So today we, we have three student lectures.
Unfortunately one of them, one of that we,
you will see on the program that there are actually four names.
Unfortunately Chow Wang could not make it, so
we'll just have three, three talks this afternoon.
And so the first talk that we're going to hear form is from Christina,
Christina Lee.
Christina is a PhD candidate, advised by Matt Cannon, and
we heard from Matt earlier in the conference and
she's in the department of chemistry at Stanford University.
She received a BA in chemical and physical biology from Harvard University,
and is now here at, at Stanford.
And so Christina will talk on CO2 and CO reduction on oxide derived copper.
>> Thanks Rich, for the introduction.
It's a tough act to follow, Tom Freedman.
So, unfortunately, I don't have anything profound to say about the world, but
I hope to share with you some of our work on,
electrocatalysis to generate liquid fuels from CO2 and CO.
So, the main goal of our lab has been to implement a CO2 recycling scheme.
So, we heard a lot about CO2 emissions, and
the possibility of doing CO2 capture and sequestration.
Well, what we would like to do is, rather than pump the CO2 into the ground,
is actually just convert it back to a fuel, and create a closed cycle for CO2.
Such that we can constantly reuse it and not emit it into the atmosphere.
And so the scheme that we imagined is shown here on the slide.
We can input renewable energy to convert this CO2 to some form of liquid fuel,
and at the same time, we'll be converting water to oxygen.
And then that liquid fuel can just be fed directly into a car.
It can be fed into a fuel cell.
And then we can get renewable energy back out again.
Specifically, we're looking at an electrochemical process and so I've showed
a range of products here that we can generate from CO2 and those include.
Molecules like CO and formate but also molecules like ethanol,
which are already known fueled products.
And so the reactions at the cathode and the anode,
which are the reactions that we're interested in, are shown down here.
And, for example, at the cathode, we can do two-electron and
two-proton reduction of CO2 to generate CO and water, and at the anode,
we'll be doing water oxidation So there are some key challenges
in CO2 reduction and some key metrics that I'll be referring to throughout my talk.
So I just want to introduce those metrics to you now.
The first is again these two reactions at the cathode and
at the anode that I mentioned.
And this determines the amount of energy that we can store in the form of fuel.
Unfortunately, we can only store that much energy if we have perfect
catalysts that can operate at the thermodynamic minimum.
Generally speaking, we need to apply a certain amount of overpotential.
And so, overpotential is a really crucial metric.
And we want to keep that overpotential as low as possible, so
that we have the highest energetic efficiency.
The other main issue is a selectivity issue, so
we're doing all of these electrolysis in water, and water reduction,
it turns out, is right around the same energy as CO2 reduction.
And it's also a kinetically quite facile reaction.
And so, whenever we do CO2 reduction, we almost certainly also see hydrogen.
And so that's a reaction that we're trying to minimize when we design our catalysts.
So to summarize, we're trying to get the best activity out of our catalysts and
that includes overpotential as well as current density.
We're trying to get good selectivity of
our CO2 reduction products over hydrogen and we're also aiming for
good stability so that we can run these electrolyzers for a long time.
Some of the work that's been done on CO2
was done in the mid 80s to early 90s by a Japanese chemist.
And a lot of that focus was on polycrystal and copper.
So copper's kind of a special material.
It's capable of making a lot of different products.
All of them are shown here.
And those products include hydrocarbons like methane and ethylene.
However, copper also has quite a lot of issues in terms of its stability and
its activity.
And I just want to highlight here some of the,
the reasons that we're working on copper, is to try to improve these deficiencies.
So the first is, on this plot we have.
The curr, the potential versus a scale that we call RAG.
Zero is where the thermodynamic minimum is at.
And then we have efficiency for a variety of different products.
So at low potential, you can see that all we're making is hydrogen.
That's the product we don't want.
However, as you ramp up the potential, the hydrogen starts to go down.
And we start to see some of these other products, including CO and formate.
And then you ramp up the potential a little bit more, and you see ethylene and
methane, which are our target products.
