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Article

Large-Scale and Highly Selective CO2

Electrocatalytic Reduction on Nickel Single-Atom Catalyst

Tingting Zheng, Kun Jiang, Na

Ta, Yongfeng Hu, Jie Zeng,

Jingyue Liu, Haotian Wang

[email protected]

HIGHLIGHTS

A facile and scalable synthesis of

Ni single-atom catalysts on low-

cost carbon blacks

Current densities over 100 mA

cm�2 with nearly 100% selectivity

for CO2-to-CO conversion

A practical CO generation rate of

3.34 L hr�1 was achieved in a 10 3

10-cm2 unit cell

Earth-abundant Ni single atoms on commercial carbon black were synthesized in

large quantities via an economic and scalable protocol, with record-high selectivity

and activity toward CO production. Scaling up the electrodes into a 10 3 10-cm2

modular cell achieves a high overall current over 8 A while maintaining a nearly

exclusive CO evolution.

Zheng et al., Joule 3, 1–14

February 20, 2019 ª 2018 Elsevier Inc.

https://doi.org/10.1016/j.joule.2018.10.015

mailto:[email protected]


Please cite this article in press as: Zheng et al., Large-Scale and Highly Selective CO2 Electrocatalytic Reduction on Nickel Single-Atom Catalyst,Joule (2018), https://doi.org/10.1016/j.joule.2018.10.015

Article

Large-Scale and Highly SelectiveCO2 Electrocatalytic Reductionon Nickel Single-Atom CatalystTingting Zheng,1,2 Kun Jiang,1 Na Ta,3 Yongfeng Hu,4 Jie Zeng,2 Jingyue Liu,3 and HaotianWang1,5,6,*

Context & Scale

Electrochemical reduction of CO2

to fuels and chemicals carries

extraordinary significance for

industry and is highly competitive

with water electrolysis and

downstream gas-phase CO2

reduction for addressing energy

problems. Single-atom materials

endowed with maximum atom

efficiency, tunable coordination

environments, and electronic

structures have emerged as highly

SUMMARY

The scaling up of electrocatalytic CO2 reduction for practical applications is still

hindered by a few challenges: low selectivity, small current density to maintain a

reasonable selectivity, and the cost of the catalytic materials. Here we report a

facile synthesis of earth-abundant Ni single-atom catalysts on commercial car-

bon black, which were further employed in a gas-phase electrocatalytic reactor

under ambient conditions. As a result, those single-atomic sites exhibit an

extraordinary performance in reducing CO2 to CO, yielding a current density

above 100 mA cm�2, with nearly 100% selectivity for CO and around 1% toward

the hydrogen evolution side reaction. By further scaling up the electrode into a

10 3 10-cm2 modular cell, the overall current in one unit cell can easily ramp up

to more than 8 A while maintaining an exclusive CO evolution with a generation

rate of 3.34 L hr�1 per unit cell.

active catalysts for converting

CO2 to CO. However, practical

application of single-atom

catalysts still seems to be too far

away due to their complicated

and high-cost materials synthesis,

as well as low performance

metrics. In this work, Ni single

atoms on a low-cost carbon

nanoparticle support are

developed via a simple and

scalablemethod, with record-high

selectivity and activity toward CO

production. Moreover, scaling up

the electrodes into a modular cell

achieves a high overall current

while maintaining an exclusive CO

evolution.

INTRODUCTION

The intensive consumption of fossil fuels along with excessive emission of carbon di-

oxide (CO2) acceleratingly exacerbate global environmental problems, which

severely limit the potential of a sustainable progress of human civilization.1,2 Devel-

oping clean energy conversion technologies becomes extremely urgent to circum-

vent these challenges. Electrochemical CO2 reduction reaction (CO2RR) under

ambient conditions, coupled with renewable electricity sources, represents a prom-

ising approach to curb CO2 emissions while generating value-added fuels and chem-

icals.3–13 In a variety of CO2RRpathways such asC1 (CO, formate,methane, etc.),14–21

C2 (ethylene, ethanol, etc.),22–28 or C3 (n-propanol, etc.),

29,30 the reduction of CO2 to

CO is currently one of the most promising practices due to its relatively high selec-

tivity and large current density, as well as the facile separation of gas product from

liquid water. More importantly, CO as a fundamental chemical feedstock such as

the component of syngas, holds a largemarket compatibility and a wide range of ap-

plications in bulk chemicals manufacturing, medicine, and so on. Despite recent

breakthroughs on exploiting various selective catalysts for reduction of CO2 to CO,

the ultimate practical viability of this technology, however, is contingent upon the

scaling up of CO2RR process, which is still in its infancy with challenges in catalyst

cost, product selectivity, scalable activity, as well as long-term stability.

