The role of biochar pretreatment on the ORR and OER electrocatalytic performance of nitrogen-doped biochars decorated with cobalt

Abstract

The development of bifunctional ORR and OER electrocatalysts is fundamental to the application of electrochemical energy conversion devices (e.g. fuel cells). However, these electrocatalysts must be sustainable and cost-effective. Herein, we prepared several shrimp shells-derived biochars (directly obtained from the pyrolysis – CC; obtained by pretreatment before pyrolysis: hydrothermal carbonization - CCh and acid washing- CCw), which were simultaneously doped with N and decorated with Co nanoparticles via wet impregnation followed by pyrolysis. The resulting composites (Co/N-CC, Co/N-CCh and Co/N-CCw) presented distinct physical-chemical properties, as assessed by ICP-OES, SEM, N2 adsorption-desorption isotherms, Raman spectroscopy, XRD and XPS. While the biochar support used had an important effect on composite electrocatalytic activity, in general, the introduction of N functionalities and Co nanoparticles led to enhanced ORR and OER performances. The effect of the pyrolysis temperature during the doping/Co incorporation procedure was also evaluated on the most promising biochar support (CC) and the resulting composite Co/N-CC_600 displayed enhanced performance towards both reactions, as it led to higher content of pyridinic-N, graphitic-N and N coordinated to Co, higher degree of graphitization and more exposed active sites. Indeed, it achieved comparable E1/2 to Pt/C (0.79 V vs. RHE), Eonset of 0.85 V vs. RHE, jL of −3.80 mA cm−2, nO2 of 3.29 and TS of 52 mV dec−1 for ORR. In OER, it presented similar η10 and TS (0.48 V and 79 mV dec−1, respectively) to benchmark IrO2. Moreover, the electrocatalyst showed excellent stability for ORR and a moderate current retention in OER.