EES Blog

Nanophotonic concepts, ranging from photonic crystals to nanowire arrays, are of tremendous interest in both solid state and photoelectrochemical (PEC) energy conversion devices.

Rinnerbauer et al, in 2012, presented a review on an emerging direction in photonics, high-temperature nanophotonics using large area 2D metallic photonic crystals aimed at solid-state energy conversion. Recently, Hu et al have shown impressive optical results in GaAs nanowire array photoanodes (for PEC devices) that occupy <5% of the fractional area of the electrode, due to efficient incoupling into the resonant waveguide modes. Despite the differences in their intended application, both papers share a common theme: use of fundamental yet scalable nanophotonic concepts in energy conversion.

Photonic crystals (PhCs) are periodically nanostructured metamaterials with unique optical properties and flexibility in the design of their density of states. In their review, Rinnerbauer et al demonstrate the efficiency of selective emitters based on 2D photonic crystals in refractory metals (like Tantalum and Tungsten). These PhCs are paving the way for high efficiency high temperature thermal-to-electricity conversion schemes based on selective emitters and absorbers, like TPV, solar thermal and solar TPV.

Work by Hu et al shows that enhanced optical absorption, in conjunction with radial carrier-collection, results in high external quantum yields (EQYs) when the periodic nanowire arrays of GaAs are used as energy-conversion devices. The key to the absorption properties of these arrays is guided and leaky optical waveguide modes determined through a combination of experimental and theoretical analysis.

Nanophotonic device design certainly has a big role in the future of energy-conversion. A variety of schemes have been proposed and demonstrated in both solid state and PEC systems and these papers represent critical steps in nanophotonic energy conversion.

By Prineha Narang

Read the papers here:

Recent developments in high-temperature photonic crystals for energy conversion
Veronika Rinnerbauer, Sidy Ndao, Yi Xiang Yeng, Walker R. Chan, Jay J. Senkevich, John D. Joannopoulos, Marin Soljacic and Ivan Celanovic
DOI: 10.1039/c2ee22731b

Optical, electrical, and solar energy-conversion properties of gallium arsenide nanowire-array photoanodes
Shu Hu, Chun-Yung Chi, Katherine T. Fountaine, Maoqing Yao, Harry A. Atwater, P. Daniel Dapkus, Nathan S. Lewis and Chongwu Zhou
DOI: 10.1039/c3ee40243f

Interested in photonics research? See our collection of articles on photonics that are free to access for a limited time here.

A novel method developed by energy researchers offers a way to assess sustainability of chemical processes at an early stage.

When developing novel chemical conversions, researchers have faced the challenge of assessing and predicting the broader environmental and economic impact of their work. Previously, no defined method was available for comparing criteria for overall sustainability of the new processes at an early state of development. Patel et al. have developed a multi-criteria method to assess future sustainability that can be applied at an early stage to help guide innovation.

Early stage process development is a time when there is flexibility to pursue sustainable options. In a recently published EES paper, Patel et al. propose that their method can help researchers analyze the processes they are developing within a broader economic, environmental, and social context. The method described in the paper is a data-based assessment tool for chemists to determine comparative sustainability of different processes. The criteria assessed are environmental constraint, environmental impact of raw materials, process costs and environmental impact, EHS index, and risk aspects. The paper describes the method for applying these criteria to chemical processes to create a score by which different processes can be compared.

Patel et al. apply their method to a catalytic process for the production of but-1,3-diene from ethanol that is currently being developed, and compare it to the dominant conventional method for production of but-1,3-diene from naphtha in a steam cracker. When they applied their method to these processes, the bioethanol-based process scored better than the petrochemical process overall, and the scores for the various criteria were in line with known data. While the input data may be less well defined for novel processes, that this method seems to be predictive and provides a comparison of different processes suggests that in most cases it can be useful for providing a broad assessment of sustainability factors, and can therefore aid early stage decision making.

