Friday, 3 November 2017

Energy of the Future - Solid Oxide Fuel Cells

FMD PhD student, Julia Ramirez Gonzalez gives us an overview of how Solid Oxide Fuel Cells operate and the importance of such materials in meeting future energy production challenges.

Can you imagine our daily life without electricity? Maybe we can deal with it for a couple of hours, but this commodity has become one of the engines of our generation [1]. Nevertheless, the environmental implications of producing energy by fossil fuel combustion, is the driving force to look for more efficient and environmentally friendly alternatives of electricity generation.

Fuel cells are one of the alternatives. These devices generate electricity by an electrochemical reaction of gaseous reactants. One of the reactants is the oxygen in the air, and the other is hydrogen or a hydrocarbon [2]. The configuration of this devices resembles a sandwich. It has two electrodes, one in contact with fuel (anode), and the other in contact with oxygen (cathode): These two interfaces are separated by and electrolyte. There are many types of fuel cells, which are classified by its type of electrolyte, such as polymer electrolyte membrane (PEM), alkaline (AFC), phosphoric acid (PAFC), molten carbonate (MCFC), and solid oxide (SOFC) [3]. Each type differs in its operation temperatures, useful fuel, efficiency and therefore its applications.

Research on SOFC showed that these materials can work with higher hydrocarbons, giving them the advantage of fuel flexibility. And the most important feature is that all of its components are solid, which avoids the risk of spillages, gives it the freedom for stacking configuration, and it is a quiet system as it does not have any moving parts [2][4] . 

But how does it work? On one side of the cell there is a high concentration of oxygen and on the other side there is none. An electrical potential gradient across the electrolyte is built up. However, the electrolyte does not allow electrons or gas to flow through, but its crystal structure has oxygen vacancies, which allows the migration of oxygen ions. Therefore, at the three-phase boundary, cathode-electrolyte-air, oxygen will be reduced, by obtaining electrons from the cathode [2], Equation 1. 




Eq. 1
Thus, oxygen ions can hop through the electrolyte and can reach the anode-electrolyte-fuel boundary, where the fuel oxidizes; where the products will be steam and electrons, Equation 2 [2].




Eq. 2


If the anode and cathode are connected by an external circuit, the cycle is repeat again as long as the two gases are present, and this is how electricity can be harvested through the external circuit. As shown in the video. 







These devices operate in the temperature range between 500-1000°C, which adds challenges to the material requirements. The materials need to have chemical stability, to avoid reaction with the reactants, a similar thermal expansion coefficient, to reduce the possibility of cracks during cycling; strength and toughness, no one likes a broken device; but also, it has to be easy to fabricate and have a low cost [4].

As you can see there are many requirements, but material scientists like these challenges and have come up with several options.

Yttria-stabilised zirconia is the most widely used as an electrolyte, which is zirconia doped with yttria (Zr1-xYxO2-x/2; x: 0.08). It can also be doped with calcium oxide, magnesium oxide, scandium oxide, neodymium oxide and ytterbium oxide. In addition, cerium oxide doped with samarium (SDC), gadolinium (GDC), and calcium (CDC); lanthanum gallate; bismuth yttrium oxide; barium cerate and strontium cerate, can be also used. The reason of the high operation temperatures of these devices is to promote the oxide ion conduction within the electrolyte [4].

Both electrodes have to be able to distribute the hydrogen and oxygen respectively, serve as catalysts and allow the flow of electrons. Therefore, the anode is a porous ceramic-metallic composite. The metal provides the electronic conduction pathway for the electrons, the ceramic made from the same material as the electrolyte assures a similar thermal expansion coefficient and good compatibility. The popular choice is Ni-YSZ, but there is also Ni-SDC and Ni-GDC [4]. 

For the cathode also a porous structure to allow the flow of gas usually made from a perovskite-type lanthanum strontium manganite (LSM), and lanthanum calcium manganite (LCM); also provides a similar thermal expansion. It is been discovered that by making a composite of perovskite and electrolyte increases the active sites for the electrochemical reactions [4]. 

These electricity generation systems have many possibilities and applications. It can be used as a combined heat and power plant, distributed generation, but also used in remote areas as the generation can be at the point of consumption, reducing the transmission costs. It has an efficiency of ~60% [3]. From an environmental point of view the CO2 emissions will be considerable reduced, Mike Manson an expert in SOFC from Manchester said to The Guardian that a 35% reduction in CO2 emission could be possible using this technology, comparing it with consuming electricity from gas power plant and hot water from a boiler [5]. There is also the idea of a hybrid/gas turbine cycle, to take advantage of the high temperatures of the existing power plants [3]. There are many companies that see the potential of this technology and have and R&D area dedicated to it, as Rolls-Royce and Bloomenergy [6][7]. 

