EES Blog

By reviewing both current laboratory and industrial-scale reactors and emerging technologies, Baytekin, Baytekin and Grzybowski offer strategic alternatives to incineration that usefully could harness the giant potential energy found in discarded synthetic polymers through their Energy and Environmental Science article, Retrieving and converting energy from polymers: deployable technologies and emerging concepts.

Plastics constitute many millions of tonnes of waste globally each year – 28 million tonnes in the United States alone, a waste approaching a trillion MJ of energy. Less than 10% of polymers are recycled effectively and 12% are incinerated, but incineration of plastics merely substitutes one pollutant for many; these authors offer real, clean, and effective alternatives that make better use of this energy reserve, some presently feasible on the industrial scale and others still in development. Anyone with an interest in emerging energy technologies, energy policy, industrial chemistry, active polymers, or green chemistry would certainly read this article with great interest.

One high-value recycling strategy is chemical degradation of polymers – mainly by heating under inert, partial O₂, or H₂ atmospheres – to surprisingly useful ends. In one instance, PMMA yields its monomer, methylmethacrylate, at a yield of 97%. Polymers with high levels of impurities can be converted into fuels such as diesel, coke, and hydrogen at high qualities, with high efficiency, and on the industrial scale. The authors also discuss more exotic energy interconversions involving polymers, presently being developed, among them mechanical to electrical energy interconversion via triboelectric and piezoelectric generators and heat to electrical energy interconversion via polymer thermoelectrics, adding a vision of future possibilities to this already enthralling read.

Read the article in EES:

Retrieving andconverting energy from polymers: deployable technologies and emerging concepts
Bilge Baytekin, H. Tarik Baytekin, and Bartosz A. Grybowski
DOI: 10.1039/C3EE41360H

Researchers led by Professor Christoph Langhammer have developed a novel tool based on indirect nanoplasmonic sensing for in situ studies of dye-sensitized solar cells (DSSCs). Their work elucidates the kinetics of dye impregnation into mesoporous TiO₂, known to be a critical step in fabrication of DSSCs and therefore important for scale-up of DSSCs.

DSSCs offer a potentially low-cost, aesthetically appealing alternative to conventional silicon based technologies. The key components of a cell are a TiO₂ film filled with a densely packed monolayer of photon absorbing dye molecules and an electrolyte. Mechanism-oriented tools and studies are needed to understand how to reproducibly form an optimal dye monolayer on the TiO₂ and how to make the DSSC fabrication process compatible with industrial demands. Langhammer’s group has done exactly that by using a new method to follow the dye impregnation process in detail.

The researchers use Hidden Interface-Indirect Nanoplasmonic Sensing (HI-INPS), a technique that uses the localized surface plasmon resonance of Au nanoparticles (coated with a thin dielectric layer). When illuminated with near-visible light, their sensitivity to dielectric changes. This sensitivity is short- ranged, typically within 50–100 nm from the sensor particle surface. Therefore if a thick layer of material, like mesoporous TiO₂, is deposited onto such a sensor chip, the plasmonic Au sensor particles probe the hidden interface region between the sensor chip surface and the sample material. HI-INPS is a neat way to monitor the dye molecule adsorption without disturbing the DSSC.

Researchers at TU Braunschweig under the direction of Professor Uwe Schröder have demonstrated highly selective electrochemical hydrogenation of two furanics, common biomass derivatives.

Chemical hydrogenation of biomass substrates can be a difficult prospect.  High pressures of H₂ at high temperature would make any chemist with a reasonable expectation of longevity understandably squeamish.  According to research from the University of Technology at Braunschweig, a potentiostat may be the organic chemist’s best friend.

In their recent article in EES, the authors, Schröder and Nilges, demonstrate the electrochemical conversion of furfural and 5-hydroxymethylfurfural to 2-methylfuran and 2,5-dimethylfuran, respectively.  These substrates are derived from lignocellulosic biomass or from polysaccharides such as cellulose and starch.  By demonstrating selective electroorganic reduction at copper and lead electrodes, this work opens the possibility of inexpensive electrochemical reactors that could improve the value of biomass-derived compounds.

One main problem is the competing hydrogen evolution reaction (HER).  This is a common hurdle for reductive electrochemistry on most electrode surfaces and can severely limit Faradaic efficiency.  The authors report that for this system, running high concentrations (500 mM) of substrate can reduce percentage of electrons lost to HER.  While this presents a challenge for scaling-up, these products actually separate from the aqueous electrolyte solution, forming another phase that is relatively easy to remove.

Looking to the future, this work represents an intriguing combination of renewable energy strategies – using renewable sources of energy to convert biomass to more useful or at least more energy dense chemicals.  Certainly, the energy future poses some daunting challenges that resist any single silver bullet.  By combining renewable energy storage with biofuel substrates, some challenging steps in both fields might be avoidable.

By Michael Doud

Read the article in EES:

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

Exciton diffusion in organic photovoltaic cells
S. Matthew Menke and Russell J. Holmes
DOI: 10.1039/C3EE42444H, Review Article

A carbon quantum dot decorated RuO2 network: outstanding supercapacitances under ultrafast charge and discharge
Yirong Zhu, Xiaobo Ji, Chenchi Pan, Qingqing Sun, Weixin Song, Laibing Fang, Qiyuan Chen and Craig E. Banks
DOI: 10.1039/C3EE41776J, Paper

First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction
Desheng Kong, Judy J. Cha, Haotian Wang, Hye Ryoung Lee and Yi Cui
DOI: 10.1039/C3EE42413H, Communication

A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solar-to-hydrogen efficiency
T. Jesper Jacobsson, Viktor Fjällström, Martin Sahlberg, Marika Edoff and Tomas Edvinsson
DOI: 10.1039/C3EE42519C, Paper

Researchers from Sweden have demonstrated a process where the crucial 10 % solar-to-hydrogen (STH) efficiency threshold – required for a device to be considered commercially viable – is met by connecting a number of solar-absorbers in series along with a Pt-based catalyst.

Harnessing the power of the sun to directly split water in order to produce hydrogen is anticipated to be an important process in post-carbon, green economies.

Normally there is a trade-off between high solar-absorption and high water splitting efficiency due to the mismatch between the energy required for water splitting (about 2 eV) and the most efficient band-gap for harvesting solar light (about 1.35 eV).

In their recent article, the research group from Uppsala University have overcome this problem by interconnecting 3 cells, based on the semiconductor CuIn_xGa_1-xSe₂ (CIGS), into a single monolithic device. By placing the semiconductors in series, and tuning them for efficient absorption of the solar spectrum (achieved by varying the In:Ga ratio), their device can have both a high solar absorption efficiency and a sufficiently high electrochemical potential to drive the water splitting reaction.

Due to the relative simplicity of the approach, the authors suggest that this may be an economically compatible route to green H₂ production. What’s more, they claim that this novel approach has room for an increase of several percentage points in STH efficiency, and that it has opened the door to many other photoabsorbers, which were previously disregarded due to too-low band gaps, being re-investigated. In any case, the outlook for this field certainly looks bright.

By Aled D. Roberts

Take a look at the article in EES:

A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10 % solar-to-hydrogen efficiency
Jesper Tor Jacobsson, Viktor Fjällström, Martin Sahlberg, Marika Edoff and Tomas Edvinsson
DOI: 10.1039/C3EE42519C, Paper

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