Activating Nitrogen-doped Graphene Oxygen Reduction Electrocatalysts in Acidic Electrolytes using Hydrophobic Cavities and Proton-conductive Particles.

Although pyridinic-nitrogen (pyri-N) doped graphene is highly active for the oxygen reduction reaction (ORR) of fuel cells in alkaline media, the activity critically decreases under acidic conditions. We report on how to prevent the deactivation based on the mechanistic understanding that O 2 + p y r i - N H + + e - → O 2 , a + p y r i - N H ${{{\rm O}}_{2}+{\rm p}{\rm y}{\rm r}{\rm i}{\rm { -}}{\rm N}{{\rm H}}^{+}+{{\rm e}}^{-}{\to }_{\ }^{{\rm \ }}{{\rm O}}_{2,{\rm a}}+{\rm p}{\rm y}{\rm r}{\rm i}{\rm { -}}{\rm N}{\rm H}}$ governs the ORR kinetics. First, we considered that the deactivation is due to the hydration of pyri-NH+ , leading to a lower shift of the redox potential. Introducing the hydrophobic cavity prevented the hydration of pyri-NH+ but inhibited the proton transport. We then increased proton conductivity in the hydrophobic cavity by introducing SiO2 particles coated with ionic liquid polymer/Nafion® which kept the high onset potentials with an increased current density even in acidic media.

Active Pt-Nanocoated Layer with Pt-O-Ce Bonds on a CeO x Nanowire Cathode Formed by Electron Beam Irradiation.

A Pt-nanocoated layer (thickness of approx. 10-20 nm) with Pt-O-Ce bonds was created through the water radiolysis reaction on a CeO x nanowire (NW), which was induced by electron beam irradiation to the mixed suspension of K2PtCl4 aqueous solution and the CeO x NW. In turn, when Pt-nanocoated CeO x NW/C (Pt/C ratio = 0.2) was used in the cathode layer of a membrane electrode assembly (MEA), both an improved fuel cell performance and stability were achieved. The fuel cell performance observed for the MEA using Pt-nanocoated CeO x NW/C with Pt-O-Ce bonds, which was prepared using the electron beam irradiation method, improved and maintained its performance (observed cell potential of approximately 0.8 V at 100 mW cm-2) from 30 to 140 h after the start of operation. In addition, the activation overpotential at 100 mA cm-2 (0.17 V) obtained for MEA using Pt-nanocoated CeO x NW/C was approximately half of the value at 100 mA cm-2 (0.35 V) of MEA using a standard Pt/C cathode. In contrast, the fuel cell performance (0.775 V at 100 mW cm-2 after 80 h of operation) of MEA using a nanosized Pt-loaded CeO x NW (Pt/C = 0.2), which was prepared using the conventional chemical reduction method, was lower than that of MEA using a Pt-nanocoated CeO x /C cathode and showed reduction after 80 h of operation. It is considered why the nanocoated layer having Pt-O-Ce bonds heterogeneously formed on the surface of the CeO x NW and the bare CeO2 surface consisting of Ce4+ cations would become unstable in an acidic atmosphere. Furthermore, when a conventional low-amount Pt/C cathode (Pt/C = 0.04) was used as the cathode layer of the MEA, its stable performance could not be measured after 80 h of operation as a result of flooding caused by a lowering of electrocatalytic activity on the Pt/C cathode in the MEA. In contrast, a low-amount Pt-nanocoated CeO x NW (Pt/C = 0.04) could maintain a low activation overpotential (0.22 V at 100 mA cm-2) of MEA at the same operation time. Our surface first-principles modeling indicates that the high quality and stable performance observed for the Pt-nanocoated CeO x NW cathode of MEA can be attributed to the formation of a homogeneous electric double layer on the sample. Since the MEA performance can be improved by examining a more effective method of electron beam irradiation to all surfaces of the sample, the present work result shows the usefulness of the electron beam irradiation method in preparing active surfaces. In addition, the quantum beam technology such as the electron beam irradiation method was shown to be useful for increasing both performance and stability of fuel cells.

Design of Active Sites on Nickel in the Anode of Intermediate-Temperature Solid Oxide Fuel Cells using Trace Amount of Platinum Oxides.

Invited for this month's cover is the group of Prof. Dr. Toshiyuki Mori at National Institute for Materials Science (NIMS), Japan. The front cover picture shows the formation of new active sites on Ni in the anode of a solid oxide fuel cell (SOFC), which displays high performance at intermediate temperature. The combination of processing route design, microanalysis, and surface atomistic simulation provides us with a new design paradigm for fabrication of high-performance SOFCs. Read the full text of the article at 10.1002/cplu.201800170.

Design of Active Sites on Nickel in the Anode of Intermediate-Temperature Solid Oxide Fuel Cells using Trace Amount of Platinum Oxides.

In recent years, the lowering of the operation temperature of solid oxide fuel cells (SOFCs) has attracted much attention owing to the trade-off between the best performance and the life span of SOFCs. For this challenge, new active sites on the Ni surfaces in a Nickel-Yttria-Stabilized Zirconia (Ni-YSZ) cermet anode of SOFCs have been created by deposition of trace amounts of platinum oxide (PtOx ) followed by an activation step of the anode at 1073 K in a hydrogen flow. The internal resistance (IR) free value (185 mA cm-2 at 0.8 V) observed for the single cell with an anode sputtered with a trace amount of PtOx (Pt content in anode: from 9 to 91 ppm) at 973 K is conspicuously higher than that of a similar single cell with a nonsputtered cermet anode (85 mA cm-2 ) at 0.8 V and 1073 K. Transmission electron microscopy microanalysis shows that the defect structure is formed on a partially oxidized Ni surface by active Pt species. Also, surface atomistic simulation on NiO (111) predicts the formation of Frenkel defect clusters with Pt cations, which partially cover the Ni surface. The formation of Frenkel defect clusters on the partially oxidized Ni surface (i.e., creation of new active sites for formation of water molecules) promotes the anode reaction, resulting in improvements in the anode performance of SOFC single cells at 973 K. Design of the aforementioned new active sites on Ni through sputtering of trace amounts of PtOx provides a great opportunity for "radical innovation" in the design of intermediate-temperature SOFCs.