Well-defined boron/nitrogen doped polycyclic aro-matic hydro-carbons are active electrocatalysts for the oxygen reduction reactionRachel J. Kahan, Wisit Hirunpinyopas, Jessica Cid, Michael J. Ingleson* and Robert A. W. Dryfe*School of Chemistry, University of Manchester, Manchester, M13 9PL, United KingdomABSTRACT: Top down synthesized B and B,N-doped carbons (e.g. graphenes) have been previously re-ported as catalysts for the oxygen reduction reaction (ORR), with activity superior to Pt electrocatalysts also previously reported in some cases. Such doped carbon materials are, however, chemically complex and contain multiple sites which complicates the development of structure activity relationships and thus subsequent catalyst optimisation. Herein, a number of well-defined B and B,N-doped polycyclic aro-matic hydrocarbons (PAHs), prepared by a “bottom up” approach, are shown to be active catalysts for the ORR in alkaline solution when deposited on carbon electrodes in contrast to the all carbon based PAH perylene. Six dissimilar B-doped-PAHs have been tested on three working electrodes and the merits of each electrode for assessing ORR catalytic activity determined. A boron doped diamond electrode was found to have the lowest background activity (relative to glassy carbon and HOPG) and thus proved op-timal for determining the ORR catalytic activity of the PAHs. Of the six B doped-PAHs studied the two PAHs with the highest LUMO energy were found to be inactive, while the other PAHs with lower LUMO en-ergies were found to be active catalysts for the ORR. Proximal doping of two heteroatoms, doubly B doped and a B,N co-doped PAH containing separate (non-bonded) B and N atoms, was found to lead to the most active ORR catalysts from this set. This suggests that two proximal electrophilic sites improve the ORR activity of doped carbons. This is the first study, to the best of our knowledge, which uses well defined doped PAHs as models to identify potential ORR electrocatalytic moieties present in heteroatom-doped carbon materials: this approach thus enables definitive structure activity relationships to be de-veloped in this important area.

The oxygen reduction reaction (ORR) is a crucial process in metal-air batteries and in fuel cells.1

While platinum-based catalysts are currently used for the ORR in fuel cells a combination of high cost, poor long term durability and poisoning (e.g. by CO / MeOH) has hampered mass commercial-isation of this technology.1,2 The development of carbon materials that are electrocatalysts for the ORR has attracted significant interest particularly over the last decade as the need for renewable energy alternatives to fossil fuels has increased.3-6

A range of earth abundant p block element-doped carbon materials have been previously reported as ORR catalysts,7,8 including B/N-doped carbons, some of which are reported to display comparable or superior electrocatalytic performance to plat-inum without the aforementioned drawbacks.9-13

The preparation, characterisation and perform-ance of heteroatom-doped carbon electrocata-lysts has been reviewed extensively.8, 14-20 How-ever, the “top down” synthetic approaches pro-duces chemically complex materials containing a range of different functional groups. Furthermore, these approaches do not allow for precise control over dopant content, dopant atom proximity or

the chemical nature of the dopant. This has been noted using X-ray photoelectron spectroscopy (XPS), whereby boron-doped carbons typically feature BC3, BC2O, BCO2 and B2O3 functionalit-ies;21–25 additional functionalities are observed for boron nitrogen co-doped materials (such as B-N bonds and pyrrolic, pyridinic and graphitic nitro-gen functionalities).26–32 Furthermore, it has been demonstrated that oxo-terminated defect and edge sites can contribute to the ORR activity of graphenes,33-36 and trace metal impurities (in par-ticular iron from commercial graphite and man-ganese species from the synthesis of graphene oxide by the Hummers method) also can signific-antly contribute to the ORR catalytic activity of carbon materials.37-39 Indeed, unequivocally dis-counting metal-impurity catalysis in carbon ma-terials is challenging and often cannot be pre-cluded.40 Combined, these factors have complic-ated understanding of the precise nature of the key ORR active sites in heteroatom-doped carbon materials. Consequently, the field has had to rely on computational studies to provide insight into potential active sites and the mechanism for the ORR.21, 41-44 However, there still remains significant

uncertainty over the identity of the most active sites in doped carbons for catalysing the ORR.

An alternative approach to obtain structure-activity relationships in this important area would be to use precisely defined carbon materials con-taining a small number of dopant chemical sites. In recent years there has been sufficient progress in the preparation of boron-doped and boron/ni-trogen co-doped polyaromatic hydrocarbons (PAHs) by bottom up synthetic methods to enable this approach to be explored.45-49 As these doped-PAHs are structurally well characterised and have only a small number of heteroatom functionalit-ies, they can be used as well defined models for the functionalities present in larger doped carbon materials. This allows for the correlation of relat-ive ORR catalytic activity to specific dopant site structure and properties.

Herein we report our initial investigations into determining the ORR electrocatalytic activity in alkaline solution of a range of well-defined boron-doped and boron/nitrogen co-doped PAHs. Analys-ing this series revealed that a minimum LUMO en-ergy is essential, with only the compounds with lower LUMO energies showing electrocatalytic activity for the ORR. Furthermore, out of the four active compounds the two which contain two proximal dopant atoms show highest catalytic activity.

Figure 1: The well-defined doped PAHs used herein, and the comparison compounds quinone A and perylene (compound B). Mes = mesityl a boron “protecting group”.

The compounds 1 – 6 have been previously synthesised and were selected as they are among

the largest well-defined B-and B,N-doped PAHs re-ported to date. Furthermore, they are relatively chemically robust for di- (and tri-) organoboranes, for example they do not undergo protodeborona-tion (C-B cleavage) readily with H2O and are stable to silica gel (unlike most triarylboranes).50-54 This is crucial as it maximises the probability of these compounds remaining intact under ORR re-action conditions, essential for determining struc-ture-activity relationships. These compounds also feature comparable boron-containing functionalit-ies to those postulated to be ORR active in B-doped graphenes (all compounds feature BC3(aryl) units, with the exception of 5 which has a C2BN unit). However, both the number of dopant atoms (1 - 3 boron atoms, 0 - 1 nitrogen atoms) and the proximity of the dopant atoms vary between the compounds. These compounds therefore repres-ent some of the different configurations available to BC3 and C2BN units within boron-doped and boron/nitrogen co-doped carbon materials.

The key goal of this initial investigation is to determine the relative activity of these well-defined compounds, thereby enabling correlation of activity with structure. Therefore electrocata-lytic conditions that have been used widely in the literature for doped graphenes were employed in this study in order to determine compound activ-ity (see SI). For validation of the methodology, 9,10-phenanthrenequinone (A) was tested along-side compounds 1 – 6. Compound A is known to be an active PAH catalyst for the ORR and its catalytic activity is well understood under the conditions used in this study, making it a suitable reference PAH catalyst.55-58 Furthermore, an all carbon PAH, perylene (compound B) was selected for comparison as it is structurally related to the B,N-doped PAH 6 thereby enabling further prob-ing of the effect of heteroatom doping on cata-lysis of the ORR. In addition, carbon electrodes have been shown to have varying degrees of ORR catalytic activity: this includes glassy carbon,59,60

boron doped diamond (BDD),59-62 and highly ordered pyrolytic graphite (HOPG).58, 63-65 There-fore the background activity of these electrodes was analysed prior to compound deposition to un-derstand the contribution to ORR activity from these carbon substrates.

Compounds 1 – 6 were synthesised as previ-ously reported and deposited onto a glassy car-bon electrode (glassy carbon was selected for ini-tial studies as it is the most commonly used sub-strate in literature ORR studies) by drop-casting from a toluene solution and drying at ambient temperature. To facilitate evaporation of toluene, the coated electrode was subjected to reduced pressure (10-2 mbar) for several minutes prior to commencing ORR testing. The CVs of compounds 1 – 6 (Figure 2a and 2b) loaded onto glassy car-bon show that in air saturated KOH(aq) (0.1 M) ORR activity is actually diminished to varying extents with respect to the bare glassy carbon electrode.

Repeating the measurements in N2 saturated KOH(aq) led to featureless voltammograms for these compounds (Figure S1 – S3), illustrating that the observed activity in each case is due to oxygen reduction. It is noteworthy that the ORR activity of the glassy carbon electrode diminishes on compound deposition, presumably due to the PAHs blocking ORR active surface sites. This hy-pothesis was supported by performing multiple electrocatalysis cycles (Fig. 2c). Cycling results in desorption of the compound and a return to the

CV of the original (pre-deposition) glassy carbon. It also should be noted that attempts to use Nafion as a binder to form stable (with respect to compound desorption during cycling) compound deposition on the electrode surface did not lead to reproducible results in our hands (see Support-ing Information), therefore in this work all of the voltammagrams are for the first ORR cycle after deposition and drying (as even on cycle 2 and 3 significant compound desorption has occurred – Fig 2c).

Figure 2. (a) CVs of 1 – 4 and (b) 5 – 6 on a glassy carbon electrode in air saturated 0.1 M KOH(aq) at a scan rate of 50 mVs-1. The CVs of the bare GC electrode are shown by the grey dashed lines. (c) CVs of 4 on a GC electrode in air saturated 0.1 M KOH(aq) at a scan rate of 50 mVs-1 illustrating desorption of the compound from the electrode. GC = glassy carbon.

With initial work using these PAHs deposited on glassy carbon proving inconclusive (with re-spect to any PAH ORR activity) due to the rela-tively high background activity of the glassy car-bon substrate other substrates with lower in-trinsic ORR activity were investigated. Other car-bon substrates that have been studied extens-ively for the ORR include BDD58-62 and HOPG,58, 63-65

and both of these have significantly reduced ORR activity in comparison to glassy carbon. Boron doped diamond is reported to be completely in-active for the ORR (any ORR activity in BDD is at-tributed to traces of sp2 carbon present in the substrate).62,66 Therefore BDD was studied to de-termine its suitability as an electrode for the as-sessment of the six doped PAHs as catalysts for the ORR. The CVs of two commercial BDD elec-trodes show minimal ORR activity and import-antly, show no reductive features around -0.4 V in air or O2 saturated KOH (Figure S7). Notably, the ORR activity of PAH A deposited on BDD is similar (although with a slightly lower onset potential) to the intrinsic ORR activity of glassy carbon consist-ent with surface quinone groups in glassy carbon being involved in the ORR (Figure 3). Therefore BDD electrodes are more useful for this study as they provide a background with lower intrinsic activity than the glassy carbon electrode, thereby facilitating the determination of ORR activity of compounds 1 – 6.

Figure 3. Left, CVs of A on a glassy carbon electrode in N2 and O2 saturated 0.1 M KOH(aq) at a scan rate of 50 mVs-1. Right, CVs of A on a BDD electrode in N2 and O2 saturated 0.1 M KOH(aq) at a scan rate of 50

mVs-1. The CVs of bare glassy carbon and bare BDD are shown by the dashed lines.

The six compounds again were deposited on the BDD electrodes by drop-casting a toluene solution of the compound followed by evaporation of toluene at low pressure. The CVs after deposit-ing compounds 1 – 6 onto BDD all show no fea-tures in N2 saturated KOH(aq) (see Figure S11), but in O2 saturated KOH solutions significant differ-ences were observed (Figure 4a-b). Compounds 1, 3, 4 and 6 loaded onto BDD gave CVs with in-creased ORR activity relative to the bare BDD electrode. In contrast, BDD deposited with 2 and 5 showed less, or effectively identical, ORR activ-ity to that of the bare BDD electrode. This indic-ates for the first time that well defined B-doped and B,N-co-doped PAHs are active catalysts for the ORR. Of the active doped PAHs, 3 and 4 showed the lowest increase in activity with 3 hav-ing a higher onset potential than the other active compounds. By comparison, 1 and 4 have lower onset potentials than 3, while 1 has a higher cur-rent density than both 3 and 4 (Table S3). The CV of compound 6 showed the highest ORR activity of all six compounds investigated in this study with a low onset potential and a comparable cur-rent density to 1 at -1.0 V vs Hg/HgO.67 Perylene (B) can be viewed as a structurally related all car-bon PAH analogue of compound 6. However, with perylene deposited onto the BDD substrate the ORR activity (figure 4c) is poor and not signific-antly increased compared to the background activity of the BDD substrate. This clearly high-lights the importance of heteroatom doping in these systems for catalysing the ORR.

It should be noted that whilst the compounds selected for this study are bench stable (due to steric protection of the organoborane by the mes-ityl group or by structural constraint around the boron centre)50-52 these species may still undergo decomposition (e.g. protodeboronation) under more forcing conditions such as in the presence of aqueous bases.68-70 To provide support that the ORR activity observed is due to the boron doped PAHs and not any decomposition products, the

cba

adsorbed compounds were extracted from the electrode surface (using toluene) after elec-trocatalytic testing and analysed by mass spec-trometry (see SI). This confirmed the presence of the molecular ion (e.g. for compounds 1 and 4) or (M+OH) species (e.g. for compounds 5 and 6).

Ions correlating to anticipated decomposition products were not observed for any of the com-pounds, suggesting that under these ORR condi-tions these compounds do not undergo significant B-C cleavage or other decomposition.

Figure 4. (a) CVs of 1 – 4 and (b) 5 – 6 on a BDD electrode in O2 saturated 0.1 M KOH(aq) at a scan rate of 50 mVs-1. (c) CVs of B on a BDD electrode in N2 and O2 saturated 0.1 M KOH(aq) at a scan rate of 50 mVs-1. The CVs of the bare BDD electrode are shown by the grey dashed lines in a-c. (d-e) CVs of 6 and (f) CV of A on HOPG in air saturated 0.1 M KOH (aq) at a scan rate of 50 mVs-1. The CVs of the bare HOPG electrode are shown by the grey dashed lines.

Each of the doped PAHs are synthesised via at least one metal mediated step prior to purifica-tion by column chromatography, however, the ab-sence of any significant trace metal impurities was confirmed by ICP-MS, (including Fe, Pt, Mn, Co and Ni all of which are documented to catalyse the ORR).37-39 Furthermore, the purity level of these PAHs is > 99% (by multinuclear NMR spec-troscopy) further disfavouring impurity catalysis. Combined these observations support the conclu-sion that the observed ORR activity on BDD is due to the B-doped PAHs.

Figure 5: A RRDE voltammogram obtained at Pt/Pt ring-BDD disk assembly in 0.1 M KOH at 1600 rpm at room temperature. The experiment was carried out

in saturated oxygen condition. The BDD disk poten-tial was scanned from 0 V to 1.2 V at 5 mV/s and the ring potential was 0.1 V.

Repeated attempts to determine the value of n for the ORR process were frustrated by a lack of dependence on scan rate (CV), however for the best performing model catalyst, 6, RRDE voltam-metry with a bespoke apparatus (BDD/catalyst disk and Pt ring) yielded stable disk and ring cur-rents. The number of electrons transferred in the catalytic process using 6 was 2.65, indicating that mixed pathways operate (potentially dependent on compound surface coverage), but a portion of the oxygen is reduced to water under these con-ditions (Figure 5).

PAH 6 has the highest observed electrocata-lytic activity on BDD, therefore the ORR perform-ance of this compound was also studied on HOPG. Bare HOPG displays a small ORR feature around -0.4 V vs Hg/HgO with an onset potential in the re-gion of -0.3 V, which is consistent with other re-ports of ORR catalysed by HOPG.61 The adsorption of 6 onto HOPG results in a negative shift in the reduction potential (to a value consistent with that obtained for 6 from the BDD electrode). Thus 6 is less active than surface groups on HOPG, but the activity of 6 can clearly be determined, with it suppressing the intrinsic activity of surface HOPG sites. This demonstrates that ORR active PAHs can also be investigated on this substrate (Fig 4d and e), although BDD remains the preferred sub-strate due to its lower and more negative back-ground activity. For comparison compound A was also adsorbed onto HOPG and the onset and peak

a b c

d e f

potentials of this compound (-0.284 V and -0.421 V, respectively) further support evidence in prior literature that the ORR active groups on HOPG are quinone-based.57

From the above studies it also can be con-cluded that BDD is the preferred substrate for de-termining the electrocatalytic activity of PAHs to-wards ORR. While glassy carbon is ubiquitous in the literature and offers a stable, reproducible background over a series of measurements, the relatively high intrinsic ORR activity of this sub-strate can complicate unequivocal determination of the ORR activity for any compounds with a higher onset potential and/or lower current dens-ity than catalytically active surface groups. HOPG offers a background with lower intrinsic activity than glassy carbon substrates; however the back-ground activity observed is dependent on the number and type of defect and edge sites.61, 63

Furthermore, the variation between measure-ments on HOPG (due to removal of the surface layer(s) in between each use) can complicate ac-curate comparison of electrocatalytic activity between compounds deposited on HOPG (e.g. see background CVs in Figure 4d, e and f, and Table S4). Therefore BDD offers the best (least ORR act-ive) background of the three carbon substrates and provides reproducible background values over a series of measurements. Table 1: Select properties of compounds 1 – 6 and A.

No. HOMO (eV)a

LUMO (eV)a

Epeak

1st red (V)b

DipoleMoment

(D)a

Onset Potential

(V)c

1 -6.47 -2.28 -1.56d 0 -0.352 -6.91 -1.81 -2.10e 1.00 -3 -6.91 -2.35 -1.56e 0.39 -0.334 -6.80 -2.37 -1.63f 6.37 -0.315 -7.34 -1.30 - 0.04 -6 -7.52 -2.08 -1.80e 5.13 -0.32A -8.07 -2.36 -1.25 7.68 -0.27

a = calculated frontier orbital energies. b voltage at peak current for the first reduction process, poten-tials are given relative to the Fc/Fc+ redox couple. C = onset potential on BDD for ORR active com-pounds. The onset potential in the absence of sub-strate for the intrinsic activity of the BDD substrate was -0.63 V c = from ref 50, d = from ref 51, e = from ref. 54. For reference the E1/2 of perylene (B) is reported as -2.12 V relative to Fc/Fc+.72

The relative ORR catalytic activity of the six doped PAHs analysed in this study can be grouped into three subsets, compounds 1 and 6 = most active, 3 and 4 = some activity, and 2 and 5 = not-active for the ORR. It should be noted that while a number of these B and B,N-doped PAHs are active catalysts for the ORR they are less active than A and previously reported B/N doped graphenes. Nevertheless, these com-pounds do reveal important structure activity re-

lationships.8 Key properties of these six PAHs are summarised in Table 1 with DFT studies per-formed at the M06-2X/6311-G(d,p) level, with this functional chosen as it was parameterised to give accurate data for p block element containing compounds.71 Charges are from Natural Bond Or-bital (NBO) calculations. From inspection of the frontier orbital energies (from calculations and cyclic voltammetry) it can be seen that com-pounds 2 and 5 have the highest LUMO energies. Therefore the lack of ORR activity can be attrib-uted to the high energy LUMOs which will result in low Lewis acidity of these two PAHs (i.e. the bind-ing energy of O2 and O2 derived species can be expected to be low), and a more negative reduc-tion potential. This is consistent with previous computational studies on heteroatom doped car-bons which suggest that oxygen reduction pro-ceeds via binding of oxygen to the heteroatom (for heteroatoms such as boron)44 or a neighbour-ing electrophilic carbon site (for heteroatoms such as nitrogen).41, 73,74 Therefore in doped-PAHs it is proposed that for oxygen binding/reduction to occur the PAH must have a sufficiently low in en-ergy LUMO. This is also consistent with the ORR activity of quinone A which has the lowest LUMO energy (by cyclic voltammetry) and the lowest onset potential for the ORR. However, comparing the four active B-doped PAHs, although com-pound 6 has the highest ORR activity this com-pound does not have the lowest LUMO energy of the four active B-doped PAHs. It is therefore clear that once the LUMO energy is sufficiently low other factors, presumably the presence of specific chemical functionality, also significantly contrib-ute to the ORR activity.

Figure 6: LUMO (at isosurface value =0.04) and se-lect NBO charges for compounds 1, 3 and 6. Inset, the product from reaction of a NHC-stabilized 9,10-dibora-anthrene with O2.

The importance of specific functional groups/dopant atom proximity is exemplified by compar-ison of the ORR activity of 1 and 3. These two PAHs are similar in terms of boron content, PAH size (excluding the OMes groups on 1 which do not contribute to the frontier molecular orbitals, Fig. 6, top left) and LUMO energy, but have differ-ent ORR activities. Compounds 1 and 3 have sim-ilar charge distribution (positively charged boron centres and negative, ca. -0.4 e, carbon centres); with a LUMO that has significant boron character in both cases. The higher activity of 1 relative to 3 (and 4) therefore is proposed to be due to the proximity of the two Lewis acidic positively charged boron centres which can feasibly both bind to a single O2 molecule (or an O2 derived species) during the ORR. Indeed an O2 adduct of a related 9,10-dibora-anthrene compound has been recently reported with one O2 molecule binding to both proximal boron sites (inset, Fig. 6).75 Calcula-tions on boron-doped graphene nanoribbons also found that the proximity of multiple boron sites positively affects the ORR activity, with boron atoms in a 1,4-configuration in a six membered ring (such as in 1) showing increased ORR activity due to higher affinity for oxygen adsorption, again presumably due to O2 (or O2 derived spe-cies) binding to both boron atoms facilitating the ORR.44

The relatively high activity of compound 6 is also consistent with a proximal heteroatom co-doping effect, with one boron centre and a posit-ively charged carbon (either ortho or para to N) being effective proximal Lewis acidic sites. In-spection of the LUMO for 6 revealed it has signi-ficant character on boron and the carbon para to nitrogen, suggesting that these two are potential sites for bidentate O2 binding. Furthermore, the incorporation of N into 6 leads to more charge po-larisation with the magnitude of positive charge on boron now greater than that found in 1, 3 and in 5 where boron is directly bound to nitrogen (see Supporting Information). Boron / nitrogen co-doped carbon materials are documented to be ex-cellent catalysts for the ORR and it has been sug-gested that separated (not directly bonded) boron and nitrogen dopants have high ORR activity partly due to the redistribution of electron density leading to more charge polarised systems.12 Previ-ous calculations have found the strength of the co-doping synergistic effect decreases as the dis-tance between dopant atoms increases, thus proximal B and N are potentially required for con-certed binding of O2 (or O2 derived species) and thus effective ORR catalysis.13 The higher elec-trocatalytic activity of 1 and 6 in this series sup-ports the hypothesis that two proximal het-eroatom dopants helps to promote high ORR activity by generating two proximal electrophilic sites.

In summary, these initial studies have shown for the first time that well-defined boron and

boron nitrogen co-doped PAHs are active elec-trocatalysts for the ORR in alkaline solutions provided the LUMO energy is sufficiently low. Fur-thermore, in this series of compounds the pres-ence of two dopant atoms in relatively close prox-imity leads to enhanced ORR activity for both doubly B doped (in compound 1) and B,N co-doped (in compound 6) PAHs. Notably, a related all carbon PAH, perylene, that is structurally re-lated to 6 displays essentially no electrocatalytic activity under these conditions. Multiply het-eroatom doped PAHs therefore have significant potential as model compounds for exploring the effects of specific functionalities on the electro-activity of doped-carbon materials. It is acknow-ledged that for more in-depth understanding of the nature of active functional groups, more com-pounds (with different functionalities/arrange-ment of dopant atoms / LUMO energies / LUMO distributions to those studied herein) need to be studied, but this work suggests that larger prox-imally co-doped-PAHs with lower LUMO energies are attractive targets. Furthermore, the study presented herein illustrates that an effective screening method for assessing doped PAHs for ORR activity requires understanding of the relat-ive activity of the electrode substrate and the compound of interest. This has led to B and B/N doped PAHs being confirmed as active catalysts for the ORR for the first time.

ASSOCIATED CONTENT Supporting Information. The Supporting Information listed below is available free of charge on the ACS Publications website at DOI:Materials and methods, electrochemical measure-ments, characterization of boron doped diamond electrodes and computational details (PDF).

AUTHOR INFORMATIONCorresponding Author* Michael J. Ingleson michael.ingleson@manch-ester.ac.uk* Robert. A. W. Dryfe robert.dryfe@manchester.ac.uk

Author ContributionsThe manuscript was written through contributions of all authors. All authors have given approval to the fi-nal version of the manuscript.Funding SourcesThe EPSRCThe Leverhulme TrustThe European Research Council.

ACKNOWLEDGMENT We are grateful to the EPSRC (EP/K03099X/1), the ERC (Grant No. 305868) and the Leverhulme Trust (RPG-2014-340). Additional research data supporting this work are available as supplementary informa-tion accompanying this publication.

REFERENCES(1) Dey, S.; Mondal, B.; Chatterjee, S.; Rana, A.; Amanullah, S. Dey, A. Molecular Electrocatalysts for the Oxygen Reduction Reaction. Nature Reviews, 2017, 1, 0098 (and references therein).(2) Sealy, C. The Problem with Platinum. Mater. Today 2008, 11, 65–68.(3) Aricò, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Van Schalkwijk, W. Nanostructured Materials for Ad-vanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4, 366–377.(4) Hu, M.; Yao, Z.; Wang, X. Graphene-Based Nanoma-terials for Catalysis. Ind. Eng. Chem. Res. 2017, 56, 3477–3502.(5) Gong, K.; Du, Feng.; Xia, Z.; Durstock, M.; Dai, L. Ni-trogen-Doped Carbon Nanotube Arrays with High Elec-trocatalytic Activity for Oxygen Reduction. Science, 2009, 323, 760-764.(6) Guo. D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active sites of nitrogen-doped carbon ma-terials for oxygen reduction reaction clarified using model catalysts. Science, 2016, 351, 361-365(7) For a very recent example in this area see: Chen, S.; Chen. Z.; Siahrostami, S.; Higgins, D.; Nordlund, D.; Sokaras, D.; Kim, T. R.; Liu, Y.; Yan., X.; Nilsson, E.; Sin-clair, R.; Nørskov, J. K.; Jaramillo, T. F. Designing boron nitride islands in Carbon MAterials for Efficient Electro-chemical Syntheiss of Hydrogen Peroxide, J. Am. Chem. Soc., 2018, 140, 7851-7859. (8) For more extensive examples see references in the review: Zhang. J.; Dai, L. Heteroatom-Doped Graphitic Carbon Catalysts for Efficient Electrocatalysis of Oxygen Reduction Reaction. ACS Catalysis, 2015, 5, 7244-7253.(9) Yang. L.; Jiang, S.; Zhao, Y.; Zhu, L.; Chen. S.; Wang, X.; Wu. Q.; Ma, J.; Ma, Y.; Hu, Z. Boron-Doped Carbon Nanotubes as Metal Free Electrocatalysts for the Oxy-gen Reduction Reaction. Angew. Chem. Int. Ed. 2011, 50, 7132-7135.(10) (a) Wang, S.; Iyyamperumal, E.; Roy, A.; Xue, A.; Yu, D.; Dai. L. Vertically Aligned BCN Nanotubes as Effi-cient Metal-Free Electrocatalysts for the Oxygen Reduc-tion Reaction: A Synergetic Effect by Co-Doping with Boron and Nitrogen. Angew. Chem. Int. Ed., 2011, 50, 11756. (b) Wang, S.; Zhang, S.; Xia, Z.; Roy, A.; Chang, D. W.; Baek, J.-B.; Dai, L.; BCN Graphene as Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Re-action. Angew. Chem. Int. Ed. 2012, 51, 4209-4212.(11) (a) Ozaki, J.; Kimura, N.; Anahara, T.; Oya, A. Pre-paration and oxygen reduction activity of BN-doped car-bons. Carbon, 2007, 45, 1847-1853. (b) Ishii, T.; Maie, T.; Kimura, N.; Kobori, Y.; Imashiro, Y.; Ozaki, J. Int. J. Hy-drogen Energy, 2017, 42, 15489-15496.(12) Zhao, Y.; Yang, L.; Chen, S.; Wang, X.; Ma, Y.; Wi, Q.; Jiang, Y.; Qian, W.; Hu, Z. Can Boron and Nitrogen Co-doping Improve Oxygen Reduction Reactivity of Carbon Nanotubes? J. Am. Chem. Soc., 2013, 135, 1201-1204.(13) Zheng, Y.; Jiao, Y.; Ge, L.; Jaroniec, M.; Qiao, S. Z. Two-Step Boron and Nitrogen Doping in Graphene for Enhanced Synergistic Catalysis. Angew. Chem. Int. Ed. 2013, 52, 3110–3116.(14) Higgins, D.; Zamani, P.; Yu, A.; Chen, Z. The Applic-ation of Graphene and Its Composites in Oxygen Reduc-tion Electrocatalysis: A Perspective and Review of Re-cent Progress. Energy Environ. Sci. 2016, 9, 357–390.(15) Zhou, X.; Qiao, J.; Yang, L.; Zhang, J. A Review of Graphene-Based Nanostructural Materials for Both Catalyst Supports and Metal-Free Catalysts in PEM Fuel

