Mesa+ Meeting

Energy storage and conversion chaired by Guido Mul/Mark Huijben

Abstracts

Elucidating the mechanisms behind the facet-selectivity photo-deposition of metals and metal oxides on BiOBr particles, Igor Siretanu (PCF)

Facet-engineering and the deposition of co-catalysts lead to improvement in efficiency of semiconductors in photocatalytic applications. While facet-based charge separation is a known phenomenon and it is reported for a range of semiconductors, there is still a limited understanding of it and the mechanism behind remain elusive. Moreover, the proposed simplistic mechanism based on an offset of the valence band and conduction band of different facets is debated. Here, we show that facet-selective, photo-induced reductive or oxidative deposition of co-catalysts onto plate-like bismuth-oxy-bromide (BiOBr) nanoparticles is strongly pH-dependent. High resolution atomic force microscopy and spectroscopy measurements demonstrate that the effect of pH is caused by a reversal of the surface charge of the {001} facets upon increasing the pH from 3 to 9 (isoelectric point 5), while the side facets remain (increasingly) negatively-charged. We propose a different mechanism and we suggest that differences in surface charge/band bending that vary for different surface orientations is the dominant element causing selective charge transport between different facets and metal-oxide deposition.

3-D Vertically Aligned Graphene Electrodes for High Performance Lithium–Sulfur Batteries, Deepak Singh (IMS-NECS)

As the next-generation energy storage materials, lithium–sulfur (Li–S) batteries have become increasingly attractive owing to their high gravimetric density (2600 W·h·kg⁻¹) and specific capacity (1671 mA·h·g⁻¹), additionally; sulfur is a highly cost-effective and environmentally benign element. However, the overall performance of current Li–S batteries is impeded by inherently poor electronic and ionic conductivity of sulfur and the dissolution of higher-order polysulphides phases (Li₂S_n (8 ≥ n ≥ 2)) during potential cycling which causes irreversible loss of active material. Here, we present novel binder-free 3-D vertically aligned electrodes of few layered graphene (FLG) nanoflakes with interconnected micro voids/channel, filled with partially reduced graphene oxide-sulfur (PrGO-S) nanocomposites for high performance Li–S batteries^[1].

[1] 3-D vertically aligned few layer graphene – partially reduced graphene oxide/sulfur electrodes for high performance lithium–sulfur batteries, D.P. Singh, N. Soin, S. Sharma, S. Basak, S. Sachdeva, S.S. Roy, H.W. Zanderbergen, J.A. McLaughlin, M. Huijben and M. Wagemaker, Sustainable Energy Fuels 1, 1516 (2017).

Promoting Photocatalytic Overall Water Splitting by addition of Mg to SrTiO_3, Kai Han (PCS)

Directly converting solar energy into chemical energy through water splitting is a promising way to store solar energy^[1]. SrTiO₃ is a well-known photocatalyst inducing overall water splitting under UV light irradiation. However, solar-to-hydrogen efficiencies are low (less than 1%). Here, we introduce a simple solid state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO₃. As shown by structural characterization, variation of Mg-content results in the synthesis of materials consisting of various phases. Further characterization of the Mg:SrTiO_x composites revealed that Mg is most likely substituting the tetravalent Ti-ion, leading to a favorable space-charge layer. After deposition of appropriate co-catalysts like Pt or Ni/NiO_x, the overall photocatalytic water splitting efficiency of the co-catalyst modified Mg-modified SrTiO_x (Mg:SrTiO_x) composite is up to 20 times higher as compared to conventional co-catalyst modified SrTiO₃, and thus an apparent quantum efficiency (AQE) of 10 % at 300 - 400 nm illumination can be achieved^[2].

[1] F. E. Osterloh, Chem. Soc. Rev., 2013, 42, 2294-2320.

[2] K. Han, T. Kreuger, B. Mei. G. Mul, ACS Catal., 2017, 7, 1610-1614.

H₂O + hν = H_2, Wouter Vijselaar (MNF)

Photovoltaic (PV) cells become increasingly popular and constitute a perfect example of a renewable energy source. However, the efficient conversion of sunlight into electricity is not the complete answer to the current energy crisis: we need to be able to store and transport energy as well. In this regard, many have championed the hydrogen economy, using hydrogen (H₂) as a clean fuel, although H₂ is currently derived from methane and other petroleum-based products, and only 4% is derived in a renewable fashion. A possible solution is photoelectrolysis, whereby light-harvesting and electrochemical processes are performed in a single, integrated device known as a photoelectrochemical (PEC) cell. Such a device is widely investigated, since it requires advances in new materials and the design of the device to become economically competitive. Here we present an overview of the technological challenges and their possible solutions to achieve a low-cost PEC device.