the design of efficient 
 water oxidation catalysts ( wocs ) based 
 on nonprecious materials remains an important challenge for achieving 
 a clean and sustainable solar fuels - based energy economy . we have previously shown that active wocs can be formed by anodic 
 electrodeposition of metal - oxides from neutral and near - neutral buffered 
 aqueous solutions of cobalt , nickel , and recently , manganese . in particular , the cobalt 
 oxygen - evolving catalyst ( co - oec ) has been studied in detail , resulting 
 in an understanding of the electrochemical kinetic mechanisms of its 
 formation , catalysis , and charge transport . the 
 structural and electronic properties of co - oec have been clarified 
 using xas , x - ray pdf , epr , and x - ray gid . these studies have revealed that the electrodeposited catalyst 
 films comprise molecular to nanoscale - sized metalate clusters composed 
 of edge - sharing coo6 octahedra with a mixed valence co(iii / iv ) 
 resting state . the development of soluble molecular wocs based 
 on co as well as other transition metals , such as ir , ru , cu , and fe have also been a subject 
 of intense focus . molecular wocs are attractive research targets because 
 they provide a tractable means to characterize catalytic mechanisms 
 and to identify reactive intermediates , thus forming the basis for 
 the continued development of new wocs . however , the true identity 
 of the active catalyst must be clarified prior to a detailed interrogation 
 of the woc mechanism . indeed , some molecules that were thought to 
 be wocs have subsequently been shown to be precursors of heterogeneous 
 or colloidal materials , which are the active catalysts . proper catalyst identification is especially challenging for the 
 study of molecular cobalt wocs because extremely small amounts of 
 co - oec may be produced from the decomposition of the molecular catalyst . an exemplar of this challenge is the all - inorganic cobalt polyoxometalate 
 [ co4(h2o)2(pw9o34)2 ] ( co4pom ) , which was 
 suggested as a woc . re - examination of 
 the molecule showed that electrochemically driven oxygen evolution 
 arose from the formation of co - oec on glassy carbon ( gc ) electrodes 
 at 1.1 v vs ag / agcl . because the co4pom was unstable at higher potentials , water oxidation activity 
 could not be conclusively attributed to the co4pom , as 
 opposed to its role as a molecular precursor to co - oec . the co4pom has now been suggested 
 to exhibit water oxidation activity but under specific photochemical 
 conditions where ru(bpy)3 is the oxidant . ( left ) 
 molecular structure of co4o4 cubane 
 structure 1 and ( right ) thermal ellipsoid representation 
 at the 50% probability level of the one - electron oxidized cubane , 1[pf6 ] . atoms 
 are color - coded : gray ( carbon ) , blue ( nitrogen ) , red ( oxygen ) , dark 
 blue ( cobalt ) , green ( fluorine ) , and yellow ( phosphorus ) . against this backdrop , cubane co4o4 clusters , 
 such as co4o4(oac)4(py)4 , ( 1 , figure 1 ) , have come under 
 investigation as a class of molecular cobalt complexes that are potential 
 wocs . we had previously investigated 1 , first synthesized 
 by das and co - workers , and a related 
 co4o4 cubane of christou , in order to gain valuable insights into the electronic 
 characteristics and proton - coupled electron transfer ( pcet ) behavior 
 of co(iii / iv ) in a co - oec environment . the structure 
 of 1 has been previously reported ( figure s1 , supporting information ) ; the crystal structure of the oxidized cubane 1 was known as a perchlorate salt and is now obtained as a pf6 salt , as shown in figure 1 . in our studies , 
 we did not find any evidence that these cubanes were active wocs . motivated by the recent reports to the contrary and subsequent computational 
 work outlining a detailed mechanistic pathway for 1 as 
 a woc , we renewed our investigation of 
 these molecules . a comparison of the h nmr spectra of ( black 
 line ) 10 
 mm crude 1 and ( red line ) 10 mm pure 1 in 
 d2o . herein , we report that 
 a co(ii ) impurity in as - synthesized cubane 1 is primarily 
 responsible for the reported catalytic water 
