the production of fuels 
 or fine chemicals from water , co2 , and sunlight is a promising 
 way to store solar energy and alleviate 
 co2 accumulation in the atmosphere , one of the main causes 
 of global warming since the industrial revolution . research activities 
 on ( photo)electrocatalytic reduction of co2 have therefore 
 increased exponentially , especially in the past few years . different 
 reaction products have been observed depending on , e.g. , the nature 
 of the electrocatalyst , the electrolyte , the ph , etc . unfortunately , the existing electrocatalysts for the co2 reduction still suffer from the competing hydrogen evolution reaction 
 ( her ) , high overpotentials , and poor selectivity toward a desired 
 product . boron - doped diamond ( bdd ) is a popular electrode material 
 in electrochemistry , 
 because of its interesting properties such as a wide potential window , 
 high stability , robustness under extreme conditions ( potential , temperature , 
 pressure ) , and low background capacitive currents . bdd finds its use 
 in electrochemistry mainly as a substrate for electrodeposition or 
 nanoparticles and for electroanalytical purposes . in 
 the field of electrocatalytic co2 reduction , bdd has been 
 used as a substrate for catalysts as diverse as ruo2 layers , cu nanoparticles , and metal complexes . bdd has not been 
 investigated as an electrocatalyst for co2 reduction until 
 recently , when nakata et al . reported high faradaic efficiency toward 
 formaldehyde ( hcho ) as well as a small amount of formic acid ( hcooh ) . the standard equilibrium potentials for hcooh , 
 hcho , and co are obtained from formation energies in aqueous media 
 at atmospheric pressure and 25 c and given in table 1 . note that eeq0 for hcooh is ph - dependent for ph > 4 , due 
 to the dissociation of hcooh into hcoo and h. it is well - known that the local ph at the electrode  electrolyte 
 interface plays a crucial role for numerous electrochemical reactions . in aqueous media , cathodic potentials lead to a local alkaline environment 
 caused by the consumption of protons and/or production of hydroxide 
 ions ( eqs 1 and 2).12the influence of the local 
 ph variations is 
 often neglected , although it is very important , as the local ph change 
 may lead not only to a shift in the effective overpotential but also 
 to the formation of other species by base - catalyzed chemical or disproportionation 
 reactions . it is evident that the effect of the local ph should be 
 known for a correct interpretation of the data , especially for mechanistic 
 studies . possible organic transformations related to the reduction 
 of co2 have recently been discussed from a general point 
 of view . in this context , aldehydes are 
 the most important intermediates / products , because of their diverse 
 reactivity . the cannizzaro reaction is a base - catalyzed disproportionation 
 reaction of an aldehyde , devoid of -h atoms , into the corresponding 
 carboxylic acid and alcohol . aldehydes with -h atoms 
 do not undergo the cannizzaro disproportionation , as the aldol reaction 
 is much faster . in the aldol reaction , c  c bond formation occurs 
 by addition of the -carbon of one aldehyde / ketone molecule 
 to the carbonyl carbon of another molecule under the influence of 
 a base . in this work we elaborate on the utilization 
 of bdd as electrocatalyst 
 for the electrochemical co2 reduction under ambient conditions 
 in acidic media . we will discuss the importance of disproportionation 
 and chemical reactions on product distributions often encountered 
 with co2 electroreduction also on other electrode materials . we will show that a supposedly inactive material such as bdd is active 
 for the reduction of co2 to methane and the concomitant 
 formation of methanol and formic acid , as a result of base - catalyzed 
 cannizzaro reactions . the electrochemical experiments were carried out in a conventional 
 three - electrode cell where the working electrode and counter electrode 
 compartments were separated by a nafion membrane ( nafion 115 ) . bdd 
 discs of 3 mm or 10 mm diameter ( windsor scientific ltd . , uk ) were 
 embedded in teflon and used as working electrodes . the counter and 
 reference electrodes were a platinum gauze and a reversible hydrogen 
 electrode ( rhe ) , respectively . for correct measurements versus the 
 rhe scale , the luggin capillary and the rhe compartment were filled 
 with co2-saturated electrolyte before co2 reduction . electrolyte solutions were prepared with high - purity perchloric acid 
 ( merck suprapur ) , sodium perchlorate ( sigma - aldrich , acs reagent ) , 
 and ultrapure water ( millipore milli - q gradient a10 system , 18.2 mcm ) . 
