Active Site Formation for Oxygen Reduction Reaction on Carbon-Support-Free Titanium Oxynitride with Boosted Activity in Acidic Media

Abstract

An active site formation process for recently reported carbon-support-free titanium oxynitride (TiO x N y ) catalysts is investigated to reveal the critical factors for enhancing the oxygen reduction reaction (ORR) activity in acidic media. Neither the titanium source nor the presence of a conductive oxide support is a key to displaying activity whereas an oxidized TiN surface is revealed to be necessary. The ORR activity is successfully enhanced without using a carbon support to the level of the best carbon-supported oxide-based catalyst, zirconium oxynitride on multiwalled carbon nanotube in 0.1 mol dm–3 H2SO4 solution, mainly by optimizing hydrochloric acid concentration in the precursor dispersion to increase the surface nitrogen content. The catalyst is stable; i.e., the activity remained unchanged after 20 000 potential cycles between 0.6 and 1.0 V versus reversible hydrogen electrode.

Efficient Phosphorus Doping into the Surface Oxide Layers on TiN to Enhance Oxygen Reduction Reaction Activity in Acidic Media

“…42 The activity of these catalysts was similar to that of ZrO x N y -MWCNT, which is the best oxide-based catalyst, by optimizing the nitrogen content on the oxidized TiN surface, TiO x N y , without using supports. 43 The source of both conductivity and activity was revealed to be TiO x N y , not the carbon residues from the urea precursor. 43 The crystal structure of the oxidized surface was amorphous 42 or rutile 42−44 TiO 2 , and oxygen vacancies formed by substitutional nitrogen doping were necessary to produce the active sites.…”

“…These catalysts were first synthesized on carbon black, but the ORR activity was dramatically improved by replacing carbon black with Ti 4 O 7 . The activity of these catalysts was similar to that of ZrO x N y -MWCNT, which is the best oxide-based catalyst, by optimizing the nitrogen content on the oxidized TiN surface, TiO x N y , without using supports . The source of both conductivity and activity was revealed to be TiO x N y , not the carbon residues from the urea precursor .…”

“…The source of both conductivity and activity was revealed to be TiO x N y , not the carbon residues from the urea precursor . The crystal structure of the oxidized surface was amorphous or rutile − TiO 2 , and oxygen vacancies formed by substitutional nitrogen doping were necessary to produce the active sites . Without altering the number of the oxygen vacancies, pentavalent phosphorus atoms were recently doped into surface rutile TiO 2 on TiN to reduce the charge transfer resistance .…”

Nanometric Fe-Substituted ZrO2on Carbon Black as PGM-Free ORR Catalyst for PEMFCs

“…), have been reported to be ORR active. [17][18][19][20] The intrinsic thermodynamic stability against dissolution in an acidic environment of valve metal oxides and their reported ORR activity makes this class of materials very attractive. However, in our previous work, we concluded that ORR activities of pure carbon supported ZrO 2 nanoparticles are still far too low for practical PEMFC applications.…”

Nanometric Fe-Substituted ZrO2_Madkikar et al.

“…Another class of PGM-free catalysts is based on valve metal compounds (nitrides, carbonitrides, and oxides of Group 4 and 5 metals); in particular valve metal oxides (ZrO2, Ta2O5, etc. ), have been reported to be ORR active [17][18][19][20]. The intrinsic thermodynamic stability against dissolution in an acidic environment of valve metal oxides and their reported ORR activity makes this class of materials very attractive.…”