 oxidation activity . we present a series of experiments that are useful 
 for determining whether a small amount of a co(ii ) impurity may lead 
 to formation of a heterogeneous woc . we further emphasize the utility 
 of differential electrochemical mass spectrometry ( dems ) for clarifying 
 how anodic potentials affect the decomposition of glassy carbon electrodes , 
 which are commonly used in the study of wocs . the reported experiments 
 are aimed at establishing a standardized approach to evaluate the 
 presence of co(ii ) impurities in molecular complexes under investigation 
 as water oxidation catalysts . we synthesized 
 and isolated 1 by precisely following the one - pot procedure 
 developed by others . despite satisfactory elemental 
 analyses for 1 ( table s1 ) , 
 we determined that this as - synthesized 
 material , which was isolated by concentrating a dichloromethane ( dcm ) 
 extraction , was not pure . the presence of impurities was indicated 
 by the observation of many small peaks in the h nmr spectrum 
 ( figures 2 and s2s4 ) and by the presence of slowly moving bands that eluted behind the 
 product band on a silica thin layer chromatography ( tlc ) plate ( figure s5 ) . on the basis of the tlc result , purification 
 of the compound was performed by column chromatography on silica , 
 eluting with a gradient of 210% meoh in dcm . along with the 
 slowly moving green bands , a comparison of h nmr spectra 
 for crude ( i.e. , as - synthesized ) and purified 1 is shown 
 in figure 2 . several peaks that are observed 
 in the aromatic region in the nmr of the crude sample are absent in 
 the nmr of the purified sample . molecular impurities are also indicated 
 by many peaks in the m / z range of 
 300700 in the esi - ms of crude 1 ; these peaks 
 are absent in the purified sample ( figure s6 ) . a structural variant , 1-coome , was also synthesized 
 according to das original procedure ; the final product was isolated by precipitation and filtration . no diamagnetic impurities were detected in the h nmr spectra 
 of the precipitated 1-coome . however , to remove possible 
 paramagnetic impurities , the precipitated 1-coome was 
 subject to further purification by chromatography . interestingly , 
 the same h nmr spectrum was obtained for precipitated 
 and chromatographed 1-coome ( figure 
 s7 ) , though the former was observed to have impurities that 
 were not removed by precipitation . background corrected cvs of crude ( black 
 dotted ) and purified ( red 
 solid ) samples of 1 ( 0.852 mg / ml ) in 0.2 m kpi buffer , ph = 7 . two scans are presented for the crude sample demonstrating 
 the loss of activity upon the second scan . the reported water oxidation activity 
 of 1(36,39 ) could not be replicated using 
 purified samples . figure 3 compares the cvs 
 of crude and purified 1 ( 0.852 mg / ml , 1 mm assuming 100% 
 purity ) . the catalytic current , peaking at 1.3 v ( all potentials are 
 referenced to ag / agcl ) , in the crude sample is consistent with the 
 woc activity that has been previously reported of 1 in 
 the presence of proton accepting electrolytes . however , a similar 
 catalytic wave in the purified sample is completely absent ; only a 
 reversible co(iii)3co(iv)/co(iii)4 couple centered 
 at e1/2 = + 1.05 v is observed . interestingly , 
 the catalytic current detected with the crude sample is only prominent 
 in the first scan of the cv . a similar behavior is observed for 1-coome where precipitated samples exhibit a large catalytic 
 current in the cv and chromatographed samples show only the reversible 
 co(iii)3co(iv)/co(iii)4 couple , as shown in figure s8 . the e1/2 of the reversible couple is at a more positive potential than for 1 , due to the electron withdrawing nature of the methyl ester 
 substituents on the pyridine ligands . the 
 crude sample also showed a catalytic current ( ep = 1.4 v , figure s9 ) , which 
 was absent in the purified sample . the only observed difference between 
 the cvs in carbonate and phosphate electrolyte is that the catalytic 
 peak current of the crude sample occurs at a more positive ( 80 
