Engineering surface states of hematite based photoanodes for boosting photoelectrochemical water splitting

Hematite-based photoanodes are promising candidates for photoelectrochemical water splitting. However, the performance of pristine hematite semiconductors is unsatisfactory due to charge recombination occurring at different interfaces: Back contact, bulk and semiconductor/electrolyte interfaces. Increasing efforts have been focused on enhancing the performance of hematite based photoanodes via nanostructure control, doping, heterojunction construction, and surface modification with a secondary semiconductor or an oxygen evolution electrocatalyst. Most of the previous studies attributed the enhanced PEC water splitting performance to the changes in the donor density via doping, the formation of type II heterojunction via a secondary semiconductor coating and the improved water oxidation kinetics via coating oxygen evolution electrocatalysts. However, the role of surface states presented at the semiconductor/electrolyte interfaces of hematite-based photoanodes has been overlooked in previous investigations, which virtually play a critical role in determining the photoelectrochemical water oxidation process. In this review, we summarize the recent progress of various techniques employed for the detection of surface states of hematite photoanodes and highlight the important role of modifying surface states in the development of high performance hematite based photoanodes for photoelectrochemical water splitting application. The challenges and future prospects in the study of hematite based photoanodes are also discussed

Engineering the Interfacial Microenvironment via Surface Hydroxylation to Realize the Global Optimization of Electrochemical CO Reduction

The adsorption and activation of CO on the electrode interface is a prerequisite and key step for electrocatalytic CO reduction reaction (eCO RR). Regulating the interfacial microenvironment to promote the adsorption and activation of CO is thus of great significance to optimize overall conversion efficiency. Herein, a CO-philic hydroxyl coordinated ZnO (ZnO-OH) catalyst is fabricated, for the first time, via a facile MOF-assisted method. In comparison to the commercial ZnO, the as-prepared ZnO-OH exhibits much higher selectivity toward CO at lower applied potential, reaching a Faradaic efficiency of 85% at −0.95 V versus RHE. To the best of our knowledge, such selectivity is one of the best records in ZnO-based catalysts reported till date. Density functional theory calculations reveal that the coordinated surficial −OH groups are not only favorable to interact with CO molecules but also function in synergy to decrease the energy barrier of the rate-determining step and maintain a higher charge density of potential active sites as well as inhibit undesired hydrogen evolution reaction. Our results indicate that engineering the interfacial microenvironment through the introduction of CO-philic groups is a promising way to achieve the global optimization of eCO RR via promoting adsorption and activation of CO

NiSn bimetallic nanoparticles as stable electrocatalysts for methanol oxidation reaction

Nickel is an excellent alternative catalyst to high cost Pt and Pt-group metals as anode material in direct methanol fuel cells. However, nickel presents a relatively low stability under operation conditions, even in alkaline media. In this work, a synthetic route to produce bimetallic NiSn nanoparticles (NPs) with tuned composition is presented. Through co-reduction of the two metals in the presence of appropriate surfactants, 3–5¿nm NiSn NPs with tuned Ni/Sn ratios were produced. Such NPs were subsequently supported on carbon black and tested for methanol electro-oxidation in alkaline media. Among the different stoichiometries tested, the most Ni-rich alloy exhibited the highest electrocatalytic activity, with mass current density of 820¿mA¿mg-1 at 0.70¿V (vs. Hg/HgO). While this activity was comparable to that of pure nickel NPs, NiSn alloys showed highly improved stabilities over periods of 10,000¿s at 0.70¿V. We hypothesize this experimental fact to be associated to the collaborative oxidation of the byproducts of methanol which poison the Ni surface or to the prevention of the tight adsorption of these species on the Ni surface by modifying its surface chemistry or electronic density of states.Peer ReviewedPostprint (author's final draft

Superior methanol electrooxidation performance of (110)-faceted nickel polyhedral nanocrystals

