Doping Studies of Perovskite-related Materials for Potential Applications as

Fuel Cell Cathodes

Peter Slater University of Birmingham, UK

Surrey, July 2013

Outline

• Perovskite and K2NiF4 Materials Structural Aspects, hexagonal versus cubic

perovskites, interstitial sites in K2NiF4 phases

• Alternative doping and hydration strategies

AMnO3: Doping strategies to enhance electronic conductivity

Hydration of La2MO4 (M=Ni, Cu) systems:- indirect methods

• Summary

Introduction:-Perovskites Perovskite materials are attracting considerable interest for all components of Solid Oxide Fuel Cells (electrolyte and electrodes): also growing interest in their potential as low cost electrode catalyst materials for alkaline fuel cells Perovskite: ideal formula: ABO3 A= large cation (e.g. La, Sr, Ba) B= small cation (e.g. transition metal) The properties can be controlled by doping with aliovalent cations to introduce mixed valency/oxide ion vacancies

Perovskite Tolerance Factor

A key parameter to consider is the tolerance factor t = (rA+rO)/[√2(rB+rO)] For t= 1, we have an ideal cubic perovskite For 0.9 <t<1, distortion from cubic symmetry observed For t>1, face sharing of octahedra occurs hexagonal

perovskites (these tend to be poorly conducting)

Hexagonal

Face Sharing

“Cubic”

Corner Sharing

The K2NiF4 structure

Structurally related to perovskite is the K2NiF4 structure adopted by a number of transition metal containing systems. This system has the capability to accommodate either anion vacancies or interstitials e.g. Ln2MO4 (Ln= rare earth; M=Ni, Cu); (Ln/A)2MO4 (A= Ca, Sr, Ba: M= transition metal)

Our aim

1. To investigate novel doping strategies to optimise the performance of perovskite electrode materials for both high and low temperature fuel cell applications

2. To investigate strategies to incorporate anion interstitials in

La2NiO4 and related phases, with a view to fuel cell electrode applications

2. Doping studies of perovskite

manganates

SrMnO3

SrMnO3 is a hexagonal perovskite:- low electronic and ionic conductivity:- unsuitable for fuel cell applications Dope with SiO4

4- a “cubic” perovskite and a substantial increase in the conductivity

Hexagonal

Face Sharing

Cubic

Corner Sharing

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5 3 3.5 4

1000K/T

log σ

/Scm

-1

SrMn0.85Si0.15O3-x

SrMnO3-x

Dalton Trans 40, 5599, 2011 + 42, 5421, 2013.

Similar doping strategies can be employed for other perovskite systems, e.g. CaMnO3 (here no change in cell symmetry on doping)

CaMnO3

Low level Silicate, doping leads to enhanced conductivity due to electron doping

SiO2 + 3MnMn + OO SiMn + 2MnMn' + VO●● + ½ O2 + MnO2

CaMnO3

CaMn1-xSixO3-y

0.8 1.5 2.2 2.9-2

-1

0

1

2

logσ

(S·c

m-1)

1000/T (K-1)

Samples show promising performance as solid oxide fuel cell cathodes

(i.e. Oct. Mn replaced by tet. Si O vacancies reduction of Mn oxid state)

Ru doping also high conductivity (higher valent Ru possible mixed valency) Initial ORR experiments under alkaline conditions showed the best onset potentials for the CaMnO3 systems Ru doped CaMnO3 showed the better initial performance that the Si doped samples, and so was examined in more detail

Other dopants analysed

CaMnO3

CaMn0.85Ru0.15O3-x

Both undoped and Ru doped CaMnO3 showed an initial low onset potential, but a decline in performance on cycling

ORR polarisation curves under alkaline conditions

undoped Ru doped

Performance on addition of Vulcan XC-72R carbon powder

ORR polarisation curves under alkaline conditions

More favourable onset potential, but greater decline in performance:- partial decomposition

undoped Ru doped

3. Indirect hydration methods

K2NiF4 structure: a lot of interest in Ln2NiO4 (Ln= La, Pr, Nd) systems as solid oxide fuel cell cathode materials

Evidence for water incorporation in Pr2NiO4, but only low

levels (e.g. Tailledes et al. Fuel Cells 10, 166, 2010; Grimaud et al; JECS 159, B683, 2012)

Can we increase water incorporation through indirect

methods an oxide hydroxide with potential as an AFC catalyst ?

Initial results showed very good stability in alkaline

conditions for La2NiO4, even in the presence of peroxide

Hydration of K2NiF4 phases

It is possible to partially substitute O2- by F- leading to high anion excess:

i.e. La2NiO4-xF2x (x≈0.6)

Indirect introduction of hydroxide into La2NiO4

Reaction with PVDF (-CH2CF2-)n

at 350-400°C

Hydrothermal treatment of La2NiO4-xF2x with KOH(aq) replacement of F- with OH-

High levels (≈La2NiO3.4(OH)1.2) of water incorporation (lost at temperatures above 250-300°C)

(large volume expansion: La2NiO4 : 377.1 Å3, hydrated phase : 382.0 Å3)

OH-/F- exchange

However, no improvement in the onset potential for the ORR

Summary

• A range of doping strategies have been investigated to introduce mixed valency/increase the electronic conductivity in perovskite manganates- isovalent doping with Si is a novel way of achieving this

• Ru doping in CaMnO3 was shown to improve the onset potential for the ORR, but these phases were shown to be unstable towards partial decomposition

• A novel La-Ni oxide hydroxide has been prepared by an ion exchange route, although no improvement in the ORR onset potential was observed.

• Currently investigating a range of other perovskite and related systems

Acknowledgements University of Birmingham: Jose Porras, Phil Keenan

University of Surrey: Cathryn Hancock, John Varcoe, Bob Slade

Funding: