electrochemical water 
 oxidation , also known as the oxygen evolution 
 reaction ( oer ) , is a key energy conversion reaction in a number of 
 clean energy technologies , including rechargeable metal 
air 
 batteries , electrolysis cells , and solar fuel production . widespread commercialization of these technologies is limited in 
 part by the scarcity and high cost of the best known catalysts for 
 the oer , ruthenium and iridium oxides .
nickel oxides 
 ( niox ) present a viable alternative to 
 precious metal oxides in alkaline environments and are currently used 
 in commercially available alkaline electrolyzers .
other 
 nonprecious metal oxides , including manganese oxides ( mnox ) and cobalt oxides ( coox ) , have also demonstrated promising oer activity under alkaline conditions . to facilitate the development of nonprecious metal oer catalysts 
 with improved activities , it is necessary to identify the specific 
 catalytic sites that participate in the reaction and accurately determine 
 their turnover frequencies .
although isolating site - specific turnover 
 frequencies can be challenging in oxide electrocatalysts , the likelihood 
 of success can be improved if the catalytic activity is studied on 
 well - defined materials deposited on inert supports .
there have been conflicting reports in the recent 
 literature regarding 
 the role metal supports play in the oer activity of metal oxide catalysts . 

a study , which used pt(111 ) and au(111 ) single crystal surfaces as 
 supports for oer catalysts , demonstrated that the oer activity of 
 four first row transitional metal oxides did not vary with the nature 
 of the metal support and was linearly dependent on the coverage of 
 the support by the metal oxide .
other 
 reports , however , have shown that the oer activities of nickel , cobalt , and manganese 
 oxides were influenced by the nature of the underlying 
 support , and that the oer turnover frequency of a bulk metal oxide 
 was inferior to that of a submonolayer amount of the same metal oxide 
 deposited on a metal support .
our study , which focuses 
 on one particular metal  supported transition metal oxide system , 
 mnox / au , conclusively demonstrates that 
 adding a noble metal to a metal oxide oer catalyst can have a significant 
 impact on its electrocatalytic activity .
we show that this impact 
 can not be explained simply by surface area effects and we identify 
 interesting changes in the red - ox properties of both mnox and au when the two materials are present in one 
 composite catalyst . using our experimental data and previous literature 
 results ,
preparation 
 of rotating disk electrode substrates : rotating disk 
 electrode substrates were prepared from 200 mm long glassy carbon 
 ( gc ) rods ( diameter 5 mm , sigradur g htw hochtemperatur - werkstoffe 
 gmbh ) . before deposition of mno or au nanoparticles ,
the rods were 
 processed by stanford crystal shop to produce 4 mm long pieces with 
 one side lapped and chamfered and the other side polished to a root 
 mean square ( rms ) surface roughness of less than 50 nm .
preparation 
 of substrates for in situ x - ray absorption spectroscopy 
 ( xas ) characterization : electrode substrates for in situ xas characterization 
 were prepared from 200 mm long gc rods ( diameter 10 mm , sigradur g 
 htw hochtemperatur - werkstoffe gmbh ) . before deposition of mno or au 
 nanoparticles ,
100200 m thick wafers with one side 
 lapped and the other side polished to a surface rms roughness of less 
 than 50 nm .
synthesis of catalyst materials : mno nanoparticles 
 were prepared 
 with a sputtering system ( nanosys500 , mantis deposition ltd . ) using 
 a glow discharge magnetron sputtering with an inert gas condensation 
 technique , as described previously .
briefly , the system consists of a nanoparticle 
 source and the quadrupole mass filter , which filters sputtered nanoparticles 
 by mass in situ .
mn nanoparticles were size selected at approximately 
 10 nm and deposited at a pressure of 0.3 mtorr with a rate of 0.16 
 s , monitored by a quartz crystal microbalance 
 ( qcm ) .
these size selected nanoparticles were channeled to the main 
 chamber and deposited on substrates . after deposition
, samples were 
 transferred to the load lock chamber , which was vented with ar .
gold nanoparticles 
 were prepared using an electron beam evaporator to deposit 8  
 gold at a rate of 0.10.2 s monitored by a qcm .
electrochemical characterization : the 
 oer activity of the catalyst 
 samples was characterized using cyclic voltammetry in a three electrode 
 electrochemical cell in a rotating disk electrode ( rde ) configuration . 

