ARTICLE

Understanding activity and selectivity ofmetal-nitrogen-doped carbon catalysts forelectrochemical reduction of CO2Wen Ju1, Alexander Bagger2, Guang-Ping Hao 3, Ana Sofia Varela1,4, Ilya Sinev5, Volodymyr Bon 3,

Beatriz Roldan Cuenya5,6, Stefan Kaskel3, Jan Rossmeisl2 & Peter Strasser1

Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity

has attracted significant attention partly due to the fundamental challenges related to

reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas

diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to

CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically

active MNx moieties (M=Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic

reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that FeNC

and especially NiNC catalysts rival Au- and Ag-based catalysts. We model the catalytically

active MNx moieties using density functional theory and correlate the theoretical binding

energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-

scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from

the MNx moieties and it provides predictive guidelines for the rational design of selective

carbon-based CO2 reduction catalysts.

DOI: 10.1038/s41467-017-01035-z OPEN

1 Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Berlin 10623, Germany. 2 Department of Chemistry, University ofCopenhagen, Universitetsparken 5, Copenhagen 2100, Denmark. 3 Department of Inorganic Chemistry, Technical University Dresden, Dresden 01062,Germany. 4 Institute of Chemistry, National Autonomous University of Mexico, Mexico City 04510, Mexico. 5 Department of Physics, Ruhr UniversityBochum, Bochum 44801, Germany. 6 Interface Science Department, Fritz-Haber-Institut der Max-Planck Gesellschaft, 14195 Berlin, Germany. Wen Ju andAlexander Bagger contributed equally to this work. Correspondence and requests for materials should be addressed toG.-P.H. (email: guangping.hao@manchester.ac.uk) or to J.R. (email: jan.rossmeisl@chem.ku.dk) or to P.S. (email: pstrasser@tu-berlin.de)

NATURE COMMUNICATIONS |8: 944 |DOI: 10.1038/s41467-017-01035-z |www.nature.com/naturecommunications 1

http://orcid.org/0000-0001-5849-9965http://orcid.org/0000-0001-5849-9965http://orcid.org/0000-0001-5849-9965http://orcid.org/0000-0001-5849-9965http://orcid.org/0000-0001-5849-9965http://orcid.org/0000-0002-9851-5031http://orcid.org/0000-0002-9851-5031http://orcid.org/0000-0002-9851-5031http://orcid.org/0000-0002-9851-5031http://orcid.org/0000-0002-9851-5031mailto:guangping.hao@manchester.ac.ukmailto:jan.rossmeisl@chem.ku.dkmailto:pstrasser@tu-berlin.dewww.nature.com/naturecommunicationswww.nature.com/naturecommunications

D irect electrochemical reduction of CO2 (CO2RR) is apromising early-stage technology to produce commoditychemicals and synthetic fuels. Electricity from renewablesources can provide the input power needed to react waterand waste CO2 to produce carbon-based chemicals or fuels in asustainable manner1. The ultimate technological viability of thisprocess, however, is contingent upon the identification ofaffordable catalyst materials that overcome the challengesregarding the poor product selectivity and poor voltage andenergy efficiency2.

Metals have been the most common choice as electrocatalystsfor the CO2RR. In early studies, copper was found to be theunique metal able to reduce CO2 into relevant amounts ofhydrocarbons3. This is why catalysis studieswhenever hydro-carbon products were of primary interesthave invariablyfocused on Cu or Cu-derived materials. From these we now knowthat the reaction conditions such as electrolyte46 and appliedpotential3, 7 can have a significant effect on activity and selectivityof Cu during the catalytic reaction process. In addition,more recent work evidenced that the morphology of the copperelectrode8, its oxidation state9, 10, the geometric shape1114 andsize15 of the Cu nanoparticles, the interparticle distance16, 17, aswell as the presence of a second metal1820 also play a crucial rolefor the resulting catalytic performance.

In contrast to hydrocarbon formation, the CO2 reduction toCO requires only two electron/proton transfers, which makes it asubstantially less hindered process. The formation of CO isusually accompanied by HER resulting in syngas production,which can be used as feedstock in synthetic fuels production viathe catalytic Fischer-Tropsch process. The chloralkalineelectrolysis-based polyurethane and polycarbonate industries,however, strive to adopt an electrocatalytic cathodic reduction ofCO2 to pure CO streams for production of phosgene intermediatefurther downstream. Such innovative CO2-depletion cathodes

coupled to the anodic chlorine production electrode are still inearly-stage research and currently require first and foremostfundamental advances in our understanding of the catalyticmechanism and the identification of suitable efficient catalysts.