 the reported current density is ir corrected and normalized by the 
 geometric surface area of the bdd disc . during voltammetry , online 
 electrochemical mass spectrometry ( olems ) was utilized for the detection 
 of volatile reaction products and online high - performance liquid chromatography 
 ( online hplc ) for the analysis of nonvolatile reaction products , as 
 described before . the reported concentrations of 
 liquid products are an average of two or three independent measurements . additionally , the data points of each specific experiment are recorded 
 as the average of two injections of one sample in hplc , since a slightly 
 different chromatogram also adds an uncertainty to the calculated 
 concentration . the latter source of error is more important for low 
 concentrations ( < 0.05 mm ) , while the deviation in concentration 
 resulting from repeated experiments is dominant for high concentrations . prior to each experiment , the bdd electrode 
 was cleaned by ultrasonication 
 in concentrated hno3 and water . a raman spectrum of the 
 bdd electrode is given in the supporting information ( figure s.1 ) . after ultrasonication , blank cyclic voltammograms 
 were recorded in 0.001 m hclo4 + 0.099 m naclo4 at a scan rate of 500 mv s until a stable voltammogram 
 was obtained ( typically around 50 cycles ) to ensure a clean surface , 
 as shown in figure s.2a . experiments on 
 bdd in a co2-saturated electrolyte show higher current 
 densities compared to those using an ar - saturated electrolyte , implying 
 that bdd is active for co2 reduction ( figure s.2b ) . figure 1 shows the formation of volatile species during ( a ) co2 reduction and ( b ) co reduction , and ( c ) the formation of 
 nonvolatile species during co2 reduction . besides h2 , co2 , and co , the mass fragment of hcho is observed , 
 which is in agreement with the literature and our hplc data ( figure 1c ) , except for lower faradaic efficiencies for hcho and hcooh , 
 as shown in figure s.3 . the lower faradaic 
 efficiencies are the result of an electrolyte with higher proton concentration , 
 leading to the formation of a significant amount of h2 compared 
 to the work by nakata et al . remarkably , figure 1b shows the formation 
 of ch4 during co reduction on bdd . the current measured 
 during co2 reduction is much higher compared to the current 
 measured during co reduction , probably due to suppression of the her 
 by co. the reason why no ch4 is detected during co2 reduction is assumed to be related to the small amount of 
 co produced , needed for further reduction to ch4 . the production 
 of ch4 during co reduction is not limited to ph 3 , but 
 is also observed in electrolytes with a ph between 1 and 7 . our previous 
 experiments and dft calculations suggest that ch3oh is 
 an intermediate in the further reduction toward ch4 on 
 cobalt protoporphyrins immobilized on pyrolytic graphite . however , a similar pathway is not likely for co2 or co 
 reduction on bdd , since ch4 is not detected with olems 
 during methanol reduction on bdd . even though the amount of ch4 is not expected to be high , this finding is very intriguing very recently it was shown that metal impurities 
 in catalysts and electrolytes can have significant effects on the 
 catalytic activity for co2 reduction . since our experiments are conducted on high surface area bdd electrodes 
 and on relatively small time scales , the catalyst poisoning by metal 
 ion impurities in the electrolyte as discussed by wuttig et al . moreover , the promotion of co2 reduction by impurities in carbon materials , especially trace levels 
 of copper leading to ch4 , is assumed not to affect our 
 experiments on bdd , since a cleaning procedure is followed similar 
 to one suggested by lum et al . needed for removal of these metallic 
 impurities and the voltammogram recorded afterward does not show additional 
 redox peaks ( figure s.2a inset ) . for these 
 reasons we believe that the formation of ch4 on bdd is 
 not the result of metallic impurities promoting the co2 reduction toward ch4 , but can be ascribed to the electrochemical 
 activity of bdd for co2 reduction . as pristine pyrolytic graphite is not active for co2 reduction and does not produce hcho , hcooh , ch3oh , or 
 ch4 , the activity of bdd is likely associated with its 
 sp - hybridized carbon atoms . volatile products detected 
 during reduction of ( a ) co2 and ( b ) co , and liquid products 
 detected during ( c ) co2 reduction on bdd in 0.001 m hclo4 + 0.099 m naclo4 electrolyte . scan rate : 1 mv 