In summary, SOFC are a great alternative for the generation of electricity, further research needs to be done to reduce its operations temperatures, but it represents a step closer to a greener electricity technology era.

[1]          “Key world energy statistics,” Int. Energy Agency, 2017.
[2]          R. J. Kee, H. Zhu, and D. G. Goodwin, “Solid-oxide fuel cells with hydrocarbon fuels,” Proc. Combust. Inst., vol. 30, pp. 2379–2404, 2005.
[3]          D. of Energy, “Comparison of Fuel Cell Technologies | Department of Energy.” [Online]. Available: https://energy.gov/eere/fuelcells/comparison-fuel-cell-technologies. [Accessed: 01-Nov-2017].
[4]          A. B. Stambouli and E. Traversa, “Solid oxide fuel cells (SOFCs): a review of an environmentally clean and efficient source of energy,” Renew. Sustain. Energy Rev., vol. 6, pp. 433–455, 2002.
[5]          D. Clark, “Manchester Report: Ceramic fuel cells | Environment | The Guardian,” The Guardian, 2009. [Online]. Available: https://www.theguardian.com/environment/2009/jul/13/manchester-report-fuel-cells. [Accessed: 30-Oct-2017].
[6]          Rolls-Royce, “Rolls-Royce grows global fuel cells capability with US acquisition – Rolls-Royce.” [Online]. Available: http://www.rolls-royce.com/media/press-releases/yr-2007/rr-grows-global-fuel-cells.aspx. [Accessed: 02-Nov-2017].
[7]          Bloomenergy, “Fuel Cell Energy - Solid Oxide Fuel Cells SOFC | Bloom Energy.” [Online]. Available: http://www.bloomenergy.com/fuel-cell/solid-oxide/. [Accessed: 02-Nov-2017].



Friday, 6 October 2017

Sustainability of Operation Research and Management

Lucy receiving her award from the Conference Chairs,
Prof. Ying Fan and Prof. Ernesto DR Santibanez.
The Second Global Conference on The Applications of Operations Research and Operations Management for Sustainability (GCTAOS) was held from the 6th to the 8th September at Beihang University in Beijing, China.

Lucy Smith was able to attend after receiving the Research Student’s Travel Grant from the Worshipful Company of Armourers and Brasiers and present research on the life cycle assessments of high and intermediate temperature Solid Oxide Fuel Cell material structures. In addition to this, the Advance Resource Efficiency Centre, led by Prof. Lenny Koh, were able to showcase the SCEnATi (Supply Chain Environment Analysis Tool) to an international audience.

A wide range of key note speakers gave their thoughts on issues such as smart systems, relief chain management, energy investment and technology evaluation. This paved the way for an interesting collection of presentations on similar topics from other academics.

The conference was attended by academics, student and industry representatives allowing for interesting discussion and future collaboration opportunities.

Lucy was awarded the Best Paper Award, in recognition of the professional excellence of the paper for ‘Life cycle assessments and environmental profile evaluations of high and intermediate solid oxide fuel cells’ by L. Smith, F. Yang, T. Ibn. Mohammed, I. M. Reaney, D. C. Sinclair and S. C. L. Koh.


Tuesday, 1 August 2017

Next Generation Force Fields - Designing force fields in an age of cheap computing - Perspective


This past week at Halifax Hall, the University of Sheffield was host to a workshop of experts and world leaders on computational chemical simulations. The aim of the workshop was to present the advances in this field, for the atomistic simulation of bulk materials, surfaces, interfaces, solutions, biomolecules, and more. It was also a chance for the delegates to discuss the next challenges facing the discipline given the increasing power of computers.

Chaired by Dr Colin Freeman, Dr Chris Handley, and Prof. John Harding, the invited speakers included Prof. Nohad Gresh, Prof. Jorg Behler, Prof. Bernd Hartke, Prof. Stefan Goedecker, Dr Peter Brommer, Prof. Paul Popelier, Dr Paul Richmond, and Dr David Mobley.

From the talks and contributions it was clear there are common challenges that all researchers face in the coming years. In terms of the technologies available, the push to GPU computing, driven by machine learning, means that the software used for chemical simulations must be reassessed if they are going to exploit the potential computer power on offer.

Another key issue is how machine learning is used to design new chemical simulations. Machine learning, such as neural networks, or Gaussian process regression, can be trained to extract the underlying non-linear relationship between atomic positions and some property of the system (we are often looking for the relative energy differences for a given configuration of atoms, and the associated forces as these are used to drive the molecular dynamics simulation). The machine learning methods use for training data quantum mechanical simulations. The danger however, is that while the non-linear model is discovered, we retain no information about the true physics at play, which is critical if we wish to have a deeper understanding of the interactions at play within a simulation.

