# Ni-SAC-2.5 CO2-to-CO performance: research briefing v2

## Executive summary

Ni-SAC-2.5 achieved a peak CO Faradaic efficiency (FE_CO) of 98.9% at −1.2 V versus RHE in CO2-saturated 0.1 M KHCO3. This measures charge selectivity to CO, not energy efficiency or CO2 conversion. Durability was assessed separately at −0.8 V versus RHE: current density decreased only slightly over 20 h, and before/after Ni K-edge XANES spectra were described as nearly unchanged. The two operating points should not be merged; the cited evidence does not establish that the 98.9% FE_CO persisted during the 20 h test. Overall, the study supports high CO selectivity under a laboratory H-cell condition and short-duration current retention, while practical conversion, complete energy performance, statistical reproducibility, and longer operation remain unresolved by the cited evidence.

**Source:** [Yang et al. 2019](https://doi.org/10.1038/s41467-019-12510-0), *Nature Communications* 10, 4585. DOI: 10.1038/s41467-019-12510-0.

## Operating points

| Metric | Value or duration | Potential | Test context | Exact PDF locator |
|---|---:|---:|---|---|
| Peak CO selectivity | FE_CO = 98.9% | −1.2 V vs RHE | Ni-SAC-2.5; CO2-saturated aqueous 0.1 M KHCO3 | Main PDF p. 6, Fig. 6b; Main PDF p. 7, Fig. 6e |
| Current retention | 20 h; only slight decrease | −0.8 V vs RHE | Separate fixed-potential stability test; before/after XANES also compared | Main PDF p. 6, Fig. 6c–d; interpretation continues on Main PDF p. 7 |

## Evidence assessment

| Evidence | Supported interpretation | Boundary | Exact PDF locator |
|---|---|---|---|
| FE_CO reached 98.9% at −1.2 V; the authors state FE_CO exceeded 90% from −0.7 to −1.5 V. | High electron selectivity to CO under the reported half-cell conditions. | FE_CO does not establish energy efficiency or inlet CO2 conversion. | Main PDF p. 6, Fig. 6b; Main PDF p. 7, Fig. 6e |
| H2 FE was measured across potentials; no liquid products were detected by NMR after CO2RR. | CO was the dominant detected reduction product. | The cited passages and captions do not establish a complete carbon balance, universal detection limits, or outlet CO2 accounting. | Main PDF p. 6; Supplementary PDF p. 47, Supplementary Fig. 51; Supplementary PDF p. 48, Supplementary Fig. 52 |
| Current density decreased only slightly over 20 h at −0.8 V. | Short-duration current retention at that potential. | This does not establish 20 h retention of the 98.9% FE_CO obtained at −1.2 V. | Main PDF p. 6, Fig. 6c |
| Before/after Ni K-edge XANES spectra were described as almost identical. | Similar ensemble-averaged Ni XANES response before and after CO2RR. | This does not exclude local or minority-site restructuring, support changes, or catalyst loss. | Main PDF p. 6, Fig. 6d; Main PDF p. 7, first paragraph |
| Tests used a sealed three-electrode H-cell with catalyst-coated glassy carbon, Ag/AgCl reference, Pt-wire counter electrode, and 0.1 M KHCO3. FE holds lasted 30 min; gas products were analyzed by GC. | Defined laboratory half-cell assessment. | The cited method does not establish complete-electrolyzer voltage efficiency, membrane-electrode-assembly performance, or flow-reactor conversion. | Main PDF p. 8, “Electrochemical measurements” |
| FE_CO at −1.2 V was 98.9%, 98.5%, 98.0%, and 95.3% for 2.5, 3.4, 4.5, and 5.3 wt.% Ni. | Ni-SAC-2.5 led this loading series. | The cited comparison does not quantify run-to-run variability. | Main PDF p. 7, Fig. 6e; structural context: Main PDF p. 5, Fig. 5 |

All rows refer to DOI 10.1038/s41467-019-12510-0.

## Metric distinctions

**Faradaic efficiency to CO** is the fraction of transferred charge assigned to detected CO formation; it asks where the electrons went among measured products.

**Energy efficiency** also requires electrical-energy input, operating voltage or overpotential, and a thermodynamic reference. FE_CO alone does not establish it.

**CO2 conversion** is the fraction of supplied CO2 consumed, often reported as single-pass conversion. FE_CO and the absence of detected liquid products do not determine inlet-to-outlet conversion.

## Methodological limits

Selectivity came from 30 min constant-potential FE measurements in a three-electrode H-cell, whereas durability was a 20 h current-density trace at another potential. The cited evidence supports two distinct claims, not “98.9% for 20 h.”

Within the cited main-PDF performance discussion and methods and cited supplementary figures, the evidence does not establish a complete carbon balance, outlet CO2 conversion, full-electrolyzer energy efficiency, replicate-based uncertainty, or durability beyond 20 h. This is bounded to the cited evidence, not a claim about every page.

Current retention and before/after XANES do not resolve selectivity drift, local active-site changes, support corrosion, or catalyst detachment.

## Proposed follow-up checks

1. **Proposal — selectivity during durability:** Repeatedly measure FE_CO and FE_H2 during a replicated 20 h test at −0.8 V vs RHE. This would test whether current retention coincides with stable selectivity and quantify run-to-run uncertainty.
2. **Proposal — conversion and carbon balance:** Use a controlled-flow reactor with measured inlet/outlet CO2 and quantified gaseous and liquid products. This would resolve single-pass conversion and carbon utilization.
3. **Proposal — practical energy and structure:** Test a complete electrolyzer or membrane-electrode assembly while recording full-cell voltage, CO rate, and energy efficiency, paired with spatially resolved characterization after substantially longer operation. This would connect selectivity to device energy use and structural durability.

## Bottom line

Ni-SAC-2.5 reached 98.9% FE_CO at −1.2 V vs RHE. Separately, at −0.8 V vs RHE, current density decreased only slightly over 20 h and before/after Ni K-edge XANES was nearly unchanged. The cited evidence supports high CO charge selectivity and short-duration current retention, but not a 20 h peak-selectivity claim.