characterization was performed in 0.1 m koh electrolyte using a scan 
 rate of 20 mvs , at room temperature .
a carbon 
 rod ( ted pella ) was used as the counter electrode , while ag|agcl ( fisher scientific ) was used as the reference 
 electrode .
the potential scale was calibrated to a reversible hydrogen 
 electrode ( rhe ) , and all the potentials were ir - compensated 
 to 85% and reported vs rhe .
the average measured resistance between 
 the working and reference electrodes was 40  for all 
 samples .
the oer activity was determined by scanning the potential 
 from 0.05 v to 1.71.8 v in a n2 saturated environment . 
 to prepare surfaces for ex situ xas characterization , the potential 

was scanned from 0.05 v to a vertex potential of 1.65 v at 20 mvs and held at 1.65 v for 30 min .
physical and 
 chemical characterization of nanoparticles : the size 
 and morphology of the catalytic materials were investigated using 
 scanning electron microscopy ( sem , fei magellan 400xhr ) .
the images 
 were obtained using a secondary electron detector , a beam current 
 of 25 pa , and beam voltage of 5 kv .
the oxidation state of the mnox nanoparticles was characterized using ex 
 situ and in situ x - ray absorption spectroscopy ( xas ) at stanford synchrotron 
 radiation lightsource ( ssrl ) .
ex situ measurements were performed 
 on the 31-pole wiggler beamline 
 101 at the ssrl using a ring current of 350 ma and a 1000 
 lmm spherical grating monochromator with 
 40 m entrance and exit slits , providing 10 phs at 0.3 ev resolution in a 1 mm beam spot .
all data were acquired in a single load at room 
 temperature and under ultrahigh vacuum ( 10 torr ) 
 in total or auger electron yield ( tey , aey ) modes .
the measurements 
 were performed on mnox nanoparticles and 
 five powder standards ( mnf2 , mno , mn3o4 , mn2o3 , and -mno2 ) attached 
 to an aluminum sample holder using conductive carbon .
-mno2 powder was prepared by dissolving 0.5 g of kmno4 in 30 ml of millipore water , followed by dropwise addition of ethanol 
 under stirring , drying the resulting powder at 60 c overnight , 
 and calcining the powder at 400 c for 3 h. mnf2 , 
 mno , mn3o4 , and mn2o3 powders 
 were purchased from sigma - aldrich and used as received .
the energy 
 was carefully calibrated in two steps , by first correcting the energy 
 scale for the drift in the beam energy and then aligning the energy 
 of the first peak of the mn3o4 powder control 
 with a literature value of 639.6 ev .
hard x - ray xas measurements were carried out at the 
 ssrl 
 at beamline 6 - 2 es2 .
the beamline was operated with a double - crystal 
 si(311 ) monochromator with a rh - coated mirror to reject the high order 
 harmonics .
the x - ray beam was focused with a parabolic mirror to a 
 spot size of 230 m fwhm ( v ) by 400 m fwhm ( h ) at the 
 sample position .
the x - ray energy was calibrated to the pre - edge absorption 
 peak ( 6543.3 ev ) of potassium permanganate .
the mn k1 signals 
 were resolved by 6 spherically bent analyzer crystals ge 333 
 installed on the high energy resolution x - ray emission spectrometer and detected by a silicon drift detector in 
 photon counting mode .
all spectra were normalized 
 to have an edge jump of unity after linear backgrounds were subtracted 
 from the raw data .
the error bars reported in the spectra and the 
 residuals represent one standard deviation based on poisson statistics 
 and standard error propagation . in situ
completely 
 separate electrochemical cells ( custom built cells , adams & chittenden ) , 
 reference electrodes ( ag|agcl , basi ) , and counter electrodes ( 100 
 mm length/5 mm diameter glassy carbon rod , si gradur g htw hochtemperatur - werkstoffe 
 gmbh ) were used with the mno - gc and mno / au - gc samples to avoid the 
 possibility of contaminating the mno - gc sample with trace amounts 
 of au . the working electrodes ( gc wafers with deposited nanoparticulate 
 catalysts ) were glued to the window of the electrochemical cell using 
 epoxy .
the schematic of the in situ electrochemical cell is shown 
 in supporting information , figure s1 .
prior 
 to performing xas measurements , cyclic voltammograms were collected 
 at 20 mvs in nitrogen - saturated 0.1 m koh 
 electrolyte by first scanning the potential from 0.05 to 1.1 v and 
 then from 0.05 to 1.65 v. to record in situ mn xanes spectra at an 
 oer relevant potential , the potential was scanned from 0.05 v to a 
 vertex potential of 1.65 v at 20 mvs and 
 held at 1.65 v for 15 min .
all measurements were ir - compensated to 85% and are reported vs rhe by assuming a potential 
 shift of 0.960 v. the average measured resistance between the working 
 and the reference electrodes was 70 . effect 
 of au : to further study the effect of au on the oer activity 
 of mno - gc nanoparticles , additional electrochemical experiments were 
 carried out . for these experiments ,
the potential was scanned on a 
 mno sample and bare gc support from 0.05 v to 1.8 v in a n2 saturated environment .
then 0.1 mm hau(iii)cl4 hydrate 
 ( sigma aldrich , 99.999% metals basis ) was added to each electrolyte 
 and the samples were repeatedly scanned starting in the oer region , 
 1.2 to 1.8 v , with the cathodic scan progressively reaching more reductive 
 potentials with each cycle to deposit increasing amounts of au on 
 the surface of the catalyst .
schematic ( a ) and sem images ( b ) of the three 
 catalytic samples , 
 mno - gc , mno / au - gc , and au / mno - gc , illustrating morphology and coverage 
 of glassy carbon support by mno and au nanoparticles .
because the 
 influence of the underlying substrate on the oer activity 
 of mnox has not yet been clearly established 
 in literature , we prepared nanoparticulate catalysts on inert gc supports 
 to directly evaluate the impact of au .
the first sample consisted of only mno deposited 
 on gc ( mno - gc ) .
the second and third samples consisted of both mno 
 and au nanoparticles : one sample with mno sputtered on top of au nanoparticles 
 evaporated onto gc ( mno / au - gc ) , while in the other au was evaporated 
 on top of the mno - gc catalyst ( au / mno - gc ) .
schematic representations 
 and sem images of the mno - gc , mno / au - gc , 
 and au / mno - gc catalysts are shown in figure 1a , b . from the images
, we can estimate that the mno loading is less 
 than 1 g ( calculations are presented in the supporting information ) .
total electron yield ( tey ) mn l - edge 
 xas characterization of the mn surface oxidation state in the three 
 catalysts is presented in figure 2 . in the 
 figure , the xas spectra of the composite samples consisting of both 
 mno and au
are compared to the spectra of mno - gc and two powder controls : 
 mnf2 and mno . in these experiments ,
mnf2 was 
 used as a mn powder standard in addition to commercially 
 purchased mno powder because of the known surface oxidation of the 
 mno phase in air .
comparison of the spectra of 
 the powder standards to the spectra of the as - deposited mno / au - gc 
 and au / mno - gc catalysts using this surface - sensitive technique illustrates 
 that after the addition of au , mno nanoparticles exhibit the oxidized 
 form rather than the reduced form of the mn phase .
it 
 is not clear whether this slight difference in the starting surface 
 oxidation state results in a change in the crystal structure of the 
 material to form a different bulk phase .
our attempts to characterize 
 the crystallinity of the catalysts with and without the addition of 
 au using conventional and synchrotron grazing incidence x - ray diffraction 
 ( xrd ) yielded only reflections corresponding to the gc support ( supporting information , figures s2 and s3 ) .
this 
 indicated that either both types of samples were amorphous or that 
 the crystallinity of the nanoparticles could not be detected using 
 grazing incidence xrd .
total electron yield of mn l - edge xas of the three catalytic 
 samples , 
 mno - gc , mno / au - gc , and au / mno - gc , and two powder standards , mnf2 and 
 mno . the surface of mno , which is known to oxidize in air , was not 
 sputtered prior to characterization and represents oxidized  mno  .
