA-2.1 containing 50 mol% C6 composition shows 16.6 times higher H2 permeability and 22.3 times higher O2 permeability than that of QC60BA-2.1 without C6 groups. Through-plane swelling of QC650BA-2.1 decreases to 12.5% from 31.1% (QC60BA-2.1) while OH- conductivity slightly decreases (64.9 and 56.2 mS cm-1 for QC60BA-2.1 and QC650BA-2.1, respectively, at 30 °C). The water electrolysis cell using the highly gas permeable QC650BA-2.1 ionomer and Ni0.8Co0.2O in the anode catalyst layer achieves two times higher performance (2.0 A cm-2 at 1.69 V, IR-included) than those of the previous cell using in-house ionomer (QPAF-4-2.0) (1.0 A cm-2 at 1.69 V, IR-included). During 1000 h operation at 1.0 A cm-2, the QC650BA-2.1 cell exhibits nearly constant cell voltage with a decay rate of 1.1 µV h-1 after the initial increase of the cell voltage, proving the effectiveness of the highly gas permeable and dimensionally stable ionomer in AEMWEs.

NiFe Alloy Integrated with Amorphous/Crystalline NiFe Oxide as an Electrocatalyst for Alkaline Hydrogen and Oxygen Evolution Reactions.

The rational design of efficient and low-cost electrocatalysts based on earth-abundant materials is imperative for large-scale production of hydrogen by water electrolysis. Here we present a strategy to prepare highly active catalyst materials through modifying the crystallinity of the surface/interface of strongly coupled transition metal-metal oxides. We have thermally activated the catalysts to construct amorphous/crystalline Ni-Fe oxide interfaced with a conductive Ni-Fe alloy and systematically investigated their electrocatalytic performance toward the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline solution. It was found that the Ni-Fe/oxide material with a crystalline surface oxide phase showed remarkably superior HER activity in comparison with its amorphous or poorly crystalline counterpart. In contrast, interestingly, the amorphous/poorly crystalline oxide significantly facilitated the OER activity in comparison with the more crystalline counterpart. On one hand, the higher HER activity can be ascribed to a favorable platform for water dissociation and H-H bond formation, enabled by the unique crystalline metal/oxide structure. On the other hand, the enhanced OER catalysis on the amorphous Ni-Fe oxide surfaces can be attributed to the facile activation to form the active oxyhydroxides under OER conditions. Both are explained based on density functional theory calculations. These results thus shed light onto the role of crystallinity in the HER and OER catalysis on heterostructured Ni-Fe/oxide catalysts and provide guidance for the design of new catalysts for efficient water electrolysis.

Electronic States and Transport Phenomena of Pt Nanoparticle Catalysts Supported on Nb-Doped SnO2 for Polymer Electrolyte Fuel Cells.

Semiconducting oxide nanoparticles are strongly influenced by surface-adsorbed molecules and tend to generate an insulating depletion layer. The interface between a noble metal and a semiconducting oxide constructs a Schottky barrier, interrupting the electron transport. In the case of a Pt catalyst supported on the semiconducting oxide Nb-doped SnO2 with a fused-aggregate network structure (Pt/Nb-SnO2) for polymer electrolyte fuel cells, the electronic conductivity increased abruptly with increasing Pt loading, going from 10-4 to 10-2 S cm-1. The Pt X-ray photoemission spectroscopy (XPS) spectra at low Pt loading amount exhibited higher binding energy than that of pristine Pt metal. The peak shift for the Pt XPS spectra was larger than that of the Pt hard X-ray photoemission spectroscopy (HAXPES) spectra. For all of the spectra, the peaks approached the binding energy of pristine Pt metal with increasing Pt loading. The Sn XPS spectral peak proved the existence of Sn metal with increasing Pt loading, and the peak intensity was larger than that for HAXPES. These spectroscopic results, together with the scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDX) spectra, proved that a PtSn alloy was deposited at the interface between Pt and Nb-SnO2 as a result of the sintering procedure under dilute hydrogen atmosphere. Both Nb spectra indicated that the oxidation state of Nb was +5 and thus that the Nb cation acts as an n-type dopant of SnO2. We conclude that the PtSn alloy at the interface between Pt and Nb-SnO2 relieved the effect of the Schottky barrier, enhanced the carrier donation from Pt to Nb-SnO2, and improved the electronic transport phenomena of Pt/Nb-SnO2.

Electrochemical oxidation of hydrolyzed poly oxymethylene-dimethyl ether by PtRu catalysts on Nb-doped SnO(2-δ) supports for direct oxidation fuel cells.

