In Situ Observations of Oxygen Evolution Cocatalysts on Photoelectrodes by X-ray Absorption Spectroscopy: Comparison between Cobalt-Phosphate and Cobalt-Borate

Abstract

The differing activities of cobalt-phosphate (Co-P i ) and cobalt-borate (Co-B i ) oxygen evolution cocatalysts photodeposited on SrTiO 3 photoelectrodes under the same conditions for use in water splitting were investigated using in situ X-ray absorption fine structure (XAFS) spectroscopy. Prior to XAFS analyses, the photoelectrochemical water oxidation activities of both cocatalysts were assessed by linear sweep voltammetry, with results demonstrating that the Co-B i cocatalyst enhances oxygen evolution to a greater degree than the Co-P i . Co Kedge XAFS spectra were acquired for both cocatalysts on SrTiO 3 photoelectrodes during photoelectrochemical water splitting. The XAFS spectrum of the Co-B i was significantly more intense than that of the Co-P i , indicating that a greater concentration of the Co-B i cocatalyst was present on the photoelectrode compared with the Co-P i . The results of this study demonstrate that both the Co-P i and Co-B i cocatalysts are able to efficiently promote water oxidation, and that the Co-B i functions more effectively than the Co-P i because it generates a greater abundance of reaction sites.

In Situ Formation of a 3D Amorphous Cobalt‐ Borate Nanoarray: An Efficient Non‐Noble Metal Catalytic Electrode for Non‐Enzyme Glucose Detection

“…Meanwhile, amorphous materials have received wide interest for exposing a large number of catalytic active sites and easy accessibility to target molecules ,. Cobalt‐borate (Co−Bi) as one of an interesting amorphous film has usually been developed to improve catalytic activity for water oxidation . So, we estimate that the amorphous Co−Bi nanoarray as catalyst electrode can offer us a superior performance for non‐enzymatic glucose sensing, which, however, has not been reported before.…”

Mixed Valent, Distorted Cobalt Ludwigite (Co3BO5/Co3O2BO3) and Its Composite with Reduced Multiwalled Carbon Nanotubes (R‐MWCNT) in Enhancing the Domain Edge‐Sharing Oxygen as Superior Water Oxidation Electrocatalysts

“…Herein, we report a Co 3 BO 5 ‐reduced‐multiwalled carbon nanotubes (R‐MWCNTs) hybrid (overpotential, η=270 mV@10 mA cm −2 ) as an OER electrocatalyst which even outperforms the outstanding activity of its Co−B i ‐graphene nanosheet analogue (η=290 mV@10 mA cm −2 , in alkaline medium) and standard RuO 2 (η=370 mV@10 mA cm −2 ). The earlier reports on OER mechanism and active sites of these metal‐borate or phosphate systems suggest that Co−P i and Co−B i are composed of assemblies of several nanometer nanoclusters, with a domain consisting of edge‐sharing CoO 6 octahedra ,. The domain edge sharing oxygen groups are responsible for the high activity of Co−P i and Co−B i compared to domain inside oxygen groups.…”

“…So the extensive researches in this area have been fully focused on the earth‐abundant metal‐based electrocatalysts with/without carbon support especially on Co‐based electrocatalyts owing to their benign operating conditions, robustness and self healing properties . Extensive spectroscopic studies on Cobalt and its derivatives like Cobalt phosphate (Co−P i ), Cobalt boride (CoB/Co 2 B), Cobalt borate (Co−B i ), Cobalt oxide (CoO), Cobalt hydroxide (Co(OH) 2 ), Cobalt oxy‐hydroxide and Co 3 O 4 have been devoted with or without conducting carbon support like graphene and CNTs for OER ,,. Spectroscopic tools are important for identifying the intermediates for mechanistic studies.…”

“…Hence, in‐situ X‐ray absorption fine structure (XAFS) spectroscopy and X‐ray pair distribution function (PDF) studies have enlighten the understanding of OER mechanism and active sites ofmetal‐borate or phosphate systems (Co−B i /Co−P i /Ni−P i /Ni−B i ) tremendously ,. Among all this class of materials, Cobalt and nickel borates (Co−B i : Co 3 B 2 O 6 ; Ni−B i ) have been explored recently for their outstanding performance in OER in neutral medium by in‐situ synthesis (η: Co−B i : 315 mV, Co−P i : 390 mV@10 mA cm −2 , η: Co−B i : 363 mV, Co−P i : 442 mV@100 mA cm −2 ),,, and also alkaline medium along with graphene support (η: Co−B i : 345 mV, η: Co−B i ‐nano graphene sheets: 290 mV@10 mA cm −2 ) . Here the Co−B i is found to be more active than that of Co−P i .…”

“…Among all this class of materials, Cobalt and nickel borates (Co−B i : Co 3 B 2 O 6 ; Ni−B i ) have been explored recently for their outstanding performance in OER in neutral medium by in‐situ synthesis (η: Co−B i : 315 mV, Co−P i : 390 mV@10 mA cm −2 , η: Co−B i : 363 mV, Co−P i : 442 mV@100 mA cm −2 ),,, and also alkaline medium along with graphene support (η: Co−B i : 345 mV, η: Co−B i ‐nano graphene sheets: 290 mV@10 mA cm −2 ) . Here the Co−B i is found to be more active than that of Co−P i . Despite the several researches on Co−B i (Co 3 B 2 O 6 , Orthorhombic crystal structure, space group: P nmn ), another analogue of Co−B i , called cobalt boron oxide/ cobalt oxyborate with the empirical formula Co 3 BO 5 (also known as Cobalt‐ludwigite, Orthorhombic crystal structure, space group: P bam ) has not been explored yet for OER till date although the magnetic propertiesand structural features have been studied well after incorporating iron in the lattice ,…”