On the way of scaling up CO2RR for practical CO2 electrolysis, mass production

of high-performance catalysts with cost efficiency is the cornerstone and first

step. However, there are only a few known catalysts to date, including Au and Ag

noble metals,31–34 developed to deliver a significant selectivity toward CO evolu-

tion. As a cost-effective substitute and for the continuous efforts in our group,35,36

Joule 3, 1–14, February 20, 2019 ª 2018 Elsevier Inc. 1

1Rowland Institute, Harvard University,Cambridge, MA 02142, USA

2Hefei National Laboratory for Physical Sciencesat the Microscale, Department of ChemicalPhysics, University of Science and Technology ofChina, Hefei, Anhui 230026, P.R. China

3Department of Physics, Arizona State University,Tempe, AZ 85287, USA

4Canadian Light Source Inc., University ofSaskatchewan, 44 Innovation Boulevard,Saskatoon, SK S7N 2V3, Canada


earth-abundant single-atom catalysts (SACs) provide an intriguing paradigm for

CO2-to-CO conversion, with projected high atomic efficiency, superior activity,

and selectivity.37–41 The Ni single atoms coordinated in graphene vacancies, with/

without neighboring N coordination, have been demonstrated to be highly selective

to CO.42–45 Nevertheless, the commonly pursued strategies for preparing SACs,46

e.g., core-shell strategy, confined pyrolysis strategy, and polymer encapsulation

strategy are not as straightforward to scale up, and sometimes lack general applica-

bility: most of the carbon precursors, including graphene oxides,36,45 carbon nano-

tubes,47 and metal organic frameworks (MOFs),43 are either not economically viable

for large-scale production, or involve relatively complicated preparation steps; in

addition, some of the carbon matrix with nanosheet structures suffer from gas diffu-

sion limit when piled up layer by layer on the electrode, greatly hindering the reduc-

tion current density for practical implementation. In this sense, developing a facile

process for massive production of SACs becomes an important stepping-stone for

practical CO2 electrolysis.

Another critical challenge that goes beyond the nature of the electrocatalysts re-

volves around the low current density needed to maintain a high CO selectivity. In

a traditional H-cell device where the catalysts were immerged in liquid water, the

maximal CO evolution current was limited by the following two factors: (1) the solu-

bility of CO2 in water is relatively low, and beyond some point the CO2RR current

density will be dominated not by the reaction kinetics but by the mass diffusion lim-

itation, and (2) due to the concentrated water molecules around the catalyst surface,

once the overpotential is gradually increased for larger current density, the

hydrogen evolution side reaction (HER) can take off and eventually dominate the re-

action as observed in previous studies.43,44,48 Fuel cell technology emerges as a

platform for maximizing the throughput of CO2RR as reflected in the current and

selectivity boost, via preventing the catalyst from direct contact with liquid water,

as well as facilitating CO2 gas diffusion.36,49–54 In addition, the compact design of

cell andmembrane electrode assembly (MEA) can further boost a practical CO2 elec-

trolyzer system with scalable stacks and gas flow system.55

Herein, we report the synthesis of high-performance Ni SACs with commercial

carbon black particles as the support via a simple and scalable method. The Ni

single-atomic sites exhibit excellent performance for CO2RR in a traditional H-cell,

with a CO faradic efficiency (FECO) of �99% at �0.681 V in 0.5 M KHCO3 aqueous

electrolyte. More importantly, large current densities above 100 mA cm�2 with

nearly 100% CO generation, which are �10-fold higher than the current densities

in H-cell, were demonstrated on an anion MEA. An ultra-high CO/H2 ratio of 114,

which we define as the ‘‘relative selectivity’’ when the CO selectivity is close to

100% and H2 below 1% by gas chromatography (GC), was achieved while maintain-

ing a significant current of 74 mA cm�2. In addition, after 20 hr continuous operation

with an average current density of�85 mA cm�2, the CO formation FE was still main-

tained around 100%, while H2 below 1%. When the Ni SACs were further integrated

into a 10 3 10-cm2 modular cell, the CO evolution current in one unit cell can be

scaled up to as high as 8.3 A with an FECO of 98.4%, representing a large CO

generation rate of 3.34 L hr�1 per unit cell.

