My hard

work years.

Just out that is undergraduate was

doing master's in aerospace
engineering I guess.

So by a great way back to school here.

Thank you very much Tom and
I really appreciate the invitation to come

to talk to you today have
had a really great day.

Last night I went out for
to a very nice Greek restaurant and

have some advice to before you if you ever
go on an interview don't drink the Greek

coffee the night before
your interview you will.

I was told not to drink the dregs but even
if you stay away from the dregs it can be

a challenge but
I've had lots of good conversations Doc

starting with Dr Lou this morning and
and ing with Bill chorus this afternoon.

So I've said many times today.

I well talk about mention that in my talk
so why don't I give you my talking see if

I can mention everything the topic
is mediated electrode systems

who are interested in using enzymes as
catalysts primarily for fuel cells which

is an example of a system where the rate
of electric tells us is important.

This is distinguished from sensors.

But there's other examples.

This is my musical interlude.

There are other examples of systems
where high rate is important.

So the.

The fuel cell examples
are pictured here this is sort of

a classical I made this picture when I was
a post-doc of a implanted fuel cell and

a blood vessel where the fuel cell
can be catalyzed at the cathode

by an enzyme like lack ace at the end and
I'm like glucose oxidase and

the and the blood which
contains generally glucose and

oxygen can conserve as the reactance for
for the system.

This is a little bit far fetched
from a surgical standpoint but

it that it acts as a sort of
an initial motivation more simple

systems might include implanting
a small bio fuel cell in.

A fruit like this is a great
this is worked on by man.

What Heller and University of Texas and
ultimately more near term

one can imagine using the selectivity
of electric catalysts in more

practical direct methanol fuel cells
where the cathode can be catalyzed

by a selective lack a lack a sense I'm and
because and the anode can be placed in

the same plane as a cathode so that the
feed can be a mixed feed of both fuel and

an oxidant and it relies on
the selective any of these devices so

this is an example of a way
to to intensify the the fuel

cell system using the selectivity of
the enzymes a more general approach might

be bioreactors using enzymes to do
chemical conversions from low value.

Fee's like glucose using like
mentality hydrogenated to

format a tall which is a high value
sugar one can take listener all which is

a byproduct of biofuels production using.

A DI hydrogenation enzyme
to produce the ha.

Which is done had dropsy acetone which
is a tanning agent that you do use

in self tanning.

OK so.

So we can also consider chemical
conversions in addition to

energy generating devices to make.

You added products that
are really only obtainable using

catalyze reactions and we can couple
these reactions so this is a and

oxidation reaction this is a reduction
reaction so that we can actually do these

types of the conversion simultaneously
in the same reactor the common themes

here are that the these reactions have to
be operated at high rate in order to have

you know efficient production of electric
electrical energy or chemical products and

in order to make these devices
practical stability is a big concern.

So enzymes have some advantages
in terms of selectivity etc but

they are challenged compared
to conventional catalysts by

a lack of stability and
a relatively low turnover rate.

So a common question.

In the context of using And
sometimes as catalysts electric catalyst

is whether they have what it takes
to do what's probably the highest

power density application for fuel cells
which is the automotive application.

And so I dressed this sort of
doing a very brief analysis

using a criteria that was
established two thousand and

five by gas starter a doll which is
the the requirements for a free catalyst.

So if one were to replace a precious
metal such a platinum with.

With a catalyst ideally that
replacement would be free

because that would overcome the cost
issues with platinum and so

the activity of a costless Cathy Cathy
catalyst really is defined

mostly by whether the fuel cell that
one would design with this catalyst.

Has sufficient energy per unit volume in
other words can you put enough catalyst

into the fuel cell to get the power that
you want and so that the definition that

the guest worker came up with is
that the activity per unit volume.

It's sort of quantified as
current per unit volume for

automotive applications needs to.

About.

The order of a tenth of
the industrial plant and

activities in other words the activity of
a platinum catalysts supported on carbon

as implemented in an automotive fuel cell.

So here's a little table that
sort of does the analysis for

an enzyme catalyst as
compared to platinum.

The this is a supported platinum catalyst
on carbon forty seven wait percent

the conditions for the platinum
catalyst and this is a lack ace

oxygen reducing catalyst are are at
their ideal conditions so

the platinum is at roughly eighty
degrees C. the lack a says at forty C.

the PH for lack a says five the PH for
Platinum is around zero

at at reasonable high potentials
where you want to operate a cathode

the turnover frequency for a platinum
catalyst in terms of electrons for

sight per second is around twenty five
the the enzyme itself the lack of self

has a turnover frequency as measured in
our experiments of around two hundred so

just based on the two or
frequency the lackeys looks pretty good.