 mv ) potential in carbonate electrolyte . to confirm that the 
 catalytic current in the crude sample was associated 
 with the oxygen evolution reaction , electrochemical oxidation was 
 performed in a dems experimental setup , which allows for the immediate 
 and simultaneous detection of all gaseous products formed at the electrode 
 surface . the catalytic current from an 
 unpurified sample shown in the red trace of the top of figure 4a is accompanied by the production of o2 , as shown in the middle panel of figure 4a . purified 1 and 1-coome were also investigated 
 using dems under the identical conditions employed for that of the 
 crude sample . as shown in the top panels of figure 4b , c for purified 1 and 1-coome , 
 respectively , the faradaic current density decreases by over an order 
 of magnitude from that of the crude sample . the waveform of the cvs 
 in figure 4 are different than those of cvs 
 taken on stationary gc electrodes ( e.g. , figure 3 , red trace ) owing to the flow conditions of the dems experiment ; 
 similar waveforms are observed , for instance , at rotating disk electrodes 
 where there is forced solution flow across an electrode surface . the signal from the mass channel of o2 for the purified samples ( middle panels in figure 4b , c ) shows no o2 production for applied potentials 
 below 1.4 v ; at potentials of 1.4 v or greater , an extremely small 
 amount of o2 is observed ( pa intensities as opposed to 
 na intensities of crude samples ) . we note that for all three samples , 
 the mass channel of co2 exhibits a sizable signal when 
 the electrode potential surpasses 1.2 v. the high level of 
 evolved co2 is observed even in the background scans of 
 blank gc electrodes ( black lines in the bottom panels in figure 4a  c ) . dems experimental data for three samples : 
 ( a ) crude 1-coome , ( b ) purified 1 and ( c ) 
 purified 1-coome . top panels display the faradaic current 
 density vs potential ; middle 
 and bottom panels display the current collected for mass channels 
 32 ( o2 ) and 44 ( co2 ) m / z , respectively . red lines are representative data from 
 the samples , and black lines are the data from the corresponding blank 
 gc electrodes . we sought to place a 
 limit on the level of o2 produced 
 by the cubane cluster within the error of our measurements . the middle 
 panel of figure 4b indicates that there is 
 a small but non - negligible amount of o2 produced in purified 
 samples of 1 at applied potentials > 1.4 v. we therefore 
 wished to quantify the amount charge passed with the current associated 
 with the slight downturn in the red cv trace at potentials above 1.4 
 v in figure 3 . three separate voltammograms 
 ( using three independently prepared gc electrodes ) were collected 
 with a sample of purified 1 ( black traces in figure s10b  d ) . a simulated cv ( figure s10a ) was subtracted from the background 
 corrected raw data to remove the current that is due to the reversible 
 co(iii)3co(iv)/co(iii)4 couple , thus leaving 
 only the current that may be attributed to oxygen evolution ( red traces 
 in figure s10b  d ) . from these data , the average current density was 0.11  0.04 ma / cm at 1.5 v. assuming that all of this current leads to the production 
 of o2 , then a tof of 0.06 mol o2/mol catalyst 
 is calculated at an overpotential of 0.89 v ( see si for details ) . this low current density and tof is consistent 
 with catalysis from ppb concentrations of co(ii ) produced from decomposition 
 of the cubane ( see discussion ) . solution [ o2 ] measurements during illumination of crude 
 samples of 1 ( black ) , purified 1 ( red ) , 
 and without added 1 ( green ) . photochemical reactions 
 were performed in the presence of 0.5 mm ru(bpy)3 , 35 mm na2s2o8 , and 100 mm kpi 
 ph = 7 buffer . the concentration of crude and purified 1 was 0.33 mm , assuming 100% purity for the crude material . cvs of 2 mm ( assuming 100% purity ) crude 1 and [ edta ] 
 = 0 ( black ) , 0.10 ( red ) , 0.25 ( blue ) and 0.50 ( green ) mm in 0.2 m 
 kpi ( ph = 7 ) . arrow and cross indicates the initial point and direction 
 of scan . to exclude the possibility of 
 chemistry specific to a 1:gc interaction , pt , au , and 