So what we sought to do, was to improve the performance of copper by
lowering that overpotential towards the products that we want.
By making nano structured materials and so our synthetic method is very, very simple.
We start with that bulk copper foil, whose activity I just showed,
we oxidize it up in air and it forms this copper oxide material so
you can see these rods and then by XRD we see that it is in fact copper oxide and
then we do an electro reduction.
And we reform copper zero,
but our copper zero is now very different from the copper zero we started with.
You can see in this SEM image that it's composed of very small
particles that are aggregated together into these sort of larger structures.
And we can, in fact, confirm that the copper oxide has been completely removed,
and copper zero remains.
So I'm going to show you, here,
sort of our typical, data that we get out of C02 reduction electrolysis.
So, plotted here is a bulk like electrolysis that was run at minus 0.5
volts versus RHE.
So we're talking about 500 millivolts of overpotential
in a saturated bicarbonate electrolyte.
So, water with bicarbonate.
And we're running this over about a six hour period, to test for
sort of nominal stability.
And we have current density on this left hand axis, and the efficiency for CO.
So, C02 to CO, on the right hand axis.
So, polycrystalline copper, that foil that I showed you,
is our sort of comparison sample.
And you can see.
That the current density overall is very, very low at this potential.
So we're at low over potential,
polycrystalline copper just can't do a whole lot.
The efficiency for CO2 reduction is also very poor.
We get about 10% CO, that drops down quite quickly.
We get 3% formate.
And we get nothing else.
Now if we put oxide derived copper into that mix.
We see that immediately the current density jumps up by a lot.
So initially we see a very high current density, that's due to the copper oxide
reduction to copper zero, and then once that stabilizes,
we get CO2 reduction at 30 to 40% CO production.
And 33% for mint production.
So we've improved the activity of copper from about 10 total percent
CO2 reduction to 70% total CO2 reduction.
We can then compare over the whole range of overpotentials.
So, similar to the plot I showed earlier and I've just re-plotted the data for
the polycrystalline copper.
That I showed.
And if we put the Oxide derived Copper onto the same plot,
you can see that the overpotential for peak formation of CO and
formate has shifted to the left by four to five hundred millivolts.
So we're now able to do the same reactions,
with higher efficiency, at significantly lower applied energies.
We're obviously interested in these more reduced products as well, so
the hydrocarbons and on oxide derived copper, we don't see any methane,
which was the dominant product on polycrystalline copper,
we see ethylene and ethane, so we see only these C2 products, so
in our case this CC coupling is happening very quickly.
We wanted to understand a little bit more from the kinetic perspective
about what was going on in our system, and so we made what we call a Toffle/?
plot.
And this is simply the data that I showed earlier,
but now I've put the partial current density for
CO2 reduction on the left hand axis, and potential is on the bottom axis.
And so I plotted oxide derived copper in red and polycrystalline copper in black
and as we expected, oxide derived copper has significantly higher current density
for CO2 reduction, so that's, that was pretty obvious from the previous plot but
what we did next is we did a surface area correction because we know also.
Oxide derived copper is nanostructured.
It has high surface area and it turns out that when you
do this surface area correction, these two plots now fall on the same line.
So it turns out that oxide derived copper's
absolute ability to reduce CO2 is quite similar to that of polycrystalline copper.
So that raised the question, what is the origin of this enhanced selectivity?
You know, how, how are we getting good Faraday coefficiency?
And it turns out that the reason we have good Faraday coefficiency
is because oxide derived copper just doesn't do hydrogen evolution very well.
So again, I've made the same plot, but
now this is partial current for hydrogen evolution.
We do the surface area correction and you can see that I have a 35 times suppression
of hydrogen evolution at low over potential on oxide derived copper.
So really the origin of this high Faraday co-efficiency that we've observed
is due to hydrogen suppression rather than enhancement of CO2 reduction.
So the obvious question here was you know, are the nanoparticles important?
