A Facile Electrochemical Synthesis for Nanostructured Au for Improved Photo- and Electrochemical CO₂ Reduction Activity

Direct CO2 conversion using renewable energy source (e.g. solar energy) to value-added chemicals has attracted much attention for alternative fuel production. An efficient and durable electrocatalyst is essential to achieve practical level of CO2 conversion. In the recent years, nanostructuring of metal catalysts (e.g. Au, Ag, Cu) has been reported for enhancing CO2 reduction reaction (CO2RR) activity in aqueous solutions. In particular, nanostructured Au formed on a thick Au foil by electrochemical treatment has been intensively focused due to the superior properties regarding low overpotential and enhanced stability.[1] However, the utilization of thick Au foils can increase the cost and is also unavailable for photoelectrochemical (PEC) CO2RR which is the simplest approach for solar CO2 conversion. Accordingly, we recently demonstrated that mild electrochemical treatment enabling the formation of nanoporous Au thin film on Si photocathode for PEC CO2RR.[2] Although it shows exceptional CO2RR activity with only 20 nm-thick nanoporous Au layer, stability issue still remains to be solved for practical CO2 conversion. Here, we propose a novel electrochemical synthesis strategy to form Au nanostructured thin film for further improved CO2RR activity. The electrochemical synthesis is simply performed with 160 nm-thick Au thin film on Si substrate by anodization of Au thin films to grow Au oxide at constant potential (2.5 V (vs. RHE) for 40 min) followed by electro-reduction of the grown Au oxide at constant current density (-0.5 mA cm-2 for 10 min) in 0.2 M KHCO3 solution. Nanoporous Au nanostructures with height of ~180 nm were formed as a result of the electrochemical treatment. The CO2RR activities (overpotential, durability) for the nanostructured Au is significantly improved as compared to bare Au thin film. For example, our CO selectivity is to 66% and 98% at -0.35V and -0.39V, respectively, (bare Au: 0 and 20%, respectively). Nanostructured Au also shows better stability at -0.49V (~2 hours for over 90% selectivity) as compared to bare Au (20% selectivity at an hour). In order to further improve the stability of our nanostructured Au during CO2RR, we introduce additional electrochemical oxidation and reduction cycles. Interestingly, declined CO2RR activity (CO selectivity: ~70%) is revived to have 97% CO selectivity after an additional cycle. Furthermore, after the 4th electrochemical cycles, our Au nanostructures are stable over 6 hours while producing over 90% CO selectivity. We performed a systematic morphological and microstructural investigation of Au nanostructures during the repeated electrochemical treatment cycles. In particular, the height of Au nanostructured increases to about 390 nm and roughness factor is doubled after the 4th treatment. TEM investigations confirmed that remarkably increased number of grain boundaries (GBs) on the 4th treated Au samples. In addition, most GBs disappear after 15 hours of CO2RR. These results indicate that extended stability by repeating cycle of electrochemical synthesis is attributed to increased GBs on a roughened Au nanostructure. Finally, we applied our Au nanostructures to Si photocathodes for PEC CO2RR with Si photocathodes by forming Au micro-mesh. Micro-mesh Au on Si enables electrochemical treatments selectively on Au surface without deteriorating the underlying Si. Our Si photocathode with nanostructured Au co-catalyst remarkably achieves ~82% CO selectivity at -0.07 V which is more positive than CO2/CO redox potential under 1 sun illumination with extended stability. Therefore, our electrochemical synthesis would provide an efficient strategy to improve and recycle nanostructured Au activity for photo- and electrochemical CO2reduction. [1] Yihong Chen, Christina W. Li, and Matthew W. Kanan, J. Am. Chem. Soc. 134, 19969 (2012) [2] Jun Tae Song, Hyewon Ryoo, Minhyung Cho, Jaehoon Kim, Jin-Gyu Kim, Sung-Yoon Chung, and Jihun Oh, Adv. Energy Mater. 7, 1601103 (2017)

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