 fto a similar behavior was obtained as 
 for the gc experiments : the cvs of the crude 1 showed 
 significant water oxidation current , which was absent in the cvs of 
 the purified material ( figure s11 ) . in addition to electrochemical woc activity , 
 photochemical water oxidation has been reported for as - synthesized 
 samples of 1 using the ru(bpy)3/s2o8 sacrificial oxidant 
 system . the photochemical assay was performed in triplicate according 
 to the literature procedure , with the 
 exception that phosphate buffer was used instead of carbonate ( see si for details ) . the concentration of o2 was measured for samples of crude 1 , purified 1 , and without added catalyst . a fluorescence - based o2 sensor was immersed into n2 purged solutions containing 
 [ ru(bpy)3 ] = 0.5 mm , [ s2o8 ] = 35 mm , and   = 0.33 mm , and 
 the cuvettes were photolyzed with a hg / xe arc lamp ( exc > 400 nm ) . the yield of o2 over 400 s of photolysis 
 decreased 
 from 167  15 m for the crude samples to 31  6 m 
 for the purified samples ( figure 5 ) . ( a ) p nmr spectra of the phosphate signal of a 0.5 
 mm solution ( 0.426 mg / ml ) of purified 1 in 0.2 m kpi 
 ( ph = 7 ) with added co(ii ) at the indicated concentrations . ( b ) the 
 measured full - width at half - maximum ( fwhm ) of the phosphate p nmr signal is linearly dependent on the concentration of added 
 co(ii ) . the equation of the linear calibration curve is fwhm = { ( 936 
  8)  [ co(ii ) ] } + ( 7.2  0.4 ) . ( c  e ) p nmr spectra of the phosphate signal for three separate batches 
 of 0.426 mg / ml of crude 1 in 0.2 m kpi at ph = 7 . using 
 the calibration curve of ( b ) , the amount of line broadening corresponds 
 to a co(ii ) concentration of 0.086  0.004 mm , 0.091  0.008 
 mm , and 0.065  0.006 mm for samples ( c ) , ( d ) and ( e ) , respectively . to 
 identify and quantify the impurity found in the crude samples of 1 , a series of spectroscopic and the epr spectrum of a solid sample of crude 1 reveals a broad paramagnetic signal over the range g = 10 to 2 , which is absent in the purified sample ( figure s12 ) . to confirm 
 the presence of a co(ii ) impurity , edta was titrated into a cv solution 
 of the crude sample . figure 6 shows the cvs 
 for the addition of edta ( 00.5 mm ) into a 2 mm solution ( assuming 
 100% purity ) of crude 1 in 0.2 m kpi ph = 
 7 . nearly complete suppression of the catalytic current was observed 
 at 0.5 mm edta addition . as a control , a 50 m solution of purified 1 was treated with 10 mm edta in 0.2 m kpi at ph 
 = 7 for 1 h , and no changes in absorbance were observed ( figure s13 ) , confirming that 1 is 
 kinetically stable in the presence of edta . the cv wave of the co(iii)/co(iv ) 
 couple of purified 1 with addition of edta ( figure s14 ) is fully reversible , indicating that 1 is also stable to edta on the time 
 scale of the cv experiment . the amount of co(ii ) introduced 
 by dissolving the crude preparation of 1 in aqueous media 
 could be quantified by applying p nmr line broadening 
 analysis , which we previously employed to quantify the self - healing 
 properties of co - oec . a calibration curve 
 was constructed by adding increasing amounts of a 1:1 mixture of co(oac)2:pyridine to a 0.5 mm solution ( 0.426 mg / ml ) of purified 1 in 0.2 m kpi buffer ( figure 7 , see si for experimental details ) . this calibration curve was used to determine the amount of co(ii ) 
 in batches of crude 1 . although cv experiments were performed 
 with 1 at a concentration of 0.852 mg / ml , at this concentration 
 of crude 1 , the broadening of the phosphate signal is 
 too great to construct a calibration curve over a wide enough range . thus , we performed p nmr line broadening experiments 
 at half the concentration used for cv experiments . figure 7 shows the p nmr signals of phosphate 
 upon dissolving 0.426 mg / ml of crude 1 for three separately 
 prepared batches . per the calibration curve , we determine that the 
 co(ii ) ion concentration in solution is [ co(ii ) ] = 0.086  0.004 
 mm , 0.091  0.008 mm and 0.065  0.006 mm for samples ( c ) , 
 ( d ) and ( e ) , respectively ( [ co(ii)]avg = 0.08  0.01 