The problem of course is that the density
of sites for Platinum around three point

two So point two times ten to
the twenty percent cubic centimeter

is almost an order of magnitude
bigger than the density for lack A So

the reason there's a lack a says
a hundred kill a dog molecule.

It's very large so
the site density is really limiting.

So if you multiply these two
numbers together you get more or

less the current per unit volume and if
we see just on a theoretical ideal basis

black ace is certainly within the order
of magnitude requirement for platinum.

So if you if you if we
can utilize black ace

in an ideal way high utilization high
turnover then weren't in the ballpark.

However if we really look at a practical
device you know this is a practical

platinum catalyst we look at
a practical mediated lack a system

at the same condition.

Same more or less same potential a little
bit lower the turnover frequencies

the same but the additional components
that one has to add in order to really

obtain high utilization of the electrode

lowers the site density by
about an order of magnitude

which brings the current per unit volume
down again by about an order of magnitude.

So that sort of brings us out of the range
for power application so from a power

density standpoint lactase the you
know what we can envision doing with

like case now is not what you'd expect to
do with it for automotive applications and

we haven't even addressed the stability
aspects which are also a big detriment.

However it's easy to show
this is work by Matthew and

Heller from two thousand and three.

It's easy to show that that
enzyme electrodes have some

advantages compared to
platinum on the on the left or

polarization curves where the current
increases with a decrease in potential for

platinum and the same thing for
for this is for a bill

ribbon oxidase also an oxygen reducing and
I mean we can see that the current

densities under ideal conditions can be
quite high as compared to platinum and

most importantly the potentials at which
the cattle to tell says Can occurs.

This is a minus point two versus
the reversible potential of oxygen

are higher than the potentials for
for play platinum so

there's a third a Namak advantage to
using an enzyme in the sense that we

can catalyze the reaction at a higher
potential So there are some advantages.

There is commercial interest in bio fuel
cells primarily for portable power so

for smaller devices where the power
density is not as important and

energy density becomes important.

This is a system that's
being developed by Sony.

This is a.

This is the size of the system it's you
know this is an M P three player and

this is the size of the system is roughly
you know four or five times the size and

these are for glucose oxidizing and
air reducing.

Fuel cells that glucose is
is oxidized by an energy age

based enzyme which requires additional
components to do mediation which

we'll talk about a moment in the same
thing is true on the Oxygen side and

these devices generate on the order
of microwatts per cubic center right.

Centimeter right now really just
barely enough if you have a large

device to power the M P three player and
so the power density of these devices is

really at issue and what is also true
is that this device only works for

about a minute and
then it's pretty much done so.

So power density instability or
it's are still very much.

At issue.

So there's two ways that we can extract or
inject electrons into an enzyme.

One is direct light electron transfer and

the other is mediated electron transfer
in direct electron transfer one orients

the enzyme as closely as possible directly
on the surface of an electrode and

if the active center is close enough
you can have electron tunneling

between the electrode surface and the
enzyme and that's the most the simplest

possibly the most efficient way
to to achieve electron transfer.

But generally speaking this is
hard as I'll show in a moment and

the the alternative is to introduce
a mobile species a mediator that can and

has good kinetics with respect
electron transfer at the electrode and

then can transport the the electrons
to the enzyme and generally

speaking at least in our hands these sorts
of approaches are more effective in terms

of utilization of the enzyme then
the direct electron transfer approaches.

Having said that there are several
examples in the literature where people

have used direct like trying transfer to
get reasonably high current densities

this is a fructose to hydrogenation.

And I'm from the condo group in Japan
where they have actually measured

the order of six million per square
centimeter at pretty high over potentials

for fructose oxidation.

Just to give you a reference this sort of
current answer on the order of five or

six millions per square centimeter for

an enzyme electrode is quite high for
a platinum based fuel cell if it's a less

than an amp per square centimeter
It's not that interesting.

So those are the sorts of scales that
we're working on this is a lack a scale

cathode where the current increases
with decreasing potential in the current

densities I know you can't read this that
they measured on the order of tens of

millions per square centimeter
quite impressive for

a direct electron transfer and then this
is a bilirubin oxidase electrode which has

similar performance in the million
percent of your squared range so

it can be done but the mediated systems
we've found have been more effective and

this is why this is
the structure of lack ace in

the yellow balls represent
the active centers of a lack ace and

this is from the the this yellow
ball is a copper center called the T

one site that's responsible for electron
transfer between a CO substrate and

the enzyme and so the CO substrate in our
case would be the electrode itself and

then these three coppers are the T two
T three site that's responsible for

oxygen reduction all the way to water and
so ideally one would immobilize these

lack aces and here the coppers are in blue
with respect to the electrode surface

such that single copper site is oriented
perfectly respect to the surface.

