MATLAB: Subscript indices Error for a Plot

indexingindices

Hi I am trying to calculate power but I cant seem to find out what is wrong (im a novice in matlab) The error is in the line for P_out.
%%An example of Matlab code
clc;clear;
format long;
%%Constants
hf_H2   = 0;
hf_O2   = 0;
hf_H2Og = -244820;
sf_H2   = 130.68;
sf_O2   = 205.14;
sf_H2Og = 188.72;
cp_H2O  = 33.7;
cp_H2   = 29.5;
cp_O2   = 329.0;
Tref    = 298;
Ru      = 8.314;
T       = 353;
n       = 2;
F       = 96487;
p_H2    = 1;
p_O2    = 0.21;
a_H2O   = 1;
po      = 1;
delT    = T-Tref;
%%Calculate Nerst Reversible Cell Potential
% Calculate Gibbs Free Energy
delH = hf_H2Og+cp_H2O*delT - (hf_H2+cp_H2*delT) - 0.5*(hf_O2+cp_O2*delT);
delS = sf_H2Og+cp_H2O*log(T/Tref) - (sf_H2+cp_H2*log(T/Tref)) - 0.5*(sf_O2+cp_O2*log(T/Tref));
delG = delH - (Tref * delS);
% Reversible voltage
Ero = -delG/(n*F);
% Nerst Expression for voltage
Eo=Ero+(Ru*T/n/F)*log((p_H2/po)*(p_O2/po)^0.5);
%%Populating matrix for calculations
V_mdl = @(a,I)(Eo-Ru*T/F*log((I+a(4))/a(1))+Ru*T/2./F*log((a(2)-(I+a(4)))/a(2))-I*a(3));
I_exp = [0,2.5,5,10,20,40,60,80,100,200,400,600,800,1000,1200];
V_exp = [0.986615,0.950774,0.951704,0.916295,0.910731,0.90338,0.857798,0.853067,0.851497,0.799984,...
    0.730907,0.689608,0.638525,0.57848,0.440924];
Irange = min(I_exp):1:max(I_exp);
%%Initial guess & Brute-force minimize error function
a(1)=0.5; %Io: Reference exchange current density mA/cm2
a(2)=1200.00; %I_lim: Limiting current density mA/cm2
a(3)=2.5e-4; %total resistance kohm*cm2
a(4)=21.0; %cross-over current density mA/cm2
g = @(a) norm(V_mdl(a,I_exp)-V_exp);
ahat = fminsearch(g,a); % Values Calculated
V_cell = @(I) V_mdl(ahat,I);
%%Plotting and figures
hold on
figure(1)
scatter(I_exp,V_exp,'ok','MarkerFaceColor','b')
plot(Irange,V_cell(Irange),'r','linewidth',2)
grid on
title('Cell Voltage vs Current Density')
xlabel('Current Density [i, mA/cm^2]')
ylabel('Voltage [\eta, V]')
legend('-- Polarization Curve')
%%Calculation of power
I_t=I_exp';
_P_out(I_exp) = (V_cell(I_exp))* I_t;_
figure(2)
title('Fuel cell power')
xlabel('Current density (A/cm^2)');
ylabel('Power(Watts)');
plot(I_exp,P_out,'*');
grid on
hold on

Best Answer

Replace:
   P_out(I_exp) = (V_cell(I_exp))* I_t;
With
   P_out = (V_cell(I_exp))* I_t;
Your I_exp are real numbers, you cannot use real numbers like indices. Only +ve integers are used to index arrays/ matrices.
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