 s. in addition to the formation of hcho , figure 1c shows the formation of hcooh at more cathodic 
 potentials ( <  1.3 vrhe ) . this could be 
 due to the fact that bdd has some activity for co2 reduction 
 to hcooh , but the same trend for hcooh 
 is observed during direct reduction of hcho , as shown in figure 2a . importantly , the 
 onset potential of hcooh production is the same as the onset of the 
 her . the formation of hcooh , an oxidation product of hcho , under reducing 
 conditions is believed to be the result of a disproportionation reaction 
 such as the cannizzaro reaction . the h2 evolution at less 
 negative potentials ( approximately between 0.5 and 1.3 
 vrhe ) is the result of proton reduction , and at more negative 
 potentials ( < 1.3 vrhe ) caused by direct water 
 reduction similar to results obtained previously on pyrolytic graphite 
 in perchloric acid of ph 3 . the direct 
 water reduction produces oh ions in the vicinity 
 of the electrode surface ( eq 2 ) , which promote the disproportionation reaction . the cannizzaro 
 reaction is expected to form ch3oh as well which is not 
 detected with hplc due to sensitivity limitations . however , as shown 
 in figure s.4 , ch3oh is detected 
 with olems and with hplc when the hcho concentration is increased . liquid 
 products detected during reduction of ( a ) 100 mm formaldehyde , 
 ( b ) 100 mm acetaldehyde , and ( c ) 100 mm propionaldehyde on bdd 
 in 0.001 m hclo4 + 0.099 m naclo4 electrolyte . 
 scan rate : 1 mv s. we also investigated the formation of liquid products from 
 the 
 c2 and c3 aldehydes , acetaldehyde and propionaldehyde , 
 under reducing conditions as shown in figure 2b , c . both , the primary alcohols and carboxylic 
 acids , are observed commencing together with the her akin to hcho 
 reduction . this observation justifies and generalizes the involvement 
 of a disproportionation reaction at negative potentials . the product 
 distributions suggest a cannizzaro type disproportionation reaction , 
 although the classical cannizzaro reaction does not take place with 
 aldehydes having -h atoms . however , cannizzaro products from 
 aldehydes with -h atoms have been observed before . when the c1c3 aldehydes 
 are treated with different concentrations of naoh , the carboxylic 
 acids and alcohols are formed , as displayed in figure s.5 . it can be seen that the amount of alcohol and 
 carboxylic acid formed depends on the concentration of naoh analogous 
 to a higher local concentration of oh at more negative 
 potentials . if self - disproportionation is the dominant mechanism , 
 the expected yields of alcohol and carboxylic acid should be around 
 50:50% , which is not obtained in our aldehyde reduction experiments . 
 a factor 10 higher yield is observed for propionic acid compared to 
 1-propanol during propionaldehyde reduction ( figure 2c ) , which suggests the involvement of at 
 least one other reaction pathway or a mechanism not corresponding 
 to the classical cannizzaro mechanism . the formation of c2c3 alcohols and 
 carboxylic acids is probably associated with a catalytic or kinetic 
 effect , where near the bdd surface the aldol reaction is suppressed 
 or the cannizzaro reaction is preferred over the aldol reaction . ethanol 
 and acetic acid as products of disproportionation reactions of acetaldehyde 
 have been reported before . cook et al . have reported a homogeneously 
 catalyzed disproportionation of acetaldehyde into acetic acid and 
 ethanol . found several products 
 including acetic acid and ethanol formed by a noncatalytic reaction 
 pathway of acetaldehyde under hydrothermal conditions . a catalytic effect of bdd for the disproportionation 
 reactions is unlikely as the reduction of the c1c3 aldehydes on a pyrolytic graphite electrode in the same electrolyte 
 also leads to the formation of carboxylic acids and alcohols as shown 
 in figure s.6 . additionally , the cannizzaro 
 products for formaldehyde disproportionation , hcooh and ch3oh , are also observed on gold and copper electrodes as depicted in figure s.7 . the fact that no aldol reaction products 
 are detected suggests that the presence of the electrode surface plays 
 a role . even though the classical cannizzaro and aldol reactions were 
 originally reported for homogeneous alkaline solutions , a variety 
 of heterogeneous and catalytic systems for these reactions have been 
 investigated for formaldehyde and acetaldehyde as well . several groups have 
