EES Blog

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

Researchers, led by Dr. Volker Presser in Germany, present a methodology to directly predict the desalination performance of carbon-based electrodes for capacitive deionization (CDI), an important first step towards directed CDI design.

Carbon-based electrodes offer an energy-efficient water desalination technology that involves removal of ions from water by electrosorption in charged porous carbon electrodes. The family of carbon materials ranging from activated carbons, carbon nanotubes, exfoliated grapheme electrodes to templated carbons and carbide-derived carbons have been extensively studied for desalination by CDI. To achieve optimal performance, components of the CDI system need to be tuned to achieve both high salt electrosorption capacity and fast kinetics, simultaneously. Therefore, tools to predict the performance of a certain carbon material and CDI design are essential for device design.

Given the inherent non-linearity of desalination by porous carbons, Porada et al. in their paper, Direct Prediction of the Desalination Performance
of Porous Carbon Electrodes for Capacitive Deionization, follow a two-prong approach: i) predict the desalination performance of a carbon material based on its pore size distribution and ii) use a two-dimensional porous electrode CDI transport model to predict the actual salt electrosorption kinetics. The authors have convincingly demonstrated that there is no direct relationship between salt electrosorption capacity and typically cited pore metrics and that the salt electrosorption capacity can be predicted by analysis of the pore size distribution and the pore volume correlated with incremental pore size ranges.

Despite the complexity of CDI, their work has shown the feasibility of predicting performance of different carbon-based materials in a passionately debated field. Looking ahead, the rational device design of carbon electrodes is certainly on the cards.  This paper is a must-read for anyone working in the field of carbon materials!

Read the paper here:

Direct Prediction of the Desalination Performance of Porous Carbon Electrodes for Capacitive Deionization
Slawomir Porada, Lars Borchardt, Martin Oschatz, Marek Bryjak, Jennifer Atchison, Karel J. Keesman, Stefan Kaskel, Maarten Biesheuvel and Volker Presser
DOI: 10.1039/C3EE42209G

By Prineha Narang

The “working horse” of current organic photovoltaics research are the polymers P3HT and PC(61)BM. Both materials have been characterized extensively in single layers as well as in devices. To date, a wide range of power conversion efficiency values have been reported and linked to various factors including material sources and processing conditions.

A new article in the RSC Journal Energy & Environmental Science by Westacott et al. examines P3HT and PCBM with respect to photon absorption. They compare the influence of a high and low average molecular weight P3HT (H-P3HT/L-P3HT type) and temperature treatments upon exciton dissociation yield. This comparison reveals that a highly intermixed phase of both materials improves charge generation which is promoted by a high average weight of P3HT. The underlying mechanism is found to be the formation of a finely intermixed phase between both materials.

A distinct increase in dissociated excitons was found for the high weight P3HT even under different concentration levels for both devices.

To distinguish between different mechanisms of phase formation in both types, pristine layers of each molecule type were deposited and laminated. Upon short heating (30s, 150°C), similar phase intermixing and subsequent reduction of crystallinity was observed for both types.

The L-P3HT is assumed to aggregate easier as the average chain length is smaller. Upon heating, separation of both materials is induced which in turn leads to a loss of the finely intermixed phase. The H-P3HT is assumed to incorporate less mobile amounts of PCBM due to the macromolecular structure of the folded molecules. Thus the H-P3HT type yields higher exciton dissociation rates and thus possibly larger efficiencies.

By Sebastian Axmann

Read the article in EES:

On the role of intermixed phases in organic photovoltaic blends
Paul Westacott, John R. Tumbleston, Safa Shoaee, Sarah Fearn, James H. Bannock, James B. Gilchrist, Sandrine Heutz, John deMello, Martin Heeney, Harald Ade, James Durrant, David S. McPhail and Natalie Stingelin
DOI: 10.1039/C3EE41821A