Taner Akbay

Ni-Fe Nitride Nanoplates on Nitrogen-Doped Graphene as a Synergistic Catalyst for Reversible Oxygen Evolution Reaction and Rechargeable Zn-Air Battery

Obtaining bifunctional electrocatalysts with high activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is a main hurdle in the application of rechargeable metal-air batteries. Earth-abundant 3d transition metal-based catalysts have been developed for the OER and ORR; however, most of these are based on oxides, whose insulating nature strongly restricts their catalytic performance. This study describes a metallic Ni-Fe nitride/nitrogen-doped graphene hybrid in which 2D Ni-Fe nitride nanoplates are strongly coupled with the graphene support. Electronic structure of the Ni-Fe nitride is changed by hybridizing with the nitrogen-doped graphene. The unique heterostructure of this hybrid catalyst results in very high OER activity with the lowest onset overpotential (150 mV) reported, and good ORR activity comparable to that for commercial Pt/C. The high activity and durability of this bifunctional catalyst are also confirmed in rechargeable zinc-air batteries that are stable for 180 cycles with an overall overpotential of only 0.77 V at 10 mA-2 .

Development of Intermediate-Temperature SOFC Module Using Doped Lanthanum Gallate

An intermediate temperature solid oxide fuel cell (SOFC) module was developed using electrochemically active cells composed of (La, Sr)(Ga, Mg, Co)O 3 electrolyte, Ni-(Ce, Sm)O 2 anode, and (Sm, Sr)CoO 3 cathode. Seal-less planar type stack design was employed. The first generation module successfully provided the output power of I kW with thermal self-sustainability below 800°C. Maximum electrical efficiency obtained with this module was 43%[LHV] together with the corresponding fuel utilization of 78%. Dynamic performance tests demonstrated the capability of output power alteration from 0.6 to 1 kW while maintaining a high electrical conversion efficiency. Further testing and modification of the module for methane fuel utilization are in progress.

The interaction of molecular oxygen on LaO terminated surfaces of La2NiO4

Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as air electrode materials for intermediate to high temperature electrochemical device applications. The surface chemistry of these materials plays a critical role in determining the kinetics of oxygen reduction and exchange reactions. Among various perovskite-structured oxides, certain members of the Ruddlesden–Popper series, e.g. La2NiO4, have been identified as significantly active for surface oxygen interactions. However, the challenge remains to be the identification of the structure and composition of active surfaces, as well as the influence of these factors on the mechanisms of surface exchange reactions. In this contribution, the changes in the electronic structure and the energetics of oxygen interactions on the surfaces of La2NiO4 are analysed using first principles calculations in the Density Functional Theory (DFT) formalism. As for the surface chemistry, LaO termination rather than NiO2 termination is presumed due to recent experimental evidence of the surfaces of various perovskite structured oxides after heat treatment in oxidizing environments being transition metal free. Our findings substantiate the fact that the LaO-terminated surface can indeed participate in the formation of surface superoxo species. Detailed charge transfer analyses revealed that it is possible for such a surface to be catalytically active owing to the enhanced electronic configurations on the neighbouring La sites to surface species. In addition, positively charged oxygen vacancies, relative to the crystal lattice, can act as active sites and catalyse the O–O bond cleavage.

SOFC Module and System Development by Means of Sealless Metallic Separators with Lanthanum Gallate Electrolyte

The third-generation 1-kW e -class module was developed with an automatic control system. A conversion efficiency of 48% ac/lower heating value [ac/LHV] was achieved with an exhaust heat recovery unit. An endurance test using the third-generation 1-kW e module was done for over 1000 h and no degradation of the power generation performance was observed. In parallel, a single-cell unit, which includes one cell and two metallic separators, was tested for over 10000 h and the degradation rate of the terminal voltage was found to be 1-2%/1000 h. In the direction of scale-up, a triple-stack module of 3-kW e output was developed. A partial load as well as excess loads on the module were tested and the output power of 1-5 kW e was attained under thermally self-sustainable conditions. It was found that a high efficiency of 55% dc/lower heating value [dc/LHV] was obtained under stable operation. Ongoing research of the fourth-generation 1-kW e module has resulted in the conversion efficiency of 58% [dc/LHV].

Oxide ion and electronic conductivity in Co doped La0.8Sr0.2Ga0.8Mg0.2O3 perovskite oxidePresented at the 78th International Bunsen Discussion Meeting on ?Complex Oxides: Defect Chemistry, Transport and Chemical Reaction?, Vaals, The Netherlands, October 6?9, 2002.

