EES Blog

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

Researchers from China have found an effective strategy for filling the pinhole defects of organic dye-grafted titania films, enhancing the performance of organic dye-sensitized solar cells employing non-corrosive cobalt redox electrolytes.

Dye-sensitized solar cells (DSCs) use organic dyes to enhance the light-harvesting of inorganic solar cells, altering the wavelengths of incoming light to those better absorbed by the cell in a process known as up- or down-conversion. When dyeing a cell, a good solvent must be used to prevent the dye aggregating, the aim being to create only a monolayer of dye on the surface. However, this creates an incomplete coating of the inorganic surface with the dye, the gaps called ‘pinhole defects,’ which act as sites where the charge-hole pairs created by the absorption of photons recombine easily, significantly reducing current generation and thereby the effectiveness of the cell.

Zhang et al. in the paper Judicious selection of a pinhole defect filler to generally enhance the performance of organic dye-sensitized solar cells detail their quest to solve this problem, creating two bulky model dyes for coating a titania support and three very similar but less bulky ‘fillers.’ The dyes were coated onto the titanium with good solvents and the fillers with poor solvents to maintain the integrity of the dye coating. In every case, the filler reduced open-current voltage, a fundamental measure of efficiency. The net result was an impressive solar-to-electricity energy efficiency of 10.5% under non-concentrated light for their best cell.

The exciting results of this study make it a must-read for those involved with organic solar cells. Additionally, though this is a technical paper specific to dye-sensitized organic solar cells, anyone with interest in solar cells would find interest in this article because the general strategy of defect-filling could be used to reduce interfacial charge recombination in other types of solar cells.

Read the article in EES:

Judicious selection of a pinhole defect filler to generally enhance the performance of organic dye-sensitized solar cells
Min Zhang, Jing Zhang, Ye Fan, Lin Yang, Yinglin Wang, Renzhi Li and Peng Wang
DOI: 10.1039/C3EE42431F, Communication

By Benjamin Britton

Take a look at this week’s selection…

Ni3S2 nanorods/Ni foam composite electrode with low overpotential for electrocatalytic oxygen evolution
Weijia Zhou, Xue-Jun Wu, Xiehong Cao, Xiao Huang, Chaoliang Tan, Jian Tian, Hong Liu, Jiyang Wang and Hua Zhang
DOI: 10.1039/C3EE41572D, Communication

Electrochemistry for biofuel generation: production of furans by electrocatalytic hydrogenation of furfurals
Peter Nilges and Uwe Schröder
DOI: 10.1039/C3EE41857J, Communication

Judicious selection of a pinhole defect filler to generally enhance the performance of organic dye-sensitized solar cells
Min Zhang, Jing Zhang, Ye Fan, Lin Yang, Yinglin Wang, Renzhi Li and Peng Wang
DOI: 10.1039/C3EE42431F, Communication

Researchers from the Chinese Academy of Sciences harvest energy from the environment to achieve a self-powered fluorescence switch system.

Electronically powered response systems are frequently hindered by their external power sources. These external power sources increase the size of the system and make independent and sustainable operation difficult. Focusing on electrical stimuli-responsive fluorescence systems, Bai et al. addressed the problem of system size and sustainability by exploring a switch system based on biofuel cells.

By using the electroactive prussian blue (PB) to control fluorescence change and biocatalysis, the authors were able to build a fluorescence switch system that operates on one biofuel cell. This kind of enzymatic biofuel cell extracts bio-energy from biochemical reactions to produce electricity, meaning the system is fully integrated and requires no external power source. Essentially self-powered, the fluorescent switch system described in a recent EES paper is reversible, reproducible, and power-dense (up to 87 μW/cm2).

The device functions by controlling the redox states of PB with a membrane-less, mediator-less biofuel cell. The fluorescence of the hybrid film is then switched with the absorbance change of the PB. By combining the electrochromatic PB controlling fluorescence switch with the biocatalytic reaction, a functioning self-powered switch system is achieved.

The idea of electronics that can operate by harvesting energy from the environment is certainly exciting. This kind of technology appeals to the imagination and would undoubtedly have huge applications in consumer goods. As someone without a technical background, it is exciting to learn about research with possible game-changing applications for everyday items.

Feeling electrified? Read the full Energy and Environmental Science article here:

Self-powered fluorescence controlled switch systems based on biofuel cells
Lu Bai, Lihua Jin, Lei Han and Shaojun Dong
DOI: 10.1039/C3EE41028E

By Paige Johnson

Organic solar cells have attracted wide interest in the scientific community as a possible alternative for silicon based photovoltaics in certain areas. While laboratory efficiency of organic devices went beyond 10 % within the last years, lifetime issues such as rapid performance degradation remain to be solved.

In a recent article, Mateker et al. examined the performance degradation of solar cells made of the commonly used polymers PBDTTPD and PC61BM. Earlier findings indicated that cells made with PBDTTPD of high average molecular weight (M_w) degraded even in inert atmosphere and darkness while those of low M_w did not.

By intentional contamination with the small molecule TPD, the researchers demonstrated the influence of such impurities onto device performance. As a consequence, high weight PBDTTPD was thoroughly purified. Devices utilizing this filtered polymer demonstrated shelf lifetimes beyond 111 days.

The performance reduction of the unfiltered high M_w polymer is attributed by the researchers to small molecules which form a layer at the cathode contact of the cell. This layer was indicated by the widely known S-shaped JV-characteristic.  Such features are developed within less a week of storage in darkness. By removing the old and evaporating a new cathode layer, device performance was partially recovered and the standard solar cell JV-curve shape was re-established.

Intentional introduction of TPD (a building block of PBDTTPD and thus a possible residue of the synthesising reactions) into the low M_w polymer created the same behaviour as for the unfiltered high M_w counterpart. In consequence, the authors removed small molecule impurities from the high M_w polymer by size exclusion chromatography (SEC) and demonstrated the excellent improvement of device lifetime.

Read more detail in the article:

Improving the long-term stability of PBDTTPD polymer solar cells through material purification aimed at removing organic impurities
William R. Mateker, Jessica D. Douglas, Clément Cabanetos, I. T. Sachs-Quintana, Jonathan A. Bartelt, Eric T. Hoke, Abdulrahman El Labban, Pierre M. Beaujuge, Jean M. J. Fréchet and Michael D. McGehee
DOI: 10.1039/C3EE41328D

By Sebastian Axmann