Nitrogenated‐Graphite‐Encapsulated Carbon Black as a Metal‐Free Electrocatalyst for the Oxygen Evolution Reaction in Acid

Abstract

No metal needed: A core-shell-like carbon catalyst consisting of a few layers of nitrogen-doped graphite on a carbon black core was prepared by pyrolyzing an ionic liquid (IL) with carbon black (CB) particles. The catalyst has a noticeably improved OER performance of the nanocarbon in acid solution. The resilience of the catalyst in acid solution is due to the greater protection of the nitrogen-containing OER active sites in a graphitic shell. The oxygen evolution reaction (OER) in acid solution is very challenging for metal-free catalysts based only on heteroatomdoped carbon nanomaterials which have thus far shown relatively low catalytic activity and poor stability. Herein, we report a core-shell-like carbon catalyst consisting of a few layers of nitrogen-doped graphite on a carbon black core prepared by pyrolyzing an ionic liquid (IL) with carbon black (CB) particles. The catalyst prepared as such has a noticeably improved OER performance of the nanocarbon in acid solution (1.70 V at a current density of 10 mA cm À2 ) in comparison with the few metal-free OER electrocatalysts reported to date. The experimental results suggest that the resilience of the catalyst in acid solution is due to the greater protection of the nitrogencontaining OER active sites in a graphitic shell.Electrochemical water-splitting and aqueous rechargeable metal-air batteries have drawn growing interest in recent years as clean energy technologies. [1] The oxygen evolution reaction (OER) is a technology choke point; and the development of active and durable OER electrocatalysts is the most effective solution. [2] Cost considerations for large installations naturally exclude the use of precious metal-based OER catalysts such as IrO x and RuO x , which have the best OER catalytic performance. [3] Some transition-metal oxides have shown activities close to the those of IrO x and RuO x , but only in alkaline media. [4] In general OER in acid solution has the advantage of a higher voltage efficiency [5] and higher energy density. [6] OER in acid is however extremely challenging for the transition metal-based catalysts because of the instability of their active sites (MÀOH, MÀO, MÀOOH) in an acidic environment.There is also a parallel interest in the development of heteroatom-doped/functionalized nanocarbons as metal-free catalysts, [7] initially for the oxygen reduction reaction (ORR). [8] Some of these catalysts have also been examined as OER electrocatalysts or bifunctional (i. e. ORR and OER) electrocatalysts in alkaline solution. [7a,9] By comparison, the use of nanocarbon materials as metal-free OER electrocatalysts in acid solution is less explored. [10][11] In 2016, the Xu group reported that a nitrogen, phosphorus and oxygen triply-doped/functionalized carbon cloth was active and stable for OER in acid solution. [11] The study reported the property of the triplydoped/functionalized carbon cloth as an integrated catalyst system. Since the contribution of the carbon cloth to the measured OER activity and stability has not been...

Ultrathin Nitrogen‐Doped Holey Carbon@Graphene Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions in Alkaline and Acidic Media

“…A large Tafel slope of 297.5 mV dec −1 (Figure S32) suggests a sluggish acidic OER kinetic process of N‐HC@G‐900 . N‐HC@G‐900 shows lower acidic stability than Ir/C (Figure e), but better than other reported metal‐free electrocatalysts …”

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

“…[162,[220][221][222][223][224] Here, we list some carbon-based materials, such as multiwall carbon nanotubes [220] and C 60 -SWCNT. [221] Besides, these carbon materials can be modified by other atoms (N, O, P) to form, such as, metal-free amino-rich hierarchical-network carbon (aminoHNC) framework, [162] nitrogenated-graphiteencapsulated carbon black, [222] ultrathin nitrogen-doped holey carbon@graphene, [223] and nitrogen, phosphorus, oxygen tridoped porous graphite carbon@oxidized carbon cloth (ONPPGC/OCC). [224] The electronic and surficial structure of carbon-based catalysts can be tuned by these heteroatoms and thus influencing their activity toward OER in acid.…”

“…[224] The electronic and surficial structure of carbon-based catalysts can be tuned by these heteroatoms and thus influencing their activity toward OER in acid. [222][223][224] Note a striking drawback of carbon structure is the thermodynamic instability against the electrochemical oxidation, which is a great challenge of the development of carbon-based materials toward OER.…”

Ultrathin Nitrogen‐Doped Holey Carbon@Graphene Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions in Alkaline and Acidic Media

“…Compared to N-C@G-900, the higher acidic OER activity of N-HC@G-900 confirms once again the critical role of HC structures and high pyridinic-N contents.Alarge Tafel slope of 297.5 mV dec À1 ( Figure S32) suggests asluggish acidic OER kinetic process of N-HC@G-900. [26] Acidic ORR/OER bifunctionality of N-HC@G-900 is shown in Figure 3f.A lthough the measured E ORR1/2 and E OER10 gap (0.93 V, separately tested) is 0.13 Vhigher than Pt/ C + Ir/C (0.80 V), it is still the best performing carbon-based bifunctional electrocatalyst in acidic media reported so far. [26] Acidic ORR/OER bifunctionality of N-HC@G-900 is shown in Figure 3f.A lthough the measured E ORR1/2 and E OER10 gap (0.93 V, separately tested) is 0.13 Vhigher than Pt/ C + Ir/C (0.80 V), it is still the best performing carbon-based bifunctional electrocatalyst in acidic media reported so far.…”

“…[23] N-HC@G-900 shows lower acidic stability than Ir/C (Figure 3e), but better than other reported metal-free electrocatalysts. [26] Acidic ORR/OER bifunctionality of N-HC@G-900 is shown in Figure 3f.A lthough the measured E ORR1/2 and E OER10 gap (0.93 V, separately tested) is 0.13 Vhigher than Pt/ C + Ir/C (0.80 V), it is still the best performing carbon-based bifunctional electrocatalyst in acidic media reported so far.…”