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The application of Co_2‐xRh_xP nanoparticles as electrocatalysts for the hydrogen evolution reaction (HER) and overall water splitting in basic media is reported. The experimental design seeks to dilute rhodium with earth‐abundant cobalt as a means to lower the cost of the material and achieve catalytic synergism, as reported for related bimetallic phosphides. The HER activity of Co_2‐xRh_xP is found to be composition‐dependent, with the rhodium‐rich compositions being more active as compared to their cobalt‐rich counterparts, with overpotentials (η) at 10 mA/cm²_geometricof 58.1–63.9 mV vs. 82.1–188.1 mV, respectively. In contrast, Co‐rich Co_2‐xRh_xP nanoparticles are active for the oxygen evolution reaction (OER) process in basic media, with η= 290 mV for x=0.25. A full water electrolysis cell was created using the most active compositions for OER and HER as the anode and cathode, respectively, generating an overall η= 390 mV. Notably, the cell became more active over a 50 h stability test, increasing by 2 mV/cm²_geometricat a constant applied voltage of 1.62 V vs NHE. This enhanced activity correlates with nanoscale phase segregation of Rh in the anode. Thus, the lower overpotential achieved for Co_1.75Rh_0.25P relative to Co₂P, and the augmented activity over time in the former, may be a consequence of restructuring of the anode driven by Rh phase‐segregation. The augmentation in activity at the anode more than compensates for small losses at the cathode.

 

Relative to conventional chemical approaches, electrochemical assembly of metal chalcogenide nanoparticles enables the use of two additional levers for tuning the assembly process: electrode material and potential. In our prior work, oxidative and metal-mediated pathways for electrochemical assembly of metal chalcogenide quantum dots (QDs) into three-dimensional gel architectures were investigated independently by employing a noble-metal (Pt) electrode at relatively high potentials and a non-noble metal electrode at relatively low potentials, respectively. In the present work, we reveal competition between the two electrogelation pathways under the condition of high oxidation potentials and non-noble metal electrodes (including Ni, Co, Zn, and Ag), where both pathways are active. We found that the electrogel structure formed under this condition is electrode material-dependent. For Ni, the major phase is oxidative electrogel, not a potential-dependent mixture of oxidative and metal-mediated electrogel that one would expect. A mechanistic study reveals that the metal-mediated electrogelation is suppressed by dithiolates, a side product from the oxidative electrogelation, which block the Ni electrode surface and terminate metal ion release. In contrast, for Co, Ag, and Zn, the electrode surface blockage by dithiolates is less effective than for Ni, such that metal-mediated electrogelation is the primary gelation pathway. 

The ternary manganese pnictide phases, MnAs 1− x Sb x , are of interest for magnetic refrigeration and waste heat recovery due to their magnetocaloric properties, maximized at the Curie temperature ( T C ), which varies from 580–240 K, depending on composition. Nanoparticles potentially enable application in microelectronics (cooling) or graded composites that can operate over a wide temperature range, but manganese pnictides are synthetically challenging to realize as discrete nanoparticles and their fundamental magnetic properties have not been extensively studied. Accordingly, colloidal synthesis methods were employed to target discrete MnAs x Sb 1− x nanoparticles ( x = 0.1–0.9) by arrested precipitation reactions of Mn 2 (CO) 10 with (C 6 H 5 ) 3 AsO and (C 6 H 5 ) 3 Sb in coordinating solvents. The MnAs x Sb 1− x particles are spherical in morphology with average diameters 10–13 nm (standard deviations <20% based on transmission electron microscopy analysis). X-Ray fluorescence spectroscopy measurements on ensembles showed that all phases had an excess of Sb relative to the targeted composition, whereas energy dispersive spectroscopic mapping data of single particles revealed that the nanoparticles are inhomogeneous, adopting a core–shell structure, with the amorphous shell rich in Mn and O (and sometimes Sb) while the crystalline core is rich in Mn, As, and Sb. Magnetization measurements of the nanoparticle ensemble demonstrated the presence of both ferromagnetic and paramagnetic phases. By combining the magnetization measurements with precision chemical mapping and simple modeling, we were able to unambiguously attribute ferromagnetism to the MnAs x Sb 1− x crystalline core, whereas paramagnetism was attributed to the amorphous shell. Magnetization measurements at variable temperatures were used to determine the superparamagnetic transition of the nanoparticles, although for some compositions and particle sizes the blocking temperature exceeded room temperature. Preliminary magnetic studies also revealed a conventional dependence between core size and coercivity, in spite of variable compositions of the nanoparticles, an unexpected result.