Developing highly active single-atom catalysts for electrochemical reactions is a key to future renewable energy technology. Here we present a universal design principle to evaluate the activity of graphene-based single-atom catalysts towards the oxygen reduction, oxygen evolution and hydrogen evolution reactions. Our results indicate that the catalytic activity of single-atom catalysts is highly correlated with the local environment of the metal centre, namely its coordination number and electronegativity and the electronegativity of the nearest neighbour atoms, validated by available experimental data. More importantly, we reveal that this design principle can be extended to metal–macrocycle complexes. The principle not only offers a strategy to design highly active nonprecious metal single-atom catalysts with specific active centres, for example, Fe-pyridine/pyrrole-N4 for the oxygen reduction reaction; Co-pyrrole-N4 for the oxygen evolution reaction; and Mn-pyrrole-N4 for the hydrogen evolution reaction to replace precious Pt/Ir/Ru-based catalysts, but also suggests that macrocyclic metal complexes could be used as an alternative to graphene-based single-atom catalysts.

A universal principle for a rational design of single-atom electrocatalysts

Due to the Fenton reaction, the presence of Fe and peroxide in electrodes generates free radicals causing serious degradation of the organic ionomer and the membrane. Pt-free and Fe-free cathode catalysts therefore are urgently needed for durable and inexpensive proton exchange membrane fuel cells (PEMFCs). Herein, a high-performance nitrogen-coordinated single Co atom catalyst is derived from Co-doped metal-organic frameworks (MOFs) through a one-step thermal activation. Aberration-corrected electron microscopy combined with X-ray absorption spectroscopy virtually verifies the CoN4 coordination at an atomic level in the catalysts. Through investigating effects of Co doping contents and thermal activation temperature, an atomically Co site dispersed catalyst with optimal chemical and structural properties has achieved respectable activity and stability for the oxygen reduction reaction (ORR) in challenging acidic media (e.g., half-wave potential of 0.80 V vs reversible hydrogen electrode (RHE). The performance is comparable to Fe-based catalysts and 60 mV lower than Pt/C -60 μg Pt cm-2 ). Fuel cell tests confirm that catalyst activity and stability can translate to high-performance cathodes in PEMFCs. The remarkably enhanced ORR performance is attributed to the presence of well-dispersed CoN4 active sites embedded in 3D porous MOF-derived carbon particles, omitting any inactive Co aggregates.

Nitrogen-Coordinated Single Cobalt Atom Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells.

Replacing precious platinum with earth-abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last 10 years, the fastest-growing branch in this area has been carbon-based metal-free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals. Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most up-to-date results are presented, along with remarks and perspectives for the future development of carbon-based metal-free ORR electrocatalysts.

Carbon-Based Metal-Free ORR Electrocatalysts for Fuel Cells: Past, Present, and Future.

The urgent need to address the high-cost issue of proton-exchange membrane fuel cell (PEMFC) technologies, particularly for transportation applications, drives the development of simultaneously highly active and durable platinum group metal-free (PGM-free) catalysts and electrodes. The past decade has witnessed remarkable progress in exploring PGM-free cathode catalysts for the oxygen reduction reaction (ORR) to overcome sluggish kinetics and catalyst instability in acids. Among others, scientists have identified the newly emerging atomically dispersed transition metal (M: Fe, Co, or/and Mn) and nitrogen co-doped carbon (M-N-C) catalysts as the most promising alternative to PGM catalysts. Here, we provide a comprehensive review of significant breakthroughs, remaining challenges, and perspectives regarding the M-N-C catalysts in terms of catalyst activity, stability, and membrane electrode assembly (MEA) performance. A variety of novel synthetic strategies demonstrated effectiveness in improving intrinsic activity, increasing active site density, and attaining optimal porous structures of catalysts. Rationally designing and engineering the coordination environment of single metal MNx sites and their local structures are crucial for enhancing intrinsic activity. Increasing the site density relies on the innovative strategies of restricting the migration and agglomeration of single metal sites into metallic clusters. Relevant understandings provide the correlations among the nature of active sites, nanostructures, and catalytic activity of M-N-C catalysts at the atomic scale through a combination of experimentation and theory. Current knowledge of the transferring catalytic properties of M-N-C catalysts to MEA performance is limited. Rationally designing morphologic features of M-N-C catalysts play a vital role in boosting electrode performance through exposing more accessible active sites, realizing uniform ionomer distribution, and facilitating mass/proton transports. We outline future research directions concerning the comprehensive evaluation of M-N-C catalysts in MEAs. The most considerable challenge of current M-N-C catalysts is the unsatisfied stability and rapid performance degradation in MEAs. Therefore, we further discuss practical methods and strategies to mitigate catalyst and electrode degradation, which is fundamentally essential to make M-N-C catalysts viable in PEMFC technologies.