 mm ) . translating this result to the concentrations used for cv experiments , 
 a sample of 0.852 mg / ml of crude 1 introduces an average 
 concentration of [ co(ii ) ] = 0.16  0.02 mm into solution . the results of the p nmr experiments were confirmed 
 by an electrochemical titration , in which [ co(ii ) ] was correlated 
 with the catalytic current observed by cv ( figure 
 s15 ) . with increasing [ co(ii ) ] , the peak current of the catalytic 
 wave at 1.3 v increases linearly . a calibration curve was again constructed 
 and used to assess [ co(ii ) ] in the three batches of crude 1 at the concentration used for cv experiments . the results of this 
 assay shows excellent agreement with the p nmr experiment , 
 albeit with larger error bars , giving 0.153  0.019 mm , 0.178 
  0.020 mm , and 0.120  0.016 mm for the three samples , 
 with a [ co(ii)]avg = 0.15  0.03 mm . because 
 of the insolubility of co3(po4)2 in 
 aqueous media , the measured co(ii ) concentration could 
 be diminished due to loss of cobalt in the form of a co3(po4)2 precipitate . however , at these low [ co(ii ) ] , 
 precipitation of co(ii ) by phosphate is negligible due to the slow 
 kinetics of formation of the co3(po4)2 on the time scale of the electrochemical or photochemical experiments , 
 which take minutes to complete . to experimentally verify that no co3(po4)2 formed under our experimental 
 conditions , a 0.15 mm solution of a 1:1 mixture of co(oac)2:pyridine in the presence of 0.2 m kpi at ph = 7 was monitored by p nmr line broadening over a 4 h period ( figure s16 ) . the p nmr spectrum establishes that 
 the concentration of co(ii ) in solution does not significantly decrease 
 over this time period . the p nmr line broadening 
 experiment is also a sensitive 
 measure of compound stability . solids of purified 1 can 
 be stored on the benchtop for at least 25 days without decomposition . the p nmr line broadening analysis of 0.5 mm 1 in 0.2 m kpi buffer solution shows that the presence 
 of co(ii ) ions after 25 days is negligible ( figure 
 s17 ) . in addition , comparison of the h nmr spectra 
 of 1 , hours after purification and after 25 days are 
 identical ( figure s18 ) . since crude 1 introduces 
 co(ii ) into the solution , we would expect that 
 at anodic potentials , co - oec will be deposited . indeed , bulk electrolysis 
 of a 1 mm solution of crude 1 at 1.2 v for 5 min resulted 
 in the deposition of co - oec material on the electrode surface , which 
 was readily observed by scanning electron microscopy ( sem ) and energy 
 dispersive x - ray spectroscopy ( eds ) ( figures s19a 
 and s20a , respectively ) . bulk electrolysis of the crude sample 
 at a higher potential of 1.4 v results in significantly less co - oec 
 detected on the electrode surface ( figures s19c 
 and s20c ) , despite more charge being passed ( figure s21 ) . per the dems experiment , current is redirected 
 from water splitting ( o2 production ) to degradation of 
 the gc electrode ( co2 production at higher potential ) . although bulk electrolysis performed over 300 s of a purified sample 
 at 1.2 v resulted in an eds spectrum that is indistinguishable from 
 that of a blank sample ( figure s20b , d ) , 
 the sem images of the pure and blank samples showed a subtle difference . the density of light contrast material was increased in the pure sample 
 as compared to that of the blank sample . we therefore pursued further 
 characterization of the electrode surface by xps analysis , which is 
 more selective to analysis of surface materials than eds . a comparison 
 of high - resolution co 2p xps spectra of crude , pure , and blank gc 
 electrodes after 300 s of bulk electrolysis at 1.2 v is shown in figure s22 . a trace signal at the co 2p3/2 peak of the pure sample is barely distinguishable over background , 
 whereas a large co 2p3/2 signal is observed for electrodes 
 removed from bulk - electrolyzed solutions of crude 1 . as - synthesized samples of 1 contain 
 significant amounts 
 of impurities in two forms . the many aromatic peaks in the h nmr spectra of crude 1 ( figure 2 ) and the slowly eluting bands on tlc plates are likely co(iii ) clusters 