It’s great that we’re at a point where there is encouragement for researchers to consider sustainability issues at all stages of development, and that people are thinking about ways to make it practical to do so. I think this new method will prove to be a concrete tool for researchers developing novel chemical conversion processes to start thinking about sustainability on the front end. Not only will it provide a relatively quick way to assess new processes in terms of sustainability, it can help guide research targets and goals, prompt consideration of potential alternatives, and encourage researchers to think about their results in a broader context.

Read about the details of this new approach in the full EES article here:

Sustainability assessment of novel chemical processes at early stage: application to biobased processes
Akshay D. Patel, Koen Meesters, Herman den Uil, Ed de Jong, Kornelis Blok and Martin K. Patel
Energy Environ. Sci., 2012, 5, 8430
DOI: 10.1039/c2ee21581k

By Paige Johnson

Take a look at this week’s selection…

Direct prediction of the desalination performance of porous carbon electrodes for capacitive deionization
S. Porada, L. Borchardt, M. Oschatz, M. Bryjak, J. S. Atchison, K. J. Keesman, S. Kaskel, P. M. Biesheuvel and V. Presser
DOI: 10.1039/C3EE42209G, Paper

The identification, characterization and mitigation of defect states in organic photovoltaic devices: a review and outlook
John A. Carr and Sumit Chaudhary
DOI: 10.1039/C3EE41860J, Review Article

Efforts made at Clarkson University have demonstrated implantable biofuel cells that can be used to one day generate power for medical implants in humans.

In their most recent feature article, Katz and coworkers have showcased their work towards an implantable energy generation device that could one day be made compatible in humans. One of the largest problems in making implantable biofuel cells practical is generating enough voltage and current to power a device. Using electrodes comprised of compressed multi-walled carbon nanotubes, the researchers have been able to generate 470 mV open circuit voltage, at a current of 5 mA short circuit. Coupled with a DC-DC converter, it was enough power to operate a pacemaker from a single device.

The article goes into detailed explanation, refreshingly, about the current constraints and considerations to be made in order to advance this technology further. One such aspect is to improve the amount of active enzymes on the electrode surface, which is currently only measured at 6% electrically active on the electrodes. By improving this single factor, it is believed that the current density could be large enough for a small (0.5 cm² electrodes) implantable device.

It is postulated in the paper that one day miniature devices could be implanted in the human brain, where a constant source of glucose fuel could be delivered in order to power devices. Who knows, we may soon be pouring over literature from the comfort of our own cranium computers!

Interested? Read the full communication in Energy and Environmental Science here:

Implanted biofuel cells operating in vivo – methods, applications and perspectives
Evgeny Katz, Kevin MacVittie
DOI: 10.1039/C3EE42126K

By David Novitski

Thermoelectric power generation technology aims to harvest and convert waste heat into electricity. Researchers are looking for not toxic, abundant and inexpensive new materials with good thermal and chemical stability in air at high temperatures.

Quaternary oxyselenides (BiCuSeO) have been addressed as a promising thermoelectric system to use in sustainable energy generation from waste heat. Thermoelectric conversion efficiencies for these materials is described by the dimensionless figure of merit, ZT, which has reached a maximum value of about 1.4. Textured Bi_0.875Ba_0.125CuSeO has outperformed this value due to its low thermal conductivity and improved electrical transport properties, which has been achieved by increasing carrier concentration through optimising dopants.

In a recent article by Li-Dong Zhao and co-workers, a hot-forging process was used to produce grains oriented along a preferential axis. A higher carrier mobility along the direction perpendicular to the pressing direction was also obtained. The microstructure was then observed using X-ray diffraction (XRD) and scanning electron microscopy (SEM).

The highest ZT value ever reported for oxygen containing materials was achieved making oxyselenides a valid candidate for medium temperature thermoelectric applications.