Partial electronic and hole conductivity in Co doped LaGaO3 based perovskite oxide was investigated with the ion-blocking method. Typical S-shaped polarization curves were observed on La0.8Sr0.2Ga0.8Mg0.2−XCoXO3 (0 < X < 0.1). The oxygen partial pressure (PO2) dependence of the electronic and hole conductivity is estimated to be PO2−1/4 and PO21/4, respectively, at temperature higher than 1173 K. However, these decreased to PO2−0.12 and PO20.06 respectively at 873 K. It is considered that the electronic and hole conductivities, that are intrinsic to LSGM are dominant at high temperature, however, the extrinsic electronic and hole conductivity caused by doped Co becomes dominant with decreasing temperature. The estimated transport number of the Co doped sample was higher than 0.95 over the PO2 range from 1 to 10−30 atm, which is slightly higher than that estimated by the H2–O2 cell. The partial electronic and hole conductivities in Co doped LaGaO3 based oxide increased with increasing the amount of Co, in particular, increase in the electronic conductivity is significant at Co content higher than 8.5 mol% to Ga site. PO2 dependence for electronic and hole conductivity is much smaller than that of PO2−1/4 and PO21/4, respectively, suggesting that the electronic and hole conductivity which is extrinsic to LSGM is dominant with increasing Co amount and the specimens behaves like an intrinsic semiconductor. The estimated theoretical efficiency of the electrolyte reaches a maximum value of ca. 0.90 around a thickness of 100 μm in 5 mol% Co doped sample at 0.8 A cm−2 and 1073 K.

Computational Fluid Dynamic Analysis of a Seal-Less Solid Oxide Fuel Cell Stack

Combined heat and power generation systems accommodating intermediate temperature (600-800°C) solid oxide fuel cell (SOFC) modules have been developed by Mitsubishi Materials Corporation and The Kansai Electric Power Co., Inc. High overall efficiency system units are designed in such a way that their output power can be modularized by altering the number of stacks inside the SOFC modules. The seal-less design concept is adopted to build generic stacks made up of stainless steel separators and disk-type planar electrolyte-supported cells. Innovative stack design together with its precise integration with the hot balance of plant components inside the SOFC module requires a number of design iterations supported by carefully planned experiments. In order to achieve improved levels of efficiency and reliability via optimum number of iterative cycles, we believe that the computational techniques offer significant advantages. In this work, a commercial computational fluid dynamics code is employed for solving the conservation of mass, momentum, and energy equations with an additional electrochemical submodel to simulate the coupled multiphysics processes in a generic SOFC stack. This approach proved to be effective in providing necessary guidance for identifying problem areas in the stack design and estimating the stack performance via less expensive numerical experiments. The results of the computational model are also compared with data I obtained by experimental measurements.

Development of IT-SOFC Based on Lanthanum Gallate Electrolyte

The Kansai Electric Power Company, Inc. and Mitsubishi Materials Corporation started a joint research project for developing intermediate temperature (600{degree sign}C to 800{degree sign}C) solid oxide fuel cells (IT-SOFC) in 2001. High performance disk-type cells with lanthanum gallate-based electrolyte and a seal-less cell-stack design enable high efficiency operation at temperatures below 800{degree sign}C. The development of 10 kW-class CHP system under a project supported by NEDO (New Energy and Industrial Technology Development Organization) was started in FY2004. In 2007, the performance test of the 10 kW-class system using town gas as fuel has been undertaken. In this test, the electrical efficiency of 41%HHV on AC power output of 10 kW and the overall efficiency of 82%HHV by recovering 60aC hot water have been achieved. And the voltage degradation rate per 1,000 hours was 1.08% after 3,000 hours operation has been observed during the field test of the 10 kW-class system. Furthermore, after making some refinements to enhance the performance of 10 kW-class module, the second long-term stability test for 3,000 hours were carried out in 2008. Efforts to enhance cell and module durability have been made with 1 kW-class modules operated for 5,000 hours, and with the cell-stack units operated for 15,000 hours. The voltage degradation rates per 1,000 hours were 0.51 % for the 1 kW-class modules, and 0.23 - 0.48% for the cell-stack units.

Development of Intermediate-Temperature Solid Oxide Fuel Cells Using Doped Lanthanum Gallate Electrolyte

Intermediate-temperature(IT) solid oxide fuel cells(SOFCs) were developed using lanthanum gallate electrolyte, samarium cobaltite cathode and the cermet anode of nickel and ceria. High efficiency operation below 800°C was enabled using planar disk type cells with unique seal less stack design. The first 10 kW-class combined heat and power (CHP) system provided AC output power of 10 kW with electrical and overall efficiency of 41 and 82 %HHV, respectively. Optimization of cell-stack components to increase the output power density is in progress.

A Unique Seal-Less Solid Oxide Fuel Cell Stack and Its CFD Analysis

Combined Heat and Power (CHP) generation units based on intermediate temperature (600∼800°C) solid oxide fuel cell (SOFC) modules have been collaboratively developed by Mitsubishi Materials Corporation and The Kansai Electric Power Co., Inc. Currently, hydrocarbon fuel utilising units designed to produce modular power outputs up to 10 kWe-AC with overall efficiencies greater than 80% (HHV) are being tested. A unique seal-less stack concept is adopted to build SOFC modules accommodating multiple stacks incorporated of stainless steel separators and disk-type planar electrolyte-supported cells. In order to advance the current technology to achieve improved levels of efficiency and reliability, through design iterations, computational modelling tools are being heavily utilised. This contribution will describe the results of coupled computational fluid dynamics (CFD) analysis performed on our fourth-generation 1 kW class SOFC stack. A commercially available CFD code is employed for solving the governing equations for conservation of mass, momentum and energy. In addition, a local electrochemical reaction model is coupled to the rest of the transport processes that take place within the SOFC stack. It is found that the CFD based multi-physics model is capable of providing necessary and proper guidance for identifying problem areas in designing multi-cell SOFC stacks. The stack performance is also estimated by calibrating the computational model against data obtained by experimental measurements.Copyright