Nanostructured electrochemical reactors for NOx/PM decomposition and micro SOFCs

Transcription

1 OECD Conference on Potential Environmental Benefits of Nanotechnology: Fostering Safe Innovation-Led Growth, Session 3. Clean Car Technology Paris, July 15-17, 2009 Nanostructured electrochemical reactors for NOx/PM decomposition and micro SOFCs Masanobu Awano Institute of Advanced Industrial Science and Technology(AIST) Nagoya , JAPAN Ceramic electrochemical reactors are expected for their high performance on conversions of energy and substances; for examples, electric power generation (solid oxide fuel cells: SOFCs), synthesis of hydrogen, and decomposition and purification of environmental pollutants. Development of novel electrochemical modules for denox/pm reactor by nanostructure control Combination with thermoelectric ceramic module for harvesting of waste heat energy New type MicroSOFC development of high performance APU unit for vehicles by nano-micro structure control as clean energy source

2 Various applications of electrochemical reactor (O 2- -conducting ceramics ) Air O 2- e - 1 H 2 2 CH 4 e - H 2 O (SOFC) H 2 O + CO 2 (SOFC) e - Air 3 NO N 2 (de-no X ) 4 H 2 O H 2 (H 2 generation) O 2- O 2- e - O 2 5 CH 4 CO + H 2 (Syngas) 6 O 2 pumping 7 CH 4 CH 3 OH (GTL) 8 C CO 2

3 6th Pacific Rim Conference on Ceramic and Glass Technology, September 15th 2005, Hawaii, USA Nitrogen oxides ( NOx ) in exhaust gas are known - to cause air pollution problems (acid rain, photochemical smog) - to give damage to human nerves and respiratory organs The reduction of NO X emission has become one of the greatest challenges in environment protection. 100 % NO 2 50 H C C O 0 Japanese regulation A/F Active at higher PO 2 atmosphere near zeroemission

4 Environment purifying / Saving energy NO x N 2 +O 2 NOx/PM DECOMPOSITION NANOSTRUCTURED DE-NOx REACTOR ELECTROCHEMICAL/THERMO ELECTRIC MODULE

5 Contents 1. Ceramic electrochemical reactor for NOx decomposition 2. Ceramic electrochemical reactor for PM (particulate matter) decomposition 3. Thermoelectric ceramic module for enhanced denox property by using waste heat energy

6 1. Ceramic electrochemical reactor for NOx decomposition

7 Example: scheme of NOx purifying by an electrochemical cell (under excess oxygen coexistence such as diesel engine exhaust gas) 高温排ガス クリーンガス N O x 高温における N O x 高選択性 Oxygen as an inhibitor to de-nox reaction O 2 e N 2 Catalytic Activation Site ガス分子吸着サイト 多孔質触媒電極層 Porous Cathode 熱電変換による電力供給 e O 2- O 2 Solid 固体電解質 Electrolyte Oxygen ( 酸素イオン ion conductor 伝導体 ) Porous 多孔質電極 Anode Large amount of electrical current supply is required difficulty to the application

8 Proposed Mechanism of Selective DeNOx Reaction high conc. oxygen defects layer nano particles 2nm N Ox m olecules Ni nano particles Exhaust gas O-N N 2 YSZ YSZ N io Ni NiO nano pores e - nano redox-reaction zone O-ion O 2 O 2- ( O 2 ) N 2 TEM image of an NiO and YSZ interface, and reaction model of selective NO molecule decomposition. The expected mechanism nano-space - of the absorption and decomposition of N to Ni, and oxygen capturing and pumping in the region of high defects concentration is also displayed

9 Improvement of de-nox / current efficiency NOx decomposition (%) NOx Conversion (%) t=600 0 C oxygen 2% 1000ppm-NO EC electrode mesoscale control nanoscale Energy efficiency of ordinary catalyst oxygen 2% t=700 0 C 1000ppm-NO Pt C Ag C Pd C Pd C Pd-Pt C Literature previous results Current (ma) C ellcurrent (m A) Improved de-nox efficiency for applied current by nano- and meso-scale structurally controlled electrochemical cells in comparison with previous results.

10 Microstructure development of electro-catalytic electrode by the factors of applied voltage and temperature YSZ(covering layer) NiO+YSZ (catalytic electrode) YSZ+Pt(electrode) YSZ(electrode)

11 Optimization of Nano-space reaction zone (applied voltage) Microstructure development of electrocatalytic electrode at the interface of NiO-YSZ grain boundaries as a function of applied voltage; (a)before applying current, (b)voltage 1V, (c)1.5v,(d)2v,(e)2.25v, (f)2.5v,(g)2.75v,(h)3v.