Every time to enhance electron
transfer and there are some chemistry

that are being developed to do this but
it's very hard and so generally when one

does a random approach the orientation
is not ideal and only some of the lack

a sense I'm zeroing in to properly
to achieve directly try to transfer.

So this ends up being a less active
electrode as compared to this one.

Now the alternative is to not worry about
orientation by introducing the mediator so

what we do is we immobilize the enzyme
in a and a hydrogen else so

this these lines are intended
to be the hydrogen.

Backbone and the hydrogen also
has complex to it are redux

complex that I'll describe in a moment and
if the concentration

of the Redux complex these purple
balls are significantly higher than

the concentration of the enzyme then
electron transfer via the mediator can can

be efficient in terms of getting
electrons to these a single copper site.

So one it cheese a situation where
the orientation of the enzyme

is not important one.

Also achieves a situation where the does
not have to be immobilized directly on

the surface and so you can have multilayer
the mobilization of the enzyme and

this leads to the possibility of
higher utilization of the enzyme and

higher loading of the enzyme on
the electrode surface the challenges

are the transport
limitations at the electron

ultimately is transported to the enzyme by
electron hopping between these readouts

complex sites and this hopping process
is the few gentle in nature and can be

characterized by diffusion coefficients on
the order of ten to the minus eight to ten

the minus ten which is very low
especially compared to for example

the diffusion coefficients of oxygen which
are on the order of ten of the minus five.

So the transport limitation
both in terms of electrons and

if this ends up being a very thick system
transport of the oxygen ends up being

the limiting process in these systems.

So these are the Palmers the Redux
polymers that we've we've synthesize

in order to study the interaction
between the enzyme and the mediator.

These are all polyvinyl emitters all
backbones to which we've complex and

complex and we can choose the Libyans.

These are all either by peer again or tour
period leggins and sometimes there's two

by purity Liggins and a chlorine leg and
we can choose these these

subgroups on the period eans in order to
tune the potential that which this cause.

Plex can oxidize So

if you use chloride groups
the oxidation potential is quite high.

If groups X.
today some potential is quite low.

And so we can span the potential
range of almost one full

down to almost zero volts to to
control the the way that that of

that mediator oxidizes and we can use
that parameter to study electron transfer

rate kinetics between the mediator and
lack a sense of.

So just to give you an idea of
the electrochemistry of the mediators.

These are cyclical down time of
Graeme's where we scan rather quickly.

The potential of the of an electrode
on which the this mediators

are immobilized and then we return in the
oxidation reduction peak sort of define

the Redux potential of the electrode
in these range over a broad range for

each of the end of the mediators we
can also use the peak of the cycle for

time gram to estimate activity and we can
use these sorts of measurements to also

measure transport rates of
electrons in the meter.

The way this can be put into
a fuel cell is described here.

This is a bill.

Reuben oxidase cathode where
action is reduced to water.

The electrons are transported
from the electrode

through the mediator to the enzyme and
then to to the to the oxygen where it's

reduced the water and so they're
traveling up a potential gradient So

the electrons like to go
towards positive potentials and

the rate of electron transport in this
direction depends on these potential jobs.

The larger the difference in potential
between the mediator and the larger

the jump and the same thing is true out
and at the anode where our glucose is

oxidized and those free electrons also
travel up the potential gradient and

then go to the cathode and so
when you look at the system.

There's a.

Several different potential
differences involved.

There's a thermodynamic potential
difference between the glucose in

the oxygen the two reactance which is
fairly large there's the potential

difference between the two enzymes which
is slightly smaller and then small Saval

is a potential difference between the two
mediators and this potential difference in

operating fuel cell really defines
the potential of the operating fuel so.

So in order to drive electron
transfer between the mediator and

the enzyme one needs a large potential
difference between the mediator in

the enzyme in order to have a high of cell
potential and remember power in a fuel

cell is potential times current one
has to have a large difference.

In tension here and so there's a tradeoff
between the kinetics of electron transfer

and the potential of a cell which
is a classic trade off and.

In electro chemical solve side.

Also there's other issues involved for

example the mediator is competing
with oxygen if it's present for

the electrons glucose oxidase which
is a typical glucose oxidizing

and uses oxygen as its natural course
substrate and so there's a natural ability

for glucose oxidase to reduce oxygen to
proc side which scavenge is electrons.