5Department of Chemical and BiomolecularEngineering, Rice University, Houston,TX 77005, USA

6Lead Contact

*Correspondence: [email protected]


RESULTS AND DISCUSSION

Materials Synthesis and Characterizations

Instead of starting with well-defined graphene matrix or precursors such as polymers

or MOFs,35,36,43,56 we used commercially available carbon blacks with activated

2 Joule 3, 1–14, February 20, 2019

mailto:[email protected]


Figure 1. Schematic of Synthetical Procedure of Ni-NCB

A facile ion adsorption process by mixing Ni salts with activated carbon black dispersed in aqueous solution is carried out and followed with further

pyrolysis, enabling gram-scale SACs produced in a one-batch synthesis.


surface to trap Ni single atoms and thus form a similar coordination environment and

active sites for CO2-to CO-conversion. Compared with that of graphene nanosheets

where the layer-by-layer stacking could block the gas diffusion pathways,36 the

nanoparticulate morphology of the carbon black support further facilitates the

CO2 diffusions across the gas diffusion layer to ensure a high local concentration

of reactants. An illustration of the synthetic process for the catalyst is shown in

Figure 1. In a typical preparation (see Experimental Procedures), 1 g of activated car-

bon blacks was well dispersed in water, followed with drop-by-drop addition of Ni2+

solution under vigorous stirring. Due to the presence of defects and oxygen-contain-

ing functional groups on the surface as well as the high surface areas, the activated

carbon black possesses a high adsorption capacity to metal cations in aqueous

solution. To ensure a full, but not excess, adsorption of Ni2+ on the carbon black,

the solution was stirred overnight and then centrifuged to collect the products

denoted as Ni2+-adsorbed carbon black (Ni2+-CB). Subsequently, the Ni2+-CB was

mixed with certain amount of urea as the N source and annealed at elevated temper-

atures (800�C) in Ar for 1 hr, with gram scale catalysts (denoted as Ni-NCB)

produced.

The high-resolution transmission electron microscopy (HRTEM) image of Ni-NCB in

Figure 2A shows the onion-like, defective graphene layers in CB particles, which can

serve well as the coordination matrix for Ni single atoms. The corresponding aber-

ration-corrected high-angle annular dark-field scanning transmission electronmicro-

scopy (HAADF-STEM) image reveals the individually and uniformly dispersed Ni

atoms as bright spots on the CB nanoparticle (Figure 2B). The individual Ni atoms

were well separated from each other and were relatively stable under electron

beam irradiation, suggesting strong anchoring (Figure 2C). In supplementation, a

large area TEM image confirms that no Ni nanoparticles or clusters were formed

on the CBs (Figure S1). Elemental mapping by energy-dispersive X-ray spectroscopy

Joule 3, 1–14, February 20, 2019 3

Figure 2. Structure Characterization of Ni-NCB

(A) Aberration-corrected bright-field STEM image. Scale bar represents 2 nm.

(B) Aberration-corrected HAADF-STEM image. Scale bar represents 2 nm.

(C) Zoom-in HAADF-STEM image shows the isolated Ni single atoms confined in carbon matrix as represented by these high-contrast dots. Scale bar

represents 0.5 nm.

(D) Ni 2p region XPS spectra.

(E) Ni K-edge XANES spectra.

(F) Ni K-edge Fourier transformed EXAFS spectra in R space.

See also Figures S1–S4.


(EDS) demonstrates that Ni and N species are homogeneously distributed

throughout the carbon framework (Figure S2). The mass loading of Ni was

determined to be �0.27 wt % by inductively coupled plasma atomic emission

spectrometry (Experimental Procedures). X-ray photoelectron spectroscopy (XPS)

characterization of Ni-NCB was performed to further elucidate the profile of

elemental composition and related chemical states (Figure S3). The Ni 2p spectrum

of Ni-NCB shows a positive Ni 2p3/2 binding energy (854.9 eV) relative to Ni

metal (852.6 eV), indicating the positive oxidation states of Ni single atoms

(Figure 2D). The XPS N 1s spectrum deconvoluted into pyridinic (�398.3 eV), Ni-N

(�399.5 eV), pyrrolic (�400.5 eV), quaternary (�401.3 eV), and oxidized (�403.0

eV)-like N species (Figure S4).45,57 The atomic concentrations of Ni and N in Ni-NCB

determined by XPS is 0.28 at % and 1.81 at %, respectively. Synchrotron-based X-ray

absorption near-edge spectroscopy (XANES) and extended X-ray fine structure

(EXAFS) were used to determine the electronic and local coordination of the

single-atomic sites in Ni-NCB (Experimental Procedures). The Ni K-edge XANES

profiles in Figure 2E indicate that Ni species in Ni-NCB were in a higher oxidation

state than Ni foil and lower than NiO, according to the near-edge position, which

is consistent with the XPS results. As shown in the EXAFS results in R space (Fig-

ure 2F), Ni-NCB exhibits prominent peaks at 1.4 and 1.9 A arising from the first shell