Cell Oxygen Reduction Reactions. Adv. Energy Mater. 2014, 4, 1301523.(16) Zhou, M.; Wang, H.-L.; Guo, S. Towards High-Effi-ciency Nanoelectrocatalysts for Oxygen Reduction through Engineering Advanced Carbon Nanomaterials. Chem. Soc. Rev. 2016, 45 (5), 1273–1307.(17) Liu, J.; Song, P.; Ning, Z.; Xu, W. Recent Advances in Heteroatom-Doped Metal-Free Electrocatalysts for Highly Efficient Oxygen Reduction Reaction. Elec-trocatalysis 2015, 6, 132–147.(18) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060–2086.(19) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823–4892.(20) Stacy, J.; Regmi, Y. N.; Leonard, B.; Fan, M. The Re-cent Progress and Future of Oxygen Reduction Reaction Catalysis: A Review. Renew. Sustain. Energy Rev. 2017, 69, 401–414.(21) Van Tam, T.; Kang, S. G.; Babu, K. F.; Oh, E.-S.; Lee, S. G.; Choi, W. M. Synthesis of B-Doped Graphene Quantum Dots as a Metal-Free Electrocatalyst for the Oxygen Reduction Reaction. J. Mater. Chem. A 2017, 5, 10537–10543.(22) Lei, Z.; Chen, H.; Yang, M.; Yang, D.; Li, H. Boron and Oxygen-Codoped Porous Carbon as Efficient Oxy-gen Reduction Catalysts. Appl. Surf. Sci. 2017, 426, 294–300.(23) Baik, S.; Suh, B. L.; Byeon, A.; Kim, J.; Lee, J. W. In-Situ Boron and Nitrogen Doping in Flue Gas Derived Carbon Materials for Enhanced Oxygen Reduction Reac-tion. J. CO2 Util. 2017, 20, 73-80.(24) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Origin of the Electrocatalytic Oxygen Reduction Activity of Graphene-Based Catalysts: A Roadmap to Achieve the Best Performance. J. Am. Chem. Soc. 2014, 136, 4394–4403.(25) Sheng, Z.-H.; Gao, H.-L.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. Synthesis of Boron Doped Graphene for Oxy-gen Reduction Reaction in Fuel Cells. J. Mater. Chem. 2012, 22, 390–395.(26) Patil, I. M.; Lokanathan, M.; Ganesan, B.; Swami, A.; Kakade, B. Carbon Nanotube / Boron Nitride Nanocom-posite as a Significant Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Chem. Eur. J. 2017, 23, 676–683.(27) Zehtab Yazdi, A.; Fei, H.; Ye, R.; Wang, G.; Tour, J.; Sundararaj, U. Boron/Nitrogen Co-Doped Helically Un-zipped Multiwalled Carbon Nanotubes as Efficient Elec-trocatalyst for Oxygen Reduction. ACS Appl. Mater. In-terfaces 2015, 7 (14), 7786–7794.(28) Jiang, S.; Sun, Y.; Dai, H.; Hu, J.; Ni, P.; Wang, Y.; Li, Z.; Li, Z. Nitrogen and Fluorine Dual-Doped Mesoporous Graphene: A High-Performance Metal-Free ORR Elec-trocatalyst with a Super-Low HO2- Yield. Nanoscale 2015, 7, 10584–10589.(29) Fei, H.; Ye, R.; Ye, G.; Gong, Y.; Peng, Z.; Fan, X.; Samuel, E. L. G.; Ajayan, P. M.; Tour, J. M. Boron- and Ni-trogen-Doped Graphene Quantum Dots / Graphene Hy-brid Nanoplatelets as Efficient Electrocatalysts for Oxy-gen Reduction. ACS Nano. 2014, 8 (10), 10837–10843.(30) Jin, J.; Pan, F.; Jiang, L.; Fu, X.; Liang, A.; Wei, Z.; Za-hgn, J.; Sun, G. Catalyst-Free Synthesis of Crumpled Boron and Nitrogen Co-Doped Graphite Layers with Tun-able Bond Structure for Oxygen Reduction Reaction. ACS Nano. 2014, 8, 3313–3321.(31) Xue, Y.; Yu, D.; Dai, L.; Wang, R.; Li, D.; Roy, A.; Lu, F.; Chen, H.; Liu, Y.; Qu, J. Three-Dimensional B,N-Doped

