Takanori Tamaki

Enhanced activity and durability for the electroreduction of oxygen at a chemically ordered intermetallic PtFeCo catalyst

We have designed a chemically ordered face-centred tetragonal intermetallic PtFeCo (trimetallic) (fct-TM) alloy catalyst using a simple solid-state impregnation method for the oxygen–reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). The fct-TM catalyst has demonstrated both enhanced activity and durability, unlike many Pt alloys. The chemical ordering of the fct-TM was verified by high-angle annular dark-field scanning transmission electron microscopy. The ORR activity of fct-TM was examined using the rotating-disk electrode (RDE) technique and the results are compared with those for a chemically disordered face-centred cubic (fcc), fcc-TM catalyst, and a commercial catalyst from Tanaka Kikinzoku Kogyo, TKK-PtC. The fct-TM displayed superior catalytic (mass) activity relative to disordered fcc-TM and TKK-PtC. The mass activity of fct-TM (0.505 A mgPt−1) is 2.5 times higher than that of TKK-PtC (0.23 A mgPt−1). The durability of these catalysts was evaluated over 5000 (5k) potential cycles in the lifetime regime. The fct-TM retained 80% of its initial mass activity and electrochemically active surface area (ECSA); however, fcc-TM and TKK-PtC maintained about 50% and 70% activity, respectively. The fct-TM also retained the chemically ordered structure after 5k durability cycles. This was confirmed using selected-area electron-diffraction (SAED) patterns. Furthermore, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) line scans of the fct-TM catalysts after 5k durability cycles revealed that Fe and Co were found similar to as before cycling, which signifies that the dissolution of Fe and Co was impeded by the fct-TM catalysts. The observed enhancement in durability might be due to the ordered arrangement of Pt and Fe/Co within the alloy.

Connected nanoparticle catalysts possessing a porous, hollow capsule structure as carbon-free electrocatalysts for oxygen reduction in polymer electrolyte fuel cells

We employ connected nanoparticle catalysts with a porous, hollow capsule structure as carbon-free electrocatalysts for the cathode in polymer electrolyte fuel cells (PEFCs) or proton exchange membrane fuel cells (PEMFCs). The catalysts consist of fused ordered alloy platinum–iron (Pt–Fe) nanoparticles. This unique beaded network structure enables surprisingly high activity for the oxygen reduction reaction, 9 times that of the state-of-the-art commercial catalyst. Because the connected nanoparticle catalysts are formed without sacrificing the high surface area of the nanoparticles and can conduct electrons, the catalysts show good performance in an actual PEMFC without a carbon support. Moreover, the elimination of carbon intrinsically solves the problem of carbon corrosion. Thus, the connected nanoparticle catalysts with a unique structure are a significant advancement over conventional electrode catalysts and will lead to an ultimate solution for PEMFC cathodes.

Beneficial Role of Copper in the Enhancement of Durability of Ordered Intermetallic PtFeCu Catalyst for Electrocatalytic Oxygen Reduction

Design of Pt alloy catalysts with enhanced activity and durability is a key challenge for polymer electrolyte membrane fuel cells. In the present work, we compare the durability of the ordered intermetallic face-centered tetragonal (fct) PtFeCu catalyst for the oxygen reduction reaction (ORR) relative to its counterpart bimetallic catalysts, i.e., the ordered intermetallic fct-PtFe catalyst and the commercial catalyst from Tanaka Kikinzoku Kogyo, TKK-PtC. Although both fct catalysts initially exhibited an ordered structure and mass activity approximately 2.5 times higher than that of TKK-Pt/C, the presence of Cu at the ordered intermetallic fct-PtFeCu catalyst led to a significant enhancement in durability compared to that of the ordered intermetallic fct-PtFe catalyst. The ordered intermetallic fct-PtFeCu catalyst retained more than 70% of its mass activity and electrochemically active surface area (ECSA) over 10 000 durability cycles carried out at 60 °C. In contrast, the ordered intermetallic fct-PtFe catalyst maintained only about 40% of its activity. The temperature of the durability experiment is also shown to be important: the catalyst was more severely degraded at 60 °C than at room temperature. To obtain insight into the observed enhancement in durability of fct-PtFeCu catalyst, a postmortem analysis of the ordered intermetallic fct-PtFeCu catalyst was carried out using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) line scan. The STEM-EDX line scans of the ordered intermetallic fct-PtFeCu catalyst over 10 000 durability cycles showed a smaller degree of Fe and Cu dissolution from the catalyst. Conversely, large dissolution of Fe was identified in the ordered intermetallic fct-PtFe catalyst, indicating a lesser retention of Fe that causes the destruction of ordered structure and gives rise to poor durability. The enhancement in the durability of the ordered intermetallic fct-PtFeCu catalyst is ascribed to the synergistic effects of Cu presence and the ordered structure of catalyst.

The proton conduction mechanism in a material consisting of packed acids

Proton conduction due to acid–acid interactions is an important topic in a variety of fields, from materials science to biochemistry. We observed a distinctive proton conduction phenomenon for a material consisting of packed acids at the interface of zirconium sulphophenylphosphonate (ZrSPP) and sulphonated poly(arylene ether sulphone) (SPES). The proton in the composite was found to be active, while water, a general proton carrier, remained immobile. Moreover, the conductivity of the composite material was higher than the sum of the individual conductivities of ZrSPP and SPES, which can be attributed to the packed acids present at the interface. We propose a “packed-acid mechanism” based on the results of ab initio calculations in order to explain such a significant and interesting behaviour of protons. During common proton conduction, pseudo-shuttling of a proton between a proton donor and acceptor is a general event that disrupts the reorientation phenomenon, which is an important process associated with common proton conduction. Based on our results, it could be inferred that in packed acid materials, the acid–acid interaction eliminates the pseudo-shuttling (interception) and facilitates reorientation, resulting in successive proton conduction.

Enhanced oxygen reduction reaction by bimetallic CoPt and PdPt nanocrystals

Excellent oxygen reduction reaction (ORR) activity of Pt-based bimetallic alloys like CoPt and PdPt has been studied in detail. A simple solution-phase synthetic procedure for the bimetallic alloys of CoPt and PdPt is described here. This method, involving a single-step reaction, is believed to be suitable for large-scale synthesis of both monometallic and multimetallic nanocrystals with required size and shape. Among various catalyst compositions, Pd22Pt78 and Co15Pt85 catalysts have shown excellent ORR activity compared with Pt/C and commercial PtCo/C (PtCo/C(comm)) samples. At 0.9 V, Co15Pt85/C and Pd22Pt78/C exhibit mass activities with values ∼3.8 times and ∼2.1 times higher than that of Pt/C, and Co15Pt85/C exhibits 1.8 times higher activity than that of PtCo/C(comm). Quantification and the effect of surface-oxygenated species generated at higher potentials were studied by stripping voltammetry, in the form of total charge.

Enzymatic Biofuel Cells Based on Three-Dimensional Conducting Electrode Matrices

Enzymatic biofuel cells can use a variety of fuels such as glucose and ethanol, and they have the potential to power portable devices. This article summarizes recent advances made in the use of three-dimensional conducting materials as electrode matrices of enzymatic biofuel cells from the point of view of the current density and the power density.

Biomolecule-recognition gating membrane using biomolecular cross-linking and polymer phase transition.

We present for the first time a biomolecule-recognition gating system that responds to small signals of biomolecules by the cooperation of biorecognition cross-linking and polymer phase transition in nanosized pores. The biomolecule-recognition gating membrane immobilizes the stimuli-responsive polymer, including the biomolecule-recognition receptor, onto the pore surface of a porous membrane. The pore state (open/closed) of this gating membrane depends on the formation of specific biorecognition cross-linking in the pores: a specific biomolecule having multibinding sites can be recognized by several receptors and acts as the cross-linker of the grafted polymer, whereas a nonspecific molecule cannot. The pore state can be distinguished by a volume phase transition of the grafted polymer. In the present study, the principle of the proposed system is demonstrated using poly(N-isopropylacrylamide) as the stimuli-responsive polymer and avidin-biotin as a multibindable biomolecule-specific receptor. As a result of the selective response to the specific biomolecule, a clear permeability change of an order of magnitude was achieved. The principle is versatile and can be applied to many combinations of multibindable analyte-specific receptors, including antibody-antigen and lectin-sugar analogues. The new gating system can find wide application in the bioanalytical field and aid the design of novel biodevices.