 of smaller nuclearity , which are known to be stable compounds . of greater significance , as demonstrated by 
 epr spectroscopy ( figure s12 ) , electrochemical 
 measurements in the presence of the ion scavenging edta ( figure 6 ) and p nmr line broadening analysis 
 ( figure 7 ) , a co(ii ) impurity is present in 
 crude samples of 1 . electrochemical titration experiments 
 and p nmr line broadening experiments quantify significant 
 amounts of co(ii ) in as - synthesized preparations of 1 . repeated experiments on different batches of as - synthesized 1 show that the concentration of co(ii ) is 16% of the expected 
 concentration of the cubane molecule , 1 . because the 
 co(ii ) impurity is soluble in dcm , the ligation of the co(ii ) ion 
 likely involves solubilizing organic groups , such as the acetate or 
 pyridine reactants of 1 , as salts of co(ii ) with outer - sphere 
 anions , such as acetate or nitrate , are unlikely to have significant 
 solubility in dcm . ligation of the solubilizing groups appears to 
 be sufficiently weak that co - oec is easily formed ( vide infra ) . as 
 a cautionary note , the absence of line broadening in the h nmr spectra of 1 does not provide sufficient evidence 
 that co(ii ) is not present in solution . the lack of significant line broadening in the h nmr 
 spectra upon titrating 1:1 co(oac)2:pyridine into a sample 
 of purified 1 ( figure s23 ) 
 indicates that this is not a sensitive measure of paramagnetic impurities , 
 presumably because 1 ( a neutral , weakly basic molecule ) 
 does not interact significantly with the co(ii ) ion . as figure 7 demonstrates , the phosphate p nmr 
 signal is a much more sensitive measure of the presence of co(ii ) 
 impurities . the co(ii ) ion impurities do not elute on silica and thus 
 are easily removed from 1 . the same behavior is observed 
 for 1-coome , where silica gel chromatography can be used 
 to remove co(ii ) impurities from as - synthesized or precipitated samples . the co(ii ) impurity acts as a source for the formation of the known 
 water oxidation catalyst , co - oec . the formation of heterogeneous co - oec 
 occurs from solutions of co(ii ) with any proton accepting electrolyte , 
 as long as the concentration of the electrolyte is sufficiently high 
 to control ph . moreover , co - oec will be formed 
 from co(ii ) either electrochemically or ( photo)chemically as long 
 as the potential is sufficient to oxidize co(ii ) to co(iii ) in the 
 presence of electrolytes such as phosphate or carbonate . consistent 
 with the formation of co - oec , the catalytic wave in figure 3 has the same peak potential and onset current as 
 found for a cv of co(ii ) solutions from which co - oec electrodeposits 
 ( figure s24 ) . however , unlike a well - behaved 
 catalytic process , as is typical of co - oec on fto , a peak response 
 is observed in the cyclic voltammogram . a peak in the catalytic wave 
 will result from either depletion of substrate or catalyst deactivation . since the solvent , h2o , is the substrate , 
 ph is maintained by a high concentration of phosphate , and current 
 densities are low , we can safely rule out substrate depletion as the 
 cause for the peak in figure 3 . however , a 
 peak will result if the catalyst were to be removed from the electrode 
 in a parasitic side reaction , or as in this case , if oxidative degradation 
 of the electrode is significant ( vide infra ) . once the impurities 
 are removed by column chromatography , the large 
 catalytic waves in cvs of solutions of unpurified 1 ( figure 3 ) and 1-coome ( figure s8 ) disappear completely . this behavior is observed 
 on other electrode materials ( pt , au , and fto , figure s11 ) as well . crude 1 shows higher currents 
 at anodic potentials than purified 1 , providing further 
 evidence that an impurity is responsible for the woc , as opposed to 
 spurious activity arising from a specific deleterious interaction 
 between the cobalt cubane molecule , 1 , and a gc electrode . sem , eds , and xps support the formation of a heterogeneous co catalyst , 