4 Joule 3, 1–14, February 20, 2019


Ni-N or Ni-C coordination.58 No other typical peaks for Ni-Ni contribution at longer

distances (2.2 A) were observed. Thus, Ni atoms were atomically dispersed

throughout the N-doped carbon blacks. Although different Ni-N andNi-C structures

have been proposed in literatures,35,36,59 the explicit coordination environment of

Ni is still not clear and awaits further exploring.

Electrocatalytic CO2RR Performance

The CO2 electrocatalytic reduction activity and selectivity of Ni-NCB were first eval-

uated in a standard three-electrode H-cell configuration with CO2-saturated 0.5 M

KHCO3 as the electrolyte. In control, an N-doped carbon black (denoted as N-CB)

and a Ni-doped carbon black (denoted as Ni-CB) were also prepared for comparison

(Figures S5 and S6). As revealed by linear sweep voltammetry in Figure S7, Ni-NCB

shows a much higher current density in CO2-saturated electrolyte than that of N2,

indicating the participation of CO2 gas in the reaction. Steady-state chronoam-

perometry of CO2 electrolysis was recorded under different potentials between

�0.3 and�1 V versus reversible hydrogen electrode (vsRHE). The FE of gas products

were analyzed by online GC (Figures 3A and S8; Supplemental Information).35,60 In

CO2-saturated 0.5 M KHCO3, Ni-NCB exhibits current densities significantly higher

than those of Ni-CB and N-CB (Figure S8). H2 and CO are the major gas products in

all these three samples. For Ni-NCB, CO signals were detectable at�0.41 V vs RHE,

suggesting that the onset overpotential of CO2 to CO is at least lower than 290mV. It

is noted that the overall FE under this potential is far less than 100%, which is possibly

due to the instrumental detection limit. As the potential becomesmore negative, the

FE of CO increases, while that of H2 decreases correspondingly. A high plateau of

CO FEs over 95% was retained under a broad potential range from �0.6 to

�0.84 V vs RHE, with a maximum CO selectivity of above 99% at �0.68 V vs RHE

while the competitive HER suppressed to 2%. No other liquid products were de-

tected by 1H nuclear magnetic resonance (NMR) (Figure S9). In sharp contrast,

NCB exhibits a faint activity for CO generation, indicating that Ni single atoms

play a critical role in activating CO2 to produce CO (Figure S8). In addition, Ni-CB

only shows a maximum FECO of 29%, which is presumably attributed to the poor

dispersion of Ni atoms on the CBs in the absence of nitrogen, as demonstrated in

our previous study.35,36 The partial current shown in Figure 3B demonstrates that

the activity of the Ni-NCB is better than, or comparable with, most of the noble-

metal-based catalysts reported to date.32,33,61 Moreover, Ni-NCB exhibits a high

intrinsic CO2 reduction activity, reaching a specific CO current of 111 A g�1. Besides,

a CO2-to-CO Tafel slope of 101 mV/decade on Ni-NCB (Figure S10) suggests that

the first electron transfer process generating surface adsorbed *COOH species is

possibly the rate-determining step for CO evolution.45 To further testify the intrinsic

activity of Ni-NCB for CO2 reduction, the CO production turnover frequency (TOF)

per Ni single-atomic site is calculated based on the total mass loaded on the elec-

trode, as a minimum value of estimation, as well as the electrochemical double layer

capacity (EDLC), as the effective surface area normalization (Figure S11). As shown in

Figure 3C, the TOF of Ni-NCB normalized by the mass and electrochemical active

surface area (ECSA) for CO production was calculated to be 3.67 and 9.66 s�1,

respectively, at an overpotential of 0.56 V, which is better than, or comparable

with, those of metal porphyrins or noble metal catalysts in aqueous solutions.7,8,31,62

Furthermore, to elucidate the influence of Ni content, N doping, as well as annealing

temperature, a series of control samples were prepared and tested for CO2 reduc-

tion (Figures S12–S18). It shows that both the partial current density and CO FE of

Ni-NCB annealed under NH3 atmosphere are slightly lower than those with urea

as N precursor. This could be due to the different vapor pressures of the N dopants,

or different radicals from N2H4 and NH3 under high temperature. It also reveals that

Joule 3, 1–14, February 20, 2019 5

Figure 3. Electrocatalytic CO2RR Performance of Ni-NCB

(A–C) FEs of H2 and CO (A), the corresponding steady-state current densities (B), and TOF (C) of Ni-NCB in an H-cell test. The catalyst mass loading

is 0.2 mg cm�2. The error bars represent three independent samples.