So the difference in potential between
the mediator and the glucose oxidase

also controls its ability to compete
with with oxygen for electrons.

So these are quote unquote steady
state polarisation curves these

are sweeps of the potential
in the presence of oxygen and

the lack a C. and
also mediated by the various mediators and

what we can see this is for a fairly
high potential mediator as we reduce

the potential the current starts becoming
negative which means that we're reducing

oxygen and it becomes negative more or
less at the potential redux potential of

the mediator and then it levels off
at some potential where where the.

MEDIATOR is fully reduced and there is
no potential dependence whatsoever.

And this plateau is controlled primarily
by the kinetics it can be controlled

by mass transfer it can also be controlled
by the kinetics of of oxygen reduction.

It can be controlled by the kinetics of
electron transfer between the mediator and

the enzyme So this this this
polarization curve is a cople pated

interpretation of all the processes
involved in the electrode and

we can show similar results for a lower
potential Palmer and we can see that for

a low potential polymer generally speaking
we have a higher current which indicates

faster kinetics at a lower potential and
we can add all of the pollers and

we can see this is a fairly messy when we
look at all the cycle full time grabs but

if we plot the plateau current as
a function of mediator potential we see

a very general trend of increasing
current density with decreasing

mediator potential it's not a complete
trend because some of the polymers.

Are not really following the trend at
all in the there's several reasons for

that having to do with transport and

other artifacts in these experiments
that have to be accounted for and so

the next step in this process is to start
a colony for those things to do that

we developed a relatively simple transport
model of this and I mediator film

just taking into account diffusion and
transport of oxygen into the film and

diffusion and transport of what Trons
from the other side we use a very simple

Ping-Pong Bye-Bye mechanism that
involves a turnover rate and

to Caylus constants and we use this to
experimentally a while before we use

this model we experimentally obtain the
fusion coefficient for the mediator and

I'll show those results next and then we
use that diffusion coefficient to actually

determine the kinetics of the reaction and
if you look at these pla.

What's the trend transport of oxygen.

This is the C.S. is the concentration of
oxygen coming comes in from the right it's

more or less flat and so that
the transport limitations do not involve

oxygen in most of these experiments on
the other hand the mediator which has

again the lower diffusion
coefficient is transport limited and

sometimes the rate the reaction rate
follows the meter concentration.

Sometimes it does not is that it really
depends on the saturation of the media

in this ratio of of K.-M. to the media.

Reduce mediator concentration.

If that concentration is high
the system is saturated and

the rate is constant if it's not
high then there's a dependence.

So we measure the diffusion coefficient of
the mediator using the control technique

where we where we plot the current as
a function of time after a potential

step and that's just a transit measurement
and connect if we plot it correctly

we can extract the slope and extract
the diffusion coefficient directly and

generally speaking the diffusion
coefficient should follow a linear

model with respect to the concentration
of the osmium redux complex

in the readouts Palmer and
so after measuring all

the diffusion coefficients we can make
a plot and we find that yes indeed.

The diffusion coefficients do
follow that linear relationship.

So what that one main conclusion that is
that not only does the Redux potential of

Palm but also the concentration
of the Redux complex in

the Palmer has a significant impact
on the diffusion coefficient and

as you might expect on the overall rate.

So having designed the model and

obtained diffusion coefficients to account
for transport within the electrode film we

can go ahead and extract kinetic constants
from the system and again we're using this

simple system where we can the main
constants that we can extract are the.

By molecular weight constants for the
mediator which is just K. Cat divide by K.

M. and then the by molecular a constant
for the substrate oxygen and these

are plotted here as a function of mediator
potential which is what one of the main

parameter that were varying and you can
see that as the potential decreases.

There the kinetics generally increases.

This is an exponential scale so
they include increase exponentially and

then flatten out and essentially what that
means is that the kinetics reach a point

as at which they are internally controlled
by electron transport processes within

the enzyme and in which case
the potential of the mediator is

not important and in contrast the by
molecular weight constant for

the substrate is more or
less flat which is what you'd expect.

So this information can be used for design
purposes and just as an illustration we

can we can predict the performance
of a hypothetical bio fuel cell

as a function of the mediator
potential we can plot the current

of the fuel cell just using
a simple exponential rate function

as a function of mediated potential and we
see that as as I mentioned as a potential

decreases the current increases up to
the point where which is this plateau

where again the kinetics are controlled
by an internal mechanism in the end zone.

But if we plot the power of the system
which is essentially this potential

multiplied by the current
We see a plateau and so

this hour we see an optimum and this
optimum defines the mediator potential for

this hype is that hypothetical cell which
is is optimal for a real fuel cell.

So we we've done a kinetic study and
use the results of the kinetic study to

actually design the mediator for
this hypothetical.

Electric.

Also there's several
prime ministers of the.

Of the beater that are important that
we've also studied in terms of synthesis

details.

For example that I mentioned that the
concentration of the mediator is important

to a point.

It can help to a point.

It starts to hurt and
also the ratio of the mediator to

the monomer of the backbone
can also be an important and

just looking at a very simple study where
we've increased the concentration of

the polyvinyl images all increase
the concentration of the osmium and

then a very at the ratio of the polymer
backbone to the mediator we

see that a very interesting result if
we look at the cyclical Tammy tree.

We see that the peak for these high
concentration mediators are more or

less the same and they both lead to
increased current density in a complete

electrode but the but where as these peak
eyes are about the same would suggest that

the diffusion coefficients are about
the same the the plateau current

density meaning the maximum current
density of the electrode is different and

so the indication there
is that there is a and

that there is an impact of
swelling of this hydrogels on

the kinetics of the electrode more
water higher osmium concentrations lead

to lower water intake into the electrode
and that has seems to have no impact on.

Electron transport but for

some unknown reason has an impact
on the the catalyst activity.

So what if what have we done so far the in
our interests is you know generating

higher power density by generating higher
current density and we've done that by

first and Hansen electron transport
to the enzyme we've we have this one

dimensional film where we have this the
hydrogels essentially the light blue air.

And it contains an enzyme worth
of which are the pink things and

then there are purple balls
which are the mediator.

And we've designed the mediator
to enhance electron transport.

But another thing that we can do is
to facilitate electron transport with

composite materials and so the next
step is to introduce composite supports

that allow electrons to be
transported via the solid

phase and increase the activity of

the enzyme by reducing the distance
over which the mediator has to act.

So that's something I'll describe next
we could also introduce these carbon

phases and we tend to make the electrodes
a lot thicker which introduces transport

limitations on oxygen and one thing that
we can do to enhance oxygen transport for

example is to introduce a gas phase for

oxygen concentration and
the defensive auction are much higher.

And so we'll show a little bit
of preliminary data on that but

going back to the carbon
materials what we like to do

is to start again with this one
dimensional film and what I

did as post-doc is to to to introduce
a carbon paper composite where we use.

Roughly ten micron diameter fibers on the
order of one hundred microns thickness and

and immobilize the ends of hydrogen within
these fibers and see that's the box.

I want but just by introducing this this
carbon material we increase the surface

area of the electrode by a factor of fifty
and the measured current density increased

by a factor of five compared to this
more or less one dimensional film.

And then the next step is to start adding
surface area so we want to maintain this

the structure of the carbon
fibers in order to maintain.

Mass transport within the structure but.

We want to enhance the surface
area of the material

by growing nanotubes on the structure and
in doing so

we increase the surface area again
by a factor of one hundred and and

the ultimate result in terms of current
density is an increase by a factor of ten.

I'll show you that in a second.

So this is the way we work with the carbon
materials we start with a with Tory paper

which is again these ten
micron carbon fibers and

we've set it up in a room temperature
chemical vapor deposition system or

we pass a current through the carbon
paper to heat it up and we Simon Taylor

simultaneously send a gas precursor
feed just typically carbon monoxide and

hydrogen through the carbon
paper in order to

facilitate chemical vapor deposition of
the growth and we have a catalyst which is

generally in our nanoparticle we have
a couple ways of delivering the particles

either prior to the to the chemical
vapor deposition we can deposit

them from hexane or we can actually use
a gas phase iron for example our input

to Carbondale to mobilize the catalyst and
this is the sort of growth that we get.

This is the bare Tory paper that
you might have seen before and

as we grow for one minute
understood our baseline conditions

we start getting a little bit of follows
on the carbon fibers and as we go for

five in ten minutes the growth is
is messy but at substantial we get

roughly a doubling of the diameter of
the fiber due to the nanotube growth.

And we can take those materials and
load them with an enzyme and

with a mediator and
measure the the kinetics of tells us and

in this case we're going to use the media.

We're going to use a low
potential polymer.

And it's going to be
used to mediate tells us.

Glucose oxidase So now we're going to
be doing making an ode that oxidizes.