electrochemical characterization 
 of the catalysts in 0.1 m koh 
 electrolyte shown in figure 3 demonstrates 
 that the composite catalysts , consisting of both mno and au nanoparticles , 
 have significant oer activity , while the oer current density of the 
 mno - gc catalyst is negligible by comparison .
the supporting information ( figure s4 ) also presents characterization 
 of au evaporated onto gc in the absence of mno ( au - gc ) to rule out 
 simple additive effects of mno - gc and au - gc . from the inset in figure s4
it is clear that the oer current of 
 the composite samples is significantly greater than the sum of the 
 individual activities of mno - gc and au - gc .
table 1 compares the oer activity metrics 
 of mno - gc , mno / au - gc , and au / mno - gc , including geometric current density , 
 mass activity , and turnover frequency ( tof ) , all calculated at either 
 300 or 400 mv overpotential (  ) , to the activity metrics of 
 some of the best mnox electrocatalysts 
 reported in literature .
although table 1 focuses purely 
 on electrocatalysts , supporting information , table s1 presents a similar comparison to exceptional manganese 
 oxide photochemical water oxidation catalysts .
the tabulated activity 
 metrics for manganese oxide catalysts reveal that the catalytic behavior 
 of mno - gc in the absence of au is consistent with expectations .
although 
 its current density normalized to the geometric surface area is low 
 in magnitude , the mass activity and tof values of the catalyst match 
 the performance of the best mnox oer catalysts . 

furthermore , normalization of mno - gc oer current by its capacitance 
 produces similar surface - area normalized activity to that of the previously 
 reported mno thin film catalyst ( supporting information , figure s6 ) deposited by atomic layer deposition ( ald ) . from tables 1 and s1 , it is also apparent the two composite catalysts 
 consisting of both 
 mno and au nanoparticles exhibit unusually high oer activities .
each 
 mass activity and tof is about an order of magnitude higher than the 
 corresponding metrics for the previously reported highly active thin 
 film mnox catalysts .
the only other mnox catalyst with similar activity was published 
 by subbaraman and co - workers , who electro - deposited mnox islands on a pt(111 ) support .
although the authors 
 did not include a cv of mnox demonstrating 
 its oer activity , they reported an oer current ( on a geometric basis ) 
 of 5 macm at ca .  = 560 ( 1.78 v ) . 

this geometric current density is similar to the geometric current 
 densities of our composite catalysts after the background oer current 
 of pt(111 ) substrate is subtracted . from the results of subbaraman 
 and co - workers , it is not clear whether or not the use of a noble 
 metal support has an impact on the measured oer activity of mnox catalyst .
direct comparison between the 
 activity metrics of mno - gc and au / mno - gc catalysts in the present 
 report , however , convincingly demonstrates that the addition of a 
 noble metal to mno nanoparticles sputtered onto a gc substrate will 
 lead to a significant enhancement in their oer activity .
the 
 influence of adding au to the oer activity of mnox could occur either through a modification of the 
 mno phase of the starting catalyst , thus creating a bulk mnox phase with intrinsically higher oer activity than 
 the original catalyst , or through the formation of active sites at 
 the interface of mnox and au . to probe 
 this further
, we characterized the catalytic surface after exposure 
 to oer conditions . in figure 4 , we present 
 ex situ tey mn l - edge xas spectra of mno - gc , mno / au - gc , and au / mno - gc 
 catalysts previously exposed to an oer relevant potential of 1.65 
 v. the corresponding chronoamperometry curves used to generate the 
 oer catalytic surfaces are shown in supporting 
 information , figure s7 .
as expected , exposure to oxidative 
 potentials leads to oxidation of mno nanoparticles in all three cases . 

