A Comparison of Electrochemical ORR Activity of Boron in Graphene Oxide; Incorporated As a Charge-Adsorbate and/or Substitutional p-Type Dopant

The present contribution aims to examine electrocatalytic oxygen reduction (ORR) performance of boron (B) in two diverse classes of B-doped graphitic materials; (i) 3-D graphene organic framework (GOF) as a descriptive of adsorbate-induced doped materials and (ii) BGs as illustration of the substitutionally boron-integrated graphene. Pillared GOF is easily synthesised under mild solvothermal condition (reaction temperature of 80 °C for 8 h) by reacting graphene oxide (GO) suspension with 1,4 phenyl diboronic acid as stitches to graphene layers. Trivalent boron is integrated into sp² graphene honeycomb structure using thermal annealing method which is believed to produce fewer layer graphene with less agglomeration, higher surface area and electrical conductivity. Here, for tuneable boron surrogation, mixtures of GO and boric acid (H₃BO₃, as precursor) are annealed at elevated temperatures of 600 and 1200 °C to form substitutionally boron-doped BG1 and BG2. X-ray photoelectron spectroscopy (XPS) analysis verified the total embedded boron contents of 1.90 and 4.19 at% corresponding to BCO₂ and BCO₂/ BC₃ structures in BG1 and BG2, respectively.

The electron transfer efficiency of the state of the art pillared boronate lattices with CBO₂ active chemistries is examined alongside the commonly literature cited boron doped materials (BGs) via cyclic voltammetry (CV) and rotating ring disk (RDE) tests. Generally, collected CV data confirms noticeable catalytic modulation of pristine GO by synergistic electron accepting-donating effect of amalgamated boron in GOF and BGs. In fact, the difference in electronegativity of boron and its neighbouring (C/O) atoms’ provides positively charged site for O₂ chemisorption. Subsequently, B enables to donate a portion of accumulated C=C π conjugation to the adsorbed oxygen from its 2p_z orbital and finally reduces the adsorbed oxygen.

The materials are also subjected to RDE test to obtain Koutechy-Levich plots for estimating the total number of exchanged electrons during O₂ reduction. According to cathodic responses, BG1 completes the ORR in a way similar to that of reduced graphene oxide (rGO) through a 2e^- pathway with H₂O₂ production. This in turn highlights the insignificant functionality of boron during O₂ reduction when it is in CBO₂ form. However, ahead of incorporation of three-coordinated BC₃ in BG2 sample, the reaction is managed by a 4e^- reduction mechanism to produce H₂O. This enlightens the frontier-controlling role of boron as an effective donating pole when it is in BC₃ configuration. Surprisingly, GOF shows an exclusive performance comparable to that of BG2 (with highly active BC3 configuration) which is synthesised under much more rigorous thermal condition.

To conclude, GOF has potential to be considered as a rival to previously introduced graphene doped materials for replacing the expensive Pt/C cathodic catalyst in fuel cells. The advantage of GOF compared to conventional BG materials through literature is considered to be due to the merits of moderate hydrothermal synthesis conditions and hence scale-up capability.