Crystal engineering for electrochemical applications

Materials research for advanced electrochemical applications involves more and more the complete design of materials and electrodes on all structural levels from the atomic level to the macroscale.^1,2 In particular, many works present hierarchical systems with tailored pore structures rather than simple particulate materials, and composites and heterostructures with increasing complexity are also extensively investigated.³ Thereby, an essential aspect that is often overlooked is the fact that the underlying electrochemical or electrocatalytic mechanisms are often linked to the crystal properties of the components used, and hence precisely controlling these properties may allow optimization of the electrochemical performance. As an example, lithium ion diffusion in many crystalline materials is strongly anisotropic, and hence the use of crystallographically oriented superstructures, or mesocrystals, of morphologically designed primary nanocrystals may provide optimum electrochemical properties in lithium-ion batteries combined with high mechanical and chemical stability.^4,5 Crystal engineering, which involves not only the design, preparation and processing of novel crystal structures but also the realization of uniform targeted morphologies, designed crystal habits, specific crystal orientations within larger structures, or the formation of mesocrystals, is thus highly promising for the development of materials and electrodes with superior performances in functional systems of future batteries, supercapacitors or electrocatalysts.⁶

This themed issue contains three highlight articles, three communications as well as 13 research articles from research groups around the world that show a clear focus on crystal properties and present novel crystal engineering strategies in the design and investigation of novel materials for energy storage and conversion applications via electrochemical and electrocatalytic strategies.

The highlight article by Dong and Koenig (DOI: 10.1039/c9ce00679f) reviews the synthesis of lithium-ion battery precursor materials by coprecipitation, providing guiding principles to determine synthesis conditions for achieving specific crystal properties by tailoring nucleation and growth. Several further works are directed towards lithium-ion battery applications; whilst Niederberger et al. present SnS/N-doped carbon composites that show enhanced Li-ion storage via controlled hierarchical structures (DOI: 10.1039/c9ce01147a), Liu et al. investigate separators from crystalline Al₂O₃ prepared via electrospinning (DOI: 10.1039/c9ce01557d). Smarsly and coworkers present the formation of Ti(OH)OF particles with hexagonal crystal structure and promising electrochemical properties (DOI: 10.1039/c9ce01536a). Poizot et al. synthesized novel coordination polymers based on Li,Na-dihydroxyterephthalates and evaluated their electrochemical performance against lithium (DOI: 10.1039/c9ce01674k). The article by Li et al. is devoted to lithium–sulfur batteries, presenting TiN–graphene composites to mitigate the well-known detrimental shuttle effect and enable long-term cyclability of the as-fabricated cells (DOI: 10.1039/c9ce01469a).

A number of articles focus on supercapacitor applications; Chandra et al. introduce the concept of synthesizing hollow metal oxide nanostructures with superior capacity via a nucleation and growth-driven mechanism (DOI: 10.1039/c9ce01547g). Crespo-Ribadeneyra, Titirici and Hérou investigated lignin-derived carbons and in particular looked at the dependence between the structure of the NaOH activation agent and the structural features as well as the capacitance of the resulting product (DOI: 10.1039/c9ce01702j). The use of Ni–Co layered double hydroxide composites as electrode materials for supercapacitors is studied in two reports; Sun et al. prepared arrays of aligned ZnO nanorods and layered double hydroxides, obtaining high capacitive performance (DOI: 10.1039/c9ce01550g), whereas Liu, Zhang et al. study the crystal growth of layered double hydroxides on potassium copper sulfide nanowires, also achieving high capacitance and good stability (DOI: 10.1039/c9ce01261c). NiS nanosheets anchored on carbon fibers are presented by Guo, Jia et al. as flexible electrode materials (DOI: 10.1039/c9ce01560d), whilst the work by Sun, Huang and coworkers is devoted to NiMn₂O₄ nanosheets assembled into hierarchical microspheres for supercapacitors with excellent cycling stability (DOI: 10.1039/c9ce01623f).

Materials and systems for electrocatalysis are targeted in several further contributions. The highlight article by Lee et al. (DOI: 10.1039/c9ce01883b) provides an overview of vacancy engineering as a highly relevant strategy to enhance the performance of catalysts for water electrolysis. The highlight article by Yang et al. is also devoted to water splitting, presenting recent insights and trends in the crystal engineering of transition metal-based catalysts (DOI: 10.1039/c9ce01533g). Two works deal with metal nanocrystals for the oxygen reduction reaction. Huang et al. present the rapid synthesis of hollow trimetallic PtPdCu octahedrons exhibiting high activity and durability in the acid-based oxygen reduction reaction (DOI: 10.1039/c9ce01422e), whilst Liu, Y. Tang et al. synthesized concave PtCo nanooctahedra making use of iminodiacetic acid as a morphology-regulating agent, with the product clearly outperforming the commercial standard in alkaline conditions (DOI: 10.1039/c9ce01488h). The hot topic of carbon dioxide electroreduction is addressed by Z. Tang et al., presenting a coupling strategy of the metal–organic framework structure ZIF-90 with cobalt phthalocyanine as a molecular catalyst to achieve both large current density and high catalyst stability (DOI: 10.1039/c9ce01517e).

Zhang et al. study the effects of morphology engineering for electrochemical sensing, presenting phosphates with superstructures of different shapes and investigating their detection limits for aromatic organic compounds such as catechol (DOI: 10.1039/c9ce01380f). The report by Rosei et al. investigates heterostructured photoanodes based on quantum dots and TiO₂ obtained via electrophoretic deposition, revealing the important role of formed surface traps on the photoelectrochemical performance (DOI: 10.1039/c9ce01729a).

Up to now, much work has been carried out focusing on the design of active electrode materials and the construction of novel electrode structures with high performance towards electrochemical applications (often associated with various properties, such as large surface area, short diffusion length, more active sites and high conductivity).^7,8 Creative work on integrated electrodes, flexible electrodes and in situ formed electrodes is in high demand to realize next-generation high-performance electrochemical devices. Again, the concepts of crystal engineering and multiscale crystallization may greatly contribute to the rational design of such systems. We hope that this issue may stimulate more exciting work in this field and be helpful to all researchers who are interested in electrochemical applications of engineered crystalline materials.

We would like to sincerely thank the editorial staff of CrystEngComm, in particular Dr. Debora Giovanelli and Dr. Michelle Canning, for their fast and efficient coordination of the invitation, reviewing and editing processes, ensuring the timely completion of this themed issue.

Notes and references

1. L. Yu, H. Hu, H. B. Wu and X. W. D. Lou, Adv. Mater., 2017, 29, 1604563.
2. L. Zhou, K. Zhang, Z. Hu, Z. Tao, L. Mai, Y.-M. Kang, S.-L. Chou and J. Chen, Adv. Energy Mater., 2018, 8, 1701415.
3. H. Wang, X. Liang, J. Wang, S. Jiao and D. Xue, Nanoscale, 2020, 12, 14–42.
4. J. Popovic, R. Demir-Cakan, J. Tornow, M. Morcrette, D. S. Su, R. Schlögl, M. Antonietti and M. M. Titirici, Small, 2011, 7, 1127–1135.
5. E. Uchaker and G. Cao, Nano Today, 2014, 9, 499–524.
6. S. Zhou, T. Mei, X. Wang and Y. Qian, Nanoscale, 2018, 10, 17435–17455.
7. D. Xue, Sci. China: Technol. Sci., 2015, 58, 1767.
8. M. Minakshi, D. R. G. Mitchell, R. T. Jones, N. C. Pramanik, A. Jean-Fulcrand and G. Garnweitner, ChemistrySelect, 2020, 5, 1597–1606.

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