12 Research for application of de-nox cell to diesel engine exhaust gas purification Large size cell (10cm square) 100 Current 実験セルの efficiency of typical 電流効率 small (1.65%) cell %NFC C NOx decomposition NOx 転換率 (%) (%) denox property of large size cells %NFC C %NFC C %NFC C(2) %NFC C データ 14 7:26: / Cell 電流密度 current (ma/cm 2 )) Gas DC Cell stack (20sheets) Image of a denox module of diesel engine exhaust gas Sequential development of the electrochemical cell from laboratory to a real application and NOx decomposition properties of a large size electrochemical cell (10cm square). Inserted photograph is a stack model of 20 cells assembled for exhaust gas purification of vehicles.

13 Measurement of denox performance of a stack by large cells Stack using 20 cells

14 70 60 Durability under operating conditions C 2 H SO 2 NO Conversion [%] : 0 % : 0.2% : 0.3% Power [mw] NO Conversion [%] : 0ppm : 3ppm :30ppm Power [mw] NOx conversion (ratio) NO 転化率 (-) 経過時間 (h) Time(h) No degradation for CO,CH / high conc.sox causing damage in the electrode Initial degradation less than 10% --- stable for prolonged operation over 200h

15 Electrochemical cell operation at low temperature by introduction of a nano-wired structure Nano-wire electrode Nano particles Exhaust gas Network of metallic nano- wires between electrolyte micron particles wire Electrochemical reaction Clean air power cathode electrolyte Electric wire to power source anode DeNOx property at 250 C under 20%O 2 coexistence

16 Selective reduction of NOx using our electrochemical cell Exhaust gas Cleaned gas NOx-selective layer O 2- conductor N 2 - DC NO - O 2 N 2

17 2. Ceramic electrochemical reactor for PM (particulate matter) decomposition

18 Oxidation of graphite on Ca 12 Al 14 O 33 / Ag composite anode Cathode : 2NO + 4e - N 2 + 2O 2- Anode : C + 2O 2- CO 2 + 4e -

19 Mechanism of Oxidation using Active oxygen on Anode of Electrochemical Reactor Microstructure of Ag/Ca 12 Al 14 O 33 /8YSZ Porous Anode Nanostructure controlled electrode C12A7(O*) Porous Anode YSZ,CGO 1µm O* CO 2 Reaction Between C and O* at Anode Supplying High Partial Voltage O Ion to Catalysts Graphite (PM) e- + - Cathode O2- Pump O ion into YSZ from NO NO e-

20 Simultaneous clean up of solid carbon (PM) and nitrogen oxide (NOx) Reducing electrode : 2NO + 4e - N 2 + 2O 2- Cathode Electrolyte Ag CGO + NiO CGO Oxidizing electrode : C + 2O 2- CO 2 + 4e - NOx decomposition (%) 100 NOx 100ppm(50ml/min) Graphite 0.8mg Anode 475 Ag CGO + Ag graphite Degree of CO 2 formation By graphite oxidation(ppm) cell surface through electrochemical reaction Voltage /V

21 Effect of C 12 A 7 addition on Reaction Rate Removed Graphite (mol) 2x10-5 1x10-5 Theoretical Value ( C + O 2- = CO 2 +2e - ) Pt+YSZ+Ca 12 Al 14 O 33 (14%) Pt + YSZ Amount of Applied Charge (c)

22 Table 1. Amount of electrochemical decomposition of graphite on the surface of anode at 2.5V and 475q C. Anode material Decomposed graphite(mol/cm 2 -h) Pt+8YSZ Ag+8YSZ Ca 12 Al 14 O 33 +8YSZ 0.3x x x CaO 7Al 2 O cc Diesel Engine Estimated Amount of PM from Exhaust Gas Electrochemical Reactor(ca 1 m 2 ) Cubic 1.9 g/h 1.199nm 1.6 g/h

23 3. Thermoelectric ceramic module for enhanced denox property by using waste heat energy

24 Application of thermoelectric energy conversion for supplying electric power from waste heat Thermoelectric conversion Heat O 2 N 2 Electrochemical Ceramic Reactor High-T electron hole heating e - NO x T Power generation for Electrochemical Reaction Thermoelectric Ceramic module Exhausted Gas ΔT Low-T N-type Total energy 100% Current P-type 15% 30% torque operation FUEL 5-10% Alternator 5% Pressure / (efficiency<50%) Friction loss 30% Exhaust gas 40% radiator Electric Battery components & system Waste heat h energy