PacSurf2018 Session EH-TuP: Energy Harvesting and Storage Poster Session

Metal oxides as electrode materials are of great potential for rechargeable aqueous batteries. However，they suffer from inferior cycle stability and rate capability because of poor electronic and ionic conductivities. Herein, taking vertically-orientated Bi₂O₃ nanoflakes on Ti substrates as examples, we found that δ-Bi₂O₃ electrode with plenty of intrinsic positively-charged oxygen defects have shown remarkably higher specific capacity (264 mAh g^-1) and far superior rate capability than that of α -Bi₂O₃ with less oxygen vacancies . Through pinpointing the existence form and the role of oxygen vacancies within the electrochemical processes, we demonstrate that oxygen vacancies in δ-Bi₂O₃ serve as central entrepots collecting OHˉ groups via electrostatic force effect, which has boosted the oxidation reaction and enhanced the electrochemical properties. Besides, oxygen vacancies can largely facilitate electronic conductivity. Our work merited an excellent Bi₂O₃ negative electrode material via giving full play to the role of oxygen vacancies in electrochemical energy storage.

Battery-based energy storage could be a useful tool for transitioning to a fully renewable-based energy economy. Current battery technologies, however, often lack the cost-effectiveness [1] and safety [2] requirements necessary for large-scale grid energy storage applications. Aluminium-ion batteries (AIBs) are a very promising alternative to Lithium-based systems, thanks to the low cost, non-flammability, and three-electron redox chemistry of aluminium. [3] AIBs could, in principle, offer better cost-effectiveness, energy density [4] and safety, [5] than currently available energy storage technologies. This poster deals with the investigation of new electrode nanomaterials and electrolyte systems for Al-ion batteries, with the aim of improving the performance and cost-effectiveness of AIBs, and facilitate their commercial feasibility.

REFERENCES:

[1] Delucchi, M. A.; Jacobson, M. Z. Energy Policy 2011, 39 (3), 1170.

[2] Barnes, F. S.; Levine, J. G. Large energy storage systems handbook; CRC press, 2011.

[3] K. Das, S.; Mahapatra, S.; Lahan, H. J. Mater. Chem. A 2017, 5 (14), 6347.

[4] Jayaprakash, N.; Das, S. K.; Archer, L. A. Chem. Commun. 2011, 47 (47), 12610.

[5] Lin, M.-C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.-Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B.-J.; Dai, H. Nature 2015, 520 (7547), 324.

One of the key points for designing high-efficiency thermophotovoltaic (TPV) systems is to match the spectral emission of the radiator with the spectral range where photons are converted into electron-hole pairs inside the photovoltaic device. Recent studies have shown the potential offered by photonic crystals made of metal/dielectric stack of layers acting as Fabry-Pérot cavity, ensuring spectral selectivity to the emitter. Another material requirement is thermal stability in order to keep the same spectral emission at the operating temperature of the radiator. The Mo/HfO₂ system is a promising candidate material in this respect, as recently reported by Blandre et al. [1].

In this work we investigate the synthesis by magnetron sputtering of W/HfO₂ four-layer films for TPV applications. The individual layer thicknesses were optimized based on fluctuational electrodynamics calculations of thermal emissivity of one-dimensional layered media. Prior to the fabrication of the multilayer stack, the structure and optical properties of the transparent HfO₂ layer were investigating as a function of process parameters: substrate temperature T_s and O₂ flow rate.

Water electrolysis as one of the prospective approach to produce hydrogen is limited by the anodic oxygen evolution reaction (OER) due to its sluggish kinetics. Considering the practical application, the development of highly efficient non-noble OER catalysts is essential. Among the reported low-cost electrocatalysts, CoFe-based oxides or hydroxides are intriguing owing to the rich redox chemistry. However, the severe aggregation and intrinsically poor electronic conductivity hinder their electrocatalytic performance. Hybridizing cobalt-iron oxides or hydroxides with carbon materials can not only enhance the conductivity of the catalyst, but also facilitate the uniform distribution of metal species within carbon matrix, thus improving the electrochemical performance towards OER. In view of this, we report here the bimetallic cobalt-iron hydroxide encapsulated in carbon layers derivedfrom organic ligand (Schiff base) via hydrothermal carbonization. In alkaline media, Co Fe/C loaded on GC electrode delivers a current density of 10 mA cm^-2 at an overpotential of 260 mV and exhibits a low Tafel slope of 45.2 mV dec^-1 as well as excellent durability. The remarkable OER performance outperforms those of CoFe-based electrocatalyst in the literature.