the presence of au , however , favors formation of less oxidized mnox nanoparticles . furthermore , in the au / mnox - gc catalyst where au is deposited on top 
 of the mno nanoparticles
, mn assumes the lowest oxidation state among 
 the three oer samples ( supporting information , figure s8 ) .
one potential explanation for this result is that after 
 exposure to oer conditions , the mn oxidation state may vary as a function 
 of a distance from au , with mnox located 
 at the interface with au assuming a more reduced state than mnox located away from au . to further study 
 the possible interface effect in the composite 
 samples , we compared mn l - edge xas spectra of mno / au - gc and mno - gc 
 samples in tey and auger electron yield ( aey ) detection modes . although 
 both aey and tey detection modes are surface sensitive , the probing 
 depth of aey is shallower than that of tey .
if an interface effect is present , one would expect the measured 
 oxidation state of the oxidized mnox nanoparticles 
 in the mno / au - gc sample to vary as a function of probe depth as illustrated in the schematic in supporting information , figure s9 , resulting 
 in different tey and aey spectra .
figure s9 also presents the aey and the tey spectra of mno - gc and mno / au - gc 
 for the as - deposited and for the oer samples .
comparison of aey and 
 tey spectra of mnox in the absence of 
 au ( the mno - gc sample ) in figure s9 demonstrates 
 a lack of variation in mn oxidation state as a function of distance 
 from the gc support , both before and after applying an oer relevant 
 potential . making the same comparisons with the mno / au - gc sample
, 
 however , shows that while the as - deposited sample has a negligible 
 difference in the mn spectra between aey and tey mode , a more significant 
 spectral difference is observed for the sample exposed to oer relevant 
 potentials . in that sample ,
the top of the nanoparticle , which is 
 located away from the au - mnox interface , 
 is likely more oxidized , resembling the spectra of the mno - gc sample 
 exposed to oer conditions .
these results support the hypothesis that 
 there is an interface effect between au and mnox after mno is exposed to high oxidative potentials . in conventional 
 heterogeneous catalysis
, such an interface effect has been previously 
 observed with mno2 and other 
 reducible metal oxides , such as tio2 and ce2o3 , in contact with au .
the ex situ nature of the mn l - edge xas characterization , 
 however , does not allow us to differentiate between an interface effect 
 that emerges when the catalyst is under oer conditions or an interface 
 effect that emerges after the catalyst is removed from the electrochemical 
 cell . to answer this question , in situ methods
cyclic voltammetry characterization 
 of the three catalytic samples , 
 mno - gc , mno / au - gc , and au / mno - gc .
characterization was performed in 
 0.1 m koh electrolyte , at 20 mv / s , and 1600 rpm . in situ characterization
was performed with a bulk - sensitive 
 technique 
 using mn k - edge xas .
our measurements focused on the x - ray absorption 
 near edge structure ( xanes ) region , which probes the bulk electronic 
 structure of the catalyst . in the experiments ,
mno and au nanoparticles 
 were deposited on 200 m thick gc wafers to allow x - ray 
 illumination of the backside of the electrodes .
we characterized two 
 samples , mno - gc and mno / au - gc , with a slightly reduced loading of 
 mno compared to the ex situ samples ( supporting 
 information , figure s10 ) .
these samples were held at 1.65 v 
 during the mn k - edge xas measurement . despite the mno / au - gc sample 
 having a higher oer current throughout the entire experiment ( figure s10 ) , our in situ xas characterization 
 shown in figure 5 does not reveal a measurable 
 difference between the xanes spectra of mn in the mno - gc and the mno / au - gc 
 samples .
the observation of the same average mn oxidation during the 
 electrochemical evolution of oxygen for both samples suggests that 
 a small subset of sites rather than the bulk phase is critical to 
 the enhanced oer catalysis in the samples containing au nanoparticles ; 
 for example , this could occur by means of some of the mnox sites locally interacting with au in such a way 
 as to form oer active sites with modified properties during electrochemical 
 characterization .
total electron yield mn l - edge xas of the three catalytic 
 samples , 
 mno - gc , mno / au - gc , and au / mno - gc , and two powder standards , mno2 and mn2o3 . to further explore this hypothesis we performed electrochemical 
 characterization of the mno - gc catalyst before and after adding 0.1 
 mm of hau(iii)cl4 to the electrolyte .
figure 6 shows that as we electrodeposit more au onto the surface 
 by sweeping to more reductive potentials , the oer activity increases , 
 eventually reaching a current density greater than 10 macm at 1.8 v. when the same experiment was performed 
 with a bare gc substrate , we also observed an increase in the oer 
 activity with increased electrodeposition of au , but the activity 
 remained low , below 1 macm at 1.8 v. these 
 experiments demonstrate that the addition of au salt to the characterization 
 electrolyte will lead to a similar enhancement in oer activity of 
 mno - gc catalyst as evaporation of au onto the catalytic surface prior 
 to electrochemical characterization .
