Development of perovskite-like structures for hydrogen production via two-step thermochemical water splitting
File(s)
Author(s)
Wilson, George E.
Type
Thesis
Abstract
Hydrogen powered technologies are proposed to help mitigate climate change as low carbonemitting
technologies. Devices such as fuel cells convert the chemical energy stored within
hydrogen molecules via electrochemical redox processes to electrical energy for work. These
technologies have the primary benefit of not emitting carbon dioxide – one of the main
contributing pollutants towards the greenhouse effect. However, current commercial hydrogen
production technologies require fossil fuel reactants and emit carbon dioxide as a product.
Therefore, research into ways of producing hydrogen from sustainable non-polluting sources
has been of keen interest within the scientific community. One such technique is high
temperature thermochemical water splitting. This process uses renewable concentrated solar
power to heat up and thermally reduce metal oxide compounds and induce an oxygen nonstoichiometry
within the lattice. The oxygen deficiency is then removed upon reoxidising with
steam and producing hydrogen gas.
Numerous thermochemical redox cycles have been proposed within the literature with the main
aim to lower the reduction temperatures and increase the hydrogen production volumes. This
has turned the attention of the field to investigate the ABO3 perovskite structures due to their
ability to support a larger oxygen deficiency at lower temperatures compared to the benchmark
material, cerium oxide, CeO2. This thesis combines theoretical first principle approaches and
a wide range of experimental techniques to understand and discuss three different families of
perovskite and perovskite-like metal oxide structures.
The main findings of this thesis can be summarised as the following:
Effect of antimony incorporation on the redox kinetics of SrCoO3-d
- Thermal analysis techniques observe large oxygen production volumes onset between
300 and 400 °C under an inert gas flow with increased antimony content lowering total
production.
- Density Functional Theory (DFT) confirms the low reduction enthalpy in the region of
0.5 eV/O atom. Increased Sb concentration and proximity to the dopant increases
vacancy formation energy.
6
- Low reduction enthalpy of the material was not favourable to drive thermochemical
water splitting, however isothermal redox cycling demonstrated good performance for
the alternative application of thermochemical oxygen separation compared to literature
materials.
- Antimony donor ions are postulated to lower the cobalt crystal field splitting to support
an intermediate spin electron configuration with more favourable orbital filling for fast
redox kinetics (eg=1).
Effect of iron incorporation in (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites for thermochemical
water splitting
- Thermal analysis used to observed increasing Fe content coincides with an increase the
oxygen production volumes and rates
- DFT used to confirm lower vacancy formation energy in positions neighbouring Fe
cations. Further predicted to have favourable thermodynamic properties for
thermochemical water splitting.
- Thermochemical water splitting observed hydrogen production rates similar to
literature materials, Ce0.75Zr0.25O2-d.
- Surface analysis techniques novel to this research field revealed increased strontium
segregation towards the surface that prevented cyclability of the compounds.
- Strontium-enriched perovskite surfaces can undergo reconstruction to form derivative
phases such as Ruddlesden-Popper oxides, An+1BnO3n+1.
Computational screening of n=1 Ruddlesden-Popper oxides for thermochemical water
splitting
- Screening study uses a combination of well-known crystallographic principles and DFT
simulations to narrow down the field of this underexplored metal oxide family for use
in thermochemical water splitting.
- From an initial 27,899 structures, this study outlines a potential 30 A2BO4 Ruddlesden-
Popper structures that have favourable reduction thermodynamics and “synthesisable”
under laboratory conditions.
- A new simpler and better fitting descriptor based on the lattice enthalpy is proposed to
assist future screening work of Ruddlesden-Popper oxides at significantly reduced
computational expense.
Investigating Ca2MnO4 Ruddlesden-Popper oxide for thermochemical water splitting
- Outputted compound from the prior screening study is explored further due its abundant
constituent elements and favourable reduction thermodynamics.
- Thermal analysis techniques observe similar oxygen production behaviour to the
(La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites investigated in a previous chapter.
- Hydrogen was successfully produced via thermochemical redox reactions cycling
between 1000 and 800 °C, thus experimentally verifying the screening study.
