Peer review of: Photocathode functionalized with a molecular cobalt catalyst for selective carbon dioxide reduction (2020)

Reviewer #1 (Remarks to the Author): 

Pati et al. reported a photocathode based on Cu(In,Ga)Se2 (CIGS) SC and cobalt quaterpyridine 
catalyst for the selective photoreduction of CO2 into CO with a maximum current density of 3.35 mA 
cm-2 at a bias potential of -0.15 V vs. RHE, which is an impressive performance with novelty. 
However, some explanation/experiments are confusing. Balancing all these considerations, I can not 
recommend its publication at this stage, but would be a reasonable candidate after addressing bellow 
concerns: 

(1) In Fig.S6, after catalyst grafting upon TiO2 surface, the UV-Vis absorption spectrum has changed 
significantly. Therefore, the factors to influence the spectrum variety should be discussed in detail. 

(2) In Fig.S3, a N1s characteristic signal at 399.5 eV is not pyridinic nitrogen, but metal-N (Mater. 
Res. Bull. 2012, 47, 2188-2192). Moreover, why P2p analysis is 1000 counts but not 2000 counts. 

(3) In Fig.3, before and after 2 h electrolysis at -0.51 V vs. RHE, the difference of Co2p spectrum is 
relatively large, please explain it. 

(4) In Fig. 4, I wonder why the current density decreases sharply during 2 h electrolysis. 



Reviewer #2 (Remarks to the Author): 

The authors report the coupling of a cobalt quaternarypyridine catalyst (Co-qPyH) to a Cu(In,Ga)Se2 
photocathode to generate a system that shows high activity and selectivity for photoelectrochemical 
CO2 reduction in aqueous neutral carbonate buffer. Although the Co quaternarypyridine catalysts have 
been extensively studied by this group for electrochemical and photochemical CO2 reduction, the 
combination with a CIGS photocathode is a novel and impactful advance. The photoelectrochemical 
system reduces CO2 to CO at -0.15 V under illumination with a current density for CO of 0.72 mA 
cm<sup>-2</sup> and a TOF of ~1.1 s<sup>-1</sup> over a 2 h experiment. 

This is a well-considered study that reports an important advance in the use of this class of Co 
complexes for CO2RR in aqueous solution, and I believe it has sufficient novelty and impact for 
publication in Nature Communications. I have only a few very minor suggested changes the authors 
should consider before publication: 

1)In the abstract, the authors state: “Yet, there is no catalytic system able to reduce selectively CO2 
into CO in aqueous media with high activity, and using earth-abundant molecular catalyst.” This is not 
true. There are several recent examples of surface-immobilized molecular catalysts that reduce CO2 to 
CO in water: Re(tBu-bpy) (Kubiak), CoPc (Wang, McCrory, Manthiram), CoTPPS (Sakai), Ni(cyclam) 
(Kubiak, Shafaat, Ren, Fujita, etc.), among other. This statement should be removed or clarified. 

2)The author should consider reporting TOF (per second) in addition to TON for their measurements in 
Table 1 and comparisons in Table 2. This might help readers make broader comparisons to the 
electrocatalysis and photoelectrocatalysis literature. 



Reviewer #3 (Remarks to the Author): 

This manuscript by Robert, Odobel, Barreau and collaborators reports on the immobilization of a 
cobalt polypyridyl CO2 reduction catalyst and its integration on a photocathode based on 
Cu(In,Ga)Se2 semiconductor. The known cobalt quaterpyridine electrocatalyst is interfaced with a 
know light harvesting system to obtain a high performing engineered system for CO2 reduction to CO. 
The key points emphasized by the authors are that this photocathode does not contain noble metals, 
and has “a good selectively for CO or formate, a near zero overpotential and large current densities 
when working in aqueous solution.” Further claims made by the authors are that this work “represents 
the first demonstration where a CIGS electrode has been successfully used for CO2 catalytic 
reduction” and is “the first PEC device demonstrating photoelectrocatalytic activity in pure aqueous 
medium with 99% of selectivity for CO2 reduction to CO” 
While every component of the assembly is known, the photoelectrochemical results obtained are 
convincing and represent a leap in performance compared to previous systems. 

Comments: 
• In the abstract the authors claim that their study uses “Atmospheric CO2 dissolved in aqueous 
solution”. Most people would take that to mean CO2 at atmospheric concentrations. This is clearly not 
the case in the paper, and language in the abstract should be edited to avoid confusion. 
• The XPS data in figure 3 shows that most of the Co signals are lost on the surface after electrolysis. 
Could the authors calculate how much was lost? The electrolysis current traces do not appear to show 
enough current loss to account for the loss of Co observed. What is the fate of the cobalt? 
• Similarly, they authors mention that “Complementary EDX mapping experiment (Fig. S7) shows a 
consistent atomic ratio of 2 to 1 between phosphate and cobalt, definitively proving the presence of 
the catalyst and confirming its intactness after grafting.” Performing this analysis again after 
electrocatalysis would be necessary to demonstrate that the cobalt complex stays intact on the surface 
during CO2 reduction and is the active catalyst. 
• ICP-OES (data not shown?) was used to determine catalyst loading on the electrode. The authors 
write “catalyst loading of 29 ± 4 nmol cm-2 on m-TiO2 film and 3 ± 1 nmol cm-2 in f-TiO2 film (see 
Methods for details). This concentration is about one third of previously reported data (85 nmol cm-2 
for a Re complex with a similar anchoring group on mesoporous TiO2),17 leaving room for 
improvement.” The catalyst loading is used to determine the turn-over-numbers reported after 2h. 
Given the small numbers involved, any error in the ICP-OES would have a huge impact on the 
reported performance of the catalyst. Coupled to the fact that the authors are surprised by the low 
catalyst loading, the experiment should be repeated to get an understanding of the potential error 
bars on the measurement. 
• The meaning of “TON” should be clarified in Table 1: it currently implies that the catalyst is 
completely degraded during the experiment. Given the current density traces, this does not appear to 
be the case. Clarifying the definition used or reporting the TOF during the 2-hour of the experiment 
would be less confusing. 
• Figure 6: why did the retention time change for 13CO2 when shifting from natural abundance to 13C 
labeled CO2? What was changed in the method? 
• Why were the electrolyte concentration, solution pH, and overpotential all changed for the 7h 
electrolysis (table 1 entry 5)? This makes for a difficult comparison to the rest of the work presented.