 which we attribute to co - oec , which deposits on electrodes from bulk 
 electrolyzed solutions of crude 1 . even in purified samples 
 of 1 , xps indicates that indeed a small amount of cobalt 
 can be detected on the electrode . the production of co - oec from purified 1 explains the small amount of o2 observed in the 
 dems experiment ( figure 4b , middle panel ) and 
 the minute amount of current beyond background ( figure 3 , red trace ) at potentials above 1.4 v vs ag / agcl . if all 
 the current at 1.5 v goes to the production of o2 , the 
 tof at this potential would be 0.06 mol o2/mol of 1 . however , only an extremely small amount of cobalt in the 
 form of co - oec is needed to support the current density associated 
 with this tof . using the tafel slope and the known dependence of the 
 exchange current density on the thickness ( i.e. , co content ) of films 
 of co - oec , it was determined that only 
 70 ppb of 1 , with its 4 cobalt atoms , would need 
 to decompose to furnish enough cobalt to form co - oec and produce this 
 observed current density ( see si for details 
 of the calculation ) . however , we note that the amount of co - oec and 
 o2 produced is negligible as compared to the co - oec formed 
 from as - synthesized samples of 1 . at potentials 
 above 1.4 v , the dems results show that the observed 
 current is predominantly due to the production of co2 when 
 a gc electrode is used as the anode . as eds and xps results show , 
 the process is so efficient at 1.4 v , that the current is largely 
 redirected from co - oec production from the co(ii ) impurity to oxidative 
 degradation of the electrode . importantly , the direct evidence of 
 co2 formation ( figure 4a , bottom ) 
 under conditions that thermodynamically favor the formation of co - oec 
 argues against the possibility that the co - oec catalyst is unstable 
 at these high potentials . if the potential is such that the rate of 
 degradation of the gc surface is rapid , as stracke et al . have noted , one can not interpret the absence of deposited heterogeneous material after electrolysis 
 as evidence of actual molecular catalysis , since surface catalyst 
 will be lost upon degradation of the underlying electrode . consistent 
 with this argument , sem and eds analysis show a decrease of observable 
 co - oec on the electrode for bulk electrolysis experiments performed 
 at 1.4 v vs 1.2 v ( figures s19 and s20 ) . 
 any carbon material ( e.g. , graphene , carbon nanotubes . etc . ) may be 
 compromised due to degradation at high anodic potentials and thus 
 water oxidation experiments performed on carbon - based anodes should 
 be subject to dems or other mass spectrometric analysis to ensure 
 that the current is not due to electrode oxidation to co2 . as in electrochemical experiments , removing the co(ii ) impurity 
 from photochemically driven woc also leads to a dramatic reduction 
 in the amount of o2 observed ( figure 5 ) . in the photolysis experiment , persulfate ( s2o8 ) is used as a sacrificial oxidant to form ru(bpy)3 upon irradiation . the reduction potential of 
 ru(bpy)3 is 1.06 v vs ag / agcl . at ph = 7 , 
 co - oec is formed from co(ii ) at potentials in the range of 0.750.80 
 v vs ag / agcl . therefore , under the conditions 
 of the photolysis experiment , co(ii ) can be oxidized to co - oec by 
 ru(bpy)3 . furthermore , the onset of woc by 
 co - oec is 0.900.95 v vs ag / agcl , and so ru(bpy)3 is thermodynamically capable of driving catalyst turnover . in addition , the quenching reaction of ru(bpy)3 by persulfate to produce ru(bpy)3 , also 
 produces so4 as a potential 
 oxidant , which has ample overpotential to drive water oxidation ( e  2.2 v vs ag / agcl ) . thus , the major pathway giving rise to water photooxidation activity 
 with as - synthesized 1 is consistent with the formation 
 of co - oec from the in situ oxidation of co(ii ) ions . although 
 all photochemical studies have used as - synthesized 1 , and thus water oxidation may be supported by co - oec , the present 
 study shows the photochemical oxidation of purified 1 also results in the production of measurable quantities of o2 ( 31  6 m ) over 400 s of photolysis , leading 