 reported that surface oh can initiate cannizzaro 
 disproportionation of surface adsorbed hcho leading to formate ions 
 and methoxy groups . a mechanism including a nucleophilic - addition transformation 
 of hcho to dioxymethylene ( ch2o2 ) which subsequently 
 reacts with a hcho molecule to form formate and methoxy groups has 
 been proposed . the importance of 
 the local ph ( gradient ) near the electrode is 
 demonstrated by the reduction of formaldehyde in different electrolytes 
 as shown in figure 3 . a small amount hcooh is observed in a 0.1 m hclo4 electrolyte 
 while trace amounts or no hcooh are observed in phosphate buffers 
 of ph 6.8 , depending on the buffer capacity . the higher the buffer 
 capacity , the less sensitive the local ph is to changes , the smaller 
 the possibility of disproportionation reactions to occur . in case 
 of the 0.1 m hclo4 electrolyte , the local environment is 
 very acidic and the local concentration of hydroxide ions near the 
 electrode surface is lower compared to perchloric acid of ph 3 , which 
 results in the formation of less hcooh . formic acid formation 
 during reduction of formaldehyde in perchloric 
 acid ( ph 1 and ph 3 ) , 0.1 m phosphate buffer ph 6.8 , and 0.01 m phosphate 
 buffer ph 6.8 . scan rate : 1 mv s. in the recent literature there are several studies 
 where formaldehyde , 
 methanol , formic acid or acetaldehyde , ethanol , acetic acid or propionaldehyde , 
 propionic acid , and 1-propanol have been observed during co2 electroreduction in different electrolytes on different electrodes . our results show that one should be careful not to misinterpret 
 the observation of carboxylic acids and alcohols to be a result of 
 direct co2 reduction . especially in the quest for novel 
 catalytic materials for co2 electroreduction toward 
 liquid products , specifically , ethanol formation always seems to go hand - in - hand 
 with acetate formation , suggesting that they may be ( partially ) formed 
 from acetaldehyde disproportionation . moreover , the use of buffer 
 solutions is recommended in order to avoid disproportionation reactions 
 and control experiments are recommended when using alkaline electrolytes 
 to evaluate the stability of products and intermediates . in conclusion , this work has illustrated the existence 
 and the 
 importance of disproportionation reactions during electroreduction 
 of co2 . these reactions lead to product distributions which 
 should be distinguished from direct co2 reduction products . 
 the formation of carboxylic acids and alcohols is shown to be the 
 result of disproportionation of aldehydes , which is promoted by the 
 local alkaline environment in the vicinity of the electrode surface , 
 caused by the her . this phenomenon is strongly affected by the ph 
 and buffer capacity of the electrolyte and can be generalized to other 
 electrode materials and to c2 and c3 aldehydes 
 as well . moreover , it is revealed that bdd has the ability to further 
 reduce co to methane . the bdd activity should therefore be taken into 
 account when bdd is used as a substrate for catalysts for co2 and co reduction . the involvement of the disproportionation reactions 
 results in a pathway leading to desirable products such as formic 
 acid , acetic acid , methanol , and ethanol during co2 electroreduction , 
 different from the direct co2 reduction pathway toward 
 these liquid products .
a seemingly 
 catalytically inactive electrode , boron - doped diamond 
 ( bdd ) , is found to be active for co2 and co reduction to 
 formaldehyde and even methane . at very cathodic potentials , formic 
 acid and methanol 
 are formed as well 
 . however , these products are 
 the result of base - catalyzed cannizzaro - type disproportionation reactions . 
 a local alkaline environment near the electrode surface , caused by 
 the hydrogen evolution reaction , initiates aldehyde disproportionation 
 promoted by hydroxide ions , which leads to the formation of the corresponding 
 carboxylic acid and alcohol . 
 this phenomenon is strongly influenced 
 by the electrolyte ph and buffer capacity and not limited to bdd or 
 formaldehyde , but can be generalized to different electrode materials 
 and to c2 and c3 aldehydes as well . 
 the importance 
 of these reactions is emphasized as the formation of acids and alcohols 
 is often ascribed to direct co2 reduction products . 
 the 
 results obtained here may explain the concomitant formation of acids 
 and alcohols often observed during co2 reduction .