Physics driven models are the alternative, but these models rely on knowing the proper functional forms for describing the interactions between atoms. Classical force fields often use functional forms that have been used over the decades that were initially chosen for computational convenience. Going forward our choice of functional forms used should be reassessed given that computational power is no longer an issue.
The effect of d-electrons on atomic configurations is an
electronic orbital effect and one that is not captured in
traditional force fields.


Related to the two previous points, is one of transferability. Force fields are often fitted, and thus simulate well, a particular chemical system in certain conditions. Knowing the limits of a particular model, and also how badly it will perform when if such conditions are met, is knowledge not often provided when a force field is published. Care must be taken that a model is capable of representing all the relevant physics that describes the system. There may be underlying physics not accounted for by the model explicitly, which hinders the transferability of the model when applied to similar chemical systems.

Transferability, data driven models, and physical driven models, ultimately are tied to how we partition the energy of a force field. By this we mean, how do we cut up the force field into different interactions, and how we even define these interactions. How do we define bonds? Can we accommodate reactivity into our models? What of electrostatic interactions? Should we use multipolar descriptions of charge?

Fitting and automation can be reliant on "wizardry". By that we mean, "to use a force field, and to design derivatives of it, how much highly specific expertise do we require - is the force field only really usable by those who designed it?" This is not ideal, as this slows the development of new models, and also hinders insight into why models work and fail for particular simulations.

Finally there is the topic of the reference data. More often than not, force fields are fitted to quantum mechanically generated reference data. We assume that this data is the "truth". Though, while many of the programs that perform quantum mechanical calculations have over time become closer in agreement with each other, they all share the same issue of accuracy. The fitting of force fields is thus a cyclic issue, where a reassessment of the training should be performed periodically.

Going forward the issues raised during the workshop will help inform CECAM and EU funding on future force field development and how it plays a critical role in computational chemistry, even in an age of high performance computing.

As organisers of the workshop, Colin, Chris and John would like to thank CECAM and CCP5 for the generous funding for the event, the invited speakers for their informative presentations, and the attendees whom we hope will have taken away new ideas and new future collaborations.



Conference Attendees

Wednesday, 12 July 2017

Advanced Resource Efficiency Centre (AREC) Showcase in the European Parliament




On 27th June 2017, the AREC team, led by Professor Lenny Koh, showcased its research and impact at the ‘Pathways to Global Policy, Industry and Societal Impact on Resource Efficiency and Sustainability’ event at the European Parliament, Brussels.

The event was hosted by John Procter, MEP for Yorkshire and Humber, through the White Rose Brussels group (https://brussels.whiterose.ac.uk/) and attended by policy makers, industry representatives and academics. The main aims of the event were to present the impact of AREC’s research and develop potential future connections to further extend AREC’s impact reach.


The event included a panel discussion from Professor Lenny Koh, Professor Panos Ketikidis (Vice Principal: Research and Innovation, International Faculty of the University of Sheffield in Thessaloniki, Greece), Jay Sterling Gregg (European Energy Research Alliance, representing “e3s”, Brussels, Belgium), Philippe Micheaux Naudet (Association of Cities and Regions for Sustainable Resource Management – ACR, Brussels, Belgium) and Maria Rincon-Lievana (Policy Officer – Circular Economy Action Plan, DG Environment, Brussels Belgium).

Professor Koh presented the SCEnATi (Supply Chain Environmental Analysis) tool to the group of industry specialists and academics. The SCEnATi tool is used within the FMD group to produce comparative hybrid life cycle assessments of functional materials and devices.

Professor Koh commented “Being resource efficient and sustainable should be embedded as a new norm in every supply chain, every business and every organisation whether these are public, private or third sectors. Policies that support this goal, industry practices that promote such implementation, technologies/tools that enable this achievement, and research and innovation that underpin the delivery of this new norm would lead to positive societal, economic and environmental impact”.


For further information on the SCEnATi tool please contact: Lucy Smith, lsmith17@sheffield.ac.uk

Thursday, 6 July 2017

Full Tilt - Why it Matters in Matter

"Tilting" in perovskites is all about the subtle arrangements of atoms in materials, which is inherently related to the properties of materials, such as capacitors and piezoelectrics. Tilt is also something we can control, by doping a material with another type of atom. By controlling tilt, we can design novel materials.

http://www.nature.com/nphoton/journal/v8/n7/full/nphoton.2014.134.html
I am not a materials scientist by training, but a computational chemist with a background in simulations of water, peptides, and using machine learning. So I don't think of atomic structures are rigid arrangements of atoms, but being dynamic. So I see perovskites not as neat octahedral units of B site atoms surrounded by oxygen atoms (or whatever the X site atom happens to be). In reality these octahedral units are irregular, and fluctuating, especially at ambient conditions and when heated. But if you only consider X-ray diffraction determined crystal structures, you may be led into thinking the opposite.