- Further improvements are suggested by including doping ions to alter the
thermodynamics or investigating the effect of perovskite/Ruddlesden-Popper
heterostructures that have previously been observed to accelerate oxidation reactions.
technologies. Devices such as fuel cells convert the chemical energy stored within
hydrogen molecules via electrochemical redox processes to electrical energy for work. These
technologies have the primary benefit of not emitting carbon dioxide – one of the main
contributing pollutants towards the greenhouse effect. However, current commercial hydrogen
production technologies require fossil fuel reactants and emit carbon dioxide as a product.
Therefore, research into ways of producing hydrogen from sustainable non-polluting sources
has been of keen interest within the scientific community. One such technique is high
temperature thermochemical water splitting. This process uses renewable concentrated solar
power to heat up and thermally reduce metal oxide compounds and induce an oxygen nonstoichiometry
within the lattice. The oxygen deficiency is then removed upon reoxidising with
steam and producing hydrogen gas.
Numerous thermochemical redox cycles have been proposed within the literature with the main
aim to lower the reduction temperatures and increase the hydrogen production volumes. This
has turned the attention of the field to investigate the ABO3 perovskite structures due to their
ability to support a larger oxygen deficiency at lower temperatures compared to the benchmark
material, cerium oxide, CeO2. This thesis combines theoretical first principle approaches and
a wide range of experimental techniques to understand and discuss three different families of
perovskite and perovskite-like metal oxide structures.
The main findings of this thesis can be summarised as the following:
Effect of antimony incorporation on the redox kinetics of SrCoO3-d
- Thermal analysis techniques observe large oxygen production volumes onset between
300 and 400 °C under an inert gas flow with increased antimony content lowering total
production.
- Density Functional Theory (DFT) confirms the low reduction enthalpy in the region of
0.5 eV/O atom. Increased Sb concentration and proximity to the dopant increases
vacancy formation energy.
6
- Low reduction enthalpy of the material was not favourable to drive thermochemical
water splitting, however isothermal redox cycling demonstrated good performance for
the alternative application of thermochemical oxygen separation compared to literature
materials.
- Antimony donor ions are postulated to lower the cobalt crystal field splitting to support
an intermediate spin electron configuration with more favourable orbital filling for fast
redox kinetics (eg=1).
Effect of iron incorporation in (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites for thermochemical
water splitting
- Thermal analysis used to observed increasing Fe content coincides with an increase the
oxygen production volumes and rates
- DFT used to confirm lower vacancy formation energy in positions neighbouring Fe
cations. Further predicted to have favourable thermodynamic properties for
thermochemical water splitting.
- Thermochemical water splitting observed hydrogen production rates similar to
literature materials, Ce0.75Zr0.25O2-d.
- Surface analysis techniques novel to this research field revealed increased strontium
segregation towards the surface that prevented cyclability of the compounds.
- Strontium-enriched perovskite surfaces can undergo reconstruction to form derivative
phases such as Ruddlesden-Popper oxides, An+1BnO3n+1.
Computational screening of n=1 Ruddlesden-Popper oxides for thermochemical water
splitting
- Screening study uses a combination of well-known crystallographic principles and DFT
simulations to narrow down the field of this underexplored metal oxide family for use
in thermochemical water splitting.
- From an initial 27,899 structures, this study outlines a potential 30 A2BO4 Ruddlesden-
Popper structures that have favourable reduction thermodynamics and “synthesisable”
under laboratory conditions.
- A new simpler and better fitting descriptor based on the lattice enthalpy is proposed to
assist future screening work of Ruddlesden-Popper oxides at significantly reduced
computational expense.
Investigating Ca2MnO4 Ruddlesden-Popper oxide for thermochemical water splitting
- Outputted compound from the prior screening study is explored further due its abundant
constituent elements and favourable reduction thermodynamics.
- Thermal analysis techniques observe similar oxygen production behaviour to the
(La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites investigated in a previous chapter.
- Hydrogen was successfully produced via thermochemical redox reactions cycling
between 1000 and 800 °C, thus experimentally verifying the screening study.
- Further improvements are suggested by including doping ions to alter the
thermodynamics or investigating the effect of perovskite/Ruddlesden-Popper
heterostructures that have previously been observed to accelerate oxidation reactions.
Version
Open Access
Date Issued
2022-03
Date Awarded
2022-08
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Aguadero, Ainara
Cavallaro, Andrea
Skinner, Stephen
Sponsor
Engineering and Physical Sciences Research Council (EPSRC)
Grant Number
EP/L015749/1
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)