So the current density will increase
with increasing potential and

what we have plotted here
the polarization curves for an electrode

immobilised on this scale material where
the negatives have been growing for

with zero one in twenty minutes so
this is Bear Tory paper the black and

we increase the current
density by a factor of ten.

By growing nanotubes for
about twenty minutes.

We can also plot the this current
density as a function of growth time and

we can compare it to the current
density surface area measurements

we can do in two ways one is
using just physics Orcs and

the other ways to use electric
chemical capacitive measurements and

those two measurements are more or less
the same the the capacitive measurements

at the high an attitude grosser are ninety
percent of the tea which means that there

is a limited electrolyte accessibility to
all the surface area that we've created.

But the current density jumps up very
quickly at fairly low growth times and

then levels off so that is
an indication that the surface area

that we're actually utilizing is really
is relatively low compared to the total

that we're growing so one of the things
we're working on now is to try to improve

the utilization of the surface area so
that we can activate more of the enzyme.

We can also immobilize a lack a sense
I'm oxygen reducing enzyme and

these materials these are the same

these the same polymers that we showed
before where these are very high.

Osmium low to high P.P.I. precursor
concentrations and we can get current

densities just as a point of comparison
the current densities for glucose oxidase

are on the order of twenty million answer
per square centimeter for lack a sense.

We've been able to cheat
on the order of ten so so

it is a rather high current densities for
an enzyme based electric.

We can make a complete cell with combining
these two electrodes and the way we

do that is to use a commercially
available flow through cell that has

a two glassy carbon electrodes on which
we can essential glue using carbon paint.

The Tory paper based Aleck TROs one for
the cathode and blue one for

the anode in red and
we can flow a mixture of oxygen and

glucose solution through the cell
in order to feed the cell and

then polarize it using
the glass to carbon electrodes.

And if we use this high
surface area material

on both the end of the catheter we can
take advantage of the high utilization.

So this is a just a quick pull or
there's likely to lots of data on this but

here's a colorization curve for
the cell the key numbers are that

we can generate about a half a mil
a lot per square centimeter.

Using the cell and
if you divide by the total extra area.

It's twenty one million watts
per cubic centimeters so

it's a fairly active system.

What's interesting is that glucose
oxidase is active primarily at PH seven

the lack a sense has its maximum
maximum activity at PH for

so I'm in order to balance these things
we have to operate the system at.

Intermediate ph and
we used five and a half.

So that's where that's where
the state it was taken.

Stability is an issue and
just to give you an indication of.

Of how unstable this sort of
system is this is the two hundred

fifty micron paper carbon paper.

It's essentially the twenty million

per square centimeter glucose oxidase
electrode that I showed you and it.

The decay in the current as a relative
to its initial current is plot.

As a function of time.

This is what I hope to
be exponential decay and

you will see if it's exponential.

But the time constant if and
we usually use the half

life of the electorate as a time
constant is on the order of

seventy sixty seventy hours which is
you know roughly three days or so.

So it's a it's short lived and
there's a variety of sources of

degradation the enzyme can
unfold the mediator is as

a redux active material that can that can
that can easily be chemically deactivated

running flow through the cell
leads to chemical stress and

other sorts of stresses for example
oxygen present at the glucose oxidase and

can generate proc side as I mention and
that can lead to chemical stress so

the sources of instability are myriad and
we're trying to tease this sort of data

apart to start looking at stability
as a key issue in bio fuel cells.

Some other key issues.

Gas phase activity I mentioned that we'd
like to deliver oxygen the substrate at

for cathode in the gas phase
to enhance its mass transport.

This is sort of the preliminary
data we have for

that this is current density as
a function of time for a cathode.

Operating in the presence
of gas phase air and

gas phase oxygen and
we also introduce unification

to both the air feed and the oxygen
feed to see how to what extent water and

in the gas fee can have a stabilizing
impact on the electrode.

Well the answer is it has a big
impact on the air electrode but

we really don't know why yet but
it has a negligible impact on the Oxygen

electrode the current densities
here are pretty comparable for

both oxygen and air but
the air Lector is much less stable.

It has a life time of about three hours or
so the oxygen electrode seems fairly.

Stable and if we go out to you know
longer times around ten hours it's

it's reasonably flat but
as you might expect we're.

One rarely designed a fuel cell system
to run on pure oxygen because it's not

readily available so
as a as enlightening as this might be

actually it's really challenging because
of the humidification has no impact.

It's really not a useful device and we
really want to be able to operate on dry

air and extend the stability out
to sea to at least several hours.

More challenges.

I've talked about power density
have talked about stability.

We'd also like to increase
the energy density of the system and

if one uses for
example a high molecular weight fuel

glucose is a good example
has twenty four electrons.

