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Nanomaterials hold key to hydrogen production
2
C. Sealy
Nanomaterials hold key to hydrogen production Cordelia Sealy Hydrogen offers the possibility of a clean-burning fuel that can be produced simply by splitting water with sunlight or electrochemically using a catalyst. Recent work has reported catalyst materials for photo- and electrochemical production of H2, which could improve the efficiency of the process. First up, Richard Eisenberg, Patrick L. Holland, Todd D. Krauss and colleagues from the University of Rochester have devised an artificial photosynthesis process using nanoparticles to split water into H2 and O2 [Z. Han, et al., Science Express (8 November 2012) doi: 10.1126/science.1227775]. In this type of process, light-absorbing molecules are used to photochemically split water into O2, two electrons and two protons (H + ). A catalyst then drives the formation of H2 from the protons and electrons. The researchers use CdSe nanoparticles capped with dihydrolipoic acid (DHLA) as the light adsorber with an inexpensive and abundant Ni2 + -DHLA catalyst. Unlike other artificial photosynthesis systems, which are typically shortlived, the nanoparticles bring much-needed stability and
Figure 1 CdSe nanocrystals absorb light and transfer electrons to a Ni catalyst (blue), which subsequently generates H2 (white). Credit: Ted Pawlicki, University of Rochester. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
robustness – meaning that H2 can be produced over two weeks (or 600,000 turnovers) without deterioration – and nor does it include any precious metals (Fig. 1). ‘‘Nanoparticles are critical,’’ says Krauss. ‘‘Typically organic dyes are used as light harvesters, which last hours or a day before degrading. The nanoparticles last weeks with no degradation.’’ The team are now looking at other nanoparticle systems that are more specifically engineered with charge separation in mind, including other Ni and inexpensive catalysts. Meanwhile, Vincent Artero and co-workers from CEA, CNRS and Universite Joseph Fourier in France have also come up with a stable catalyst for the electrochemical production of H2 from water [E.S. Andreiadis, et al., Nature Chemistry (2012), doi: 10.1038/nchem.1481]. Electrochemical production of H2 usually requires a catalyst based on Pt, which is both expensive and relatively scarce. Instead, the researchers demonstrate a catalyst based on Co grafted onto a carbon nanotube (CNT) electrode (Fig. 2). The diimine-dioxime Co electrocatalytic cathode material is very stable and can produce H2 for long periods in aqueous electrolyte—the researchers report 55,000 turnovers of H2 in seven hours.
Figure 2 Illustration of the diimine-dioxime Co electrocatalytic cathode material developed by French researchers to produce H2 electrochemically. Credit: E.S. Andreiadis, CEA.
News and Opinions The system could be made into an artificial photosynthesis system, says Artero, by adding photosensitizers. Ultimately, the advance could lead to the development of cheap systems based on earth-abundant elements to produce H2 from water, either electro- or photochemically, he suggests.
3 E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2012.11.009
Co nanoparticle–graphene catalyst could replace Pt Cordelia Sealy A new generation of polymer membrane electrolyte fuel cells and metal–air batteries rely on Pt as a catalyst, which is both scarce and expensive. The search is on for a low-cost, abundant alternative and researchers from Brown University think they may have found it in the form of Co/CoO nanoparticles on graphene [S. Guo, et al., Angew. Chem. Int. Ed. (2012), http://dx.doi.org/10.1002/anie.201206152]. The new catalyst consists of Co/CoO nanoparticles, synthesized and self-assembled through thermal decomposition of precursors onto a graphene (G) surface (Fig. 1). The material performs as a catalyst for the O2 reduction reaction (ORR), which takes place on the cathode side of a fuel cell or battery. In the reaction, O2 acts as an electron sink, drawing away electrons from the H2 at the anode and creating the potential difference that drives the current. The researchers say that the G–Co /CoO material is the first non-precious metal catalyst that performs anywhere close to Pt. ‘‘The G–Co /CoO has comparable electrocatalytic activity, but higher stability than the commercial C/Pt catalyst for ORR in 0.5 M KOH solution,’’ says lead researcher Shouheng Sun. ‘‘To the best of our knowledge, this G–Co / CoO catalyst has the best kinetic process for ORR among all the reported non-noble metal catalysts.’’ Although the G–Co /CoO takes longer to get the ORR started, once it does the reaction goes at a faster pace than if catalysed by Pt. The new catalyst also appears to be remarkably stable, degrading somewhat less than Pt over time. After 17 h of testing, report the researchers, the G–Co /CoO was performing at 70% of its original capacity compared with 60% for a Pt catalyst under similar conditions. The researchers report that size and structure of the nanoparticles is essential to optimizing the catalyst. The best performing material comprises nanoparticles with an 8 nm Co core and a 1 nm CoO shell. The Co nanoparticles are first selfassembled onto the G surface and then allowed to form a layer of natural CoO under ambient conditions. This layer is essential to protect the Co from deep oxidation. Although there is more work to be done to perfect the catalyst, the researchers are optimistic that the combination of Co and G could prove a promising replacement in the future for Pt catalysts in fuel cells. ‘‘Developing new non-Pt nanocatalysts with high activity and stability for ORR is essential for future energy applications,’’ says Sun. ‘‘The present G–Co /CoO material is the most promising non-noble metal catalyst, whose performance comes close to, or matches, Pt.’’
