Catalytic Nanoparticles in Solid Oxide Fuel Cells based on Triode Operation

Transcription

1 Catalytic Nanoparticles in Solid Oxide Fuel Cells based on Triode Operation Vicky Broadley* and David Joyce Mantis Deposition, Ltd., Thame, UK Functional Thin Films meeting, 6 th Vacuum Expo 15 th October 2015 CPERI/CERTH Thessaloniki Greece

2 Outline Outline 1. Introduction to Fuel cells and TCell project 2. Experimental techniques 3. Experimental results in-flight analysis of alloy NPs and TEM analysis 4. Simulation of fuel cell operating conditions 5. Summary and conclusions

3 Introduction to Solid Oxide Fuel Cells Operation of standard SOFC Hydrocarbon fuel, steam, & air undergo electrochemical reaction in the cell at 800 C to produce electricity and H 2 0. Problem with standard SOFC design: Cell lifetime compromised due to severe operating conditions of high temperature and humidity, which cause 1. Low activity towards reforming and oxidation 2. De-activation due to carbon deposition on the anode 3. Cell instability due to sulfur poisoning

4 TCELL Project : Solid Oxide Fuel Cell based on Triode operation To develop an improved hydrocarbon solid oxide fuel cell using two innovations: 1. Improved anode materials to include Ni, Au, and Mo catalysts to reduce C growth and S poisoning of anodes => improve cell lifetime TCell Project Objective 2. Use of additional Triode electrode forces O 2- ions to catalyst surface enhances cell performance and accesses formerly inaccessible operating potentials =>increase cell efficiency Materials cerments used: Anode: NiO/GDC or NiO/YSZ Electrolyte: YSZ New Catalyst: (3%wt Au) or (3%wt Au 3%wt Mo) Catalyst incorporated in cerment powders

5 TCELL Project : Solid Oxide Fuel Cell based on Triode operation To develop an improved hydrocarbon solid oxide fuel cell using two innovations: 1. Improved anode materials to include Ni, Au, and Mo catalysts to reduce C growth and S poisoning of anodes => improve cell lifetime TCell Project Objective 2. Use of additional Triode electrode forces O 2- ions to catalyst surface enhances cell performance and accesses formerly inaccessible operating potentials =>increase cell efficiency Materials cerments used: Anode: NiO/GDC or NiO/YSZ Electrolyte: YSZ New Catalyst: (3%wt Au) or (3%wt Au 3%wt Mo) Catalyst incorporated in cerment powders

6 Mantis s Contribution to TCell Deposition of metal nanoclusters for anode modification 1. Investigate Ni-Au-Mo nanoparticles formed by gas phase condensation Study the formation of Ni-Au-Mo tri-metallic nanoclusters in the NanoGen aggregation zone using the mass quadrupole Investigate the effect of aggregation zone temperature inside the NanoGen on the NP size distribution and alloy composition and on NP structure, using HRTEM. 2. Assess suitability of gas phase nanoparticles for catalytic doping in fuel cell anode layers Post-growth annealing of alloy NPs to mimic reduction temperature in fuel cell anode Consider stability of alloy NPs under operating conditions

7 Experimental Techniques NanoGen nanoparticle source Gas phase generation of nanoparticles from elemental or alloy targets. UHV conditions enable ultra pure nanoparticle formation Control over nanoparticle size from <1nm to 20nm through variation of aggregation conditions Metallic vapor generated Nucleation of vapor to form NPs Coalescence of nuclei to form nanoclusters Decreasing T

8 Experimental Techniques Controlled deposition of tri-metallic nanoparticles Alloy nanoparticles can be created from a single alloy target using the 2 magnetron in the NanoGen. NanoGen TRIO Enables generation of up to three 1 target species at a time. Independent control of power supplied to each target provides the user with control over the alloy composition. Up to 3 different materials can be studied without the need for venting to change targets.

9 Experimental Techniques MesoQ - In flight analysis of the NP size distribution using Mantis s mass quadrupole. AC and DC voltages applied to 2 pairs of parallel rods force NPs to follow elliptical path through the MesoQ. By scanning frequency of AC voltage, mass (diameter) of NPs allowed through MesoQ is altered. High-mass spectrometer/filter ( amu) Resolution control through adjustment of quadrupole DC/AC voltage ratio Mode of operation :- 1. Analysis of NP mass (size) spectrum 2. Filtering of charged NPs enables size selected NPs to be deposited on substrates.

10 Experimental Techniques MesoQ - In flight analysis of the NP size distribution using Mantis s mass quadrupole. AC and DC voltages applied to 2 pairs of parallel rods force NPs to follow elliptical path through the MesoQ. By scanning frequency of AC voltage, mass (diameter) of NPs allowed through MesoQ is altered. High-mass spectrometer/filter ( amu) Resolution control through adjustment of quadrupole DC/AC voltage ratio Mode of operation :- 1. Analysis of NP mass (size) spectrum 2. Filtering of charged NPs enables size selected NPs to be deposited on substrates.