Interested in a better understanding about this field? Read more from the Communication article:

Texturation boosts the thermoelectric performance of BiCuSeO oxyselenides
Jiehe Sui, Jing Li, Jiaqing He, Yan-Ling Pei, David Berardan, Haijun Wu, Nita Dragoe, Wei Cai and Li-Dong Zhao
DOI: 10.1039/C3EE41859F, Communication

By Martina Congiu

Take a look at this week’s selection…

Energy applications of ionic liquids
Douglas R. MacFarlane, Naoki Tachikawa, Maria Forsyth, Jennifer M. Pringle, Patrick C. Howlett, Gloria D. Elliott, James H. Davis, Masayoshi Watanabe, Patrice Simon and C. Austen Angell
DOI: 10.1039/C3EE42099J, Perspective

Diffusion and adsorption of dye molecules in mesoporous TiO2 photoelectrodes studied by indirect nanoplasmonic sensing
Viktoria Gusak, Leo-Philipp Heiniger, Vladimir P. Zhdanov, Michael Grätzel, Bengt Kasemo and Christoph Langhammer
DOI: 10.1039/C3EE42352B, Paper

Conductive polymer hydrogels (CPHs) are of significant interest for a broad range of applications, including biosensors, lithium batteries, supercapacitors and biofuel cells.

The cutting edge of the field is in creating 3D nanostructured CPHs for electrochemical devices, and a recent review by Lijia Pan, Guihua Yu et al. focuses on new synthetic techniques to make these nanostructured materials, along with a discussion of their potential applications.

One of the greatest strengths of the CPHs is their versatility.  Many different conductive polymers, such as PEDOT, PAni, PPy and PTH can be used as the electrical backbone of the material, and there are many templating techniques (such as inkjet printing, copolymerization and template-directed growth) that can be used to achieve the desired morphology and electrical properties.

An exciting new synthetic route to nanostructured CPHs is gelation of a PAni network using phytic acid as the gelator and dopant.  The interactions between the phytic acid molecules and the PAni chains allows for the formation of “foam-like nanostructures which are constructed with coral-like dendritic nanofibers with uniform diameters of 60–100 nm).”  The PAni chains allow for excellent electrical conductivity and the as-prepared hydrogel is suitable for spray coating and inkjet printing.  PAni hydrogels as high-performance electrodes for glucose sensors and supercapacitors have been demonstrated, and other CPHs show great promise for battery electrodes, in drug delivery devices, and in the removal of free radicals from the environment.

Current solar cell research focuses on increasing the power conversion efficiency (PCE) by employing multiple junctions. A new review by Adebanjo et al. provides an overview of the recent results.

The usage of multiple junctions to cover a broader range of the solar spectrum is used widely in III-V semiconductor solar cells. This approach has been transferred to organic polymer solar cells. By stacking several cells on top of each other, thermalisation losses are reduced and the open circuit voltage is improved. Achievable PCE values are calculated by the authors of the paper “Triple Junction Polymer Solar Cells”. Depending on the band gap of the materials used, maximum values are predicted to be 15% to 23%.

The article also reviews current materials used as active and interconnecting layers. Fullerenes are commonly employed as acceptor materials. Donor materials are most often polymers like P3HT. These can be tuned by structural modification for better absorption of different light spectra. Metal-oxides or transition-oxides are finally used for recombination and injection layers.

Current PCE’s do not reach the maximum value, e.g. 9.64% for a triple junction instead of 22.3%. Adebanjo et al. point out that the design of absorbing polymers, the selection of interconnecting layers and overall stack design are closely interconnected. In consequence, optimisation of the devices is achieved by tuning several parameters.

By Sebastian Axmann

Take a look at the review in EES:

Triple Junction Polymer Solar Cells
Olusegun Adebanjo, Purna Maharjan, Prajwal Adhikary, Mingtai Wang, Shangfeng Yang and Qiquan Qiao
DOI: 10.1039/C3EE42257G, Review Article