Graphene Foam as a Metal-Free Catalyst for Oxygen Re-duction Reaction. PCCP 2013, 15, 12220–12226.(32) Cao, C.; Wei, L.; Zhai, Q.; Wang, G.; Shen, J. Bio-mass-Derived Nitrogen and Boron Dual-Doped Hollow Carbon Tube as Cost-Effective and Stable Synergistic Catalyst for Oxygen Electroreduction. Electrochim. Acta 2017, 249, 328–336.(33) Jiang, Y.; Yang, L.; Sun, T.; Zhao, J.; Lyu, Z.; Zhuo, O.; Wang, X.; Wu, Q.; Ma, J.; Hu, Z. Significant Contribu-tion of Intrinsic Carbon Defects to Oxygen Reduction Activity. ACS Catal. 2015, 5, 6707–6712.(34) Zhao, X.; Zou, X.; Yan, X.; Brown, C. L.; Chen, Z.; Zhu, G.; Yao, X. Defect-Driven Oxygen Reduction Reac-tion (ORR) of Carbon without Any Element Doping. In-org. Chem. Front. 2016, 3, 417–421.(35) Zhang, L.; Xu, Q.; Niu, J.; Xia, Z. Role of Lattice De-fects in Catalytic Activities of Graphene Clusters for Fuel Cells. Phys. Chem. Chem. Phys. 2015, 17, 16733–16743.(36) Zhao, H.; Sun, C.; Jin, Z.; Wang, D.-W.; Yan, X.; Chen, Z.; Zhu, G.; Yao, X. Carbon for the Oxygen Reduc-tion Reaction: A Defect Mechanism. J. Mater. Chem. A 2015, 3, 11736–11739.(37) Wang, L.; Ambrosi, A.; Pumera, M. “Metal-Free” Catalytic Oxygen Reduction Reaction on Heteroatom-Doped Graphene Is Caused by Trace Metal Impurities. Angew. Chem. Int. Ed. 2013, 52, 13818–13821.(38) Masa, J.; Xia, W.; Muhler, M.; Schuhmann, W. On the Role of Metals in Nitrogen-Doped Carbon Elec-trocatalysts for Oxygen Reduction. Angew. Chem. Int. Ed. 2015, 2–21.(39) Masa, J.; Zhao, A.; Wei, X.; Muhler, M.; Schuhmann, W. Metal-Free Catalysts for Oxygen Reduction in Al-kaline Electrolytes: Influence of the Presence of Co, Fe, Mn and Ni Inclusions. Electrochim. Acta 2014, 128, 271–278.(40) See the discussion in: Liu, J.; Yu, S.; Daio, T.; Ismail, M. S.; Sasaki, K.; Lyth, S. M.; Metal-Free Nitrogen-Doped Carbon Foam Electrocatalysts for the Oxygen Reduction Reaction in Acid Solution. J. Electrochem. Soc., 2016, 163, F1049-F1054.(41) Ricke, N. D.; Murray, A. T.; Shepherd, J. J.; Welborn, M. G.; Fukushima, T.; Voorhis, T. Van; Surendranath, Y. Molecular-Level Insights into Oxygen Reduction Cata-lysis by Graphite-Conjugated Active Sites. ACS Catal. 2017, 7, 7680–7687.(42) Tang, S.; Wu, W.; Liu, L.; Gu, J. Oxygen-Molecule Adsorption and Dissociation on BCN Graphene : A First-Principles Study. ChemPhysChem 2017, 18, 101–110.(43) Zhao, Z.; Xia, Z. Design Principles for Dual-Ele-ment-Doped Carbon Nanomaterials as Efficient Bifunc-tional Catalysts for Oxygen Reduction and Evolution Re-actions. ACS Catal. 2016, 6, 1553–1558.(44) Wang, L.; Dong, H.; Guo, Z.; Zhang, L.; Hou, T.; Li, Y. Potential Application of Novel Boron-Doped Graphene Nanoribbon as Oxygen Reduction Reaction Catalyst. J. Phys. Chem. C 2016, 120, 17427–17434.(45) Agnoli, S.; Favaro, M. Doping Graphene with Boron: A Review of Synthesis Methods, Physicochemical Char-acterization, and Emerging Applications. J. Mater. Chem. A 2016, 4, 5002–5025.(46) Grotthuss, E. Von; John, A.; Kaese, T.; Wagner, M. Doping Polycyclic Aromatics with Boron for Superior Performance in Materials Science and Catalysis. Asian J. Org. Chem. 2018, 7, 37–53.(47) Ji, L.; Griesbeck, S.; Marder, T. B. Recent Develop-ments in and Perspectives on Three-Coordinate Boron Materials: A Bright Future. Chem. Sci. 2017, 8, 846–863.