(D) The CO2RR stability test of Ni-NCB on CFP (1.25 mg cm�2) in an H-cell under 0.55 V overpotential.

(E–G) The steady-state current densities (E) and the corresponding FEs of H2 and CO (F), and the CO/H2 ratio (G) of Ni-NCB (1.25 mg cm�2) in an anion

membrane electrode assembly.

(H) Long-term electrolysis under a full-cell voltage of 2.46 V (without iR compensation) for more than 20 hr continuous operation.

See also Figures S5–S24 and Table S1.

6 Joule 3, 1–14, February 20, 2019



increasing the Ni loading leads to the generation of Ni clusters, which impairs the

overall performance for CO2RR (Figure S14). Besides, appropriate temperature

and amount of N doping are required to gain the optimal performance of Ni-

NCB. More importantly, the CO2-to-CO reduction performance of Ni-NCB is

extremely stable, retaining 99% of the initial current for CO formation (�23 mA

cm�2) after 24 hr of continuous operation, with FECO remaining above 95%. Post-

catalysis HAADF-STEM imaging and EXAFS (Figure S19) show that those Ni species

still maintain the feature of well-dispersed single atoms, reiterating the excellent

chemical stability of the Ni atomic sites in Ni-NCB.

The scaling up of CO generation rate in a traditional H-cell is limited by the

following two factors: (1) a larger overpotential is usually required to deliver a

higher kinetic current, which, however, can promote strong HER competition due

to the contact between catalyst and liquid water, and (2) the reduced CO2 gas reac-

tant in an H-cell configuration is that dissolved in liquid water, therefore the reac-

tion rate beyond a certain point is limited by CO2 mass diffusion. To circumvent

this issue and inspired by fuel cell reaction mechanisms, an anion MEA was adopted

in a gas-phase electrochemical reactor to greatly boost the current density while

maintaining high CO selectivity (Experimental Procedures).36 On the cathode

side, humidified CO2 gas was supplied. This high concentration of CO2 and low

concentration of H2O vapor can block the direct contact between catalyst and

liquid water and prevent limiting of reactant diffusion. On the anode side, 0.1 M

KHCO3 solution was circulated whereby the water oxidation is taking place (Fig-

ure S20). As shown in Figure 3E, the CO2 conversion increases rapidly above

2.1 V cell voltage and reaches a significantly high current density of 130 mA

cm�2 at only 2.7 V without iR compensations. Notably, the catalyst maintains nearly

100% FE for CO formation across a broad range of current densities from 30 to

130 mA cm�2, while the FE of H2 was suppressed to a minimum of 0.9% (Figures

3F and S21). It is important to mention here that, due to the experimental errors

introduced by GC detection, the measured CO selectivity could sometimes be

slightly higher than 100%, especially when H2 was suppressed to below 1%. In

this case, we propose to define the CO/H2 ratio, which we denote as relative selec-

tivity, as an additional criterion to more accurately evaluate the high selectivity to-

ward CO evolution. As shown in Figure 3G, with the gradual increase of cell

voltage, the CO/H2 ratio increases accordingly and reaches a maximum value of

113.8, with a high CO2RR current density of 74 mA cm�2. This is to our knowledge

the highest ratio of CO/H2 under a significant current density compared with the

most active catalysts reported to date (Table S1). An impressive stability of the cata-

lyst in this gas-phase electrochemical reactor is also presented in Figure 3H, with an

average current density of 85 mA cm�2 over 20 hr continuous electrolysis, while

maintaining CO formation FEs �100% and H2 below 1%. The slight degradation

of the current density was probably attributed to several factors including the deac-

tivation of catalysts, the corrosion of the gas diffusion layer, as well as membrane

degradation (Figures S22–S24). Overall, this high performance of the Ni-NCB cata-

lyst in the gas-phase electrochemical reactor opens up great opportunities in

scaling up highly selective CO2 reduction.