some dissolution of au into electrolyte 
 is expected to occur at the highly oxidative potentials necessary 
 for the oer .
we took advantage of this 
 fact and also performed experiments with the mno - gc sample in the 
 setup previously used to characterize samples containing evaporated 
 au nanoparticles , but without cleaning the cell .
thus , trace au was 
 expected to be present to form a sample which we refer to as  trace - au / mno - gc  . supporting information
, figure s11 shows that 
 although the morphology of the mno nanoparticles in trace - au / mno - gc 
 had not changed while evaluating oer activity , the oer current increased 
 with time during a potential hold at 1.65 v for 30 min ( after first 
 cycling the catalyst from 0.05 to 1.65 v at 20 mvs ) , ultimately rising to the same level of oer activity as that for 
 the au / mno - gc sample .
this result further illustrates that the electrolyte 
 conditions can have a significant effect on the activity of mnox , with the addition of only a trace amount 
 of au leading to a significant increase in the oer activity . supporting information ,
figure s12 compares 
 cyclic voltammograms of the au - gc , au / mno - gc , and trace - au / mno - gc 
 samples .
analysis of electrochemical features of the samples confirms 
 the presence of au in trace - au / mno - gc sample and reveals that the 
 presence of mnox has an influence on the 
 red - ox properties of au .
specifically , the addition of mnox leads to formation of multiple oxidation / reduction 
 peaks as opposed to the one clear oxidation peak and the one clear 
 reduction peak observed in the au - gc sample .
the presence of multiple 
 oxidation / reduction peaks in both au / mno - gc and trace - au / mno - gc could 
 indicate that the interactions between au and mnox have led to a formation of au sites with different strengths 
 of au 
since reducible metal oxides such as mno2 are known to interact with noble metals to form a more oxidized 
 noble metal at the interface of the two materials , this result is consistent the presence of interfacial effects 
 between mnox and au .
mn k - edge xas of two 
 catalytic samples , mno - gc and mno / au - gc , and 
 the difference between the two spectra , demonstrating that mn has 
 a similar oxidation state under oer conditions in the presence and 
 absence of au . after taking into account 
 our ex situ and in situ spectroscopic 
 characterization as well as electrochemical studies
, we propose that 
 the observed enhancement in the oer activity of mnox in the presence of au is caused by the local participation 
 of au in the oer catalysis , for example through dissolution of au 
 and its re - deposition onto a subset of mnox sites , rather than a bulk change in the starting properties of mnox .
this interpretation is consistent with 
 all of the results in our study and is also supported by the published 
 literature in the area of oer catalysis for bulk mnox materials .
as described previously , table 1 convincingly demonstrates that the samples consisting of 
 both mnox and au had anomalously high 
 oer activity when compared to other active mnox oer catalysts . the only mnox catalyst 
 that had been reported to have similar oer activity was a manganese 
 oxyhydroxide prepared using a completely different method , an electrodeposition , 
 but also in the presence of a noble metal , pt(111 ) .
similar to au , pt is known to interact with transition 
 metal oxides to form a reduced oxide and an oxidized metal at the 
 interface of the two materials and dissolve at oxidizing 
 potentials relevant to the oer . therefore , 
 the presence of pt during the oer could lead to an enhancement in 
 the oer activity of mnox that is similar 
 to the enhancement observed in the presence of au .
additionally , a 
 recent work from najafpour and sedigh reports that several of mnox phases , including mn3o4 , -mn2o3 , -mno2 , and 
 k - birnessite can convert to the same layered mnox structure under water oxidizing conditions . as discussed by the authors , this observation deemphasizes 
 the nature of the starting mnox phase 
 in the oer catalysis and highlights the importance of the electrolyte 
 conditions during water oxidation . therefore , 
 the presence of a noble metal in the catalyst sample or the electrolyte 
 could have a greater influence on the formation of the oer active 
 sites than the as - deposited phase of mnox .