11 Alloy mixing or phase separation? Alloying/Mixing in Bulk In the bulk the enthalpy of mixing DH mix determines the lowest energy state for a bimetallic mix driven by lattice mismatch and electronegativity mismatch Negative DH mix enables ordering of elements (e.g. Ni 3 Mo) Positive DH mix causes phase separation (e.g. Ni-Au, Mo-Au) Binary Materials Enthalpy of mixing DH mix (mj/mol) BULK structure Ni-Au +11 Phase separation Mo-Au +5 Phase separation Ni-Mo Stable alloy formation Ni 3 Mo, NiMo

12 Alloying/Mixing at the Nanoscale At the nanoscale, surface energy (g) is an additional driving force for segregation. driven by valence electron number and surface ordering A large difference in surface energy leads to higher surface energy material being driven to the surface. Phase separation may occur even for materials systems that form stable alloys in the bulk for binary mixes with both DH mix >0 and DH mix <0. Binary Materials DH mix Ni-Au +11 Mo-Au +5 surface energy g (Jm -2 ) Ni 2.3 Au 1.5 Mo 3.3 Au 1.5 Dg BULK structure Nanoscale 0.8 Phase separation Phase separation 1.8 Phase separation Phase separation Ni-Mo Ni 3.3 Mo Stable alloy formation Ni 3 Mo, NiMo Phase separation? Investigate Ni-Mo-Au tri-metallic NPs - phase separation or alloying?

13 Mass spectrum for- Ni-Au-Mo NPs I=60mA 100Sccm Ar flow Aggregation Zone : 100mm (max) Au, Ni, and Mo run separately :- Elemental NP peaks at ~2-3nm diameter

14 Mass spectrum for- Ni-Au-Mo NPs Au, Ni, and Mo run separately :- Elemental NP peaks at ~2-3nm diameter Mo, Ni, and Au NPs together:- 2nm peak is suppressed, additional broadening around 6-7nm Larger NPs could be alloy NPs or agglomerations of elemental NPs Without bulk phase diagram it is difficult to predict structure of the NPs. TEM analysis required.

15 HRTEM carried out by TCell partners CERTH in Thessaloniki, Greece using JEOL JEM 2010 HRTEM with X-ray Microanalysis (Oxford INCA) TEM Analysis of Mo-Ni-Au NPs 5 n m 5 nm 5 0 n m TEM shows a distribution of NPs sizes. Smaller NPs are spherical in shape. Larger NPs are agglomerations of small spherical NPs. NPs are crystalline in nature. At% composition resolution ±0.5% Spot size ~30nm

16 TEM Analysis of Ni-Mo-Au NPs NP size distribution: MesoQ and TEM follow same broad distribution higher proportion of larger NPs observed by TEM due to agglomeration on surface of grid Composition Larger NPs At% analysis NP makeup: varied compositional mixes of all elements At% of smaller NPs not analysed

17 Simulation of SOFC conditions A second TEM grid was annealed under conditions to mimic SOFC reduction temperatures. Ni-Mo-Au NPs as-deposited Ni-Mo-Au NPs annealed stagnant Air at 800 C for 2hrs The annealed NPs are all spherical in shape. Appears that smaller NPs have agglomerated during annealing. EDS shows improved crystallinity post annealing.

18 TEM Analysis of Annealed Ni-Mo-Au NPs 20nm NP NP composition: Au at% Mo 0-50 at% Ni below resolution limit Higher magnification view of annealed tri-metallic NPs show distinct regions of contrast. Contrast regions look like phase separation. EDS shows Mo-Au mix suggests Mo-Au phase separation is driven by annealing. Implication of phase separation vs alloy formation on catalytic behaviour in SOFC not yet tested.

19 Nanoparticle Summary Ni-Au-Mo alloy nanoparticles: NanoGen Trio and gas phase synthesis MesoQ data show small elemental NPs for single target sputtering MesoQ data show formation of large NPs when elemental targets are run together TEM confirms presence of larger NPs Discrepancy between in-flight mass spectrum and TEM size distribution due in part to agglomeration of NPs on surface Annealing Studies to mimic SOFC operation: Migration and mixing of NPs into spherical crystalline NPs Evidence of phase separation during annealing process TEM grids sent for STEM analysis to confirm NP composition

20 Implications for SOFC Performance Agglomeration of NPs reduces active surface area Catalytic effect surface area Surface area 1/d Consider agglomeration when calculate loading required on anode surface Possible migration of catalyst NPs from surface of anode into bulk of layer Catalysts are most effective at reducing surface carbon build-up when catalyst is also on anode surface Possible reduction in catalytic effect if NPs migrate from surface Possible shorting across cell junction if NPs interconnect through the anode Test Au-Ni-Mo NP in functioning fuel cell Formation of phase separated NPs rather than true alloy NPs Catalytic behavior of alloy NPs compared to phase separated NPs is not understood Compare catalytic behavior of phase separated and alloy NPs with discrete Mo and Au NPs incorporated in modified anode powders

21 THANK YOU FOR YOUR ATTENTION

22 Thank You The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP/ ) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement number