A hybrid niobium-based oxide with bio-based porous carbon as an efficient electrocatalyst in photovoltaics: a general strategy for understanding the catalytic mechanism

Developing a high-performance catalyst and establishing a catalytic mechanism for understanding the catalytic activity are crucial to new generation photovoltaic technology. In this work, we present a feasible and general route to synthesize a bio-based porous carbon (BPC) supported ZnNb2O6 hybrid catalyst with a unique network structure, providing an effective means for electron transport between the electrode and the external circuit. Benefitting from the synergistic effect of ZnNb2O6 and BPC, a photovoltaic device assembled with the nanohybrid yields a power conversion efficiency of 8.83%, which is superior to that of pristine ZnNb2O6-based and conventional Pt-based cells (7.15% and 7.14%). Systematic electrochemical evaluations of the hybrid catalysts exhibit promising stability for practical application in photovoltaics. Contraposing the two vital functions of the counter electrode catalyst, collecting electrons and catalyzing I-3(-) reduction, we propose a general strategy to understand the potential catalytic mechanism from the band structure and surface adsorption by using first-principles density functional theory (DFT) calculations. The theoretical investigations clearly indicate that the splendid catalytic performance originates from the zero band-gap of surface metal atoms and the surface chemical adsorption interaction between I-3(-) and exposed metal atoms. The proposed general strategy in this work for synthesizing a hybrid material with a unique network structure and understanding the catalytic mechanism of the electrocatalyst can guide the design of expected catalytic nanohybrids applied in various energy fields.

Electronic Structures and Catalytic Activities of Niobium Oxides as Electrocatalysts in Liquid-Junction Photovoltaic Devices

Ulf Anders Hagfeldt, Sheng Meng, Sining Yun

Two types of nanosized niobium oxides and their composites, pseudohexagonal Nb2O5 (TT-Nb2O5), monoclinic NbO2 (M-NbO2), and the coexistence of TT-Nb2O5 and M-NbO2 (TT-Nb2O5/M-NbO2), are successfully synthesized through the urea-metal chloride route, and they exhibit excellent catalytic activity and photovoltaic performance in dye-sensitized solar cells (DSSCs). First-principles density function theory (DFT) calculations show that their catalytic activity is significantly influenced by their intrinsic electronic structures and properties. The lone-pair 4d(1) electrons of Nb4+ in M-NbO2 enhance the Nb-I interaction and promote electron transfer from the M-NbO2 counter electrode (CE) to I, thus resulting in superior catalytic properties in M-NbO2-based DSSCs. In addition, the adsorption energy of I on the M-NbO2 surface is in the optimal energy range of 0.3-1.2 eV, and the Fermi level of M-NbO2 is 0.6 eV, which is higher than the I-3(-) reduction reaction potential, and I-3(-) can be spontaneously reduced to 3I(-). Herein, a general strategy for understanding the electronic structures and catalytic activities of transition metal compounds as CE catalysts for DSSCs is provided.

WILEY-V C H VERLAG GMBH2019

Electronic transport properties of vertical and lateral graphene-based heterostructures

