電催化CO2 還原耦合HCHO 氧化反應(yīng)系統(tǒng)促進污染物同步經(jīng)濟高效資源化轉(zhuǎn)化

摘要：傳統(tǒng)電化學CO2還原（CO2RR）系統(tǒng)中陽極發(fā)生的水氧化半反應(yīng)（WOR）具有動力學緩慢、過電位大、能耗高等缺點，限制了CO2RR系統(tǒng)的經(jīng)濟效益和應(yīng)用。因此，本研究引入MnO2陽極進行甲醛氧化半反應(yīng)（FOR）以代替WOR，構(gòu)建了一種新型CO2RR/FOR耦合系統(tǒng)。與傳統(tǒng)的CO2RR/WOR系統(tǒng)相比，在相同的施加電勢下，CO2RR/FOR耦合系統(tǒng)的CO2RR電流密度和CO2RR產(chǎn)物的生成速率通常更具有優(yōu)勢。此外，在CO2RR/FOR耦合系統(tǒng)中，在合適的施加電勢下，HCHO可以選擇性地轉(zhuǎn)化為HCOOH。具體來說，兩電極CO2RR/FOR耦合系統(tǒng)中，在3.5 V的槽電壓下，近90%的HCHO可以被去除，且HCHO會選擇性轉(zhuǎn)化為HCOOH，其轉(zhuǎn)化率約為48%。更重要的是，在不同的工作電流下，F(xiàn)OR所需的電勢比WOR所需的電勢要小。在?10 mA?cm?2時，CO2RR/FOR耦合系統(tǒng)能降低約210 mV的槽電壓，并且其能耗比單獨的CO2RR系統(tǒng)和FOR系統(tǒng)的能耗之和降低45.13%。值得注意的是，當使用商業(yè)多晶硅太陽能電池作為電源時，在CO2RR/FOR耦合系統(tǒng)中的CO2RR電流密度、CO2RR產(chǎn)物的生成速率和HCHO到HCOOH的選擇性仍然可以實現(xiàn)相當?shù)母纳?。目前的工作將進一步推動研究開發(fā)新型的CO2RR耦合系統(tǒng)，以經(jīng)濟有效地將CO2和有機污染物同時轉(zhuǎn)化為有價值的化學品。

關(guān)鍵詞：二氧化碳還原；甲醛氧化；耦合電化學系統(tǒng)；銅；氧化錳

中圖分類號：O643

Abstract： Due to rapid industrial development and human activities， CO2emissions have led to serious environmental/ecological problems andclimate changes such as global warming. Due to this situation， achievingcarbon neutrality has become an urgent mission to improve the future ofmankind. The use of the electrocatalytic CO2 reduction reaction （CO2RR） toproduce higher-value fuels and chemicals is an effective strategy forreducing CO2 emissions and easing the energy crisis. The water oxidationhalf-reaction （WOR）， which occurs at the anode in a traditional CO2RRsystem， typically suffers from slow kinetics， a large overpotential， and highenergy consumption. The organic pollutant formaldehyde （HCHO） isoxidized into industrial materials （such as formic acid） under neutralconditions， which is of great significance for the sustainable production of energy and lessening environmental pollution.In addition， the number of electron transfers involved and the required potential for the HCHO oxidation half-reaction （FOR）are smaller than those of WOR， suggesting that FOR could potentially replace WOR as a coupling reaction with CO2reduction. In this study， FOR at a MnO2/CP anode is introduced to produce a novel paired CO2RR/FOR system. Thecurrent density and generation rate of CO2RR products in this paired CO2RR/FOR system are generally larger than thoseof conventional CO2RR/WOR systems at the same applied potential. Moreover， in paired CO2RR/FOR systems， HCHOcan be selectively converted into HCOOH at certain applied potentials. Nearly 90% of the HCHO can be selectivelyconverted to HCOOH with a conversion efficiency of about 48% at a cell voltage of 3.5 V in a two-electrode pairedCO2RR/FOR system. More significantly， under a different working current， the potentials required for FOR are systemicallysmaller than those for WOR. At ?10 mA?cm?2， the cell voltage of the paired CO2RR/FOR system can be reduced by 210mV， and the required electric energy for the paired CO2RR/FOR system can be reduced by 45.13% compared with thesum of single CO2RR and FOR systems. Notably， when a commercial polysilicon solar cell is used as the power supply，improvements in the current density， the generation rate of CO2RR products， and the HCHO to HCOOH selectivity can bestill achieved in the paired CO2RR/FOR system. The present work will inspire further studies for developing novel pairedCO2RR systems for the cost-effective， simultaneous conversion of CO2 and organic pollutants into valuable chemicals.