It has been shown that Ag21, 22, Au-derived2328 and bimetallicCuIn19 and CuSn20 catalysts can selectively reduce CO2 toCO at low overpotentials. However, despite their promisingperformance, alternative earth-abundant catalyst materials aredesired. Molecular catalysts based on Iron-Porphyrin showedsome CO2 to CO reactivity in DMF solution2931, and so didmetal-organic frameworks32 and immobilized porphyrins3335.Unfortunately, these material concepts severely suffer from lowelectric conductivity and hence are not suitable as CO2 reductioncatalysts at large current densities.

A promising recent alternative to expensive noble metals aresolid doped carbon-based powder catalysts, similar to thosedeveloped for oxygen reduction reaction in recent years3640. Inrecent studies, metal-free, nitrogen-doped carbon catalysts (NC)have been proven capable to efficiently reduce CO2 to single- andmulti-carbon species and both experimental and computationalstudies have pointed toward pyridinic-N as the active site4146.More recent studies evidenced that the metal centers are in factcrucial for the CO2RR to CO. Varela et al.33 have tested thePANI-derived catalysts as CO2RR catalyst and shown that theaddition of metal resulted in a strongly enhanced CO2RR activityand the generation of CO. More interestingly, trace amounts ofCH4 were detected33. Consistently, DFT studies on transitionmetal based porphyrin-like catalysts suggested that depending onthe metal center, *CO can be further reduced47, 48. A detailedfundamental mechanistic understanding of the CO2 reductionreactivity and selectivity of single-site metal-nitrogen-dopedcarbons is still missing. This contribution will change that.

Here, we explore an entire family of single site, N-coordinatedtransition metal-doped nanoporous carbon materials (henceforth

CO2 adsorptionMn-N-C

Fe-N-C

Co-N-C

Ni-N-C

Cu-N-C

6 8 10D/ nm

800600400200

A family of M-N-C catalysts model

Transtion metal (M)-N-CM=Mn, Fe, Co, Ni, Cu

N-dopedcarbon latticeCarbon lattice

Nanopores

Highly accessible

O

N

N

N

N

N

N

N

M

00

1

2

P/ Torr

CO

2 up

take

/ mm

ol g

1

3

dV/d

D

4

5

0 2 4

1 nm

a b

c

Fig. 1 Visualization, porosity and illustration of the MNC catalyst. a Typical SEM image of the family of N-coordinated metal-doped (MNC) carbonelectro-catalysts, scale bar=4 m; b CO2 physisorption isotherm (273 K); inset: the pore size distribution; c Materials model and a schematic localstructure

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-01035-z

2 NATURE COMMUNICATIONS |8: 944 |DOI: 10.1038/s41467-017-01035-z |www.nature.com/naturecommunications

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referred to as MNC catalysts) as single-site electro-catalysts forthe CO2RR. Using a combined experimental and computationalapproach we investigate the catalyst activity and product effi-ciency (catalyst selectivity) and unravel their mechanistic originsin terms of binding energies and energetic reaction paths. Thisfamily of MNC materials comprises highly accessible andhomogeneously dispersed MNx sites, displays balanced surfacewettability and low valence metal species, which exhibitimpressively high activity and remarkable selectivity at low over-potential for the CO2RR to CO and hydrocarbons. We show thatthese catalysts are comparable alternatives to Au-based catalystsin future industrial CO2-consumption gas diffusion cathodes(CCCs). Density functional theory (DFT) calculations offer first-of-its-kind mechanistic insight into the rate- and selectivitydetermining processes on the single-site metal-nitrogen centers.We show that the binding energies of intermediates to the MNxmoieties provide excellent descriptors to predict, and understandthe mechanistic details of the CO2RR activity and selectivity ofthis family of catalysts over a wide overpotential range.

ResultsSynthesis and characterization. We have synthesized a familyof MNC electrocatalysts starting with bipyridine-basedcoordinated polymers and a variety of transition metals such asMn, Fe, Co, Ni, and Cu. Materials characterization started withmorphological and gas adsorption experiments (Fig. 1a, b). TheMNC electrocatalysts showed hierarchical chemical structureswith visible macropores (Fig. 1a, Supplementary Fig. 1). The poresize distribution peaks narrowly at ca. 0.70.8 nm (2.52.9 timesof the dynamic diameter of CO2 molecules, Fig. 1b inset),enabling this family MNC m