( a ) cyclic voltammetry of mno - gc and bare gc support in the presence 
 and absence of trace amount of au ( 0.1 mm hau(iii)cl4 ) 
 demonstrating the increasing oer activity with increasing deposition 
 of au ; ( b ) 10-fold magnification of panel a focusing on the oer region . 

the effect of electrolyte conditions 
 on the oer activity of electrocatalysts 
 is not limited to mnox .
trotochaud and 
 co - workers have recently reported transformation of niox thin film catalyst into a layered hydroxide / oxyhydroxide 
 oer catalyst during electrochemical characterization . during the transformation , the catalyst scavenged fe impurities 
 from the solution incorporating them into the final active catalytic 
 structure .
it is possible that like mnox , niox could 
 also interact with noble metal additives to form oer active sites 
 with enhanced tofs .
for example , in a different study , yeo and co - workers 
 have already demonstrated a higher tof with a catalyst consisting 
 of only 0.14 monolayers of niox deposited 
 on au relative to the tof of a bulk niox catalyst , indicating that an interaction between niox and au exists .
although 
 trotochaud and co - workers have also attempted to assess the role of 
 au by characterizing niox catalysts on 
 au / ti and ito supports , their study involved conformal niox thin films with limited electrochemical accessibility 
 of the au support . in contrast , in the 
 study by yeo and co - workers , the au oxidation / reduction features were 
 still present in the submonolayer sample , which meant that au had 
 an opportunity to locally impart oer activity by either interacting 
 with the adjacent niox or by dissolving 
 into the electrolyte at the high oxidizing potentials necessary for 
 the oer and subsequently integrating 
 into the niox catalyst .
thus , the electrochemical 
 accessibility of the au likely plays a role and should be an important 
 consideration when designing catalyst morphologies for future studies .
in an investigation of catalysts for the oxygen evolution reaction 
 ( oer ) , we have shown that a composite catalyst of mno nanoparticles 
 in the presence of au exhibited an enhancement in electrocatalytic 
 activity by 1 order of magnitude over the best reported pure mnox catalysts . a difference in the mn l - edge 
 x - ray absorption spectra of as - deposited catalysts originally indicated 
 that a change in the starting properties of mnox could be responsible for the observed increase in the oer 
 current . however , subsequent electrochemical measurements as well 
 as ex situ and in situ mn xas characterization of the oer relevant 
 samples identified more likely possibilities : namely local , interfacial 
 effects between the au and mnox . even 
 trace amounts of au were sufficient to activate the catalyst for the 
 oer , further indicating that a local interaction between au and mnox , impacting only a subset of mnox sites , is the likely cause of the observed oer enhancement . 
 as the intrinsic oer activity of other nonprecious metal oxides such 
 as coox and niox ( supporting information , table 
 s2 ) are even higher 
 than that of mnox , investigating the role 
 of au in the oer activities of a number of nonprecious metal - based 
 transition metal oxide catalysts could lead to even higher - performance 
 materials
 to 
 develop active nonprecious metal - based electrocatalysts for 
 the oxygen evolution reaction ( oer ) , a limiting reaction in several 
 emerging renewable energy technologies , a deeper understanding of 
 the activity of the first row transition metal oxides is needed . 
 previous 
 studies of these catalysts have reported conflicting results on the 
 influence of noble metal supports on the oer activity of the transition 
 metal oxides . 
 our study aims to clarify the interactions between a 
 transition metal oxide catalyst and its metal support in turning over 
 this reaction . to achieve this goal 
 , we examine a catalytic system 
 comprising nanoparticulate au , a common electrocatalytic support , 
 and nanoparticulate mnox , a promising 
 oer catalyst . 
 we conclusively demonstrate that adding au to mnox significantly enhances oer activity relative 
 to mnox in the absence of au , producing 
 an order of magnitude higher turnover frequency ( tof ) than the tof 
 of the best pure mnox catalysts reported 
 to date . 
 we also provide evidence that it is a local rather than bulk 
 interaction between au and mnox that leads 
 to the observed enhancement in the oer activity . 
 engineering improvements 
 in nonprecious metal - based catalysts by the addition of au or other 
 noble metals could still represent a scalable catalyst as even trace 
 amounts of au are shown to lead a significant enhancement in the oer 
 activity of mnox . 
Introduction
Experimental Section
Results and Discussion
Conclusions