Key Words： CO2 reduction; HCHO oxidation; Paired electrochemical system; Cu; MnO2

1 Introduction

Carbon dioxide （CO2） is an important greenhouse gasses，which is released through human activities as well as naturalprocesses. In the past centuries， because of rapid industrialdevelopments and intensive human activities， a huge amount ofCO2 was emitted into the atmosphere， breaking the equilibriumof carbon cycle in the natural ecosystem， and bringing seriousenvironmental/ecological problems and climate changes， such asglobal warming 1–6. Among various methods to mitigate CO2emission and accelerate artificial carbon cycling， theelectrochemical reduction of CO2 （CO2RR） is a very promisingchoice， which are pursuing to use the renewable intermittentelectricity to convert CO2 into value-added chemicals underenvironmental conditions 4，5，7. However， from the viewpoints ofindustrial upscaling and commercial applications， the efficiencyand selectivity of available CO2RR systems are stilldissatisfying， especially for the targeted production of multicarboncompounds （e.g.， C2H4） with high energy density andhigh economical values.

To improve CO2RR efficiency and selectivity， great effortshave been devoted to exploiting efficient cathodicelectrocatalyst. Among available CO2RR catalysts， Cu-basedcatalyst has been widely studied， because it can convert CO2 tomulti-carbon hydrocarbons 4，8. Despite great advances haveachieved on modulating the composition， morphology， defects，chemical states and interface of Cu-based catalyst， choosingsuitable ion-exchange membrane and appropriate electrolytewith optimized pH and concentration， improving CO2 mass transfer by using gas-diffusion electrode （GDE） and buildingflow cells 9，10， most of CO2RR still seems to be economicallyunfeasible. Several intrinsic drawbacks restrict the economicfeasibility of traditional CO2RR system， which consists ofCO2RR half-reaction at the cathode and water oxidation reaction（WOR） half-reaction at the anode 11，12. Firstly， the waterreduction competing with CO2RR occurs on the cathode， makingCO2RR less economically attractive. Secondly， the product ofWOR is oxygen， which has limited value in mass production.Moreover， WOR is a four-electron transfer reaction requiring ahigh thermodynamic potential （1.23 V vs. RHE） 13. As a result，WOR shows slow reaction kinetics and requires high energyconsumption， which greatly affects the economics of CO2RRsystems 14. Based on the above discussions， in addition tocathode catalyst modification and system engineering， replacingWOR with a kinetically and economically more feasible anodichalf-reaction appears to be an effective strategy to improve theeconomy of CO2RR systems. Although several anodic reactionsto replace WOR have been attempted 14，15， it remains as greatchallenges to build paired CO2RR systems combining twomatching reactions， enabling the simultaneous production ofvalue-added chemicals with lower energy demand in bothcathodic and anodic compartments.

Several standards can be used to select the desired alternativereaction to replace WOR： （i） the solubility of the reactants in theaqueous solution should be sufficient， （ii） the oxidation potentialof the reactants should be lower than the WOR， （iii） the numberof electron transfers involved in the oxidation of the reactants is preferably smaller than that in the WOR. Formaldehyde（HCHO） is a notorious organic pollutant and has receivedwidespread attention due to its carcinogenic andteratogenicity 16–19. Formaldehyde is involved in manyindustries such as chemical industry， paper industry， textileprocessing， etc.， which leads to the production of a large amountof formaldehyde wastewater with a concentration of 100–10000mg?L?1 20，21. So far， most studies on electrocatalytic oxidation offormaldehyde （FOR） have been conducted in alkalineenvironments， but only a few studies on FOR were carried out inneutral environments， not to mention using noble metalelectrodes 22–25. In this sense， it is significant to develop nonnoblemetal catalysts for the FOR in neutral environments. Inaddition， the number of electron transfers involved and therequired potential for FOR are smaller than those of WOR，suggesting that it is of great potential to replace WOR by FORas a coupling reaction with CO2 reduction. In this study， a novelpaired electrocatalytic system composed of FOR and CO2RR isestablished. The optimized paired CO2RR/FOR paired systemreduces energy consumption by 45.13%， comparing to the sumof single CO2RR and FOR system. Moreover， the HCHO →HCOOH Faraday efficiency can be optimized as high as 48%，while the cell voltage can be reduced by 210 mV， and the HCHOremoval efficiency remains at 90%. When solar cells are used asthe power supply， compared with the CO2RR/WOR system， theCO and C2H4 generation rates in the paired CO2RR/FOR systemcan be increased by about three times and six times respectively，while converting HCHO to HCOOH with a conversionefficiency of about 60%. This study provides an example ofimproving the performance of CO2 reduction and reducingenergy consumption while converting pollutants into valuableproducts.