Roberto Urcuyo Solorzano

Graphene has some drawbacks, like the absence of an electronic band gap which leads to only small gate switching ratios, and its fast carrier recombination that limits its use in optoelectronics. In order to overcome these issues, the aim of this thesis was to tune the properties of graphene by two different modification approaches. The first approach involved chemical functionalization to create lateral graphene heterostructures, whereas the second approach was directed toward vertical heterostructures that combine graphene with another 2D material. For the former case, two novel chemical functionalization methods were investigated. The first method relied upon hyperthermal molecular ion collisions with 4,4¿-azobis(pyridine). This enabled the covalent functionalization of graphene with a functionalization degree of up to 3%, as demonstrated by Raman in combination with XPS. AFM studies revealed that the functionalized sheets retain their topographic integrity. Thus obtained stripe-like patterns of covalently graphene within extended graphene sheets enabled enhanced on/off ratios upon gate switching. In the second functionalization, OsO4 was used to selectively introduce hydroxyl groups to graphene. Best results were obtained using UV light activation, which yielded graphene of high covalent functionalization degree, as concluded from its optical transparency and gate-induced on/off switching ratios of up to 500. Temperature dependent electrical measurements revealed 2D hopping as the dominant transport mechanism. The first type of vertical heterostructure, gr-TiOx-Ti diodes were fabricated and their electrical properties in the dark and under visible light illumination studied. In contrast to conventional MIM diodes, the performance of the graphene-based diodes was found to increase with decreasing thickness of the oxide insulator. Bias-induced modulation of the work function of graphene was identified as the key to the operation mechanism of the graphene diodes, enabling them to reach a very high asymmetry and nonlinearity (9000 and 8). The diodes compete well with state-of-the-art MIM diodes. Furthermore, the graphene-TiOx-Ti diodes could be operated as photovoltaic cell, with a maximum open-circuit voltage of 0.3 V and a short-circuit photocurrent of 14 nA under global illuminationt. This finding constitutes the first proof-of-principle of hot carrier extraction from graphene, based on the photovoltaic effect. The second type of vertical heterostructures was based on black phosphorus (BP). As a first step, thin BP sheets were combined with n-GaAs into novel pn devices. They displayed pronounced rectification behavior that, in the low bias regime, approaches that of ideal diodes. Moreover, when operated as photodiodes they reached external quantum efficiencies (EQE) above 30% under higher reverse bias. On this basis, the bP was then combined with gr into a p+/p heterojunction. Thus obtained devices achieved a maximum internal quantum efficiency (IQE) and responsivity of 13% and 10.7 mA/W, respectively. This IQE is the highest thus far reported for diodes comprising BP and another 2D material. Moreover, with the aid of a top gate it was possible to modulate both, the photo-conversion efficiency and photocurrent generation mechanism of the black phosphorous-graphene diodes. Together with the possibility to tune the photocurrent response by the thickness of the black phosphorous, these diodes emerge as promising photodetectors.

EPFL2016

Intrinsic Origin of Superior Catalytic Properties of Tungsten-based Catalysts in Dye-sensitized Solar Cells

Ulf Anders Hagfeldt, Jing Li, Yao Liu, Sining Yun, Xiao Zhou

To clarify the intrinsic factor of the superior catalytic activity of tungsten (W)-based catalysts in DSSCs, W-based compounds, including W18O49, W2N, WC, and WO3, were prepared as CEs and tested in I-/I-3(-) redox shuttle. A systematic investigation of the catalytic properties and stability, as well as first-principle density functional theory (DFT) calculations, was performed for these W-based catalysts in DSSCs. The DSSCs assembled with the I-/I-3(-) redox shuttle, based on W18O49, W2N, WC, and WO3 CEs, exhibited power conversion efficiencies (PCEs) of 6.69%, 5.97% 5.20%, and 4.69%, respectively; these values were equal to or better than those of DSSCs with Pt CEs (6.52%) under the same conditions. The superior electrocatalytic activities of W-based compounds are determined by their electronic structure. Electronic density of states (DOS) revealed that the band gaps of W18O49, W2N, WC, and Pt were zero, whereas that of WO3 was 1.7 eV. Wider band gaps do not facilitate electron conduction as easily as Pt. In addition, the W/O ratio (1: 2.72) in W18O49 was greater than that in WO3 (1:3), and the W/N ratio (2: 1) in W2N was greater than that in WC (1:1). Higher ratios of W/(O or N) favored the electrocatalytic activity of the CE materials in the reduction of I-3(-) in DSSCs. This is the intrinsic reason why W-based compounds exhibit different catalytic properties in DSSCs under the same conditions. This work sheds light on the origin of W-based catalysts, and explains the differences in the catalytic performances of W-based compounds for this work and several published works. (C) 2017 Elsevier Ltd. All rights reserved.

Elsevier2017

Electronic transport properties of vertical and lateral graphene-based heterostructures

Roberto Urcuyo Solorzano

EPFL2016