Are we simply just making nanoparticles and
nanoparticles can do things very differently.
Than sort of a bulk copper foil.
So, we did a comparison to commercial nanoparticles that are more or
less the same crystallite size as oxide-derived copper, and
the answer is no, nanoparticles are not the same as oxide derived copper.
When we run these commercial nanoparticles, they do basically nothing.
They make less than 5% CO2 reduction products.
And they have high current density.
So, really, it's just pumping out a ton of hydrogen.
So, copper nanoparticles are not sufficient to reduce CO2.
So, so far, I've told you about CO2 reduction to CO emformate, which were.
Were the two most accessible products.
They're two electron, two proton reduction products.
And what I'd like to tell you about now is
the formation of some of these more reduced products,
products that we can actually consider as liquid fuel type molecules.
And so when we looked at oxide derived copper at these somewhat higher
overpotentials.
We noticed that there was, in fact,
formation of some of these products ethanol, acetate, and propanol.
And, in fact, there is a shift in the overpotential to lower overpotential
relative to polycrystalline copper.
So that gave us some hope that we would be able to access
these liquid fuel type molecules.
We also knew based on previous work that CO is the key intermediate.
Towards the formation of these more reduced products.
And so we wanted to ask the question,
what would happen if we fed CO directly to our electrolysis?
And if we had a greater CO concentration,
would we be able to access these more reduced products than higher yield?
And so we proposed this.
A two-step process to the formation of ethanol and acetate.
Where CO2 is first reduced to CO, it could be by a different catalyst and
then we feed CO to our catalyst to engage this more challenging transformation of
eight electrons and eight protons to make ethanol.
So that's in fact what we did.
We did electrolizes now in 0.1 molar KOH and we just saturate it with CO gas.
And I'm using these same copper nanoparticles as our comparison sample
because it's a high surface area, sort of bulk copper type material.
And you can see, on copper nanoparticles, that, really, it doesn't touch CO, at all.
So, here, again, I've shown the potential on the bottom axis, and
Faraday coefficiency up here.
And overall, there's less than 10% total efficiency for CO reduction,
everything else is hydrogen.
It does make a range of products that we see in grey,
we see some acetate at the lower overpotentials.
We get ethanol formation as we ramp up, and
then we get a little bit of ethylene and ethane.
We put oxide derived copper under the same conditions and
there is a dramatic increase.
In the CO reduction deficiency.
So you can see, at these lower overpotentials,
we can get 50 to 60% production of CO reduction products.
The primary products in all cases is ethanol and ac, acetate.
And from our best conditions we can get about 40% Faraday coefficiency for
ethanol at minus 0.3 volts.
And so that's, that's quite a low overpotential for
the production of a liquid fuel from CO.
We did the same analysis, where we wanted to understand, is it hydrogen suppression,
is it CO activation.
And so here I've plotted the surface area normalized data for
the CO reduction partial current density.
And in this case we actually see an absolute activation of CO.
So there's a 22 times enhancement of CO reduction efficiency, or
CO reduction total current density, relevant to these nanoparticles.
If we do.
So there was supposed to be a plot here that shows that there's a little bit of
hydrogen suppression but not a lot.
I'm not sure where it went.
[LAUGH] Okay.
So then we wanted to interrogate the mechanism a little bit further
by making by looking at the slope of the Tafel plot.
So, in electrochemistry,
the slope can tell us something about the rate determining step.
And this 113 a millivolt per decade tafel slope indicates that we have a rate
determining one electron reduction.
And that one electron reduction can take a number of forms,
we don't know exactly what's going on, but we hypothesize that it could either be
an electron coupled with a proton transfer or an electron coupled with another CO.
Coupling to form that CC bond.
We then believe that ethanol and acetate come from a common intermediate,
which could be this alene type surface bound intermediate.
And then the bifurcation of this intermediate is hydroxide dependent.
So what we showed then was if we increase.
The concentration of hydroxide in the system.
We can actually totally favor the formation of acetate
along this bottom pathway.