Atomically dispersed metal–nitrogen–carbon catalysts for fuel cells: advances in catalyst design, electrode performance, and durability improvement

Molecular Design of Single-Atom Catalysts for Oxygen Reduction Reaction

Dual-site oxygen reduction reaction mechanism on CoN4 and CoN2 embedded graphene: Theoretical insights

Theoretical insights on the reaction pathways for oxygen reduction reaction on phosphorus doped graphene

Non-precious metal catalysts have attracted particular interest in recent years due to their promising ORR (oxygen reduction reaction) activity in fuel cells. In this work, the structural stability and ORR mechanism of CoN3 embedded graphene have been studied theoretically in acid media. The results indicate that CoN3 embedded graphene is stable thermodynamically. The kinetically most favorable reaction pathway for the ORR is a four-electron process. The process of OOH hydrogenation to generate O + H2O is the most favorable pathway. In the rate determining step, the energy barrier is 0.38 eV, much smaller than the theoretical value of ∼0.80 eV for pure Pt. The predicted working potential is 0.4 V for the most favorite pathway. Besides the lower energy barrier, the smaller Tafel slope compared with pure Pt in both low and high overpotential regions also suggests that CoN3 embedded graphene is a promising electrocatalyst for the ORR.

CoN3 embedded graphene, a potential catalyst for the oxygen reduction reaction from a theoretical perspective

Heteroatom doped graphene has caused particular interest in recent years due to its promising ORR (oxygen reduction reaction) activity in fuel cells. Sn doped divacancy graphene (Sn–Gra) was predicted to be a good candidate as a cathode catalyst in the previous study. In this work, the detailed ORR mechanism has been studied for Sn–Gra. The calculated charge transfer indicates that Sn and its adjacent four C atoms are the catalytic reaction sites. The unstable intermediate HOOH suggests that Sn–Gra experiences a four-electron ORR process. The most favorite pathway is the hydrogenation of the O2 molecule. The rate determining step is the hydrogenation of OOH to form H2O + O with the energy barrier of 0.75 eV. This value is slightly smaller than 0.80 eV for Pt, implying that Sn–Gra is a potential cathode catalyst for ORR. The predicted working potential is 0.16 V for the most favorite pathway. We expect that this study could provide new insights for the design of low-cost and highly efficient electrocatalysts in fuel cells.

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Xiaoxu Sun

Papers

Dual-site oxygen reduction reaction mechanism on CoN4 and CoN2 embedded graphene: Theoretical insights

Theoretical insights on the reaction pathways for oxygen reduction reaction on phosphorus doped graphene

CoN3 embedded graphene, a potential catalyst for the oxygen reduction reaction from a theoretical perspective

The oxygen reduction reaction mechanism on Sn doped graphene as an electrocatalyst in fuel cells: a DFT study

Theoretical Investigation on the Reaction Pathways for Oxygen Reduction Reaction on Silicon Doped Graphene as Potential Metal-Free Catalyst