Visible‐Light‐Driven Water Oxidation with Nanoscale Co₃O₄: New Optimization Strategies

Abstract

Water oxidation is the "bottleneck" of artificial photosynthesis on the way to clean and sustainable solar fuels production. Although spinel-type Co3 O4 keeps attracting considerable research interest as a robust and low-cost water oxidation catalyst (WOC), full control over its manifold performance parameters remains a preparative and analytical challenge. The present study screens a wide spectrum of influential factors (such as surface area and adsorption processes, morphology, crystallinity, and cobalt oxidation states) with respect to straightforward WOC optimization strategies. Tuning of crystallinity and cobalt valence states emerges as a major guideline for spinel catalyst synthesis, thus adding a key complementary factor to the main design paradigm of surface area maximization. In contrast to the prevailing amorphous characteristics of electrocatalytic WOCs, crystallinity was found to be crucial for photochemical WOC development. The interplay of synthetic history, crystallinity, and cobalt valence states in hydrothermally synthesized Co3 O4 WOCs was investigated with a variety of analytical techniques, including electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and various electron microscopy and spectroscopic techniques. Complementary "top down" and "bottom up" optimization strategies introduce new Co3 O4 WOC design parameters for facile catalyst production.

MxCo3−xO4 (M = Co, Mn, Fe) porous nanocages derived from metal–organic frameworks as efficient water oxidation catalysts

“…1 It is the most challenging to develop efficient and robust WOCs with earth-abundant elements. Remarkable efforts have therefore been dedicated toward the development of less-expensive transition metal oxides, such as cobalt oxides, [4][5][6][7][8][9][10][11][12][13][14][15][16][17] iron oxides, [18][19][20] nickel oxides [21][22][23][24][25][26] and manganese oxides. Remarkable efforts have therefore been dedicated toward the development of less-expensive transition metal oxides, such as cobalt oxides, [4][5][6][7][8][9][10][11][12][13][14][15][16][17] iron oxides, [18][19][20] nickel oxides [21][22][23][24][25][26] and manganese oxides.…”

Reinvestigation of Water Oxidation Catalyzed by a Dinuclear Cobalt Polypyridine Complex: Identification of CoOx as a Real Heterogeneous Catalyst

“…4 Thus the design and synthesis of efficient water oxidation catalysts (WOCs) based on earth-abundant elements [5][6][7][8][9][10][11][12][13][14][15][16][17][18] remain great challenge. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 Recently, cobalt-based WOCs have been extensively investigated for their remarkable performance, [19][20][21][22][23][24] in which soluble cobalt complexes as homogeneous WOCs 11,[25][26][27][28][29]…”

Monitoring the Hydrothermal Growth of Cobalt Spinel Water Oxidation Catalysts: From Preparative History to Catalytic Activity

“…Quenching experimentso nh ydrothermal Co 3 O 4 formation:T he mostw idespread approach towards optimization of hydrothermally synthesized materials is to change the synthesis parameters and to investigate the products post synthesis. [21,23,42,43] Beyond this empirical approach, the above results illustrate that time-resolvedP XRD measurements are ap owerful tool to unravel growth mechanismso fs uch "black box" type synthetic methods. [26,27] When applying these insights on conventional, non-monitored hydrothermal lab setups, their frequents ensitivity towards experimental parameter adjustments and scale-up should be taken into account.…”

“…[19] Among them, hydro/solvothermal methods offer particularly attractive access to tunable nanosized Co 3 O 4 materials [20] due to their widely adjustable parameter space.I np rinciple, optimization of hydro/solvothermal precursors, oxidants, solvents, surfactants, additives,t emperatures, or reaction times, holds the key to controlling all of the above-mentioned performance parameters of nanoscaleo xide WOCs. [21][22][23] However,t of ully explore this design potential,f urther mechanistic studies are now required to enlighten the mainly empirical" black box" nature of hydro/solvothermal methods.…”

“…Our previousw ork involving the ODISC, for example, revealed the presence of intermediate layered double hydroxides phase during the solvothermal growth of CoGa 2 O 4 . [25] The provens trong dependence of the catalytic activity of Co 3 O 4 WOCs on the materials' parameters [10,17,23,36] renders it an ideal target to control them by synthesis throughi nsighti nto the underlying growth mechanisms. We thus comparer esults from time-resolvedX RD monitoring of hydrothermal Co 3 O 4 growth using the ODISC setupw ith trends of analogous quenching experiments.…”