For a molecule of fuel.

If you oxidise it all all
the way to carbon dioxide

methanol also has multiple
of transfer molecule.

If you oxidise it all
the way to C O two and

there's a no there you can
extract six electrons per mile.

CULE but from an enzymatic standpoint
that requires multiple enzymes

steps a classic methanol
oxidase oxidizing and so

relies on alcohol the i Drive
aldehyde hydrogenation for me.

Do you have drive all of which
are require the any any D.H.

co-factors So we're working on systems
in in which we can achieve this

carbon dioxide full
oxidation with a system and

ultimately what we'd like to do is to go
after something even more complicated

which would be the glycolysis
cycle in the Krebs cycle which

doesn't achieve that complete oxidation
of glucose to carbon dioxide.

This is a subset of the curb cycle
that highlights that the relatively

few electro chemical reaction steps
the malady has drugs and a step.

Situate headrush in a step there
are several other steps that are not

electrochemical that are required in order
to go through this entire process and

ultimately we'd like to add the rest
of the loop at the glycol so

cycle at the top and
achieve complete city.

Other projects.

Well a collaboration with Scott
pad to Columbia University

in which we're looking at
complete peptide based hydrogen

mediators where we can take the well
known Lucene zipper coil coil and

use that as a way to self assemble
peptide chains into a complete hydrogen.

And if we take in osmium complex and
attach it to the histidine the images

all ring on the histidine we can take
that self assembled peptide hydrogen.

And turn it into a mediating hydrogen.

Also we can take this super and
attach it to a recumbent it

ends on this is a small Lac ace or
slack that we've used for

our initial studies and with that
we can take we can we can more or

less create a fully
biologically derived and

sign hydrogen composite
the only chemical step

is the complex ation of
the Hydrogen will with the osmium.

And we can demonstrate this of all
that the these of peptide hydrogels

have redux activity by doing
cyclic full time three.

This is the the free complex.

This is the complex attached to the to
a couple of different peptide hydrogels

we can add the enzyme and
do cycle do electric chemistry as well.

The this is the polarization is a little
different that you seem because we don't

see that that plateau but

we do get current density at low
levels at real cell potential.

So just to wrap things up.

I've showed you some of
the results of using mediated by.

Trodes for high rate electro
chemical conversions primarily for

fuel cells but also for
chemical conversions.

I've talked a little bit about the hurdles
that we've been able overcome primarily in

terms of activity and the challenges that
we still face in terms of stability and

there's multiple ways to
address these problems

through you know bio chemical protein and
generic approaches but

also in terms of the mid materials that
are used to immobilize the inside.

We we achieve more or less maximum power
using mediated systems but we introduce

complicating factors insensitivity to the
environment stability are the main ones.

Just last I like to acknowledge
my students Josh Galloway.

And you have son have all
been of all graduated.

Let's talk Morty and
how I entered my first two students at

Michigan State Jim Hall and
help as a collaborated with the.

Chemical vapor deposition experiments.

I work with Shell and
tear on the experiments and

I've also mentioned Scott banter
with the peptide and there's my fun.

So I appreciate your attention and
I'd be happy to answer any questions.

You take place where you please.

If you look for

truth will go away the whole of looking
you are more wise in comparison.

So all of the experiments that
I showed you were mobilized.

So I think I only showed one
experiment that was in solution.

So the enzyme to the picture to keep.

Right there is the picture
to keep Is this one.

So these are this is our platform.

This is the hydrogen of
the blues the hydrogen.

The the purple is the mediator
it's the redox complex that's

attached to the polymer backbone.

And the enzyme isn't trapped
within the hydrogels well.

And so this is the way we do our
experiment so the question though is

how do these things operate in
a hydrogen as opposed to free solution.

Well at the answer is that
the enzyme activity is more or

less the same the activities
that we show or hundreds of.

The turnover rates are in the hundreds
very comparable to what one sees in

solution that the transport
the fact of the matter

is that in this system these little
purple balls they don't actually move.

In the system and so if you have
a diffusion a mediator where the molecule

the complex molecule actually moves then
the diffusion coefficient that one would

measure would be again on the order of
ten the minus six to minus five and so

that that is one of the big differences
is that the electron transport

which is the fusion of all is slowed
down dramatically by the immobilisation

of the complex but
if the film is thin enough and thin for

us as a micron or less then even that
transport limitation is not overwhelming.

Yes.

Yes.

So we haven't done that copulation but
we could these are multi-well nanotube So

it's a little bit messy to to
think about to to think about for

example the the volume
fraction of that layer that's

occupied by carbon because where it.