We present the synthesis of (110)-faceted nickel polyhedral nanocrystals (NCs) and their characterization as electrocatalysts for the methanol oxidation reaction (MOR). Ni NCs were produced at 180 °C through the reduction in solution of a Ni salt. They were combined with carbon black and Nafion and deposited over glassy carbon to study their electrocatalytic properties. Electrodes based on (110)-faceted Ni NCs displayed a first order reaction with KOH in the concentration range from 0.1 M to 1.0 M. These electrodes were characterized by higher coverages of active species, but lower diffusion coefficients of the species limiting the reaction rate when compared with electrodes prepared from spherical Ni NCs. Overall, electrodes based on faceted Ni NCs displayed excellent performance with very high current densities, up to 61 mA cm-2, and unprecedented mass activities, up to 2 A mg-1, at 0.6 V vs. Hg/HgO in 1.0 M KOH containing 1.0 M methanol. These electrodes also displayed a notable stability. While they suffered an activity loss of ca. 30% during the first 10 000 s of operation, afterward activity stabilized at very high current densities, ~35 mA cm-2, and mass activities, ~1.2 A mg-1, with only a 0.5% decrease during operation from 20 000 to 30 000 s.Peer ReviewedPostprint (author's final draft

A universal strategy for metal oxide anchored and binder-free carbon matrix electrode : a supercapacitor case with superior rate performance and high mass loading

Financial support from China Fund KU Leuven (ISP/13/02SJT) is acknowledged. J. Luo acknowledges the Research Foundation - Flanders (FWO) for FWO Postdoctoral Fellowship (12F5514N), a Research Grant (Project number: 1529816N) and a travel grant (V410316N) for a Visiting Professorship in Technical University of Denmark. X. Zhang is grateful to the China Scholarship Council. We thank Prof. Dirk De Vos (KU Leuven) for technical discussions, Prof. Lei Li (Shanghai Jiao Tong University) for providing nickel foams and Prof. Qingfeng Li (Technical University of Denmark) for assistance in TEM measurements. Appendix ADespite the significant advances in preparing carbon-metal oxide composite electrodes, strategies for seamless interconnecting of these two materials without using binders are still scarce. Herein we design a novel method for in situ synthesis of porous 2D-layered carbon-metal oxide composite electrode. Firstly, 2D-layered Ni-Co mixed metal-organic frameworks (MOFs) are deposited directly on nickel foam by anodic electrodeposition. Subsequent pyrolysis and activation procedure lead to the formation of carbon-metal oxides composite electrodes. Even with an ultrahigh mass loading of 13.4 mg cm, the as-prepared electrodes exhibit a superior rate performance of 93% (from 1 to 20 mA cm), high capacitance (2098 mF cm at a current density of 1 mA cm), low resistance and excellent cycling stability, making them promising candidates for practical supercapacitor application. As a proof of concept, several MOF derived electrodes with different metal sources have also been prepared successfully via the same route, demonstrating the versatility of the proposed method for the preparation of binder-free carbon-metal oxide composite electrodes for electrochemical devices

Solvothermal Synthesis, Gas-Sensing Properties, and Solar Cell-Aided Investigation of TiO2-MoOx Nanocrystals

Titania anatase nanocrystals were prepared by sol-gel/solvothermal synthesis in oleic acid at 250 °C, and modified by co-reaction with Mo chloroalkoxide, aimed at investigating the effects on gas-sensing properties induced by tailored nanocrystals surface modification with ultra-thin layers of MoO species. For the lowest Mo concentration, only anatase nanocrystals were obtained, surface modified by a disordered ultra-thin layer of mainly octahedral Mo oxide species. For larger Mo concentrations, early MoO phase segregation occurred. Upon heat treatment up to 500 °C, the sample with the lowest Mo concentration did not feature any Mo oxide phase segregation, and the surface Mo layer was converted to dense octahedral Mo oxide. At larger Mo concentrations all segregated MoO was converted to MoO. The two different materials typologies, depending on the Mo concentration, were used for processing gas-sensing devices and tested toward acetone and carbon monoxide, which gave a greatly enhanced response, for all Mo concentrations, to acetone (two orders of magnitude) and carbon monoxide with respect to pure TiO. For the lowest Mo concentration, dye-sensitized solar cells were also prepared to investigate the influence of anatase surface modification on the electrical transport properties, which showed that the charge transport mainly occurred in the ultra-thin MoO surface layer

Compositionally tuned NixSn alloys as anode materials for lithium-ion and sodium-ion batteries with a high pseudocapacitive contribution