2 Materials and methods

2.1 Electrode preparation and characterization

The cathode for CO2RR is consisting of Cu foil （99.9%）supported Cu nanowire arrays （Cu NWs）. The synthesis of CuNWs is shown in Fig. S1a. First of all， Cu（OH）2 NWs weresynthesized by a simple wet chemical method 26. Then， theprepared Cu（OH）2 NWs were annealed in air （150 °C， 2 h） togenerate CuO NWs. Cu NWs were generated by electroreducingCuO NWs in CO2 purged 0.1 mol?L?1 KHCO3 （?0.6 Vvs. RHE， 1 h）. As shown in Fig. S2a， the nanowire morphologywas successfully produced on the surface of the copper foil. Andthe result of XRD indicate clearly that the nanowires after heattreatment are mainly composed of CuO （JCPDS No. 80-0076），but not Cu2O （Fig. S2b）. After electrochemical reduction， CuONWs are reduced to Cu NWs （JCPDS No. 04-0836）.

The anode for FOR is consisting of carbon paper （Toray TGPH-060， 1.0 cm × 2.0 cm） supported MnO2 nanosheet arrays（MnO2/CP）. Previous studies have shown that MnO2/CP can begenerated through electrochemical oxidation 27. In this study，MnO2/CP were generated by simple cyclic voltammetry （Fig.S1b）. The reaction solution is 100 mL of aqueous solutioncontaining 0.05 mol?L?1 MnSO4 and 0.1 mol?L?1 CH3COONa.Firstly， a piece of carbon paper was pretreated in HNO3 at 120 °Cfor 6 h to increase hydrophilicity. After cleaning，electrodeposition was performed in 10 cycles （0–2.3 V vs.Ag/AgCl， 50 mV?s?1）. Then， the obtained MnO2/CP was heatedin air （350 °C， 2 h）. Fig. S3a shows the SEM image of MnO2/CPin which the nanosheet array structure can be seen. In addition，XRD pattern （Fig. S3b） shows that the prepared MnO2 is mainlycomposed of β-MnO2 （JCPDS No. 24-0735） and λ-MnO2（JCPDS No. 42-1169）. In order to further analyze the chemicalstate of MnO2/CP surface， XPS was used （Fig. S3c–f）. Theenergy separation between Mn 2p3/2 （642.2 eV） and Mn 2p1/2（654.0 eV） is 11.8 eV， which is consistent with the reportedvalue 28. The split peaks of Mn 2p3/2 peak at 642.2 eV can beassigned to Mn4+ （643.3 eV） and Mn3+ （642.0 eV），respectively （Fig. S3d） 29. The energy separation value betweenthe two peaks （89.0 and 84.0 eV） obtained from Mn 3s levelsplitting is 5.0 eV （Fig. S3e）， which is between the reported valueof MnO2 （4.8 eV） and Mn2O3 （5.4 eV）， indicating the existenceof Mn4+ and Mn3+ 28. The peaks in the O 1s XPS spectra （Fig.S3f） can be assigned to Oads （531.7 eV） and Olatt （529.8 eV），respectively 30.

2.2 Electrochemical apparatus and procedures

A potentiostat/galvanostat （CHI 760E， Chenhua， China） wasused to control potential and current. The schematic diagrams ofthe CO2 reduction （CO2RR） system， the HCHO oxidation （FOR）system， the three-electrode paired CO2RR/FOR system and thetwo-electrode paired CO2RR/FOR system are shown in Fig. 1.In the CO2RR system and FOR system， the counter electrode isa Pt foil （1 cm × 1 cm） and the reference electrode is an Ag/AgCl.In the two-electrode paired CO2RR/FOR system， the workingelectrode is Cu NWs， and the counter electrode and referenceelectrode are MnO2/CP.