Figure 1 Nanoparticles of Co attach themselves to a G substrate in a single layer. As a catalyst, the Co–G combination was a little slower getting the ORR going, but reduced O2 faster and lasted longer than Pt-based catalysts. Credit: Sun lab, Brown University.
Cordelia Sealy has many years’ experience as a scientific journalist and editor in areas spanning nanotechnology, energy, materials science and engineering, physics, chemistry and the environment. She is currently a freelance science writer for her own company, Oxford Science Writing, and serves as News and Opinions Editor of Nano Energy and Nano Today. She also writes on energy policy and business issues. In the past, Cordelia served as Editor of Materials Today and Nano Today and as Managing Editor of both titles. She also has experience in academic publishing as a books acquisitions editor and in business-to-business publishing as a journalist on European Semiconductor. She has a First in Physical Sciences (BSc) from University College London and a DPhil in Materials Science and Engineering from the University of Oxford, and is a Member of the Institute of Physics. E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2012.11.007
C. Sealy
Nanomaterials hold key to hydrogen production Cordelia Sealy Hydrogen offers the possibility of a clean-burning fuel that can be produced simply by splitting water with sunlight or electrochemically using a catalyst. Recent work has reported catalyst materials for photo- and electrochemical production of H2, which could improve the efficiency of the process. First up, Richard Eisenberg, Patrick L. Holland, Todd D. Krauss and colleagues from the University of Rochester have devised an artificial photosynthesis process using nanoparticles to split water into H2 and O2 [Z. Han, et al., Science Express (8 November 2012) doi: 10.1126/science.1227775]. In this type of process, light-absorbing molecules are used to photochemically split water into O2, two electrons and two protons (H + ). A catalyst then drives the formation of H2 from the protons and electrons. The researchers use CdSe nanoparticles capped with dihydrolipoic acid (DHLA) as the light adsorber with an inexpensive and abundant Ni2 + -DHLA catalyst. Unlike other artificial photosynthesis systems, which are typically shortlived, the nanoparticles bring much-needed stability and
Figure 1 CdSe nanocrystals absorb light and transfer electrons to a Ni catalyst (blue), which subsequently generates H2 (white). Credit: Ted Pawlicki, University of Rochester. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
robustness – meaning that H2 can be produced over two weeks (or 600,000 turnovers) without deterioration – and nor does it include any precious metals (Fig. 1). ‘‘Nanoparticles are critical,’’ says Krauss. ‘‘Typically organic dyes are used as light harvesters, which last hours or a day before degrading. The nanoparticles last weeks with no degradation.’’ The team are now looking at other nanoparticle systems that are more specifically engineered with charge separation in mind, including other Ni and inexpensive catalysts. Meanwhile, Vincent Artero and co-workers from CEA, CNRS and Universite Joseph Fourier in France have also come up with a stable catalyst for the electrochemical production of H2 from water [E.S. Andreiadis, et al., Nature Chemistry (2012), doi: 10.1038/nchem.1481]. Electrochemical production of H2 usually requires a catalyst based on Pt, which is both expensive and relatively scarce. Instead, the researchers demonstrate a catalyst based on Co grafted onto a carbon nanotube (CNT) electrode (Fig. 2). The diimine-dioxime Co electrocatalytic cathode material is very stable and can produce H2 for long periods in aqueous electrolyte—the researchers report 55,000 turnovers of H2 in seven hours.