Nickel tin alloy nanoparticles (NPs) with tuned composition NixSn (0.6 ≤ x ≤ 1.9) were synthesized by a solution-based procedure and used as anode materials for Li-ion batteries (LIBs) and Na-ion batteries (SIBs). Among the compositions tested, Ni₀₉Sn-based electrodes exhibited the best performance in both LIBs and SIBs. As LIB anodes, Ni₀₉Sn-based electrodes delivered charge-discharge capacities of 980 mAh g⁻¹ after 340 cycles at 0.2 A g⁻¹ rate, which surpassed their maximum theoretical capacity considering that only Sn is lithiated. A kinetic characterization of the charge-discharge process demonstrated the electrode performance to be aided by a significant pseudocapacitive contribution that compensated for the loss of energy storage capacity associated to the solid-electrolyte interphase formation. This significant pseudocapacitive contribution, which not only translated into higher capacities but also longer durability, was associated to the small size of the crystal domains and the proper electrode composition. The performance of NixSn-based electrodes toward Na-ion storage was also characterized, reaching significant capacities above 200 mAh g⁻¹ at 0.1 A g⁻¹ but with a relatively fast fade over 120 continuous cycles. A relatively larger pseudocapacitive contribution was obtained in Ni Sn-based electrodes for SIBs when compared with LIBs, consistently with the lower contribution of the Na ion diffusion associated to its larger size

Co–Sn nanocrystalline solid solutions as anode materials in lithium-ion batteries with high pseudocapacitive contribution

Co–Sn solid-solution nanoparticles with Sn crystal structure and tuned metal ratios were synthesized by a facile one pot solution-based procedure involving the initial reduction of a Sn precursor followed by incorporation of Co within the Sn lattice. These nanoparticles were used as anode materials for Li-ion batteries. Among the different compositions tested, Co0.7Sn and Co0.9Sn electrodes provided the highest capacities with values above 1500 mAh¿g-1 at a current density of 0.2 A¿g-1 after 220 cycles, and up to 800 mAh¿g-1 at 1.0 A¿g-1 after 400 cycles. Up to 81¿% pseudocapacitance contribution was measured for these electrodes at a sweep rate of 1.0 mV¿s-1, thereby indicating fast kinetics and long durability. The excellent performance of Co–Sn nanoparticle alloy-based electrodes was attributed to both the small size of the crystal domains and their suitable composition, which buffered volume changes of Sn and contributed to a suitable electrode restructuration.Postprint (author's final draft

Metal Oxide Clusters on Nitrogen-Doped Carbon are Highly Selective for CO2Electroreduction to CO

The electrochemical reduction of CO2 (eCO2RR) using renewable energy is an effective approach to pursue carbon neutrality. The eCO2RR to CO is indispensable in promoting C-C coupling through bifunctional catalysis and achieving cascade conversion from CO2 to C2+. This work investigates a series of M/N-C (M = Mn, Fe, Co, Ni, Cu, and Zn) catalysts, for which the metal precursor interacted with the nitrogen-doped carbon support (N-C) at room temperature, resulting in the metal being present as (sub)nanosized metal oxide clusters under ex situ conditions, except for Cu/N-C and Zn/N-C. A volcano trend in their activity toward CO as a function of the group of the transition metal is revealed, with Co/N-C exhibiting the highest activity at -0.5 V versus RHE, while Ni/N-C shows both appreciable activity and selectivity. Operando X-ray absorption spectroscopy shows that the majority of Cu atoms in Cu/N-C form Cu0 clusters during eCO2RR, while Mn/, Fe/, Co/, and Ni/N-C catalysts maintain the metal hydroxide structures, with a minor amount of M0 formed in Fe/, Co/, and Ni/N-C. The superior activity of Fe/, Co/, and Ni/N-C is ascribed to the phase contraction and the HCO3- insertion into the layered structure of metal hydroxides. Our work provides a facile synthetic approach toward highly active and selective electrocatalysts to convert CO2 into CO. Coupled with state-of-the-art NiFe-based anodes in a full-cell device, Ni/N-C exhibits >80% Faradaic efficiency toward CO at 100 mA cm-2.The research leading to these results has received funding from the A-LEAF Project, which is funded by the European Union’s H2020 Programme under grant agreement no. 732840. ICN2 and ICIQ acknowledge funding from the FEDER/Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación (projects ENE2017-85087-C3 and RTI2018-095618-B-I00) and the Generalitat de Catalunya (2017 SGR 327 and 2017- SGR-1406) and by the CERCA Programme / Generalitat de Catalunya. ICN2 and ICIQ are supported by the Severo Ochoa program from Spanish MINECO (grants no. SEV-2017-0706 and CEX2019-000925-S)