3 Results and discussion

3.1 Single CO2RR on Cu NWs cathode

The single CO2RR performance was evaluated in the CO2reduction （CO2RR） system， as illustrated in Fig. 1a. Accordingto the linear sweep voltammetry （LSV）， Cu NWs showedsignificantly higher CO2RR activity than Cu foil （Fig. 2a）. Thiscan be partly attributed to the fact that Cu NWs can expose moreactive sites， which contributes to the increase of CO2RR activity.In addition， as shown in Fig. 2b， Cu NWs exhibited lower H2selectivity and higher CO2 selectivity than Cu foil. This may bebecause the surface nanocrystallization significantly increasesthe surface roughness of the catalyst， which promotes themaintenance of high local pH near the active site and inhibits thegeneration of H2. Moreover， the activity and selectivity for C2H4over Cu NWs were also higher than Cu foil （Fig. 2b，c）. Theapplied potential is essential for accelerating C2H4 generation.Herein， the Faraday efficiency （FE） and partial current densityof C2H4 for Cu NWs were increased with increasing potential（Fig. 2d）. This result was related to the increased local pH and*CO coverage induced by higher potential 31. However， whenthe potential changed， the CO partial current density （jCO） wasalmost unchanged （Fig. 2e）. The coverage of *CO increases asthe potential increases， which promotes *CO coupling and theconsumption of *CO. As a result， the activity and selectivity toC2H4 increase， while the selectivity to CO decreases 32，33. At thesame time， high potential is conducive to *CO production.Therefore， jCO was almost unchanged （Fig. 2e）. Notably， thecurrent density of CO2RR remained stable at around ?12mA?cm?2 with negligible degradation within 3 h， but thecorresponding FEs of C2H4 was slightly decreased by 12%within 2.5 h （Fig. 2f）.

Tafel slope is very helpful for studying CO2RR reactionkinetics. Therefore， Tafel slopes based on the partial currentdensity of CO and C2H4 were calculated. As shown in Fig. 3a，the Tafel slope of CO2 reduction to CO catalyzed by Cu NWswas 191 mV·dec?1， which was lower than that catalyzed by Cufoil （274 mV·dec?1）， indicating that the kinetic of CO formationwas improved. Besides， the Tafel slope of CO formation at Cu NWs was close to 200 mV·dec?1， which indicates that the ratedetermination step （RDS） of reducing CO2 to CO at Cu NWswas the chemical adsorption of CO2 molecules 34. While at highpotentials， C2H4 replaced CO as the main CO2RR reductionproduct. The Tafel slope of Cu NWs catalyzing the reduction ofCO2 to C2H4 was 264 mV·dec?1 （Fig. 3b）， which was higher than191 mV·dec?1， suggesting that C2H4 was more difficult toproduce than CO， and a higher potential is necessary. In addition，it was also higher than 200 mV·dec?1， implying that the RDS ofC2H4 generation may be the formation of C―C bond rather thanthe chemisorption of CO2 35. While Cu foil had a much higherTafel slopes of 308 mV·dec?1 than Cu NWs， suggesting that CuNWs also shows better kinetics of converting CO2 to C2H4 thanCu foil. In addition， electrochemical impedance spectroscopy（EIS） is also helpful to understand the kinetics of CO2RR.Compared with Cu foil， the charge transfer resistance of Cu NWwas significantly smaller （Fig. 3c）， indicating that CO2 was moreeasily reduced at Cu NWs. This can help us understand why theactivity of CO2RR at Cu NWs is better than that at Cu foil.However， it cannot fully explain why Cu NWs can catalyze thereduction of CO2 to C2H4 more selectively than Cu foil. Previousstudies have shown that catalysts with highly curved surfaces（nanorods， nanowires， etc.） can generate local high electric fieldson the surface during the reaction， inducing cations andpromoting CO2 closer to the active sites， which can significantlypromote CO2RR 36. The highly rough surface of Cu NWs alsopromotes the formation of high local pH near the active sitesduring CO2RR， which reduces the barrier required for *COcoupling 11.

3.2 Single HCHO oxidation on MnO2/CP anode

The single HCHO oxidation performance was investigated under a series of operation conditions， in the HCHO oxidation（FOR） system， as illustrated in Fig. 1b. The influence of HCHOconcentration on HCHO removal was studied. When theconcentration of HCHO increases， the oxidation currentincreases accordingly （Fig. 4a）. This is because increasing theconcentration of HCHO can promote the mass transfer of HCHOand increase its adsorption at the active sites. However， when theHCHO concentration was doubled， the oxidation currentincreased less than doubled （Fig. 4a）. Therefore， the removalratio of HCHO decreased as the concentration of HCHOincreased （Fig. 4b）. Specifically， when the concentration ofHCHO was 2.5 mmol?L?1， HCHO was completely removedwithin 4 h， but when the concentration of HCHO was 15mmol?L?1， only 60% of HCHO can be removed within 4 h.Notably， as the concentration of HCHO continues to consumewith proceeding the reaction， the oxidation current graduallydecreases （Fig. 4c）. In addition， the conversion efficiency ofHCHO to HCOOH increased with the increase of HCHOconcentration （Fig. 4d）. Considering the balance of oxidationcurrent， the conversion efficiency and the removal ratio ofHCHO， 5 mmol?L?1 concentration of HCHO is considered themost suitable for the following studies.

