2,5-Furandicarboxylic acid (FDCA) has received increasing attention as a near-market platform chemical that can potentially replace terephthalic acid in the production of commercial and high-performance polymers, such as polyethylene terephthalate. FDCA can be obtained from the oxidation of 5-hydroxymethylfurfural (HMF), which is produced from the dehydration of C-6 monosaccharides obtained from cellulosic biomass. Recently, various heterogeneous Ni- and Co-based electrocatalysts were reported that can efficiently oxidize HMF to FDCA. The actual catalytically active species of these catalysts are most likely NiOOH and CoOOH or species related to NiOOH and CoOOH. However, the intrinsic catalytic properties of NiOOH and CoOOH for HMF oxidation have yet to be carefully and systematically investigated. In this study, we prepared thin and thick sets of NiOOH, CoOOH, and FeOOH films having comparable numbers of metal sites to systematically and methodically compare the intrinsic catalytic activity of these materials for HMF oxidation in a 0.1 M KOH (pH 13) solution. Our investigation revealed that they have distinctively different catalytic abilities for HMF oxidation. The use of extremely thin MOOH films containing limited numbers of catalytic sites allowed us to resolve anodic currents that were generated from HMF oxidation by different oxidation pathways. By comparing the voltammetric results of thin and thick films, the effect of the film thickness on the current generated by different oxidation pathways could be observed. The thick set of MOOH films was also used to compare the performances of these films for constant potential HMF oxidation and product analysis. The work herein contributes to a better understanding of the mechanisms of HMF oxidation on Ni-, Co-, and Fe-containing heterogeneous electrocatalysts whose surfaces are covered by their hydroxide and oxyhydroxide phases.

Materials that can selectively store Na and Cl ions in the bulk of their structures and release these ions with good cycle stability can enable the construction of a high capacity, rechargeable desalination cell for use in seawater desalination. In this study, the ability of a nanocrystalline Bi foam electrode to serve as an efficient and high capacity Cl-storage electrode using its conversion to BiOCl was investigated. When Bi as a Cl-storage electrode was coupled with NaTi(PO) as a Na-storage electrode, a new type of rechargeable desalination cell, which is charged during desalination and discharged during salination, was constructed. The resulting Bi-NaTi(PO) cell was tested under various salination and desalination conditions to investigate advantages and potential limitations of using Bi as a Cl-storage electrode. Slow Cl release kinetics of BiOCl in neutral conditions and an imbalance in Cl and Na storage (i.e., Cl storage requires three electrons/Cl, while Na storage requires one electron/Na) were identified as possible drawbacks, but strategies to address these issues were developed. On the basis of these investigations, optimum desalination and salination conditions were identified where the Bi/NaTi(PO) cell achieved a desalination/salination cycle at ±1 mA cm with a net potential input of only 0.20 V. The kinetics of Cl release from BiOCl was significantly improved by the use of an acidic solution, and therefore, a divided cell was used for the salination process. We believe that with further optimizations the Bi/BiOCl electrode will enable efficient and practical desalination applications.

2,5-Furandicarboxylic acid (FDCA) is a key near-market platform chemical that can potentially replace terephthalic acid in various polyesters such as polyethylene terephthalate (PET). FDCA can be obtained from oxidation of 5-hydroxymethylfurfural (HMF), which can be derived from cellulosic biomass through isomerization and dehydration of hexoses. In this study, electrochemical oxidation of HMF to FDCA is demonstrated using Cu, one of the cheapest transition metals, as the catalytic anode. The oxidized Cu surface is not catalytic for water oxidation, which is the major reaction competing with HMF oxidation in aqueous media. Therefore, a wide potential window to oxidize HMF without inducing water oxidation was available, enabling high Faradaic efficiencies for FDCA production. Cu was prepared as nanocrystalline and bulk electrodes by electrodeposition, and key differences in their surface oxidation and electrochemical HMF oxidation were investigated. The oxide and hydroxide layers formed on the nanocrystalline electrode appeared to have an intrinsically different catalytic ability for HMF oxidation from those formed on the bulk electrode. Both the HMF conversion and FDCA production by the nanocrystalline electrode were nearly perfectly proportional to the amount of charge passed with no significant accumulation of any intermediate oxidation product during the course of HMF oxidation. After the stoichiometric amount of charge was passed, the nanocrystalline electrode achieved a FDCA yield of 96.4%. In contrast, the bulk electrode accumulated a significant amount of 5-formyl-2-furancarboxylic acid (FFCA) during HMF oxidation and achieved an FDCA yield of 80.8%. The morphology and composition of the oxide and hydroxide layers formed on the nanocrystalline and bulk electrodes were investigated systematically before and after HMF oxidation.

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Sn nanofibers with a high aspect ratio are successfully synthesized using a simple electrodeposition process from an aqueous solution without the use of templates. The synthetic approach involves the rapid electrochemical deposition of Sn accompanied by the strong adsorption of Triton X-100, which can function as a growth modifier for the Sn crystallites. Triton X-100 is adsorbed on the {200} crystallographic planes of Sn in an elongated configuration and suppressed the preferential growth of Sn along the [100] direction. Consequently, the Sn electrodeposits are forced to grow anisotropically in a direction normal to the (112) or (1̅12) plane, forming one-dimensional nanofibers. As electrode materials for the Na-ion batteries, the Sn nanofibers exhibit a high reversible capacity and an excellent cycle performance; the charge capacity is maintained at 776.26 mAh g(-1) after 100 cycles, which corresponds to a retention of 95.09% of the initial charge capacity. The superior electrochemical performance of the Sn nanofibers is mainly attributed to the high mechanical stability of the nanofibers, which originate from highly anisotropic expansion during sodiation and the pore volumes existing between the nanofibers.