Here we report the use of a molecule-to-cluster strategy for preparing ultrasmall (~8 Å) trimetallic Co-Fe-W clusters with high efficiency for the OER in alkaline media. Besides the excellent performance as OER catalyst, our molecule-to-cluster strategy allows to achieve well-defined transition-metal clusters in the subnanometer regime.

Xin-Bao Han, Xing-Yan Tang, Yue Lin, Eduardo Gracia-Espino, San-Gui Liu, Hai-Wei Liang, Guang-Zhi Hu, Xin-Jing Zhao, Hong-Gang Liao, Yuan-Zhi Tan, Thomas Wagberg, Su-Yuan Xie, Lan-Sun Zheng

"Ultrasmall Abundant Metal-Based Clusters as Oxygen-Evolving Catalysts"
J. Am. Chem. Soc. (2019) DOI: 10.1021/jacs.8b09076

The oxygen evolution reaction is a crucial step in water electrolysis to develop clean and renewable energy. Although noble metal-based catalysts have demonstrated high activity for the oxygen evolution reaction, their application is limited by their high cost and low availability. Here we report the use of a molecule-to-cluster strategy for preparing ultrasmall trimetallic clusters by using the polyoxometalate molecule as a precursor. Ultrafine (0.8 nm) transition-metal clusters with controllable chemical composition are obtained. The transition-metal clusters enable highly efficient oxygen evolution through water electrolysis in alkaline media, manifested by an overpotential of 192 mV at 10 mA cm–2, a low Tafel slope of 36 mV dec–1, and long-term stability for 30 h of electrolysis. We note, however, that besides the excellent performance as an oxygen evolution catalyst, our molecule-to-cluster strategy provides a means to achieve well-defined transition-metal clusters in the subnanometer regime, which potentially can have an impact on several other applications.

We report the production of antimony (Sb5+) doped hematite photoanodes by a simple ex situ method. The Sb5+ dopant acts as effective electron donor, reduces recombination effects and improves the surface wettability. In addition, the presence of Sb5+ states in Sb-doped Fe2O3 photoanodes exhibit a 10-fold increase in carrier concentration and decreased photoanode/electrolyte charge transfer resistance.

A. Annamalai, R. Sandström, E. Gracia-Espino, N. Boulanger, J.-F. Boily, I. Mühlbacher, A. Shchukarev, T. Wågberg. ACS Appl. Mater. Interfaces.
DOI: 10.1021/acsami.8b02147

To exploit the full potential of hematite (α-Fe2O3) as an efficient photoanode for water oxidation, the redox processes occurring at the Fe2O3/electrolyte interface need to be studied in greater detail. Ex situ doping is an excellent technique to introduce dopants onto the photoanode surface and to modify the photoanode/electrolyte interface. In this context, we selected antimony (Sb5+) as the ex situ dopant because it is an effective electron donor and reduces recombination effects and concurrently utilize the possibility to tuning the surface charge and wettability. In the presence of Sb5+ states in Sb-doped Fe2O3 photoanodes, as confirmed by X-ray photoelectron spectroscopy, we observed a 10-fold increase in carrier concentration (1.1 × 1020 vs 1.3 × 1019 cm–3) and decreased photoanode/electrolyte charge transfer resistance (∼990 vs ∼3700 Ω). Furthermore, a broad range of surface characterization techniques such as Fourier-transform infrared spectroscopy, ζ-potential, and contact angle measurements reveal that changes in the surface hydroxyl groups following the ex situ doping also have an effect on the water splitting capability. Theoretical calculations suggest that Sb5+ can activate multiple Fe3+ ions simultaneously, in addition to increasing the surface charge and enhancing the electron/hole transport properties. To a greater extent, the Sb5+- surface-doped determines the interfacial properties of electrochemical charge transfer, leading to an efficient water oxidation mechanism.

This time the influence of tetravalent dopants such as Si4+, Sn4+, Ti4+, and Zr4+ on hematite nanostructure for enhanced photoelectrochemical water splitting is reported. The photoactivity of the doped photoanodes at 1.23 V RHE follows the order Zr > Sn > Ti > Si. The work was performed in collaboration with Prof. Jum Suk Jang (Chonbuk National University, Korea), and the results are published in the journal of Applied Surface Science.