Electrolyte concentration is another very important factor thataffects oxidation current and HCHO removal ratio. Theoxidation current increased with electrolyte concentration （Fig.5a）. The EIS results showed that the increase in electrolyteconcentration reduced the internal resistance without affectingthe charge transfer at the interface （Fig. 5b）. These resultsindicate that increasing the electrolyte concentration can reducethe voltage loss caused by internal resistance， thereby promotingthe oxidation reaction. Therefore， a high concentration of electrolyte should be conducive to the removal of HCHO.Specifically， when the electrolyte concentration increased from0.02 mol?L?1 to 0.1 mol?L?1， the removal ratio of HCHOincreased from 50.1% to 93.2% （Fig. 5c）. With the increase ofNa2SO4 concentration， the attenuation of the oxidation currentduring FOR was more obvious （Fig. 5d）， consisting with thehigher removal ratio of HCHO. However， compared with 0.1mol?L?1 Na2SO4， the removal ratio of HCHO in 0.2 mol?L?1Na2SO4 only slightly increased. This indicates that in 0.2 mol?L?1Na2SO4 electrolyte， the oxidation current is no longer mainlycontributed by FOR. In addition， when the electrolyteconcentration is high， the stability of the catalyst will decrease，as evidenced by the result that the dissolution of Mn ions in 0.2mol?L?1 Na2SO4 is more serious （Fig. S4）. Therefore， 0.1mol?L?1 Na2SO4 is considered appropriate.

The influence of applied potentials on HCHO removal hasalso been studied （Fig. 6）. The results show that the appliedpotential has a great influence on the removal of HCHO. As the potential increased from 1.0 V to 1.2 V vs. Ag/AgCl， the removalratio of HCHO was increased from 50.2% to 93.2% （Fig. 6a）.Further increasing the potential to 1.3 V vs. Ag/AgCl， althoughthe oxidation current still significantly increased （Fig. 6b）， theHCHO removal ratio did not increase significantly. This isbecause the WOR competing with FOR gradually dominateswith increasing potential， affecting the stability of the catalystand the removal of HCHO.

To better understand the mechanism of FOR， in situ Fouriertransform infrared （FTIR） spectroscopy and Ramanspectroscopy were used （Fig. 7）. Intermediates on MnO2/CPduring FOR can be recorded by using in situ FTIR. The peaks at1218 and 2343 cm?1 attributed to CO3 2? and CO2 intensifiedgradually with the reaction time （Fig. 7a） 37，38. This result provesthat HCHO can be finally oxidized to CO2. This result provesthat CO2 is one of the products of FOR. The gradual weakeningof the peak of H2O suggests that H2O was involved in theoxidation of HCHO 39. In addition， the peaks at 1602 cm?1 and 1720 cm?1 ascribed to *HCOO? and HCHO were alsoobserved 37，40. Therefore， we speculate that HCHO is firstadsorbed on the MnO2/CP surface and oxidized to *HCOO?，which can be further converted to CO2 41. To further understandthe changes in MnO2/CP， in situ Raman spectroscopy wasperformed （Fig. 7b–e）. According to previous studies， H2O inelectrolyte is easily adsorbed on manganese oxide and loseselectrons， forming adsorbed hydroxyl radicals （MnO2―OH） 42.Therefore， the two relatively weak peaks corresponding toMnO2―OH at 384 and 413 cm?1 observed during WOR implythat H2O was adsorbed on MnO2 （Fig. 7b） 43. The intensity of theRaman peak of MnO2―OH was stronger in WOR than in FOR（Fig. 7b–d）， suggesting that HCHO may be oxidized by MnO2―OH 44. In addition， the intensity of Raman peak （754 cm?1）attributed to MnIV＝O species in FOR was weaker than in WOR（Fig. 7e） 45， implying that MnIV＝O also played a role in FOR.At the same time， considering the active site of WOR and thecompetition between FOR and WOR， we speculated thatMnO2 ― OH oxidized HCHO to HCOO?， while MnIV ＝ Ooxidized HCOO? to CO2. In WOR， the peak （824 cm?1） ascribedto MnO4? appeared at 1.3 V （Fig. 7b） 42. This implies thedissolution of Mn and a decrease in catalyst stability. No peaksascribed to MnO4? were found in FOR （Fig. 7b）， proving that theFOR can inhibit the oxidation of MnIV＝O complex to solubleMnO4? and the dissolution of MnO2， as evidenced by the resultsof UV-Vis and ICP-MS （Figs. S5 and S6）. The Raman peak （645cm?1） attributed to MnO6 octahedra was red-shifted during theWOR rather than the HCHO oxidation （Fig. 7b）， suggesting thechange of MnO2/CP structure during WOR， which wasconsistent with the XRD patterns of the reacted catalysts （Fig.S7）.