So what do we mean by "tilt"?

Tilt means that the octahedral units that we have defined, are aligned in a manner that means the octahedral units either do, or do not, superimpose upon their neighbours. This tilt is classically defined by Glazer (using a frustrating description of rotation if you prefer Euler angles!), and from which we get different crystal structure classifications that differ by the manner the octahedral units tilt and overlap.

In the material calcium titanate, all the A site atoms are barium, the B site titanium, and X sites are oxygen. At high temperatures calcium titanate exhibits no tilt. It's cubic. But when we start to dope the material on the A site, with larger or smaller ions, such as barium, we begin to distort the structure. Or if we cool the material down, tilting emerges.



AA3B4X12 perovskite structure showing the octahedral environment of the B cation
http://pubs.rsc.org/en/Content/ArticleHtml/2010/JM/b926757c

Distorting the material has a knock on effect on the ions in the material. The titanium now no longer sits in an isotropic (so a fully symmetric and even) electrostatic field created by the oxygen atoms about it. This means the titanium atoms get shifted. The same happens with the calcium ions too.

It's this combination of distortion that generates a dipole moment - a displacement of electrostatic charge in a particular direction within the crystal structure.

So what is the challenge in materials science?

Exploring how we can dope materials, and manipulate this tilting, in a targeted manner, relies on experiment and theory working in tandem. X-ray diffraction defined structures do not show the oscillations but use structure factors to account for thermal scattering that induces oscillations of the atomic positions. From Transmission Electron Microscopy (TEM) we can generate diffraction patterns which can show this oscillation of structure. And from theory, via simulations of atoms via Molecular Dynamics, we can assess the degree of tilting (not defined by Glazer), and begin to predict TEM diffraction patterns.

The hope then is that a combination of techniques, both experimental and theoretical, can reveal further insight into the complex relationship of atomic structure and materials properties.


 Atomic resolution image of 2D halide perovskite CsPbBr 3 . (a) Structure model of cubic CsPbBr 3 perovskite unit cell. Cs (green) occupies the corner A-site while Pb (gray) occupies the body-center Bsite , and Br (brown) occupies the face-center. Pb−Br 6 octahedron is formed within the Cs cube framework. (b) Structure model of single layer 2D CsPbBr 3 NS. (c) Atomically resolved phase image of a 2D CsPbBr 3 NS obtained by reconstructing 80 low dose-rate AC-HRTEM images via exit-wave reconstruction. The [001] structure projection of a unit cell is overlaid on the image.
http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.6b03331  








Wednesday, 7 June 2017

Functional Materials for a Sustainable Future

Last month, on the 15th of May, the University of Sheffield Functional Materials and Devices group hosted a workshop centred on the topic of "Functional Materials for a Sustainable Future". A diverse number of speakers from out own group, and collaborators, and industrial partners, gave insightful and exciting talks on how a range of materials can be designed and fabricated, for use in a wide range of applications.

Presentations ranged from magnetic materials for cooling, solar power materials, and the use of computational simulations to model novel materials. The workshop was also an opportunity for the FMD group to demonstrate the value of KTP (Knowledge Transfer Partnerships) whereby researchers can pursue underpinning research to enable novel materials discovery and applications.

Guests included representatives from QinetiQ, Johnson Matthey, CeramTec, Rolls-Royce, and more (a full list can be found on the event page).

























Functional materials in Japan

The assembled attendees
Between the 29th and 31st of May, the 8th International Conference on Electroceramics (ICE) was held at Nagoya University in Japan.

Topics covered at the conference encompassed most of the oxide functionalities, including piezoelectrics, thermoelectrics and ferroelectrics. Plenary lectures included Prof John Kilner (Imperial), Prof Harry Tuller (MIT), and Dr Nava Setter (EPFL).

Attendees included academics, students and industry representatives, which enabled some interesting discussions about the future directions of functional materials.  

Becky receiving her prize
Becky attended as a speaker, talking about her work on control of morphology in barium titanate, for which she won a Young Presentation Prize.

There was also a moving memorial symposium for the late Prof Eric Cross (PSU) a pioneer in the ferroelectrics field, who passed away at the end of 2016, with contributions from former students and colleagues.