Keywords: Oxygen evolution reaction, cobalt-iron hydroxide, organic ligand, electrocatalyst

Results

As shown in Figure 1, the Co_1.2Fe/C delivers a current density of 10 mA cm^-2 at an overpotential of 260 mV and exhibits a low Tafel slope of 45.2 mV dec^-1 as well as excellent durability in 1M KOH solution.

Figure 1 (a) LSV curves of GC, Co_1.2Fe and Co_1.2Fe/C; (b) Tafel plots of Co_1.2Fe and Co_1.2Fe/C; © Chronopotentiometric measurement of Co_1.2Fe/C at 10 mA cm^-2. All tests were carried out in 1 M KOH.

Conclusions

The Co_1.2Fe/C exhibits excellent OER performance in alkaline media, which can be ascribed to the following factors: 1) the homogenous distribution of metal hydroxide within carbon matrix inhibits the aggregation of the active nanoparticles; 2) the excellent electronic conductivity enables fast electron transport during the OER process; 3) the synergetic effect between the Co_1.2Fe hydroxide and carbon species also contributes to the improvement of the electrochemical performance.

Reference

[1] S. Yin, W. Tu, Y. Sheng, Y. Du, M. Kraft, A. Borgna, R. Xu, Adv. Mater. 2017, 1705106.

[2] F. Li, J. Du, X. Li, J. Shen, Y. Wang, Y, Zhu, L. Sun, Adv. Energy Mater. 2018, 1702598.

Dye-sensitized photoelectrochemical cell (DS-PEC) water splitting into hydrogen and oxygen is a potential approach to realize the conversion solar energy to chemical energy efficiently. Ru(bda) (bda is 2,2`-bipyridine-6,6`-dicarboxylate) is one of the most efficient water oxidation catalysts in DS-PEC. Traditionally, Ru(bda) have been anchored onto the dye-sensitized photoanode by modifying phosphoric acid or carboxylic acid groups. These complexes can reduce the overpotentials of water oxidation and exhibit highest turnover. However, the efficiency of the entire system is inhibited due to the strong recombination effect of electrons injected into theTiO₂ bulk on the photosensitizer and catalyst in the oxidized state. Herein, we have replaced the anchor group of the water oxidation catalysts Ru(bda) (bda is 2,2′bipyridine-6,6′-dicarboxylate) from phosphoric acid to pyridine. Time-resolved transient absorption spectroscopy demonstrates that the pyridine anchor group have effect of inhibiting electron recombination. The photocurrent densities of 1.5-1.8 mA/cm² were achieved and produced an incident photon to current efficiency (IPCE) of 25.5% at 450 nm. Under pH=5.8 under 100mW/cm² (>400nm) illumination.

The thermal transport properties of heptane-isooctane mixtures in the liquid phase at room temperature and atmospheric pressure were measured and calculated. The thermal conductivity, k, was measured using the transient hot-wire (HW) method, and the thermal effusivity, e, was determined by the front photopyroelectric (FPPE) technique. Then, the thermal diffusivity, α, and the volume specific heat capacity (heat capacity per unit volume), C, were calculated using the well-known relationships α = (k/e)² and C = e²/k , respectively. We show that a correlation between these parameters and octane number exists, and that the obtained thermal parameters proved to be sensitive to detect molecular associations in binary hydrocarbon liquid mixtures. We also report about the thermal conductivity of mixtures of n-alkanes vapors and air showing that almost similar values are obtained for heptane and isooctane, so that octane number rating using the well-established binary mixtures of these substances becomes impossible with thermal properties measurements in the gas phase.

We demonstrate a steady-state thermoreflectance based optical pump-probe technique to measure the thermal conductivity of materials using a continuous wave laser heat source. The technique works in principle on leaving a pump laser on long enough to induce a steady-state temperature rise in a material. A probe beam is then used to detect the resulting change in reflectance, which is proportional to the change in temperature of the sample. Varying the power of the pump beam to induce larger temperature rises, Fourier’s law is used to determine the thermal conductivity. We show that this technique is capable of measuring the thermal conductivity of a wide range of materials having thermal conductivities ranging from 1 to > 2000 W m−1K−1, in excellent agreement to literature values. With a measurement length scale capable of reaching <10 microns, this techniques is capable of measuring both bulk material and films.