Cobaloxime Complex Salts: Synthesis, Patterning on Carbon Nanomembranes and Heterogeneous Hydrogen Evolution Studies

Abstract

Cobaloximes are promising, earth-abundant catalysts for the light-driven hydrogen evolution reaction (HER). Typically, these cobalt(III) complexes are prepared in situ or employed in their neutral form, for example, [Co(dmgH) 2 (py) Cl], even though related complex salts have been reported previously and could, in principle, offer improved catalytic activity as well as more efficient immobilization on solid support. Herein, we report an interdisciplinary investigation into complex salts [Co(dmgH) 2 (pyWe describe their strategic syntheses from the commercially available complex [Co(dmgH) 2 (py)Cl] and demonstrate that these double and single complex salts are potent catalysts for the light-driven HER. We also show that scanning electrochemical cell microscopy can be used to deposit arrays of catalysts [Co(dmgH) 2 (py py)Cl] on supported and free-standing amino-terminated ~1-nm-thick carbon nanomembranes (CNMs). Photocatalytic H 2 evolution at such arrays was quantified with Pd microsensors by scanning electrochemical microscopy, thus providing a new approach for catalytic evaluation and opening up novel routes for the creation and analysis of "designer catalyst arrays", nanoprinted in a desired pattern on a solid support.

Pd‐Modified De‐alloyed Au−Ni‐Microelectrodes for In Situ and Operando Mapping of Hydrogen Evolution

“…A two‐sample t‐test was performed assuming different variance, which shows that the slopes for the long‐term studies and the pH study are within a confidence level of 0.95. The developed Pd‐microsensors may be used in combination with SECM to obtain spatially resolved information on the H 2 evolution at electrocatalytic materials such as Pt microparticles, [39] molybdenum disulfide (MoS 2 ) [15] or light‐driven HER at heterogenized catalysts [7] . To map the H 2 evolution, SECM experiments were carried out in substrate generation/tip collection (SG/TC) mode [40] using the Pd‐modified Au−Ni microelectrode as the SECM probe.…”

“…For mapping the H 2 evolution at heterogenized light‐driven HER catalysts, standard methods for H 2 detection such as head‐space GC are not suitable, and the developed microsensors are a highly promising alternative, as schematically shown in Figure 5a. In a previous report, earth‐abundant cobaloxime complex salts such as the BPh 2 ‐bridged, double salt [Co(dmgH) 2 (py) 2 ] + [Co(dmgBPh 2 ) 2 Cl 2 ] − and the negatively charged TBA + [Co(dmgBPh 2 )Cl 2 ] − were therefore evaluated in respect to homogeneous and heterogeneous HER [7] . For the heterogenized system, the catalysts were deposited as micro‐ and nanoarrays on carbon nanomembranes (NH 2 ‐CNMs) prepared from 4’‐nitro‐1,1’‐biphenyl‐4‐thiol (NBPT) self‐assembled monolayers [41–43] using scanning electrochemical cell microscopy (SECCM), as recently reported in detail [7] and schematically shown in Figure S6a.…”

“…Here, we investigated in‐depth the heterogenized positively charged cobalt catalyst, [Co(dmgH) 2 (py) 2 ] + BArF − (structure is shown in Figure S6a), which was not investigated so far. This is important, as [Co(dmgH) 2 (py) 2 ] + BArF − revealed the highest TONs and TOFs in homogeneous light‐driven catalysis next to the double Co‐salt ([Co(dmgH) 2 (py) 2 ] + [Co(dmgBPh 2 ) 2 Cl 2 ] − ) [7] . We quantitatively determined the light‐driven H 2 evolution of the heterogenized [Co(dmgH) 2 (py) 2 ] + BArF − salt by depositing a microarray on NH 2 ‐CNMs via SECCM (SEM image of the array is shown in Figure S6b).…”

“…Based on the Faraday law, a H 2 concentration of 350.52+/−41.33 μmol/L (n=3) was determined for the [Co(dmgH) 2 (py) 2 ] + BArF − catalyst (Figure 5b), which is almost four times higher compared to the H 2 concentration of 89.45+/−39.29 μmol/L (n=3) obtained for the commercially available, neutral [Co(dmgH) 2 (py)Cl]. Initially performed head‐space GC measurements of homogeneous light‐driven catalysis of [Co(dmgH) 2 (py) 2 ] + BArF − resulted in a TON of 65+/−2, which reflects almost twice the value compared to the neutral catalyst, [Co(dmgH) 2 (py)Cl] with the a TON of 35+/−2 [7] . Taking the active surface area of the investigated microarrays of the two Co‐catalysts into account, which is estimated with 93.6 μm 2 for the [Co(dmgH) 2 (py)Cl] spot array and only 26.3 μm 2 for the [Co(dmgH) 2 (py) 2 ] + BArF − spot array (Figure S6b), the active surface area of the positively charged Co catalysts was only approx.…”

“…Numerous publications deal with photoelectrodes and electrocatalytic materials using nanostructures such as non‐precious nanoparticles [3–5] . More recently, light driven homogeneous and heterogenized systems for water splitting using molecular catalysts have been reported towards the development of efficient artificial photosynthesis systems [6–8] . To determine the efficiency of such heterogenized molecular catalysts during water splitting reactions, head‐space gas chromatography (GC) is the gold standard to quantify H 2 concentrations when determining turn‐over‐numbers (TONs) and turn‐over‐frequencies (TOFs) [9,10] .…”