Based on experiments and literatures， the possiblemechanisms of FOR catalyzed by MnO2/CP are proposed （Fig.7f）. First， water molecules adsorb on the surface of MnO2/CP togenerate hydroxyl radical adsorbents （MnO2 ― OH）， whichreacts with HCHO and produces *HCOO?. Due to the strongMn―OH bonding， the hydroxyl radical adsorbents can also reactwith lattice oxygen atoms to form MnIV＝O and release H+ 46. Andthe generated *HCOO? can be further oxidized by MnIV＝O toform CO2. While during the WOR， the newly formed MnIV＝Oreleases O2 and then regenerates into MnIII＝O， while a smallpart of MnIV＝O is further oxidized to form soluble MnO4? 42，45.

3.3 Coupled redox reactions in paired CO2RR/FORsystem

Based on the above results in single systems， 0.1 mol?L?1Na2SO4 and 5 mmol?L?1 HCHO were selected as the operationconditions for the anodic reaction in CO2RR/FOR paired system.Under different applied potentials， the oxidation current in thesystem with HCHO is much larger than that in the systemwithout HCHO （Fig. S8a）. In addition， under different workingcurrent， the potential required for the FOR was steadily smallerthan that for WOR （Fig. S8b）. These results suggest thatreplacing WOR with FOR as an anodic reaction， paired with CO2RR， can enhance the performance of coupled redox reactionsystems.

3.3.1 Three-electrode paired CO2RR/FOR system

The performance of the paired CO2RR/FOR system （pairedsystem， with HCHO addition） and CO2RR/WOR （systemwithout HCHO addition） was firstly tested in a three-electrodepaired system， as shown in Fig. 1c. The electrons generated byoxidizing HCHO or （and） H2O transferred to the CO2 adsorbedon the cathode. At 1.0 V vs. Ag/AgCl， the oxidation current inthe CO2RR/WOR system was 0.06 mA. But the oxidationcurrent in the CO2RR/FOR system reached approximately 2.98mA （Fig. 8a）. The oxidation current on the anode is equal to thereduction current on the cathode. Therefore， this pairedCO2RR/FOR system shows better CO2RR performance thanCO2RR/WOR system. As shown in Fig. 8a， although the currentin the three-electrode paired CO2RR/FOR system decreased withthe reaction time， it was still significantly higher than the currentin the CO2RR/WOR system. As a result， this pairedCO2RR/FOR system showed a higher generation rate of CO2RRproducts.

Specifically， at 1.0 V vs. Ag/AgCl， no CO2RR product wasdetected in CO2RR/WOR system， in contrast， in pairedCO2RR/FOR system， the generation rate of CO and C2H4 werereaching 2.94 and 0.32 nmol·cm?2·s?1， respectively （Fig. 8b）. Atthe same time， in paired CO2RR/FOR system， the selectiveHCHO to HCOOH conversion efficiency was about 60% （Fig.8e）. At 1.1 V vs. Ag/AgCl， after turning the CO2RR/WORsystem， the current density has only increased by about 5 timesin paired CO2RR/FOR system. In contrast， the CO generationrate has increased from 0.37 to 3.79 nmol·cm?2·s?1 and increasedby about 10 times （Fig. 8c）， owing to the increased COselectivity at higher applied potential. While at 1.2 V vs.Ag/AgCl， after turning the CO2RR/WOR system， the C2H4generation rate has increased from 0.14 to 1.90 nmol·cm?2·s?1（Fig. 8d）. On the other hand， while the increase in potentialbrought about an increase in the HCHO removal efficiency andthe CO2RR production rate， the selective HCHO to HCOOHconversion efficiency was decreased from ～60% at 1.0 V to～44% at 1.2 V， suggesting HCOOH tend to further oxidize intoCO2 at higher potential （Fig. 8e）. Because high potential isconducive to the further oxidation of HCOOH， when thepotential increases from 1.1 V to 1.2 V， the concentration ofHCOOH produced by HCHO oxidation does not increase （Fig.8f）.

Therefore， to increase the economy of the paired CO2RR/FORsystem， the WOR competitive reaction and the competitiveHCOOH-to-CO2 conversion reaction occurred at higherpotential shall be avoided， and thus maintaining the suitableoxidation current of HCHO at relatively lower potentials is morebeneficial.

