Understanding the Origin of Highly Selective CO₂ Electroreduction to CO on Ni,N‐doped Carbon Catalysts

Abstract

Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO2 reduction (CO2R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO2R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.

Direct Characterization of Atomically Dispersed Catalysts: Nitrogen‐Coordinated Ni Sites in Carbon‐Based Materials for CO2 Electroreduction

“…NiPACN Synthesis: Synthesis of polyacrylonitrile-derived Ni,N-doped carbon (Ni-PACN) follows previously reported work [23] and is based on synthesis of nanostructured, metal-free PACN particles. [28] In short, freeradical polymerization of acrylonitrile (2 mL) in acetone (2 mL) in the presence of azobisisobutyronitrile (AIBN, 2 mg) and a variable amount of Ni(NO 3 ) 2 6H 2 O (2-200 mg) was performed at 70 °C under N 2 protection for 12 h. The resulting white powder was vacuum dried overnight and stabilized in air at 230 °C (0.1 °C min −1 ramp rate, 2 h hold) to produce a brown, stabilized powder.…”

“…Electrochemical CO 2 R Testing: Testing protocol followed the previously reported methods. [23] NiPACN powder (13 mg) and Nafion solution (5 w%, 110 µL) were mixed in 3 mL of 3:1 IPA:H 2 O mixture and sonicated for 30 min. Ink was drop-casted on Sigracet 39BC carbon paper (13 cm 2 ) heated at 70 °C on a hot plate to achieve 1 mg cm −2 catalyst loading.…”

“…These findings are in-line with the known steep increase in activity of NiPACN at low Ni loadings and a plateau in activity at higher and saturated Ni loadings (Figure 1). [23] Although most of the values of NiN C ( ) as a function of n shows that the higher the carbon number normalization ion, the lower the predicted normalized fragment intensity ( Figure S10, Supporting Information). Due to this systematic error, we chose C 7 as the normalization ion for comparing the full set of NiN x C y fragments between different samples, as NiN C ( 7) I n x y = was comparable to the averaged I value.…”

“…[19][20][21][22] Our group has recently reported NiPACN, a Ni-N-C material derived from pyrolyzed polyacrylonitrile (PACN), catalysts that showed near-unity selectivity to CO attributed to dispersed Ni active sites. [23] Critically, these claims were made based on the observation that CO 2 R rate increased with Ni loading before flattening above 2 wt% Ni (Figure 1). This profile likely reflects the increase in dispersed active site density at low Ni loadings and then the formation of inactive Ni aggregate species after saturation of available binding sites.…”

Noble‐Metal‐Free Doped Carbon Nanomaterial Electrocatalysts

“…The general idea behind the enhanced electrocatalytic activity of Ni,N‐doped carbon nanomaterials was the coordination of Ni with the N atoms. In an effort to experimentally prove this popular allegation, a series of Ni,N‐doped carbon catalysts were synthesized in various dopant contents and extensively examined by electrochemical and physicochemical means towards the exploration of the CO 2 RR mechanism [104] . The result is a form of “vindication” for the research community, as it was found that the active sites are acquired when pyrrolic‐N moieties on the carbon lattice coordinate with the Ni atoms in a distorted square‐planar geometry that simulates the metal‐porphyrin molecular catalyst [104] .…”