We don't know the answer to that but
what we'd like to do it to answer your

question in this is ongoing research
we'd like to vary that density.

For example by varying the catalyst
site density in order to open up.

That nano scale structure and.

And hopefully increase the utilization
of that surface area with the enzyme So

we haven't done that
we're working on that and

that involves controlling the the Nano
to growth catalyst deposition process.

In addition to the growth of
the negatives themselves so

it's a very good question that we're
working on answering right now.

Now.

There was a moment.

That's very easily studied by just
looking at the ohmic resistance of

the of the system for example in the
absence of stub substrate we can do cycle

full time a tree and obtain the ohmic
resistance of the electrode and

generally speaking that
resistance is very low.

So that the comic to be of the nanotubes
is negligible compared to all the other

resistances in the system.

Yes absolutely.

That's a good that's a good question.

I'm in the case that I showed you which
was glucose oxidase immobilised on the.

Nanotubes I can't give you
an unequivocal answer but

I'd say almost certainly not and
the reason is that no one has

demonstrated direct electron transfer
using glucose oxidase at all.

There's no literature on that you can do
electron transfer between the Nano to end

the enzyme centrally charge and discharge
the enzyme that has been shown but getting

electron to go from the glucose to the
enzyme to the Nano to the direct elect.

Transfer is NOT been shown.

So in this case for
this data I don't think that is a problem.

However if we had used a different enzyme
like Ace is actually an enzyme where

direct electron transfer has shown to be
relatively efficient then it's certainly

possible that there might be a mixed
mode in which there's you know we have

a hydrogen that contains the enzyme that's
not attached to the carbon surface and

then it's tubes and then we have
some enzyme that is attached and

that that enzyme that is attached
would probably be more or

less successful in direct looked on
transfer and then the stuff that's out in

the middle of nowhere with just
a mediator would probably be mediated.

So in it with a different enzyme a system.

I think you could probably
have a mixed mode.

Yes.

I can comment on that
let's see where is that.

I have to go to that slide.

This

is.

So this is one study that we used to.

To quantify that.

So what we have here.

The main plot is just the decay in the
catalytic current density as a function of

time and it's more or

less exponential what we have in
the inset is a cycle of all time.

A gram.

In the absence of substrate.

So this is an indication of
the activity of the mediator self

not the enzyme and
what we see is that there's a after

roughly that the half life
of the electrode is reached.

In other words the activity total activity
then time has gone down by fifty percent.

We see roughly a twenty five
percent decrease in the activity of

the mediator itself so one way to think of
this and I think this is quantitative this

point is that whereas the overall
decay was fifty percent the meter

decay was twenty five percent so so
it roughly half of that decay can be

associated with the mediator half of
the decay so share with the enzyme.

I think that's a qualitative answer I
don't think it's a complete answer but

it gives an indication that both both
materials have a life time issues.

So the question is Is this a function
of the media that we use and

the answer is I don't have that
data we haven't done those studies.

At least I think I haven't seen
the results they might be available for

my student.

I'm not sure about that.

Good question.

But it

seemed like still there was a lot
of theory that we will be wrong.

Actually Or simply wrong.

Just now putting it in and
using that paper.

From the road.

Sure if the.

In our experience it.

The answer question it depends on
whether how you handle the electrode.

So an obvious.

And in my mind at least an obvious
approach would be to skip the Tory paper

skip the cart's chemical vapor deposition
and just buy some nano tubes from small

tubes and mix them in with the hydrogen
solution that put them on an electrode and

what we found is that by for

example cycle full time true we can
measure the surface area available surface

area of the nanotubes and under those
conditions and it's very low so

that means that what's happening is
the negatives are being compacted into

a fairly dense layer and because of that
the surface area availability is low and

that the activity of the electrode
is low so what the what growing.

We're still working on that approach
because it's a lot simpler than using

chemical vapor deposition and it.

It's much more scalable in terms
of actually making electrodes so

there's we're working on ways to deal with
that but what this does is allows us to

you know route the nanotubes on the carbon
surface and and provide structure

that will prevent agglomeration
unpacking and the Tory paper itself.

Also provides Micron scale

prosody that enhances transport
of substrates in and out.

So if we were to grow
the nanotubes completely fill.

We have a mail of do that I'd love to be
able to do that we're working on ways to

achieve complete filling of the Tory paper
with with nanotubes but the expectation

is that at some point you'll run into
substantial transport limitations that so

that there's an optimum there that
were there where we're reaching for.