3.3.2 Two-electrode paired CO2RR/FOR system

Based on the superior coupled electrocatalytic redoxperformance of MnO2/CP for the anodic HCHO oxidation and Cu NWs for cathodic CO2RR， a two-electrode electrolyzer （twoelectrodepaired CO2RR/FOR system） using MnO2/CP as theanode and Cu NWs as the cathode has been set up and operatedat room temperature to test the performance of the two-electrodepaired CO2RR/FOR system， as illustrated in Fig. 1d. At a cellvoltage （Ecell） of 2.0 V， the CO2RR current density in the twoelectrodepaired CO2RR/FOR system was about 3.5 times higherthan that in CO2RR/WOR system， without HCHO addition （Fig.9a）. Meanwhile， compared to the CO2RR/WOR system， thegeneration rate of H2 and CO in paired CO2RR/FOR systemincreased from 1.74 to 5.07 nmol·cm?2·s?1， and from 0.07 to 0.51nmol·cm?2·s?1， respectively （Fig. 9b）. Unfortunately， because ofthe low current density （?1.19 mA?cm?2） and low anodicpotential （Fig. 9a and Table 1）， only about 20% of HCHO wasremoved， but the selective HCHO to HCOOH conversionefficiency is as high as 64.9% （Fig. 9e，f）. The potential distribution in the electrolytic cell （Table 1） is calculated basedon Eqs. （1） and （2）.

EIR = I × Rs （1）

Ecell=Eanode -Ecathode+EIR （2）

where EIR is the voltage drop caused by the internal resistance，I is the current， Rs is internal resistance of the system， Eanode isthe anodic potential， and Ecathode is the cathodic potential.

At 2.5 V （Ecell）， the CO2RR current density in the twoelectrodepaired CO2RR/FOR system was about 1.5 times higherthan that in CO2RR/WOR system （Fig. 9a）. Meanwhile，compared to the CO2RR/WOR system， the generation rate of COand C2H4 in paired CO2RR/FOR system increased from 0.89 to1.52 nmol·cm?2·s?1， and from 0 to 0.06 nmol·cm?2·s?1，respectively （Fig. 9c）. Meanwhile， about 37.1% of HCHO wasremoved， and the selective HCHO to HCOOH conversionefficiency is remaining as high as about 61%. A higher cellvoltage is required for efficient HCHO oxidation. At 3.0 V（Ecell）， about 63% of HCHO was removed in two-electrodepaired CO2RR/FOR system （Fig. S9）. At 3.5 V （Ecell）， nearly90% of the HCHO can be removed， with a selective HCHO toHCOOH conversion efficiency of about 48% （Figs. S9 and 9e）.At higher cell voltage， the current and generation rate of CO2RRproducts in the two-electrode paired CO2RR/FOR system werekeeping greater than that in the two-electrode CO2RR/WOR system （Figs. 9a–d and S10）. Accordingly， as showed in Table1， for the cathode potential （Ecathode）， the paired CO2RR/FORsystem is generally greater than the CO2RR/WOR system， whichmeans that a larger portion of the potential is used for the CO2reduction half-reaction and improves the energy efficiency of thepaired system. However， as the cell voltage increases， thisadvantage of the two-electrode coupled CO2RR/FOR systemdiminishes （Figs. 9 and S10）. One reason is that the voltage drop（EIR） caused by the internal resistance increases as the cellvoltage increases， which reduces the difference in electrodevoltage （Eanode Ecathode ， Table 1） 14. Another reason is thatWOR occur competitively with FOR， and the correspondingpotentials required for FOR and WOR gradually approach as thecurrent density increases （Table 2）. For these reasons， improvingthe performance of FOR and reducing the internal resistance arecritical to improve the two-electrode paired CO2RR/FOR systemperformance.

Economy， cost-efficiency， is one of the key indicators toestimate the overall performance of CO2RR system. Therefore，it is necessary to compare the energy consumption （Q， in J） ofthese systems illustrated in Fig. 1. As shown in Fig. 10a， the twoelectrodepaired CO2RR/FOR system required significantlylower cell potential than the CO2RR/WOR system at the samecurrent， suggesting that pairing with FOR was beneficial to reducing energy consumption. Table 2 shows the internalpotential distribution and the comparison of cell voltage， energyconsumption and other parameters of CO2RR/WOR system andpaired CO2RR/FOR system at different current densities. Theresults show that under the given current density， the Eanode inpaired CO2RR/FOR system is generally lower than theCO2RR/WOR system. Correspondingly， the Ecell in pairedCO2RR/FOR system is generally lower than the CO2RR/WORsystem， which means that less energy is required and improvesthe overall energy utilization efficiency of the system.Unfortunately， as the current density increases， the EIR caused bythe internal resistance increases， and the Ecell difference betweenthe two systems decreases. As shown in Table 2， at ?10mA·cm?2， after replacing WOR with FOR， the energyconsumption of CO2RR was saved by a factor of 5.34%. Incomparison， at ?4 mA·cm?2， the paired CO2RR/FOR system canreduce the cell voltage by 270 mV and the energy consumptionby 9.18 %. In addition， when single FOR or CO2RR occurs， therequired energy is 189.36 and 298.8 J， respectively （Fig. 10b）.The energy required for paired CO2RR/FOR is 267.84 J， reducedby a factor of 45.13% in comparison with the sum of singleCO2RR and FOR system （488.16 J）.