(48) Stępień, M.; Gońka, E.; Żyła, M.; Sprutta, N. Hetero-cyclic Nanographenes and Other Polycyclic Heteroaro-matic Compounds: Synthetic Routes, Properties, and Applications. Chem. Rev. 2017, 117, 3479–3716.(49) Escande, A.; Ingleson, M. J. Fused Polycyclic Aro-matics Incorporating Boron in the Core: Fundamentals and Applications. Chem. Commun. 2015, 51, 6257–6274.(50) Dou, C.; Saito, S.; Matsuo, K.; Hisaki, I.; Yamaguchi, S. A Boron-Containing PAH as a Substructure of Boron-Doped Graphene. Angew. Chem. Int. Ed. 2012, 51, 12206–12210.(51) Crossley, D. L.; Kahan, R. J.; Endres, S.; Warner, A. J.; Smith, R. A.; Cid, J.; Dunsford, J. J.; Jones, J. E.; Vitor-ica-Yrezabal, I.; Ingleson, M. J. A Modular Route to Boron Doped PAHs by Combining Borylative Cyclisation and Electrophilic C–H Borylation. Chem. Sci. 2017, 8, 7969–7977.(52) Hatakeyama, T.; Hashimoto, S.; Seki, S.; Na-kamura, M. Synthesis of BN-Fused Polycyclic Aromatics via Tandem Intramolecular Electrophilic Arene Boryla-tion. J. Am. Chem. Soc. 2011, 133, 18614–18617.(53) Hatakeyama, T.; Nakamura, M.; Hashimoto, S. Poly-cyclic Aromatic Compound, US 2014/0005399 A1, Janu-ary 2, 2014.(54). Kahan, R. J.; Crossley, D. L.; Radcliffe, J. E.; Wood-ward, A. W.; Fasano, V.; Endres, S. Whitehead, G. F. S.; Ingleson M. J. Generation of a series of Bn fused oligo-naphthalenes (n= 1 to 3) from a B1 Polycyclic Aromatic Hydrocarbon. Chem. Commun. 2018, 54, 9490-9493. (55) Sarapuu, A.; Helstein, K.; Vaik, K.; Schiffrin, D. J.; Tammeveski, K. Electrocatalysis of Oxygen Reduction by Quinones Adsorbed on Highly Oriented Pyrolytic Graphite Electrodes. Electrochim. Acta 2010, 55, 6376–6382.(56) Vaik, K.; Sarapuu, A.; Tammeveski, K.; Mirkhalaf, F.; Schiffrin, D. J. Oxygen Reduction on Phenanthrenequi-none-Modified Glassy Carbon Electrodes in 0.1 M KOH. J. Electroanal. Chem. 2004, 564, 159–166.(57) Wass, J. R. T. J.; Ahlberg, E.; Panas, I.; Schiffrin, D. J. Quantum Chemical Modelling of the Rate Determining Step for Oxygen Reduction on Quinones. PCCP 2006, 8, 4189–4199.(58) Sarapuu, A.; Helstein, K.; Schiffrin, D. J.; Tam-meveski, K. Kinetics of Oxygen Reduction on Quinone-Modified HOPG and BDD Electrodes in Alkaline Solution. Electrochem. Solid-State Lett. 2005, 8, E30–E33.(59) See (and references therein): L Tryk, D. A.; Cab-rera, C. R.; Fujishima, A.; Spataru, N. Oxygen Electrore-duction on Carbon Materials. In Fundamental Under-standing of Electrode Processes in Memory of Professor Ernest B. Yeager; Prakash, J., Chu, D., Scherson, D., Enayetullah, M., Tae Bae, I., Eds.; Electrochemical Soci-ety: Orlando, 2003; pp 45–57.(60) Xu, J.; Huang, W.; McCreery, R. L. Isotope and Sur-face Preparation Effects on Alkaline Dioxygen Reduction at Carbon Electrodes. J. Electroanal. Chem. 1996, 410, 235–242.(61) Yano, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Electrochemical Behavior of Highly Conductive Boron-Doped Diamond Electrodes for Oxygen Reduction in Al-kaline Solution. J. Electrochem. Soc. 1998, 145, 1870–1876.(62) Macpherson, J. V. A Practical Guide to Using Boron Doped Diamond in Electrochemical Research. Phys. Chem. Chem. Phys. 2015, 17, 2935–2949.(63) Shen, A.; Zou, Y.; Wang, Q.; Dryfe, R. A. W.; Huang, X.; Dou, S.; Dai, L.; Wang, S. Oxygen Reduction Reac-tion in a Droplet on Graphite: Direct Evidence That the

Edge Is More Active than the Basal Plane. Angew. Chem. Int. Ed. 2014, 53, 10804–10808.(64) Hossain, M. S.; Tryk, D.; Yeager, E. The Electro-chemistry of Graphite and Modified Graphite Surfaces: The Reduction of O2. Electrochim. Acta 1989, 34, 1733–1737.(65) Yeager, E. Electrocatalysts for O2 Reduction. Elec-trochim. Acta 1984, 29, 1527–1537.(66) Bennett, J. A.; Wang, J.; Show, Y.; Swain, G. M. Ef-fect of Sp2-Bonded Nondiamond Carbon Impurity on the Response of Boron-Doped Polycrystalline Diamond Thin-Film Electrodes. J. Electrochem. Soc. 2004, 151, E306–E313.(67) Hg/HgO was used in this study as this reference electrode proved stable over long term (12 months) periodic use in aqueous alkaline solutions in contrast to other reference electrodes (see supporting informa-tion). For Hg/HgO, NaOH (0.1 M) the potential at 25oC Vs NHE = 0.165, Vs SCE = -0.076 V. See the following reference for more details: Handbook of Analytical Chemistry, Meites, L. ed., McGraw Hill, NY (1963). Sect 5.(68) Escande, A.; Crossley, D. L.; Cid, J.; Cade, I. A.; Vit-orica-Yrezabal, I.; Ingleson, M. J. Inter- and Intra-Molecu-lar C–H Borylation for the Formation of PAHs Containing Triarylborane and Indole Units. Dalt. Trans. 2016, 45, 17160–17167.(69) Zhou, Z.; Wakamiya, A.; Kushida, T.; Yamaguchi, S. Planarized Triarylboranes: Stabilization by Structural Constraint and Their Plane-to-Bowl Conversion. J. Am. Chem. Soc. 2012, 134, 4529–4532.

(70) Osumi, S.; Saito, S.; Dou, C.; Matsuo, K.; Kume, K.; Yoshikawa, H.; Awaga, K.; Yamaguchi, S. Boron-Doped Nanographene: Lewis Acidity, Redox Properties, and Battery Electrode Performance. Chem. Sci. 2016, 7, 219–227.(71) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group theremochemistry, thermo-chemical kinetics, noncovalent interactions, excited states and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Account, 2008, 120, 215-241.(72) For the reduction potential of perylene see: Good-now, T. T.; Kaifer, A. E. Does Isotopic Substitution Affect the Reduction Potential of Aromatic Molecules? J. Phys. Chem. 1990, 94, 7682-7683(73) Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduc-tion Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170–11176.(74) Kim, H.; Lee, K.; Woo, S. I.; Jung, Y. On the Mechan-ism of Enhanced Oxygen Reduction Reaction in Nitro-gen-Doped Graphene Nanoribbons. Phys. Chem. Chem. Phys. 2011, 13, 17505–17510.(75) Taylor, J. W.; McSkimming, A.; Guzman, C. F.; Hill Harman, W. N-Heterocyclic Carbene Stabilized Boran-threne as a Metal Free Platform for the Activation of Small Molecules. J. Am. Chem. Soc. 2017, 139, 11032-11035.

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