And you can see here, at minus 0.2 volts, that we have all acetate formation
in the higher hydroxide content electrolyte.
So lastly I just want to talk a little bit about structural characterizations.
So we wanted to know it's not nanoparticles.
What is the structure that's giving us high efficiency for CO2 and CO reduction?
And so we turn to TM imaging as a way to look in more detail at the structure.
And the first thing we noticed was that.
Oxide derived coppers consisted of these sort of
highly interconnected nanocrystallites.
These, these nano particles are simply particles that are sitting on top of
each other and are relatively mobile, relative to each other.
And so when you look at the grain boundaries here on oxide derived copper,
you see a very structured grain boundary and you see.
Two greens that are misoriented relative to each other and
also unable to rotate relative to each other.
And so, we feel that grain boundaries are a type of high-energy bulk defect, and
it could be a location where the binding of absorbates and
the catalicism of absorbates is highly activated.
So the last thing I'll show is just some preliminary data that we have on analyzing
these grain boundaries in a more quantitative way.
And so we do orientation mapping using TM electron defraction, and
here I've just plotted the orientations of a representative sample of oxide
derived copper.
We can then generate these grain boundary maps based on the orientation.
And then we can get quantitative statistics on the types of
orientations that we have, the angles of misorientation.
And so we're now working up this data and
trying to collect sort of a statistical amount of data to really say for
sure is the grain boundary an important aspect of this catalysis.
And the one sort of interesting thing that we've observed so
far is that there is a very high density of a special type of boundary called
the twin boundary, in our materials, that is not present in this just
like randomly statistically oriented grain boundary distribution.
So in summary,
we've shown that we can do this two step processing to make ethanol from CO2.
We've identified possibly an interesting surface sight at grain boundaries for
this CO2 reduction reaction and we've also seen that
suppression of hydrogen evolution is a really important mechanism for
catalyzing CO2 reduction with high efficiency.
So in the future we have a lot, I guess I'm pretty out of time, but we have a lot
of really exciting stuff going on in our lab, where we're trying to characterize
the CO binding strength, we're trying to characterize the structure.
And we're also trying to synthesize new materials,
sort of based on some of these principles, try to get even better catalysts and
to optimized different pathways for CO2 reduction.
So, I'd like to thank my advisor who is here as well as all of my lab mates who
have worked with me on this project and then,
all of the collaborators that helped up,
helped us with the TM work as well as some other institute studies that we're doing.
And of course the funding including GSEP.
For this project.
I'm happy to take any questions.
[APPLAUSE].
>> So I'm not quite clear on what the pH of your solution is so
what is the form of the CO2 in solution?
Is it really bicarbonate or something?
>> So at pH.
So for CO2 reduction we run everything at pH7.
And so, CO2 is dissolved.
So there is CO2 in the form of bicarbonate but
there is also about 30 millimolar dissolved CO2 in the solution.
And so we think that we're acting on that 30 millimolar dissolved CO2,
not on the bicarbonate directly.
[INAUDIBLE] experiments, when you added the OH,
you could be changing the reactants.
I mean, the reactive form.
>> Right, so in the hydroxide case, there is no CO2 in the system.
That is only feeding CO directly.
So we do maintain the PH 13, of 0.1 molar KOH.
And we're acting on the dissolved CO in that case.
I was wondering if you could explain more about where these three boundaries and
if you have any idea why that just like changes the behavior or
your surface so much?
So I guess it's known that sort of grain boundary surfaces when they,
so grain boundaries are bulk defects but when they interact with a surface you
can get a very different structure locally at the grain boundary.
And so it's actually been observed by TEM that you know, normally you have FCC
copper but at the sort of intersection of two grains you can end up having like
an HCP type structure where it's just like totally randomly oriented.
So we think that sort of just accessing these totally different lattice parameters
in the vicinity of the grain boundary can activate CO binding, for example.
And that's something we're trying to study more now.
>> Okay.
So thanks to Christina Bowers.
[APPLAUSE]