• H2020
• Special Reports

Energy harvesting from ambient vibration sources for self-powered micro/nanoelectronic devices

Harvesting energy from ambient sources in our environment has generated tremendous interest as it offers a fundamental energy solution for ‘small power’ applications, including, but not limited to, ubiquitous wireless sensor nodes; portable, flexible and wearable electronics; biomedical implants and structural/environmental monitoring devices. As an example, consider that the number of smart devices linking everyday objects via the internet is estimated to grow to 50 billion by the year 2020. Most of these Internet of Things (IoT) devices will be extraordinarily small and most likely embedded, and will wirelessly provide useful data that will make our lives easier, better and more energy efficient. The only sustainable way to power them is using ambient energy harvesting that lasts throughout the lifetime of the product.

Vibrational energy harvesting

Energy harvesting from ambient vibrations is particularly attractive as these are ever present and easily accessible, originating from sources such as moving parts of machines, fluid flow and even body movements. Indeed, we are at a unique stage in the evolution of modern electronics where the power consumption of devices has been reduced to such an extent that it is now feasible to power them from ubiquitous small scale vibrations in the environment. The time is thus ripe for innovative ways to develop and exploit this vital technology towards next-generation self-powered electronics.

In this context, piezoelectric materials offer the simplest means of directly converting mechanical vibrations into electrical power and are well suited for microscale device applications, thus offering a means of superseding traditional power sources such as batteries that require constant replacing/recharging and that do not scale easily with size. In particular, nanoscale piezoelectric energy harvesters, or nanogenerators, are capable of converting small ambient vibrations into electrical energy, thus paving the way for the realisation of the next generation of self-powered devices, with profound implications in far-reaching areas such as smart city planning, health, robotics, environmental and structural monitoring, resource management and sustainable development.

Review

A recent review article from our group¹ highlighted the fact that nanogenerator research to date has mainly focused on traditional piezoelectric materials in the form of ceramics, but these are stiff and prone to mechanical failure. On the other hand, piezoelectric polymers, although less well studied, have several advantages over ceramics such as being flexible, robust, lightweight, easy and cheap to fabricate, lead free and bio compatible. However, they do suffer from inferior piezoelectric properties in comparison to ceramics.

As such, while the field of piezoelectric nanogenerators has witnessed tremendous growth over the last few years, it is dominated by ceramics such as zinc oxide, with research in polymer nanogenerators remaining limited and restricted to just one family of polymers, namely polyvinylidene-fluoride and its copolymers. This field currently faces orthogonal difficulties associated with these two classes of materials. In order to move forward, our group aims to develop novel hybrid polymer-ceramic nanocomposites combining the best of both materials.

Such hybrid systems offer plenty of scope for innovation, and thus our strategy involves combining i) materials engineering to create novel piezoelectric hybrid nanomaterials with enhanced energy harvesting functionalities; ii) state-of-the art nanoscale characterisation to explore and exploit these novel materials; and iii) fabrication of high performance nanogenerators for implementation into commercial devices using insight gained from the computational modelling of materials and device parameters.

Prototype energy harvesting device based on piezoelectric
polymer nanowires

In terms of prototype devices, recent work from our group² on polymer-based nanogenerators has resulted in significant advances in the field, notably, the development of scalable ‘self-poled’ nanogenerators based on piezoelectric polymer nanowires grown by a simple and versatile template-wetting fabrication method. This cost-effective technique is well suited for large scale production and results in nanowires with high energy conversion efficiency. We demonstrated a small prototype nanogenerator that produces an electrical output when lightly tapped. We showed that this outputted electrical energy can be stored in a bank of capacitors and then used to light a commercial LED.

While there have been several reports in the literature on nanogenerators based on both piezoelectric ceramic and polymer nanowires, key performance parameters such as mechanical-to-electrical conversion efficiency and frequency dependence have been largely unreported, making it impossible to compare materials performance for practical applications. We have recently defined novel and bespoke figures of merit with the aim of catalysing a more focused and meaningful discussion of these promising materials and devices, thus enabling the field to mature.³

Our analysis is straightforward and covers all realisable nanogenerator-driving mechanisms, enabling a rigorous quantitative comparison between nanogenerator materials that has been hitherto absent in this highly technologically relevant field. We investigated, in detail, the energy harvesting performance of prototypical piezoelectric ceramic and polymer nanowires. Our studies revealed that even though ceramic and polymer nanowires have been found, in certain cases, to have similar energy conversion efficiencies, ceramics are more promising in ‘strain-driven’ nanogenerators, while polymers are more promising for ‘stress-driven’ nanogenerators. Our approach therefore highlights the differences in the energy harvesting performance of piezoelectric polymer and ceramic nanowires under different driving conditions, revealing that these two material classes present orthogonal opportunities which, while important, have not been elucidated before. As nanogenerators begin to make the transition from the lab to the real world, our work offers a timely and highly relevant benchmark by which piezoelectric materials and nanogenerator geometries can be designed and/or engineered for specific applications.

ERC starting grant – NANOGEN

With the recent award of an ERC starting grant on ‘Polymer-based piezoelectric nanogenerators for energy harvesting’ (NANOGEN), I aim to cement my position as a leader in the global energy harvesting community, with a world class research group that can impact the way smart technology interfaces with our lives in the future. In fact, the impact of this research goes well beyond energy harvesting as piezoelectric nanowires are extremely strain sensitive and can function as self-powered sensors.

In this context, polymer-based piezoelectric nanogenerators, as power sources for ‘reporting devices’, have applications in transportation, healthcare management, smart cities and for the community at large through passive reporting via wireless sensor networks or the IoT. Additionally, piezoelectric nanowires are particularly robust and can be stimulated by tiny physical motions/disturbances over a large frequency range, thus presenting the opportunity for a new class of strain-sensitive devices for active reporting on critical elements. The polymer based nanowires developed over the course of the starting grant will therefore be attractive for applications ranging from monitoring vital signs, early fault detection systems in buildings, and critical element reporting in aerospace applications to piconewton-scale force sensing in biological systems.

References

1. S Crossley, R A Whiter & S Kar-Narayan (2014): Polymer-based nano-piezoelectric generators for energy harvesting applications, Materials Science and Technology 30, 1613
2. R A Whiter, V Narayan & S Kar-Narayan (2014): A scalable nanogenerator based on piezoelectric polymer nanowires with high energy conversion efficiency, Advanced Energy Materials 4, 1400519
3. S Crossley & S Kar-Narayan (in press 2015): Energy Harvesting Performance of Piezoelectric Ceramic and Polymer Nanowires, Nanotechnology

Sohini Kar-Narayan
University Lecturer
Department of Materials Science
University of Cambridge

tel: +44 1223 331695

[email protected]
http://people.ds.cam.ac.uk/sk568