Novel mild conversion routes of surface-modified nano zirconium oxide precursor to layered proton conductors

Layered proton conductors of three different materials, zirconium phosphate (ZrP), zirconium sulfophenylphosphonate (ZrSPP), and zirconium sulfate (ZrS) were synthesized from the same surface-modified nano zirconium oxide precursor through novel, mild conversion routes. The conversions were performed at 80 °C under mild acidic conditions. The phase formation of ZrP, ZrSPP, and ZrS were confirmed by FT-IR and XRD techniques. Thermal stabilities of ZrSPP and ZrS were evaluated by thermogravimetric analysis and hydrothermal tests at 150 °C for 24 h. Proton conductivities of ZrP, ZrSPP, and ZrS were measured at 90 °C under various relative humidities. The mild conversion routes will enable in situ synthesis of these layered proton conductors inside a polymer matrix from the same nano zirconium oxide precursor, which has been reported to form a nanodispersed structure in polymer electrolytes such as perfluorosulfonated ionomers and aromatic hydrocarbon electrolytes.

Control of the poly(N-isopropylacrylamide) phase transition via a single strand–double strand transformation of conjugated DNA

We have demonstrated that the aggregation of DNA-conjugated thermoresponsive polymer was inhibited by the formation of double strand DNA (dsDNA) from single strand DNA (ssDNA) by the change in the number of electric charges and hydrophilic states of DNA. The polymer was prepared by conjugation of short ssDNA with poly(N-isopropylacrylamide) (PNIPAM) and subsequent hybridization of the conjugated DNA with complementary DNA to form dsDNA. The ssDNA-conjugated PNIPAM highly aggregated with an increase in temperature, while the dsDNA-conjugated PNIPAM was inhibited from aggregation by the hybridization with fully matched complementary DNA and further prevented by hybridization with longer complementary DNA with a protruding sequence. Further characterizations suggest that the difference in the degree of aggregation between the ssDNA- and dsDNA-conjugated PNIPAM was mainly driven by the change in the number of electric charges associated with the conjugated DNA.

Direct synthesis of a carbon nanotube interpenetrated doped porous carbon alloy as a durable Pt-free electrocatalyst for the oxygen reduction reaction in an alkaline medium

Direct synthesis of highly durable carbon nanotube interpenetrated porous carbon alloy electrocatalysts for the oxygen reduction reaction (ORR) from a single precursor, trimetallic zeolitic imidazole framework (t-ZIF), is reported. The use of a single precursor improves the uniform distribution of active reaction centres which is crucial for ORR catalysts. The t-ZIF has Fe, Co and Zn metal centres and 2-methylimidazole as a ligand. Carbonisation of the t-ZIF under an inert atmosphere produces nitrogen and Fe/Co–Nx doped carbon/carbon nanotubes alloyed with metal/metal oxide particles encased inside the carbon structures (FeCo-NCZ). The presence of Zn in the t-ZIF induces porosity in carbon during the carbonisation process. The peculiar morphology with a reasonably high surface area provides efficient mass transport and interpenetrated carbon nanotube assisted fast electron transport in the catalyst. X-ray photoelectron spectroscopy reveals that FeCo-NCZ is enriched with different possible active reaction centres such as pyridinic, graphitic and Fe/Co–Nx type nitrogen coordination on the catalyst surface. The ORR activity of FeCo-NCZ in oxygen saturated 0.1 M KOH was comparable to/higher than that of the reference Pt/C catalyst. The displayed onset potential (1.04 V vs. the RHE) and half-wave potential (0.91 V vs. the RHE) of FeCo-NCZ are more positive compared to those of Pt/C and other control-samples. It is noteworthy that the dioxygen reduction kinetics of FeCo-NCZ are comparable to those of Pt/C as evident from the Tafel slope and oxygen reduction follows a four electron pathway. More interestingly, FeCo-NCZ shows better fuel tolerance and electrochemical stability even at 60 °C compared to Pt/C under alkaline conditions.