 to a tof = 2.3  10 s. at the potential of ru(bpy)3 , which is 
 within the co(iii)3co(iv)/co(iii)4 wave ( figure 3 ) , no o2 is produced as measured by dems 
 ( figure 4b ) . therefore , ru(bpy)3 is not a potent enough oxidant to turn over 1 ; a greater overpotential is required , 
 if 1 is indeed a molecular catalyst under these specific 
 photochemical conditions . as noted above , the protocol of the photochemical 
 experiment produces the strongly oxidizing species so4 . this species is free not only to react directly 
 with ru(bpy)3 but also to react with 1 because the concentrations of [ ru(bpy)3 ] = 0.5 mm and   = 0.33 mm are similar . therefore , 
 the observed o2 emanating from the photolysis conditions 
 used for purified 1 in figure 5 is likely due to the interaction of 1 with so4 , which has a considerably more positive 
 reduction potential than ru(bpy)3 . h bond ( bdfe = 123 kcal / mol ) of water directly to produce the radical , oh . h bonds of the 
 ligands , and thus the molecule itself , to be thermodynamically stable 
 with respect to hydrogen atom abstraction given the extreme potentials 
 provided by the electron accepting so4 and proton accepting phosphate buffer species . if the cubane were 
 to decompose , co - oec is a likely product of the decomposition pathway . alternatively , computational investigations into the mechanism of 
 woc by 1 suggest that two oxidations of 1 to the level of co(iii)2co(iv)2 and an acetate 
 ligand dissociation were required prior to water attack and subsequent 
 o  o bond formation . we can not 
 confirm if so4 is capable of 
 oxidizing 1 because the electrochemical 
 window limits the range of potentials for investigating the behavior 
 of 1 at potentials beyond 1.5 v. if a higher oxidized 
 cubane is capable of water oxidation activity , it occurs at extremely 
 high overpotentials . without purification 
 by silica chromatography , the co(iii ) oxo 
 cubanes can be contaminated with co(ii ) impurities , which are responsible 
 for the observed water oxidation activity reported for these molecules . we have shown that an edta titration can be used to test for the presence 
 of co(ii ) and a p nmr experiment can be used for the 
 co(ii ) quantification ; these experiments are more definitive than h nmr spectroscopy for identifying paramagnetic co(ii ) impurities . 
 beyond co(ii ) as an impurity , the use of any co(ii ) complex should 
 be assessed as an authentic woc versus precursors for heterogeneous 
 catalysts such as co - oec owing to the proclivity of co(ii ) complexes 
 to undergo rapid ligand substitution . we note that 
 water oxidation activity of a catalyst should not depend on whether 
 an anodic potential is supplied electrochemically or ( photo)chemically 
 for mechanisms involving outer sphere electron transfers . in instances 
 where homogeneous and heterogeneous o2 evolution experiments 
 do not concur , it is appropriate to consider whether other species 
 are responsible for catalytic activity . finally , when inspecting carbon - based 
 electrode surfaces for the deposition of heterogeneous catalysts , 
 care must be exercised in the choice of oxidizing potentials , as extreme 
 values can give rise to spurious current that is associated with co2 evolution and electrode degradation as established by dems .
the 
 observed water oxidation activity of the compound class co4o4(oac)4(py 
 x)4 emanates 
 from a co(ii ) impurity . 
 this impurity is oxidized to produce the well - known 
 co - oec heterogeneous cobaltate catalyst , which is an active water 
 oxidation catalyst . 
 we present results from electron paramagnetic 
 resonance spectroscopy , nuclear magnetic resonance line broadening 
 analysis , and electrochemical titrations to establish the existence 
 of the co(ii ) impurity as the major source of water oxidation activity 
 that has been reported for co4o4 molecular cubanes . 
 differential electrochemical mass spectrometry is used to characterize 
 the fate of glassy carbon at water oxidizing potentials and demonstrate 
 that such electrode materials should be used with caution for the 
 study of water oxidation catalysis .