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, 6th Vacuum Expo 15th October 2015

CPERI/CERTH Thessaloniki

Greece

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

Outline

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 H20.

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

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

2. Use of additional Triode electrode forces O2- 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

TCell Project Objective

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

2. Use of additional Triode electrode forces O2- 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

TCell Project Objective

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

Mantis’s Contribution to TCell

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

Experimental Techniques

Metallic vapor

generated

Nucleation of vapor to form NPs

Coalescence of nuclei to form nanoclusters

Decreasing T

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.

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 (350 - 106 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.

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 (350 - 106 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.

Experimental Techniques

Alloying/Mixing in Bulk

Alloy mixing or phase separation?

Binary Materials

Enthalpy of mixing DHmix (mJ/mol)

BULK structure

Ni-Au +11 Phase separation

Mo-Au +5 Phase separation

Ni-Mo -6.15 Stable alloy formation Ni3Mo, NiMo

In the bulk the enthalpy of mixing DHmix determines the lowest energy state for a bimetallic mix driven by lattice mismatch and electronegativity mismatch

Negative DHmix enables ordering of elements (e.g. Ni3Mo)

Positive DHmix causes phase separation (e.g. Ni-Au, Mo-Au)

Binary Materials

DHmix surface energy

g (Jm-2)

Dg BULK structure Nanoscale

Ni-Au +11 Ni 2.3

0.8 Phase separation

Phase separation

Au 1.5

Mo-Au +5 Mo 3.3

1.8 Phase separation

Phase separation

Au 1.5

Ni-Mo -6.15 Ni 3.3

1.0 Stable alloy formation

Ni3Mo, NiMo Phase separation?

Mo 2.3

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 DHmix >0 and DHmix <0.

Investigate Ni-Mo-Au tri-metallic NPs - phase separation or alloying?

Mass spectrum for- Ni-Au-Mo NPs

• Au, Ni, and Mo run separately :- Elemental NP peaks at ~2-3nm diameter

I=60mA 100Sccm Ar flow Aggregation Zone : 100mm (max)

Mass spectrum for- Ni-Au-Mo NPs

Without bulk phase diagram it is difficult to predict structure of the NPs. TEM analysis required.

• 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

TEM Analysis of Mo-Ni-Au NPs

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.

5 0 n m5 0 n m

5 n m5 n m

HRTEM carried out by TCell partners CERTH in Thessaloniki, Greece

using JEOL JEM 2010 HRTEM with X-ray Microanalysis (Oxford INCA)

5 nm5 nm

• At% composition resolution ±0.5%

• Spot size ~30nm

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

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

• The annealed NPs are all spherical in shape. • Appears that smaller NPs have agglomerated during annealing. • EDS shows improved crystallinity post annealing.

stagnant Air at 800°C for 2hrs

TEM Analysis of Annealed Ni-Mo-Au NPs

• 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.

NP composition: Au 50-100 at% Mo 0-50 at% Ni below resolution limit

Implication of phase separation vs alloy formation on catalytic behaviour in SOFC not yet tested.

20nm NP

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

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

THANK YOU FOR YOUR ATTENTION

Thank You

The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement number 298300-10.