Figure 2 Illustration of the diimine-dioxime Co electrocatalytic cathode material developed by French researchers to produce H2 electrochemically. Credit: E.S. Andreiadis, CEA.
News and Opinions The system could be made into an artificial photosynthesis system, says Artero, by adding photosensitizers. Ultimately, the advance could lead to the development of cheap systems based on earth-abundant elements to produce H2 from water, either electro- or photochemically, he suggests.
3 E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2012.11.009
Co nanoparticle–graphene catalyst could replace Pt Cordelia Sealy A new generation of polymer membrane electrolyte fuel cells and metal–air batteries rely on Pt as a catalyst, which is both scarce and expensive. The search is on for a low-cost, abundant alternative and researchers from Brown University think they may have found it in the form of Co/CoO nanoparticles on graphene [S. Guo, et al., Angew. Chem. Int. Ed. (2012), http://dx.doi.org/10.1002/anie.201206152]. The new catalyst consists of Co/CoO nanoparticles, synthesized and self-assembled through thermal decomposition of precursors onto a graphene (G) surface (Fig. 1). The material performs as a catalyst for the O2 reduction reaction (ORR), which takes place on the cathode side of a fuel cell or battery. In the reaction, O2 acts as an electron sink, drawing away electrons from the H2 at the anode and creating the potential difference that drives the current. The researchers say that the G–Co /CoO material is the first non-precious metal catalyst that performs anywhere close to Pt. ‘‘The G–Co /CoO has comparable electrocatalytic activity, but higher stability than the commercial C/Pt catalyst for ORR in 0.5 M KOH solution,’’ says lead researcher Shouheng Sun. ‘‘To the best of our knowledge, this G–Co / CoO catalyst has the best kinetic process for ORR among all the reported non-noble metal catalysts.’’ Although the G–Co /CoO takes longer to get the ORR started, once it does the reaction goes at a faster pace than if catalysed by Pt. The new catalyst also appears to be remarkably stable, degrading somewhat less than Pt over time. After 17 h of testing, report the researchers, the G–Co /CoO was performing at 70% of its original capacity compared with 60% for a Pt catalyst under similar conditions. The researchers report that size and structure of the nanoparticles is essential to optimizing the catalyst. The best performing material comprises nanoparticles with an 8 nm Co core and a 1 nm CoO shell. The Co nanoparticles are first selfassembled onto the G surface and then allowed to form a layer of natural CoO under ambient conditions. This layer is essential to protect the Co from deep oxidation. Although there is more work to be done to perfect the catalyst, the researchers are optimistic that the combination of Co and G could prove a promising replacement in the future for Pt catalysts in fuel cells. ‘‘Developing new non-Pt nanocatalysts with high activity and stability for ORR is essential for future energy applications,’’ says Sun. ‘‘The present G–Co /CoO material is the most promising non-noble metal catalyst, whose performance comes close to, or matches, Pt.’’
Figure 1 Nanoparticles of Co attach themselves to a G substrate in a single layer. As a catalyst, the Co–G combination was a little slower getting the ORR going, but reduced O2 faster and lasted longer than Pt-based catalysts. Credit: Sun lab, Brown University.
Cordelia Sealy has many years’ experience as a scientific journalist and editor in areas spanning nanotechnology, energy, materials science and engineering, physics, chemistry and the environment. She is currently a freelance science writer for her own company, Oxford Science Writing, and serves as News and Opinions Editor of Nano Energy and Nano Today. She also writes on energy policy and business issues. In the past, Cordelia served as Editor of Materials Today and Nano Today and as Managing Editor of both titles. She also has experience in academic publishing as a books acquisitions editor and in business-to-business publishing as a journalist on European Semiconductor. She has a First in Physical Sciences (BSc) from University College London and a DPhil in Materials Science and Engineering from the University of Oxford, and is a Member of the Institute of Physics. E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2012.11.007