Due to the advantages of paired system， a commercial PV-ECsystem （Fig. 11a） was further constructed to demonstrate thephotosynthesis of CO2RR products and photoconversion ofHCHO to HCOOH. The polycrystalline silicon solar cells drivethe reactions in the paired CO2RR/FOR system and theCO2RR/WOR system. In these systems， photogenerated holesmigrate to MnO2/CP and oxidize HCHO or （and） H2O， whilephotogenerated electrons migrate to Cu NWs and then furthertransfer to reactants. Compared with the CO2RR/WOR system，the operation point of the paired CO2RR/FOR system is closerto the maximum power point （MPP） （Fig. 11b）， suggesting thatthe solar energy utilization efficiency of the paired CO2RR/FORsystem is higher. After calculation， the solar photovoltaicconversion efficiency in the CO2RR/FOR system （6.1%） washigher than in the CO2RR/WOR system （4.6%）. As shown inFig. 11c， the paired CO2RR/FOR system shows a higher CO2 reduction current density than the CO2RR/WOR system （?5.2mA·cm?2 vs. ?3.9 mA·cm?2）. In addition， the CO generation rateincreased from 1.61 to 4.73 nmol·cm-2·s?1， and the C2H4generation rate increased from 0.06 to 0.36 nmol·cm?2·s?1 （Fig.11d）. In this paired CO2RR/FOR system， within 4 h， 51.6% ofHCHO can be removed， and the selective HCHO to HCOOHconversion efficiency is about 60% （Fig. 11e，f）.

4 Conclusions

In summary， a well-designed paired CO2RR/FOR system caneffectively improve the CO2RR performance， while selectivelyconverting HCHO into high-value HCOOH. Compared toconventional CO2RR/WOR system， the current density and thegeneration rate of CO2RR products in paired CO2RR/FORsystem are generally larger at different applied potentials.Moreover， the HCHO can be efficiently removed inCO2RR/FOR paired system at suitable applied potential， withtunable HCHO to HCOOH selectivity. In addition， the potentialrequired for the FOR are steadily smaller than that for WOR，under different working current. At ?10 mA·cm?2 in twoelectrodepaired CO2RR/FOR system， using FOR instead ofWOR as the anode reaction can reduce the cell voltage of thepaired system by 210 mV， and the required electric energy forpaired CO2RR/FOR system can be reduced by 45.13%， incomparison with the sum of single CO2RR and FOR system.However， under high current density， large internal resistancewill bring considerable energy consumption， WOR may occurcompetitively with FOR， and HCOOH tend to undergo deepoxidation into CO2， thus reducing the advantage of the pairedsystem. Based on the calculated voltage distribution， when thecell voltage is not greater than 3.0 V， no competitive WORoccurs， but only FOR occurs at the anode. When commercialpolysilicon solar cell is used as the power supply， the pairedCO2RR/FOR system can show about three times higher COgeneration rate and about six times higher C2H4 generation ratethan the CO2RR/WOR system， while converting HCHO toHCOOH with a selectivity of about 60%. The HCHO oxidationpathway， via HCHO → HCOOH → CO2， as well as thedeactivation mechanism， involving dissolutive formation ofMnO4?， of MnO2/CP anodic electrode are identified using in situFTIR and Raman spectroscopy. This work may provide a newframework for the development of novel CO2RR-based pairedsystems， which can simultaneously reduce CO2 and convertpollutants into valuable chemicals with low power demand.

Author Contributions： Xudong Lv： Investigation， Writing，Original Draft Preparation; Junyan Liu， Tao Shao， and Meng Ye：Visualization， Writing， Review amp; Editing; Shengwei Liu：Conceptualization， Supervision， Funding Acquisition， Writing，Review amp; Editing.

Supporting Information： available free of charge via the internet at http：//www.whxb.pku.edu.cn.

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國家自然科學基金（51872341）， 廣東省“特支計劃”科技創(chuàng)新青年拔尖人才項目（2019TQ05L196）及廣東省科技計劃項目（2021A1515010147）資助