Additive manufacturing – The printing press meets energy storage?

At today’s energy seminar we hosted Nick Farandos, a PhD student in the Department of Chemical Engineering. His talk looked at whether 3D printing could provide the key to scalable, economical, and reproducible energy storage devices. He has written us this complementary blog post covering the same topic.

Nick Farandos

The contribution of renewably sourced electricity into distribution grids across the globe is increasing. In addition, location-flexible resources, such as solar and wind, are increasing in efficiency and rapidly dropping in cost which is all great news for the climate and national energy security.[1] Unfortunately, with the rise in renewables, we can expect a fall in their grid penetration, defined as the amount of electricity that they are capable of producing that is actually used. This is due to their inherent intermittency; i.e. solar panels only produce electricity during daylight hours. To demonstrate this, it is worth considering a real-life case.

Figure 1. Electricity supply in Portugal, by source between the 7th-11th May, 2016.

In May 2016, Portugal claimed to run on 100 % renewably-generated electricity, as shown in Figure 1. This is great, and indeed true; however, the data tells an additional story. As you can see, across the entire period, the renewable electricity supply (red line) exceeded demand (blue line). The difference between the two is the excess electricity generation. We can see that this was all exported (green line) to Spain. Interestingly, if Portugal had been able to store the excess electricity, and reconvert when required at an efficiency of 70 %, they could have run on renewables for another 22 hours, not considering any additional renewable generation over the period.

Figure 2. Operation principle of a solid oxide electrolyser. Imperial College London ©

Several energy storage technologies exist, the largest with current utility is hydroelectric storage, however this is neither location-flexible or environmentally friendly. For long-term energy storage (hours – seasons), electrochemical devices that convert excess electricity into a synthetic fuel that can be stored, and reconverted into electricity when required, are attractive options. Within this category, reversible solid oxide fuel cells and electrolysers represent attractive options for many reasons including: high power densities, efficiency, and wide fuel flexibility, combined with low electrical energy requirements due to high-temperature operation (600 – 800C) and capital costs due to the non-necessity for precious-metal catalysts. Figure 2 shows their principle of operation.

 

Our Research

Here in the Electrochemical Engineering group based in the Department of Chemical Engineering, we are researching the possibility of overcoming the limitations of solid oxide fuel cells and electrolysers using 3D printing. A significant limitation of these devices is that predicting certain performance metrics, most importantly the electrical power and device lifetime, is challenging, and currently is near-impossible to do without destroying the reactor. This limitation derives from the fabrication techniques conventionally employed to make the electrodes, which typically involve mixing powders of the constituent electrode phases and processing them to create the electrode (i.e. tape casting and screen printing). Due to the powder-mixing step, the phases within the electrode are typically randomly mixed, and therefore a random electrode microstructure results, as shown in Figure 3, when there are 2 electrode phases, e.g. the electronic and ionically conducting phases in green and orange, respectively.

Figure 3. The random electrode microstructure that results from conventional fabrication techniques (left), and the
reproducible structure possible using 3D printing (right). Imperial College London ©

We are using 3D inkjet printing to fabricate the electrodes as it is a rapid, manufacturing-compatible technique with high resolution (of the order 10s of micrometres) and low material waste. There are a number of research challenges to develop this technique, which include:

• formulation of printable inks for the electrode materials
• optimisation of inks to minimise printed feature size
• deposition of structures that survive subsequent processing steps (i.e. sintering)
• electrochemical testing of structures to validate this technique

We are focussing on the most common materials for the devices: ytttria-stabilised zirconia (YSZ) for the electrolyte (ionically-conducting) phase, nickel for the fuel-side  (electronically-conducting) electrode, and lanthanum strontium manganite for the  (electronically-conducting) oxygen-electrode. Some of the structures we have fabricated are shown in Figure 4.

Figure 4. Yttria-stabilised zirconia (YSZ) square scaffold (top) and pillars (bottom) fabricated by 3D inkjet printing from DOI:10.1016/j.electacta.2016.07.103.

On top of these structures, the LSM is added to complete the electrode, and electrochemically tested he cells in an air. Our results indicate that the performance is  reproducible between cells, which has allowed us to create finite element analysis, multi-physics models that we have used to predict how the microstructure affects current density (electrical power), and subsequently optimise the design of structures that we can print. Next, we aim to print the LSM, which will allow us to not only predict current densities, but also the exact location of the reaction sites to predict device lifetime.