Motivated by the superior activity of Ni-NCB and its facile synthesis process, it is

expected that, by increasing the catalyst loading, extending the size of the gas

diffusion layer, as well as alternatively stacking anodes and cathodes, the Ni-

NCB integrated gas-phase electrochemical reactor can be further scaled up to pro-

duce large CO generation currents for potential practical applications. Here

we customized one unit cell with a 10 3 10-cm2 anion MEA as a preliminary

Joule 3, 1–14, February 20, 2019 7

Figure 4. Electrocatalytic CO2RR Performance of Ni-NCB in a 103 10-cm2 Anion Membrane Electrode Assembly Gas-Phase Electrochemical Reactor

(A) Photographs of assembled reactor, membrane electrode assembly, and individual cell components.

(B and C) The steady-state current densities (B) and the corresponding FEs of H2 and CO (C) of Ni-NCB (1.25 mg cm–2).

(D) Long-term electrolysis under a full-cell voltage of 2.8 V and a current of �8 A. The CO selectivity maintained above 90% over the course of 6 hr

continuous operation.

(E) The accumulated CO production during 6 hr continuous electrolysis.


demonstration to justify this application possibility in the future (Figures 4A–4C).

Considering the high CO2 flow rate needed to ensure sufficient reactants, gas col-

lecting bags were used to collect the gas products which were later analyzed by

GC under different cell voltages (Experimental Procedures). As shown in Figures

4D–4F, a record-high CO2RR current of 8.3 A was achieved with a high CO selec-

tivity approximating to 99% and H2 about 1%. Delivering an average current of �8

A for stability test, our device maintained a stable CO selectivity of more than 90%

for over 6 hr continuous electrolysis with a total volume of 20.4 L CO generated

(Figure 4G). This represents a CO generation rate of 3.42 L hr�1 or 0.14 mol

hr�1 and a conversion rate of 11.33%.

8 Joule 3, 1–14, February 20, 2019


It is anticipated that this work can be further pushed forward toward commercial-

ized CO2 electrolysis by optimizing several technological aspects according to in-

dustry standards. First, stability is one of the major concerns, which suffers from

the corrosion of both anode and cathode, as well as membrane and electrode

decay, which require tremendous efforts to overcome. In addition, CO2 feed

recycling can be set up by separating CO2 from gas products to achieve a

sustained CO2 supply. The cost of the anode should also be taken into

consideration, which can be greatly reduced by replacing IrO2 with efficient

transition metal-based materials. Meanwhile, one circumstance should be paid

attention to, where the metal leaching happens from the anode to be deposited

onto the cathode, which will hamper the CO2RR by encouraging competitive

HER.

In conclusion, a highly efficient transition metal-based SAC was synthesized via an eco-

nomic and scalable protocol, and applied in CO2 electrolysis for large-scale production

of CO. The results demonstrate that it is promising to replace noble metal catalysts,

such as Au or Ag, with earth-abundant materials with remarkable CO evolution perfor-

mance approaching practical expectations, which opens an avenue for future renew-

able energy infrastructures and achieves a significant progress in closing the anthropo-

genic carbon cycle for global sustainability.

EXPERIMENTAL PROCEDURES

Synthesis

The carbon blacks were activated by dispersing 2 g carbon blacks in 100 mL of

9 M nitric acid solution followed with refluxing at 90�C for 3 hr. The Ni-NCB cata-

lyst was prepared via a facile ion adsorption process followed with further

pyrolysis. Typically, a 3-mg/mL nickel nitrate stock solution was first prepared by

dissolving Ni(NO3)2,6H2O (Puriss, Sigma-Aldrich) into Millipore water (18.2

MW,cm). A carbon black suspension was prepared by mixing 1 g activated carbon

blacks (Vulcan XC-72, purchased from Fuel Cell Store and activated in acid bath)

with 400 mL of Millipore water, and tip sonicated (Branson Digital Sonifier) for

30 min until a homogeneous dispersion was achieved. Then 40 mL of Ni2+ solu-

tion was dropwise added into carbon black solution under vigorous stirring over-

night and then centrifuged to collect the products (Ni2+-CB). The as-prepared

Ni2+-CB powder was mixed with urea with a mass ratio of 1:10, and then heated

up in a tube furnace to 800�C under a gas flow of 80 standard cubic centimeters

per minute (sccm) Ar (UHP, Airgas) and maintained for 1 hr, obtaining the final

products. NCB and Ni-CB were prepared in a similar way but with the absence

of Ni precursor and urea, respectively. Ni-NCB-NH3 was prepared by annealing

the as-prepared Ni2+-CB powder at 800�C under a gas flow of 80 sccm NH3.