We synthesized Pt and PtRu catalysts supported on Nb-doped SnO(2-δ) (Pt/Sn0.99Nb0.01O(2-δ), PtRu/Sn0.99Nb0.01O(2-δ)) for direct oxidation fuel cells (DOFCs) using poly oxymethylene-dimethyl ether (POMMn, n = 2, 3) as a fuel. The onset potential for the oxidation of simulated fuels of POMMn (methanol-formaldehyde mixtures; n = 2, 3) for Pt/Sn0.99Nb0.01O(2-δ) and PtRu/Sn0.99Nb0.01O(2-δ) was less than 0.3 V vs RHE, which was much lower than those of two commercial catalysts (PtRu black and Pt2Ru3/carbon black). In particular, the onset potential of the oxidation reaction of simulated fuels of POMMn (n = 2, 3) for PtRu/Sn0.99Nb0.01O(2-δ) sintered at 800 °C in nitrogen atmosphere was less than 0.1 V vs RHE and is thus considered to be a promising anode catalyst for DOFCs. The mass activity (MA) of PtRu/Sn0.99Nb0.01O(2-δ) sintered at 800 °C was more than five times larger than those of the commercial catalysts in the measurement temperature range from 25 to 80 °C. Even though the MA for the methanol oxidation reaction was of the same order as those of the commercial catalysts, the MA for the formaldehyde oxidation reaction was more than five times larger than those of the commercial catalysts. Sn from the Sn0.99Nb0.01O(2-δ) support was found to have diffused into the Pt catalyst during the sintering process. The Sn on the top surface of the Pt catalyst accelerated the oxidation of carbon monoxide by a bifunctional mechanism, similar to that for Pt-Ru catalysts.

Temperature- and humidity-controlled SAXS analysis of proton-conductive ionomer membranes for fuel cells.

We report herein temperature- and humidity-controlled small-angle X-ray scattering (SAXS) analyses of proton-conductive ionomer membranes. The morphological changes of perfluorosulfonic acid polymers (Nafion and Aquivion) and sulfonated aromatic block copolymers (SPE-bl-1 and SPK-bl-1) were investigated and compared under conditions relevant to fuel cell operation. For the perfluorinated ionomer membranes, water molecules were preferentially incorporated into ionic clusters, resulting in phase separation and formation of ion channels. In contrast, for the aromatic ionomer membranes, wetting led to randomization of the ionic clusters. The results describe the differences in the proton-conducting behavior between the fluorinated and nonfluorinated ionomer membranes, and their dependence on the humidity.

Effect of the state of distribution of supported Pt nanoparticles on effective Pt utilization in polymer electrolyte fuel cells.

In polymer electrolyte fuel cells, it is essential to minimize Pt loading, particularly at the cathode, without serious loss of performance. From this point of view, we will report an advanced concept for the design of high performance catalysts and membrane-electrode assemblies (MEAs): first, the evaluation of Pt particle distributions on both the interior and exterior walls of various types of carbon black (CB) particles used as supports with respect to the "effective surface (ES)"; second, control of both size and location of Pt particles by means of a new preparation method (nanocapsule method); and finally, a new evaluation method for the properties of MEAs based on the Pt utilization (UPt), mass activity (MA), and effectiveness of Pt (EfPt), based on the ES concept. The amounts of Pt catalyst particles located in the CB nanopores were directly evaluated using the transmission electron microscopy, scanning electron microscopy and corresponding three-dimensional images. By use of the nanocapsule method and optimization of the ionomer, increased MA and EfPt values for the MEA were achieved. The improvement in the cathode performance can be attributed to the sharp particle-size distribution for Pt and the highly uniform dispersion on the exterior surface of graphitized carbon black (GCB) supports.

Temperature dependence of oxygen reduction reaction activity at stabilized Pt skin-PtCo alloy/graphitized carbon black catalysts prepared by a modified nanocapsule method.

We have developed a new catalyst supported on graphitized carbon black (GCB), which exhibits higher resistance to carbon corrosion than a conventional carbon black (CB), in order to favor both high mass activity for the oxygen reduction reaction (ORR) and high durability. To protect the underlying Pt(X)Co alloy from corrosion and maintain the modified electronic structure, two monolayers of Pt-skin layer (Pt(2 ML)) were formed on the Pt(X)Co core-particles, which were of uniform size and composition. Characterization of the Pt(2 ML)-PtCo(X = 1)/GCB, both by a scanning transmission electron microscope (STEM) with an energy dispersive X-ray (EDX) analyzer and by X-ray diffraction (XRD), indicated the formation of the Pt(2 ML) on the PtCo alloy solid solution nanoparticles. The temperature dependence of the ORR activity of the Pt(2 ML)-PtCo((2 nm))/GCB catalyst was evaluated from the hydrodynamic voltammograms in O(2)-saturated 0.1 M HClO(4) solution at 30-90 °C by the channel flow double electrode (CFDE) technique. It was found that the Co dissolution from PtCo particles during the ORR was considerably suppressed by the stabilized Pt-skin structure. The kinetically controlled mass activity (MA(k)) for the ORR at the Pt(2 ML)-PtCo((2 nm))/GCB at E = 0.85 V vs reversible hydrogen electrode (RHE) was about two times larger than that for a standard commercial c-Pt/CB catalyst at 80-90 °C. The value of H(2)O(2) yield at the Pt(2 ML)-PtCo((2 nm))/GCB was found to be very low (0.2%), about one-half of that for c-Pt/CB, and it is thus better able to mitigate the degradation of the polymer electrolyte membrane and gasket material.