Wide-range work-function tuning of active graphene transparent electrodes via hole doping

Sebastián

Advanced Energy Materials

et al. 2020

and storage devices, such as fuel cells and metal-air batteries. [1][2][3][4][5][6][7][8] Especially, the polymer electrolyte membrane fuel cells, used in automotive applications, have already achieved the level of commercialization. [5] Although the Pt group metal (PGM) catalysts can efficiently address the kinetics-related challenges, the high cost and the limited resource of PGM greatly hinder the widespread commercialization of these energy conversion and storage devices. Therefore, the development of low-cost, highly active, and stable PGM-free catalysts, to replace the PGM ones, toward ORR, is highly desired. Over the past decades, extensive research was devoted to the development of alternative and low-cost catalysts, including i) non-precious Fe and Co-based metal catalysts, [3,4,[9][10][11] ii) metal oxides, nitrides, and oxynitrides, [12][13][14] and iii) metal-free catalysts such as carbon-based materials. [15] Oxides from metals belonging to groups 4 and 5 (Ti, V, Zr, Nb, Hf, Ta) have attracted interest in these fields, mainly due to their low cost compared to PGM catalysts, environmental compatibility, and excellent stability in acid medium. [12,[16][17][18][19] It has been demonstrated that these oxides can have some activity toward ORR, if their stoichiometry is conveniently tailored to generate a high concentration The oxygen reduction reaction (ORR) is one of the most important reactions in renewable energy conversion and storage devices. The full deployment of these devices depends on the development of highly active, stable, and low-cost catalysts. Herein, a new hybrid material consisting of Na 2 Ta 8 O 21−x / Ta 2 O 5 /Ta 3 N 5 nanocrystals grown on N-doped reduced graphene oxide is reported. This catalyst shows a significantly enhanced ORR activity by ≈4 orders of magnitude in acidic media and by ≈2 orders of magnitude in alkaline media compared to individual Na 2 Ta 8 O 21−x on graphene. Moreover, it has excellent stability in both acid and alkaline media. It also has much better methanol tolerance than the commercial Pt/C, which is relevant to methanol fuel cells. The high ORR activity arises not only from the synergistic effect among the three Ta phases, but also from the concomitant nitrogen doping of the reduced graphene oxide nanosheets. A correlation between ORR activity and nitrogen content is demonstrated. Deep insights into the mechanism of the synergistic effect among these three Ta-based phases, which boosts the ORR's kinetics, are acquired by combining specific experiments and density functional theory calculations.

Section: Methodssupporting

confidence: 89%

Engineering of a Low‐Cost, Highly Active, and Durable Tantalate–Graphene Hybrid Electrocatalyst for Oxygen Reduction

Sebastián

Advanced Energy Materials

et al. 2020

“…Dopants used in graphene for electrodes to date have been mainly classified into two types: (1) small molecules 3 , 8 , 25 – 35 , and (2) transition metal oxides 14 , 36 , 37 ; both exploit charge transfer on the graphene surface. Charge-transfer doping of graphene with small-molecule dopants such as inorganic small-molecule acids (e.g., HNO 3 , HCl, H 2 SO 4 ) 3 , 8 , 25 and metal chlorides (e.g., AuCl 3 , FeCl 3 ) 8 , 26 – 29 has been widely used and developed to increase the electrical conductivity of graphene; however, graphene that is doped with inorganic small-molecule acid has serious environmental instability, which has been considered as a great impediment to practical application of graphene electrodes. R sh of an HNO 3 -doped graphene gradually increases under ambient conditions due to high volatility of small-molecule acids, and high temperature accelerates significant degradation in its electrical conductivity 8 .…”

Section: Introductionmentioning

confidence: 99%

“…R sh of an HNO 3 -doped graphene gradually increases under ambient conditions due to high volatility of small-molecule acids, and high temperature accelerates significant degradation in its electrical conductivity 8 . After doping with metal chlorides, reduction of metal cations can produce metal particles on the graphene surface; these have two deleterious effects: they can decrease the optical transmittance (OT) of the graphene, and if they are large, they protrude and provide leakage paths for electrical current in thin-film devices 8 , 26 – 29 . Also, transition metal oxides are not uniformly deposited on the graphene surface because the graphene lacks dangling bonds or surface functional groups.…”

Section: Introductionmentioning

confidence: 99%

Extremely stable graphene electrodes doped with macromolecular acid

et al. 2018

Although conventional p-type doping using small molecules on graphene decreases its sheet resistance (Rsh), it increases after exposure to ambient conditions, and this problem has been considered as the biggest impediment to practical application of graphene electrodes. Here, we report an extremely stable graphene electrode doped with macromolecular acid (perfluorinated polymeric sulfonic acid (PFSA)) as a p-type dopant. The PFSA doping on graphene provides not only ultra-high ambient stability for a very long time (> 64 days) but also high chemical/thermal stability, which have been unattainable by doping with conventional small-molecules. PFSA doping also greatly increases the surface potential (~0.8 eV) of graphene, and reduces its Rsh by ~56%, which is very important for practical applications. High-efficiency phosphorescent organic light-emitting diodes are fabricated with the PFSA-doped graphene anode (~98.5 cd A−1 without out-coupling structures). This work lays a solid platform for practical application of thermally-/chemically-/air-stable graphene electrodes in various optoelectronic devices.

Journal of Alloys and Compounds

“…Based on their results, alkali metal adatoms generate the largest dipole moments and result in the largest decrease in WF of graphene. The use of Au, which is an electron acceptor element, led to an increase in the WF of graphene.Increasing the WF of graphene via the Au ion was also reported[22,26,[78][79][80].Benayad et al investigated the tuning of the electronic structure and the WF of rGO via Au treatment [78]. Increasing the WF of rGO via Au ions (from 4.5 eV to 4.9 eV) was achieved by extracting electrons from rGO sheets and p-doping of the rGO layers.…”

mentioning

confidence: 99%

Tuning the work function of graphene toward application as anode and cathode

Naghdi

Sánchez‐Arriaga

Rhee

2019

The rapid technological progress in the 21 st century demands new multi-functional materials applicable to a wide variety of industries. Two-dimensional (2D) materials are predicted to have a revolutionary impact on the cost, size, weight, and functions of future electronic and optoelectronic devices. Graphene, which shows potential as an alternative to conventional conductive transparent metal oxides, may play a central role. Since its work function (WF) is tunable, graphene exhibits the interesting ability to serve two different roles in electronic and optoelectronic devices, both as an anode and a cathode.After introducing some basic concepts, this work reviews the most important advances in controlling the tuned WF of graphene, highlighting special features of graphene electronic band structure and recognizing different methods for measuring WF. The impact of thickness, type of contact, chemical doping, UV and plasma treatments, defects, and functional groups of graphene oxide are considered and related with the applications of the modulated material. The results of the review, organized in lookup tables, have been used to identify the advantages and main challenges of the tuning methods.