Ni-NCB-1:5 and Ni-NCB-1:20 were prepared in the same way as Ni-NCB, except

by varying the mass ratio of Ni2+-CB powder and urea to 1:5 and 1:20. Ni-NCB-

600 and Ni-NCB-1000 were prepared by just varying the annealing temperature to

600�C and 1,000�C. Ni excess was prepared with a modified strategy reported

before.36

Electrochemical Measurements

The electrochemical measurements were run at 25�C in a customized gastight

H-type glass cell separated by Nafion 117 membrane (Fuel Cell Store). A BioLogic

VMP3 work station was employed to record the electrochemical response. The

set-up of the three-electrode test system can be found in our earlier reports.35,36

Typically, 5 mg of as-prepared catalyst was mixed with 1 mL of ethanol and

100 mL of Nafion 117 solution (5%, Sigma-Aldrich), and sonicated for 20 min to

Joule 3, 1–14, February 20, 2019 9


get a homogeneous catalyst ink. Ink (80 mL) was pipetted onto a 2-cm2 glassy carbon

surface (0.2 mg/cm2 mass loading). For the stability test, 500 mL of the ink was

air-brushed onto a carbon fiber paper gas diffusion layer toward a mass loading of

1.25 mg/cm2, and then vacuum dried prior to use. All potentials measured against

a saturated calomel electrode were converted to the RHE scale in this work using

E (vs RHE) = E (vs SCE) + 0.244 V + 0.0591*pH, where pH values of electrolytes

were determined by an Orion 320 PerpHecT LogR Meter (Thermo Scientific). Solu-

tion resistance (Ru) was determined by potentiostatic electrochemical impedance

spectroscopy at frequencies ranging from 0.1 Hz to 200 kHz, and manually compen-

sated as E (iR corrected versus RHE) = E (vs RHE) � Ru *I (amps of average current).

For the anion MEA test (or scale-up fuel cell test), 1.25 mg/cm2 Ni-NG and IrO2

was air-brushed onto two 2 3 2-cm2 (or 10 3 10-cm2) Sigracet 35 BC gas diffu-

sion layer electrodes as a CO2RR cathode and an oxygen evolution reaction

anode, respectively. A PSMIM anion-exchange membrane (Dioxide Materials)

was sandwiched by the two gas diffusion layer electrodes to separate the cham-

bers. On the cathode side, a titanium gas flow channel supplied 50 sccm (or

500 sccm) humidified CO2 while the anode was circulated with 0.1 M KHCO3

electrolyte at 2 mL min�1 (or 10 mL min�1) flow rate. The cell voltages in Figures

3E–3H were recorded without iR correction. The 10 3 10-cm2 MEA response was

recorded by a Sorensen DCS 33-33 power supply and is shown in Figure 4

without iR correction.

CO2RR Products Analysis

During electrolysis, CO2 gas (99.995%, Airgas) was delivered into the cathodic

compartment containing CO2-saturated electrolyte at a rate of 50.0 sccm (moni-

tored by an Alicat Scientific mass flow controller) and vented into a Shimadzu

GC-2014 GC equipped with a combination of molecular sieve 5A, Hayesep Q,

Hayesep T, and Hayesep N columns.35,60 A thermal conductivity detector was

mainly used to quantify H2 concentration, and a flame ionization detector with

a methanizer was used to quantitative analysis CO content and/or any other

alkane species. The detectors are calibrated by three different concentrations

(H2: 100, 1,042, and 49,830 ppm; CO: 100, 496.7, and 9,754 ppm) of standard

gases. The gas products were sampled after a continuous electrolysis of

�15 min under each potential. The partial current density for a given gas product

was calculated as below:

ji = xi 3 v3niFP0

RT3 ðelectrode areaÞ�1

where xi is the volume fraction of certain product determined by online GC refer-

enced to calibration curves from three standard gas samples, v is the flow rate, niis the number of electrons involved, p0 = 101.3 kPa, F is the Faraday constant, and

R is the gas constant. The corresponding FE at each potential is calculated by

FE =ji

jtotal3100%

For a 10 3 10-cm2 MEA, the FEs of H2 and CO were tested ex situ and calculated

based on the concentration normalization.

1D 1H NMR spectra were collected on an Agilent DD2 600 MHz spectrometer to test

if any liquid products present during the CO2 reduction (Figure S9). Typically, 600 mL

of electrolyte after electrolysis was mixed with 100 mL of D2O (Sigma-Aldrich, 99.9 at

% D) and 0.05 mL DMSO(Sigma-Aldrich, 99.9%) as internal standard.