A. Subramanian, E. Gracia-Espino, A. Annamalai, H. H. Lee, S Y. Lee, S. H. Choi, and J. S. Jang. Applied Surface Science (2017).
DOI: 10.1016/j.apsusc.2017.09.042

In this paper, the influence of tetravalent dopants such as Si4+, Sn4+, Ti4+, and Zr4+ on the hematite (α-Fe2O3) nanostructure for enhanced photoelectrochemical (PEC) water splitting are reported. The tetravalent doping was performed on hydrothermally grown akaganeite (β-FeOOH) nanorods on FTO (fluorine-doped tin-oxide) substrates via a simple dipping method for which the respective metal-precursor solution was used, followed by a high-temperature (800° C) sintering in a box furnace. The photocurrent density for the pristine (hematite) photoanode is ∼0.81 mA/cm2 at 1.23 VRHE, with an onset potential of 0.72 VRHE; however, the tetravalent dopants on the hematite nanostructures alter the properties of the pristine photoanode. The Si4+-doped hematite photoanode showed a slight photocurrent increment without a changing of the onset potential of the pristine photoanode. The Sn4+- and Ti4+-doped hematite photoanodes, however, showed an anodic shift of the onset potential with the photocurrent increment at a higher applied potential. Interestingly, the Zr4+-doped hematite photoanode exhibited an onset potential that is similar to those of the pristine and Si4+-doped hematite, but a larger photocurrent density that is similar to those of the Sn4+- and Ti4+-doped photoanodes was recorded. The photoactivity of the doped photoanodes at 1.23 VRHE follows the order Zr > Sn > Ti > Si. The onset-potential shifts of the doped photoanodes were investigated using the Ab initio calculations that are well correlated with the experimental data. X-ray diffraction (XRD) and scanning-electron microscopy (FESEM) revealed that both the crystalline phase of the hematite and the nanorod morphology were preserved after the doping procedure. X-ray photoelectron spectroscopy (XPS) confirmed the presence of the tetravalent dopants on the hematite nanostructure. The charge-transfer resistance at the various interfaces of the doped photoanodes was studied using impedance spectroscopy. The doping on the hematite photoanodes was confirmed using the Mott-Schottky (MS) analysis.

Hierarchical macroporous carbon foam decorated with cobalt oxide nanoparticles exhibit excellent performance for oxygen evolution reaction (OER). The observed electrocatalytic performance is rationalized by the overall 3D macroporous structure and with the firmly integrated CNTs directly grown on the foam. The work is a collaboration with Prof. Mikkola (Umeå, Sweden). The results are published in the journal of Scientific Reports.

Tung Ngoc Pham, Tiva Sharifi, Robin Sandström, William Siljebo, Andrey Shchukarev, Krisztian Kordas, Thomas Wågberg, and Jyri-Pekka Mikkola
Scientific Reports, 7, 6112 (2017) (Download)

Herein we report a 3D heterostructure comprising a hierarchical macroporous carbon foam that incorporates mesoporous carbon nanotubes decorated with cobalt oxide nanoparticles as an unique and highly efficient electrode material for the oxygen evolution reaction (OER) in electrocatalytic water splitting. The best performing electrode material showed high stability after 10 h, at constant potential of 1.7 V vs. RHE (reversible hydrogen electrode) in a 0.1 M KOH solution and high electrocatalytic activity in OER with low overpotential (0.38 V vs RHE at 10 mA cm−2). The excellent electrocatalytic performance of the electrode is rationalized by the overall 3D macroporous structure and with the firmly integrated CNTs directly grown on the foam, resulting in a large specific surface area, good electrical conductivity, as well as an efficient electrolyte transport into the whole electrode matrix concurrent with an ability to quickly dispose oxygen bubbles into the electrolyte. The eminent properties of the three-dimensional structured carbon matrix, which can be synthesized through a simple, scalable and cost effective pyrolysis process show that it has potential to be implemented in large-scale water electrolysis systems.