10 Joule 3, 1–14, February 20, 2019


Evaluation of TOF

Calculation of TOF by mass loading normalization: catalyst loading on glass carbon

electrode is 0.2 mg cm�2. The content of Ni in Ni-NCB is 0.27 wt %. The moles of

active sites per cm2:

N=0:23 10�3 3 0:273 10�2

58= 9:313 10�9 mole cm�2

TOF�s�1

�=

J3 FECO 3 0:965

23 96485:33 9:313 10�9

Calculation of TOF by ECSA normalization: according to the reported EDLC value of

graphene �21 mF/cm2(36), the electrochemical surface area of graphene layers in Ni-

NCB was calculated to be 390.5 cm2, given the 8.2 mF/cm2 EDLC value of Ni-NCB.

The moles of carbon atoms on the electrochemical surface can be calculated to be

390.5 3 10�4/2,600 3 12 = 1.25 3 10�6 mol, where 2,600 m2 g�1 is the theoretical

specific surface area of graphene. Taken together the Ni atomic content in Ni-NG

was determined to be 0.28% by XPS (Figure S1), and the number of Ni sites in the

surface was N = 3.5 3 10�9 mol. Accordingly, TOFðs�1Þ = J3FECO30:965

2396485:333:5310�9.

Instrumentation

The STEM characterization in Figure 1A was carried out using a JEOL ARM200F

aberration-corrected scanning transmission electron microscope at 200 kV with an

image resolution of �0.08 nm. All other TEM images were obtained by using a

JEOL 2100 transmission electron microscope operated under 200 kV. EDS analysis

was performed at 300 kV using Super-X EDS system in a Probe-corrected FEI Titan

Themis 300 S/TEM. Drift correction was applied during acquisition. XPS was

obtained with a Thermo Scientific K-Alpha ESCA spectrometer, using a monochro-

matic Al Ka radiation (1,486.6 eV) and a low energy flood gun as neutralizer. The

binding energy of the C 1s peak at 284.6 eV was used as reference. Thermo Avant-

age V5 program was employed for surface componential content analysis as well as

peaks fitting for selected elemental scans. XAS spectra on Ni K-edge was acquired

using the SXRMB beamline of Canadian Light Source. The SXRMB beamline used an

Si(111) double-crystal monochromator to cover an energy range of 2–10 keV with a

resolving power of 10,000. The XAS measurement was performed in fluorescence

mode using a four-element Si(Li) drift detector in a vacuum chamber. The powder

sample was spread onto double-sided, conducting carbon tape. Ni foil was used

to calibrate the beamline energy.

SUPPLEMENTAL INFORMATION

Supplemental Information includes 24 figures and 1 table and can be found with this

article online at https://doi.org/10.1016/j.joule.2018.10.015.

ACKNOWLEDGMENTS

This work was supported by the Rowland Fellows Program at Rowland Institute,

Harvard University. The Center for Nanoscale Systems (CNS) is part of Harvard

University. This research used resources of the Canadian Light Source, which is sup-

ported by NSERC, the National Research Council Canada, the Canadian Institutes of

Health Research, the Province of Saskatchewan, Western Economic Diversification

Canada, and the University of Saskatchewan. J.L. and N.T. were supported by the Na-

tional Science Foundation under CHE-1465057, and gratefully acknowledge the use of

facilities within the John M. Cowley Center for High Resolution Electron Microscopy at

Arizona State University. T.Z. and N.T. acknowledge funding from the China

Joule 3, 1–14, February 20, 2019 11



Scholarship Council (CSC) (201706340152 and 201704910441, respectively). J.Z. ac-

knowledges support from MOST of China (2014CB932700) and NSFC (21573206).

This workwas performed in part at theCNS, amember of theNationalNanotechnology

Infrastructure Network (NNIN), which is supported by the National Science Foundation

under NSF award no. ECS-0335765. H.W. acknowledges support from Rice University.

AUTHOR CONTRIBUTIONS

H.W. designed the studies. T.Z. conducted the synthesis and catalytic tests of cata-

lysts. K.J. performed the characterization of catalysts. N.T. and J.L. conducted

HRTEM characterization. Y.H. performed XAFS measurements. J.Z. provided sug-

gestions on the work. T.Z. and H.W. wrote the manuscript. All authors discussed

the results and commented on the manuscript.

DECLARATION OF INTERESTS

H.W. has submitted a patent application (US 62/486,148, 2017) regarding the tran-

sition single-atom catalyzed carbon dioxide conversion technology.

Received: August 24, 2018

Revised: October 2, 2018

Accepted: October 16, 2018

Published: November 8, 2018

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